Researchers using the first large-scale particle-in-cell simulations have found that Raman amplification of laser beams in a plasma can produce multi-petawatt peak powers – albeit in a definite parameter window – without the plasma medium being destroyed through instabilities. The control of these instabilities promises big reductions in costs and complexity for producing ultra-intense laser pulses, which in turn would allow greater access to higher intensities for use in inertial laser fusion or in laser-based particle accelerators. The process also scales to short wavelengths allowing, for example, the compression of free-electron laser pulses to durations of an attosecond.
Since the invention of the laser 50 years ago, the drive to produce increasingly intense lasers for investigating physical processes at the intensity frontier has required larger amplifying media. This progress has followed a path similar to development in accelerators as described by the Livingston curve: as one technique saturates another is discovered. At present, conventional amplifying media are based on solid-state lasers, which have already proved successful at reaching powers on the petawatt scale. Such lasers, when focused, can produce intensities of the order of 1021 W/cm2 on target, equivalent to 109 atmospheres, the pressure found inside stars. However, the intensity threshold for the breakdown of the optical components in these systems demands metre-scale beams.
A promising new technique uses a much smaller amplifying medium – millimetre-diameter plasmas, which can be 100,000 times smaller than conventional optics, making the system compact and less expensive. Raoul Trines and colleagues of the Central Laser Facility, STFC Rutherford Appleton Laboratory, St Andrews University and the Instituto Superior Técnico, Lisbon, have made the first systematic study of high-power Raman amplification. In this process a long pump pulse and a counter-propagating short probe pulse are sent into a plasma where they couple through a plasma wave and energy transfers from the pump to the probe pulse, resulting in amplification (see figure).
Using multidimensional, fully relativistic particle-in-cell simulations the team has discovered how to produce short multi-petawatt laser pulses while controlling instabilities, for example, from forward Raman scattering. They find that although increasing the pump intensity or plasma density can lead to efficiency in the amplification process, it can also increase the instabilities in both the pump and the probe. Nevertheless, they identify a parameter regime in which a 4 TW, 700 μm full-width at half-maximum (FWHM), 25-ps-long laser pulse with 800 nm wavelength can be amplified to 2 PW peak intensity with 35% efficiency, as the lower part of the figure shows.
One major application of ultra-high-power laser systems is laser-driven electron acceleration in plasma. Earlier this year, Samuel Martins and colleagues of the Instituto Superior Técnico and University of California Los Angeles, found that electrons can be accelerated to beyond 10 GeV when the driver pulse contains 300 J in 110 fs (2.8 PW peak power). Starting from a kilo-joule pump laser beam with a duration of 100 ps, such a pulse could be produced via Raman amplification in a plasma column only 15 mm long and just a few millimetres in diameter. Producing the same pulse via conventional glass optics would require gratings that are at least one metre across. This is just one of the many applications of high-power lasers that could benefit hugely from the application of the novel technique of Raman amplification.
The International Workshop on Linear Colliders (IWLC2010) recently brought together many experts involved in research and development for an electron–positron linear collider – the favoured future facility to complement the LHC. Organized by the European Committee for Future Accelerators (ECFA) and hosted by CERN, the meeting took place on 18–22 October and attracted 479 registered participants.
Two complementary technologies are currently being developed for a future linear collider: the International Linear Collider (ILC), based on superconducting RF technology in the tera-electron-volt energy range for colliding beams; and the Compact Linear Collider (CLIC), based on a novel scheme of two-beam acceleration to extend to energies of multi-tera-electron-volts. Taking advantage of a large number of synergies, the two studies are already collaborating closely on a number of technical subjects. These include: beam-delivery systems and machine-detector interfaces; physics and detectors; positron generation; beam dynamics; damping rings; civil engineering and conventional facilities; and cost and schedule.
IWLC2010 merged previously separate workshops on CLIC and ILC for the first time. Covering accelerators as well as detectors and physics, it provided an environment where researchers could exchange ideas, inform their peers about recent achievements and work together on common issues. It also took full advantage of synergies between the two studies, with common working groups to discuss the many shared challenges. The lively discussions, together with the scientific and technical presentations, showed the progress that has been made towards unifying the two study teams into a single linear-collider community.
While the opening and final plenary sessions were held at CERN to facilitate participation and information flow beyond the linear-collider community, the parallel sessions (up to a maximum of 16 simultaneously) were held in the International Conference Centre Geneva. Scheduled activities and satellite meetings started half a day before the workshop formally began and continued throughout. For example, Steinar Stapnes, CERN’s Linear Collider studies leader, presented a colloquium, “Towards a future linear collider”, as an introduction for CERN staff.
CERN’s director-general, Rolf Heuer, introduced the workshop with a presentation on “The LC roadmap” in which he referred to the active role that CERN can play towards defining global linear-collider governance and siting. A plenary session followed that reviewed the physics prospects of linear colliders, the status of the ILC and CLIC accelerators, the concepts for detectors (ILD, SiD and CLIC) and R&D activities for detectors. Another plenary session addressed the potential impact of LHC and Tevatron physics on the linear collider.
Three days were filled with parallel sessions. Some took place as separate accelerator or physics and detector sessions, but many came together in various combinations, with strong interaction between the two communities. In particular, there was a lively discussion session on the scientific imperative to vary the machine energy over a wide range and to scan over energy thresholds, while maintaining adequate luminosity. The progress of the machine designs towards this goal was reviewed and discussed. Excellent progress has been made on both studies towards their next milestones: CLIC will present its Conceptual Design Report for accelerator and detectors in 2011, while the ILC accelerator and the two validated ILC detector concepts – ILD and SiD – will publish a more advanced technical design report and a detailed baseline design, respectively, in 2012.
In particular, the ILC study has achieved its 2010 goal of demonstrating that half of the superconducting accelerating structures produced for the ILC reach the desired acceleration gradient. The successful operation of two advanced test facilities, CESR-TA at Cornell, in the US, and ATF2 at KEK, in Japan, have led to major advances across a range of subjects. The work at CESR-TA has significantly deepened understanding of electron-cloud effects, leading to several promising ways in which they could be mitigated. ATF2 continues to produce important results in many areas of beam instrumentation and beam optics.
As far as CLIC is concerned, the CTF3 test facility, which adresses the major feasibility issues of the novel CLIC technology, is near completion and is being commissioned. There has been important progress on the high-intensity drive-beam generation, which uses complex beam manipulation; on the use of this beam to produce RF power with special power-extraction and transfer structures (PETS); and on the use of this power to accelerate a probe beam, thus demonstrating the feasibility of the two-beam acceleration scheme. First measurements of the beam quality have shown current stability better than the demanding CLIC requirements. Accelerating structures that include waveguide damping features have achieved performances close to CLIC’s target in the first tests at KEK and SLAC. Measurements on a model of the mechanical quadrupole-stabilization system showed good decoupling from ground motion with a residual level consistent with the beam stability specifications.
Regarding the ILC detectors, community-wide detector R&D has led to important advances on high-precision vertex technology, highly granular calorimetry for particle flow, as well as developments in time-projection chambers based on micro-pattern gas detectors. Many of these efforts now proceed together with the CLIC study. For the CLIC detector study, recent simulations of detector performances under CLIC beam conditions have allowed the detector geometries for the CLIC_ILD and CLIC_SiD concepts to be fixed for presentation in the conceptual design report.
In all, this first joint linear-collider workshop was unanimously considered a great success, fostering mutual co-operation on both a regional and a global basis. The next joint workshop is planned to take place in Grenada on 26–30 September 2011.
The SuperB collaboration has completed a series of reports detailing the progress made since publishing the Conceptual Design Report (CDR), in consolidating the physics case and in the design, cost and schedule of the detector and accelerator. The SuperB collider consists of two rings, in which beams of electrons and positrons collide to produce tens of billions of heavy quarks and heavy leptons per year for a sensitive exploration of their decays (B O’Leary et al. 2010).
The new detector represents a substantial advance, with improved resolution, radiation hardness and background-rejection capability (Grauges et al. 2010). There have been similar advances in the accelerator design compared with the preliminary CDR (Biagini et al. 2010). It is now smaller (1250 m circumference), has a fully worked-out lattice, incorporates a polarized electron beam and includes flexibility in reaching its design goal of a luminosity of 1036 cm–2s–1.
The CMS experiment at CERN recently published first measurements of the cross-section for top-antitop pair production at a centre-of-mass energy of 7 TeV. The top quark is the heaviest known fundamental particle, with a mass of about 172 GeV, almost 185 times that of the proton. Until recently the production of top quarks was the privilege of the Tevatron at Fermilab.
Top quarks decay instantly and almost exclusively into a heavy W boson and a bottom quark (b). The b quark then “hadronizes” into a jet of particles, which can often be distinguished from a jet originating from a lighter quark or gluon by the presence of a secondary vertex in the event. The W boson can decay into either two jets or two leptons. The recent CMS analysis relies on the W bosons from both of the top quarks decaying into two leptons, i.e. either to a muon plus a neutrino, or into an electron plus a neutrino. This leads to a signature for top-antitop (tt) consisting of two b-quark jets, two charged leptons and two neutrinos. The neutrinos will pass through the detector without leaving any trace but their presence can be induced from the missing (transverse) energy in the collision.
Such a signature is very distinct and relatively free of background. The CMS collaboration performed this first analysis on a sample of data taken in the first few months of LHC operation at 7 TeV, corresponding to an integrated luminosity of around 3 pb–1. They identified 11 candidate events and calculated the number of jets per event thought to originate from bottom quarks, as shown in the figure, which also indicates the expectations for signal and background. The latter is indeed very small. The analysis of this event sample yields a top cross section of 194 ± 72 (stat.) ± 24 (syst.) ± 21 (lumi). Within the measurement uncertainties, this value for the cross-section is in good agreement with calculations in higher-order perturbative QCD.
While the top quark is an interesting object to study in itself, it will also play an important role as background in searches for new physics. Therefore an early measurement of its cross-section at this new centre-of-mass energy is an important step towards exploring the unknown.
The most precise determination yet of the mass of a heavy neutron star in a binary system has important implications on its matter content. With a mass twice that of the Sun, the millisecond-pulsar J1614-2230 rules out almost all currently proposed compositions based on hyperon or boson condensates, as well as weakly interacting quarks.
Neutron stars are the densest bodies in the universe. Their density of about 100,000 tonnes per cubic millimetre is comparable to that of atomic nuclei. They form at the heart of massive stars by the collapse of the iron core when nuclear fuel is exhausted. Their formation triggers the explosion of the outer parts of the star that is witnessed as a supernova event (CERN Courier November 2008 p11). Pulsars are neutron stars that emit regular pulses of radiation. These “lighthouses of the Milky Way” send a beam of radiation along the direction of their strong magnetic field, which rotates as the neutron star spins.
While the spin period of typical pulsars is around a second, a subclass of them exhibit pulsations on periods down to milliseconds. Such rapidly spinning neutron stars are thought to have been “spun-up” via accretion of matter from a companion star. Their rapid pulses are precise chronometers and allow astronomers to derive the orbital period of the binary star system. What makes the millisecond-pulsar J1614-2230 special is that the companion star is a white dwarf and the orbital motion of the system is seen almost perfectly edge-on from Earth. Like neutron stars, white dwarfs are remains of stellar evolution, but with a much lower density (CERN Courier October 2009 p10). Their gravitational potential well is, however, deep enough to make a significant imprint on space–time. Hence, the radio pulses from the pulsar passing by the white dwarf on their journey to the Earth will be delayed.
It is this effect of general relativity, identified by Irwin Shapiro in 1964, that was precisely measured to determine the mass of the neutron star in J1614-2230. Paul Demorest of the National Radio Astronomy Observatory (NRAO) in Virginia and colleagues from the US and the Netherlands performed a dense set of radio observations of the system in March 2010 with the NRAO Green Bank Telescope. The data were taken with a new instrument, which corrects interstellar dispersion smearing, and covered the 8.7-day orbital period of the system. These high-accuracy measurements were combined with previous long-term data to characterize the binary system in full. The “Shapiro delay” was detected with extremely high significance, a Markov-chain Monte Carlo approach being used to estimate the errors in the parameters. The derived masses for the white dwarf and the neutron star are 0.500 ± 0.006 and 1.97 ± 0.04 solar masses, respectively. This is by far the most precisely measured neutron star mass to date and allows constraints on the equation of state (EOS) of their nuclear matter.
Only a subset of all proposed EOSs are consistent with such a high mass and those are mainly the ones with standard nuclear matter. Most “exotic” hadronic models are too soft at the core of the star to sustain such a high mass. This is particularly the case for hyperons or kaon condensates. Demorest and colleagues note further that condensed quark matter at the heart of the neutron star is not ruled out, but the quarks have to be strongly interacting and are therefore not “free” quarks. This seems to indicate that neutron stars are indeed mainly composed of neutrons, although James Lattimer of the State University of New York at Stony Brook notes that depending on the radius of the star, exotic models could still be viable solutions.
The origin of this unusually heavy neutron star is another mystery. Was it so heavy when it was formed during the supernova explosion, or did it grow through efficient matter transfer from the companion star, or both?
Particle accelerators, invented during the first half of the 20th century, and lasers, invented during the latter half of the 20th century, are arguably the two most successful tools of scientific discovery ever devised. The first cyclotron, which Ernest Lawrence conceived of in 1930, and the first laser, which Theodore Maiman produced exactly 30 years later, were both palm-size devices (figure 1). Just as the cyclotron was followed by betatrons, synchrotrons and colliders, the ruby laser was followed by other solid-state, liquid, gaseous and semiconductor lasers. Since their inception, both lasers and accelerators have found applications in science, medicine and industry. Today, in contrast to their humble antecedents, the LHC at CERN and the laser of the National Ignition Facility at Lawrence Livermore National Laboratory (LLNL) are indisputably the most complex scientific instruments ever constructed for particle physics and inertial-fusion research, respectively.
The relationship between these two inventions runs deeper than this parallel history. Accelerator-based free-electron lasers (FELs) have over the past three decades been extending the capabilities of coherent light sources (Making X-rays: bright times ahead for FELs). On the flip side, intense, short-pulse lasers are being used to accelerate charged particles at a rate thousands of times greater than is possible using conventional microwave accelerators. There is little doubt that these two great inventions will continue to be the premier tools of scientific discovery in the foreseeable future.
Laser Compton scattering
Lasers began to play an important role in accelerator-based science not long after their invention. In a head-on collision between photons and electrons the Compton scattered (CS) photons are shifted up in frequency by a double Doppler shift. Within four years of the invention of the laser in 1960, a ruby laser was used to demonstrate Compton scattering of laser photons off a 6 GeV electron beam at the Cambridge Electron Accelerator (Bemporad et al. 1965). This was the first time that photons of a few electron-volts had been frequency up-shifted more than 100 million times to produce gamma-ray photons with energies of more than 100 MeV. Since then, laser CS photons at these energies have been used for nuclear physics. It was not until the late 1990s – when lasers had become sufficiently powerful – that collisions between giga-electron-volt CS photons, off relativistic electrons, and multiple laser photons were able to produce electron–positron pairs in the first demonstration of light-by-light scattering (Burke et al. 1997).
These early experiments used head-on collisions between giga-electron-volt electrons and visible laser photons to produce CS photons in the giga-electron-volt range. However, X-ray photons with an energy of a few kilo-electron-volts are needed for many scientific applications, such as to probe structural dynamics in condensed matter. In 1996, using the 50 MeV injector beam of the Advanced Light Source at the Lawrence Berkeley National Laboratory (LBNL), experimenters made a laser pulse collide with the electrons at 90° to produce the first subpicosecond pulses of X-ray photons with a wavelength of 0.04 nm (Schoenlein et al. 1996).
Compton scattering can also occur when an electron beam passes through a periodically varying magnetic field, such as in a magnetic wiggler or an undulator. Here, the static magnetic field looks like an electromagnetic wave in the frame of the relativistic electrons: the electrons Compton scatter these photons, which in classical terms is just synchrotron radiation. In an FEL, Compton scattering provides the noise photons (spontaneous emission) that are subsequently amplified by the FEL instability (stimulated emission). The subpicosecond photon-pulse facility (SPPS) at SLAC – a precursor of the Linac Coherent Light Source (LCLS) – showed that sub-100 fs X-ray pulses could be obtained via an undulator-based CS source of photons in the 10 kV range (Cornacchia et al. 2001).
It is extremely difficult for an electronic transition to provide laser action at such short wavelengths because the pumping density – required to achieve gain – scales as the frequency of the photons to the fourth power. Therefore, it is likely that above a photon energy of 1 kV, FELs are the only way to generate high-power, coherent radiation. In an FEL, the electron beam must be bunched on the scale of the photon wavelength so that the phases of the emitted photons all add coherently. This places a stringent requirement on the normalized emittance (εN) of the electron beam. However, the recent success of the LCLS has shown that accelerators are capable of producing beams of the necessary brightness (I/ε2N, where I is peak current) to produce tunable coherent photon beams in the 1–10 kV range (figure 2).
Synchrotron-radiation facilities such as the European Synchrotron Radiation Facility, France, the Advanced Photon Source, US, and Spring-8, Japan, are now being used by thousands of scientists in virtually every field. The key innovation that led to orders of magnitude increase in the brilliance of the emitted radiation was the introduction of insertion devices, i.e. magnetic wigglers and undulators. Many “pump-probe” experiments on these machines use undulator-produced X-ray photons to pump or induce change/damage in atoms/molecules, electronic and biological samples, together with a time-delayed laser pulse to probe the induced change, or vice versa. These pump-probe experiments on ultrafast phenomena using accelerator-based light sources will continue to push the boundaries of experimental science in the coming years.
How are the high-brightness electron beams needed for X-ray FELs and for future colliders for high-energy particle physics produced? Here, again, lasers have played a critical role. Until the mid-1980s, thermionic cathodes embedded in RF cavities were used to produce electron bunches. It was realized that the emittance of beams from these RF guns could be greatly improved by replacing the thermionic cathode (LaB6 for instance) by a photocathode (Cu, Mg or alkali cathode). Richard Sheffield and colleagues at Los Alamos National Laboratory carried out pioneering work on the first photo-injector gun with a Cs3Sb cathode (Sheffield et al. 1996). By illuminating the photocathode with a short laser pulse of photons with an energy just greater than the work function of the cathode material and by operating the gun at high gradients, very high currents of electron bunches with emittances less than 1 mm mrad have been produced (Akre et al. 2008). For FELs, the short duration combined with low emittance implies a beam of high brightness that can be readily bunched by the FEL instability on the wavelength scale of the emitted radiation. Almost all recent FELs, including FLASH at DESY (FLASH: the king of VUV and soft X-rays), the high-gain harmonic-generation (HGHG) FEL at Brookhaven National Laboratory and the LCLS at SLAC, use photo-injector guns as the source of electrons.
Lasers are also routinely used for alignment and diagnostics in particle accelerators. A “laser wire” is a type of beam-profile monitor used to sample nondestructively the transverse profile of intense electron or positron beams that would ordinarily destroy a thin metallic wire. The local density of the beam is sampled through collisions with a tightly focused laser pulse that has a spot size smaller than the particle beam. The relative CS yield provides a measure of the beam profile.
Future linear colliders for particle physics will bring nanometre-sized beams of electrons and positrons into head-on collision, maximizing the luminosity. Conventional techniques for measuring the size of the beam spot, such as the laser-wire scanning described above, are not suitable for measuring submicron spots. Instead, laser photon CS at 90° has been used as a noninvasive diagnostic technique for measuring ultrasmall beam sizes (Shintake 1992). Two counter-propagating laser beams produce a standing-wave interference pattern. As the focused electron bunch is scanned across this interference pattern (using a weak steering magnet) the gamma-ray yield is modulated and the depth of modulation of the CS gamma-ray flux gives the spot size. This technique has been used at the Final Focus Test Beam Facility at SLAC to measure a 47 GeV beam with a transverse size as small as 60 nm. A modified version of this technique using a propagating beat-wave interference pattern can be used as a bunch-length monitor.
Measurement of the width of highly relativistic electron bunches shorter than 100 fs poses a serious challenge. Fortunately, the transverse electric field of such bunches can itself be used to induce a change in the polarization of a synchronized laser pulse in an electro-optic crystal that is placed in the vicinity of the beam (Cavalieri et al. 2005). The duration of the photons that are affected by the polarization change can then be measured to give the pulse width of the electrons.
Accelerating with lasers
Lasers are now being used directly to produce medium-energy (100 MeV–1 GeV) electron beams (Leemans and Esarey 2009). Indeed, laser particle acceleration has grown into a distinct subfield of research since the first Laser Acceleration of Particles Workshop at Los Alamos in 1982. A short but intense laser pulse propagating through plasma can excite a wave in space-charge density, also called a wake, behind the pulse. The longitudinal electric field of this wake can be tens of giga-volts per metre, which is large enough to capture some of the plasma electrons and accelerate them (figure 3). However, the wake propagates at a phase velocity that is equal to the group velocity of the laser pulse in the plasma. Since the group velocity of a photon packet in a medium is always less than the speed of light, the accelerating electrons continuously dephase with respect to the wake. A combination of beam loading and dephasing leads to a quasi-monoenergetic beam of electrons whose energy increases as the plasma density is decreased. The transverse spread of the electrons can be a few microns and the emittance less than 1 mm mrad. Several groups are embarking on research programmes to demonstrate the coherent amplification of undulator radiation, with the eventual goal of demonstrating a tabletop, extreme-ultraviolet FEL based on a laser-wakefield accelerator (LWFA).
Although a laser-based plasma accelerator operating at the energy frontier is at this stage far into the future, the US Department of Energy (DoE) has funded the construction of a research facility called BELLA at LBNL whose goal is to demonstrate a 1 m-scale 10 GeV LWFA that can then be staged multiple times to give high energies.
An alternative approach is to use a laser pulse to produce an accelerating electromagnetic mode directly in a miniature photonic band-gap structure or a slow wave structure in a plasma medium. It is too early to say what the eventual architecture of a high-energy accelerator based on these concepts would look like but the research is fascinating in its own right.
A bright future
In the future we are likely to see even greater merging of lasers and accelerators. Laser CS has been proposed as a method for generating polarized positrons for a future e+e– collider using a high-finesse laser cavity in conjunction with an electron storage ring operating at a few giga-electron-volts. In this proposal the electron micro-bunches collide with (the circularly polarized) laser photons circulating in the cavity to produce the CS photons. These polarized multimega-electron-volt photons then collide with a target of high atomic number (Z) to produce a copious number of polarized positrons via pair production (Araki et al. 2005).
A CS-based gamma–gamma collider would be a natural second interaction region for any future e+e– collider because cross-sections for some reactions are larger for gamma–gamma collisions than for e+e– collisions (Telnov 1990). With a proper choice of laser wavelength and intensity, much of the electron energy can be converted into the gamma-ray photon and, with a net yield of about one photon per electron, the final luminosity of a gamma-gamma collider can be comparable to that of an e+e– collider (Kim and Sessler 1996). While the peak power (1 TW) and the pulse width (1 ps) required for the laser used in a gamma–gamma collider are easily obtained today, the repetition rate of such lasers is still a couple of orders of magnitude lower than in state-of-the-art lasers. There is reason for optimism, however, because diode-pumped solid-state lasers appear promising for achieving the high average powers needed.
Other possible uses of laser CS photons are for nuclear spectroscopy, where the transition energies are in the multimega-electron-volt range, as mentioned above, and for the detection of hidden fissionable materials via the observation of nuclear resonance fluorescence (NRF). If the line-width of the CS photons can be made to be less than that of nuclear transitions, then such a source could revolutionize nuclear spectroscopy much in the same way that tunable lasers have transformed atomic spectroscopy. An example of an ambitious CS source is MEGa-ray (mono-energetic gamma-ray), now under development at LLNL. It uses a state-of-the-art 250 MeV, X-band accelerator to generate an extremely bright beam of electrons at an effective repetition rate of 1 kHz, together with a high average power, picosecond laser to generate high fluxes of narrow-bandwidth mega-electron-volt photons for NRF (Gibson et al. 2010). A kilo-joule-class nanosecond laser end-station is proposed at the LCLS facility to generate matter of high energy-density that will then be probed by the highly directional X-rays from the FEL.
Laser cooling normally conjures up images of cooling atoms of low thermal energy. However, at a number of places, laser cooling has already been demonstrated on high-energy beams. For example, experiments at GSI, Darmstadt, have used laser cooling on C3+ ions at around 1.5 GeV, leading to an unprecedented momentum spread of 10–7. Laser cooling has been proposed as a method for achieving beams of ultra-low emittance for future e+e– linear colliders (Telnov 2000).
There is no doubt that lasers will play an ever increasing role in accelerators, and vice versa.
• This work was supported by the US DoE grant number DE-FG02-92ER40727. The author thanks Andy Sessler for his input to this article.
The X-ray FELs will open a new chapter in the biological and physical sciences
With these breakthrough characteristics the X-ray FEL provides a new window on dynamical processes at the atomic and molecular level. Imaging of non-crystalline matter at nanometre and subnanometre scales is also made possible by the coherence properties of the radiation. Taking an atomic-scale motion picture of a chemical process in a time of a few femtoseconds or less, and unravelling the structure and dynamics of complex molecular systems, such as proteins, are among the exciting experiments made possible by this novel source of X-rays. The LCLS – and the other hard X-ray FELs now being built at DESY in Germany and at Spring-8 in Japan – will open a new chapter in the biological and physical sciences, together with other hard X-ray FEL projects being developed in China, Korea and Switzerland.
The soft X-ray region, at wavelengths from a few nanometres to about 50 nm, has also seen a dramatic increase in performance with the equally successful operation of FLASH at DESY. Another soft X-ray FEL, Fermi@Elettra in Trieste, will be completed by the beginning of 2011 and new soft X-ray FELs are being designed at the Lawrence Berkeley National Laboratory and at SLAC, as part of LCLS II, an addition to the existing LCLS.
Another important aspect of FELs has been the successful construction and operation at the Jefferson Laboratory of an FEL with high average power, up to 9–15 kW, at wavelengths ranging from about 1.6 to 6 μm. A distinctive characteristic of this FEL, critical for high average-power operation, is the use of a recirculating superconducting linac to recover the electron energy after the beam has amplified the FEL radiation (figure 2). A similar approach to reach even higher average power is being used in an extension of the programme sponsored by the US Navy.
Soft and hard X-ray FELs represent the latest success in the development of FELs, which began a few decades ago. Many FEL oscillators have been in operation since the 1980s, at laboratories around the world, but mainly in the near- to far-infrared region. Scientists have used their unique properties of full tunability from the visible to the far-infrared and high peak power of tens to hundreds of megawatts to explore many areas of physical, chemical and biological sciences.
FEL oscillators operating in the infrared, visible or near ultraviolet part of the electromagnetic spectrum can take advantage of the existence of high-reflectivity mirrors to use an optical cavity and so operate at small gain, as John Madey first proposed (Madey 1971). In this mode of operation the undulator magnet, where the electron beam emits the radiation, can be rather short, although many electron bunches are needed to amplify the radiation in small steps up to the maximum saturation value. For an oscillator system the radiation pulse is nearly Fourier-transform and diffraction limited.
In the X-ray spectral region, research is being done to develop the mirrors and other components, in particular a very high-frequency electron gun, needed for an X-ray oscillator (Kim et al. 2008). Mirrors in general have small reflectivity and an optical cavity is complicated and, until now, impractical. The main approach – used for the LCLS, FLASH and most other new X-ray FELs – is not to use an optical cavity, but to operate in the high-gain regime, reaching saturation in a single pass of an electron bunch through a long undulator. The system can start either by amplifying the spontaneous radiation in the undulator within the bandwidth of the FEL gain (a self-amplified spontaneous emission, or SASE, FEL) or by amplifying an external laser signal (a seeded FEL) (Bonifacio et al. 1984). A SASE FEL is nearly diffraction limited but is not transform limited, except when a very short electron bunch is used to generate and amplify the radiation (Reiche et al. 2008). Seeded FELs can be nearly diffraction and transform limited.
The physics of the high-gain regime is that of a collective instability. Under the effect of an input field, external or self-generated from noise, the longitudinal distribution of the electron beam evolves from a random initial state to one in which the electrons are captured in microbunches separated by one radiation wavelength. In a SASE FEL, such as the LCLS, the collective instability leads to self-organization of the electrons in the equivalent of a 1D relativistic electron crystal propagating through the undulator at a speed near that of light. Because the crystal planes are separated by one radiation wavelength, the electrons emit in phase. As the radiation power grows exponentially, the electron longitudinal distribution changes from disorder to order. The result is that, while in the case of spontaneous radiation the total intensity is proportional to the number of electrons, Ne, in the high-gain case the total intensity is proportional to a power of Ne between 4/3 and 2. The number of electrons in a bunch is typically of the order of 109–1010, so the change in intensity can be quite large. The number of coherent photons emitted spontaneously by one electron going through an undulator is approximately given by the fine structure constant, or about 10–2. When a high-gain FEL reaches saturation the number can be as large as 103–104.
The electron beams for X-ray FELs require a new level of sophistication in their generation, handling and diagnostics. An FEL operating as an oscillator or a single-pass amplifier requires an electron beam with a large six-dimensional phase-space density, becoming larger as the wavelength decreases. However, the scaling is much more favourable than for other kinds of laser, a key reason for the success of FELs in the X-ray spectral region. In the high-gain regime, the six-dimensional density required for the electron beam scales only as the inverse of the square root of the radiation wavelength.
The electron beam for the LCLS is generated by a radio-frequency electron gun before passing down the last kilometre of the 3 km-long SLAC linac (figure 3). This beam has the highest six-dimensional phase-space density ever generated. The density is preserved through the process of beam acceleration and longitudinal compression, minimizing the effect of all of the collective instabilities that can dilute the density of a high-intensity beam. In turn, the high phase-space density is used to allow the self-organization of the beam – the FEL collective instability that leads to lasing. It is interesting to note that much of the beam physics needed to control the instabilities during acceleration and compression has evolved in the study of electron–positron linear colliders.
The main difference between the LCLS at SLAC, the European XFEL and the Spring-8 Compact SASE Source (SCSS), Japan, is in the choice of the linear accelerator to produce the electron beam – an existing S-band room-temperature linac for the LCLS, a superconducting linac for the XFEL and a C-band linac for the SCSS. While the peak intensity and power will be similar for the room-temperature and superconducting systems, a superconducting linac can produce more electrons and more X-rays per second. The wakefields in the two types of linac are also different but in both cases will produce a beam with the required characteristics.
FELs, notwithstanding their larger size, cost and complexity, offer attractive advantages over atomic/molecular lasers: complete and continuous tunability, capability of high average power and a more favourable scaling for gain at short wavelengths. These characteristics, in particular the capability of lasing in the soft and hard X-ray regions, with control of the pulse length from a few to hundreds of femtoseconds, gigawatt peak power and full tunability, are making FELs attractive to an ever wider number of scientists. On the other hand, FELs are large and expensive, justifying their use only when their characteristics are fully utilized and when atomic and molecular lasers are not available to produce radiation with similar characteristics.
With the development of high repetition-rate electron guns and by making use of continuous-wave superconducting linacs, FELs can reach repetition rates of megahertz. Using the extraordinary brightness of electron beams produced by radio-frequency or other novel types of electron sources, together with high-frequency, high–gradient room-temperature linacs, it is possible to reduce the size and cost of the accelerators driving the FELs – a particularly important factor for hard and soft X-ray devices. Ongoing research into short-period undulators, as well as the novel laser/plasma accelerators being developed in many laboratories, might lead in the future to compact, table-top FELs, at a cost and size compatible with a university-scale laboratory.
Such developments in scale hold promise not only for the FELs. The six-dimensional density of the LCLS electron beam is so large that if the beam is further compressed in the transverse direction, it generates on its surface electric fields as high as many tera–electron-volts per metre (Rosenzweig et al. 2010). Because of these properties, the electron beam itself could be used for the exploration of atomic physics or for exciting plasma wakefields exceeding 1 TV/m, thus opening the way to a table-top tera-electron-volt accelerator for use in frontier high-energy physics. While the success of short-wavelength FELs owes much to the advances in the physics of particle beams stimulated by work on electron–positron linear colliders, it is now possible to look to a future in which the development of FELs will help to continue the exploration of matter at the subatomic level.
In summary, FELs are being developed to operate in an ever larger spectral region, from hard X-ray to terahertz frequencies, generating pulses with femtosecond to attosecond duration, or pulses with extremely small line width. Longitudinal coherence can be pushed to near the Fourier-transform limit using seeding and other techniques. Such high-power, diffraction- and transform-limited X-ray pulses, with a duration that can be controlled between attoseconds and hundreds of femtoseconds, will lead to new discoveries and new knowledge in many areas of science.
The FLASH free-electron laser at DESY is the first user facility to deliver, over the past five years, short and intense light pulses in the vacuum ultraviolet (VUV) and soft X-ray region. As such, it provides the photon-science community with unprecedented opportunities to explore new territory, ranging from physics via chemistry to biology.
FLASH was initially built as a test facility for the “TESLA Linear Collider with Integrated X-Ray Free-Electron Laser Facility”. At the beginning of the 1990s, DESY initiated a vigorous R&D programme towards an e+e– linear collider. Because there were indications from the funding agencies in Germany that such a facility should be attractive not just to the particle-physics community, DESY and its international partners developed a conceptual design for a 500 GeV e+e– linear collider featuring an integrated X-ray laser facility – the TESLA project.
Based on the good experience with superconducting technology at the hadron–lepton collider HERA at DESY and the need for high luminosity at the linear collider, the challenge to realize the accelerator in superconducting RF technology was taken up by a large international effort, the TESLA collaboration. To achieve the ambitious goal of increasing the accelerating gradient while reducing the cost of the cryomodules, in each case by a factor of five, DESY built a test facility and decided to combine it with a free-electron laser (FEL) in the VUV spectral range.
Pioneering with the TTF
With excellent results achieved at the TESLA Test Facility (TTF), in March 2001 the collaboration published the technical design report for the TESLA project. In February 2003, the German government decided that the X-ray FEL part of the proposal should be realized in Hamburg as a European project, with Germany paying half of the cost. Construction of the facility, named the European XFEL, started in January 2009 and the first electron beams are expected in 2014. As for the TTF, it was expanded and converted to the soft X-ray FEL user facility FLASH, which has been serving the photon-science community at DESY since August 2005.
The FLASH experimental area. The FEL beam can be switched to five experimental stations (in the background close to the window) by moving plane mirrors. Similarly, a synchronized optical-laser beam can be transported in vacuum to each of the stations by switching plane mirrors, which are mounted in the four cylindrical vacuum vessels in the foreground. The optical laser is located in a hutch on the right, just outside the picture.
Image credit: DESY.
In its first stage of expansion, the approximately 100 m-long TTF consisted of a low-emittance laser-driven photocathode electron gun, a pre-accelerator, two superconducting accelerator modules, three sections of undulator structures (each 4.5 m long) and various electron- and photon-beam diagnostics. Lasing at saturation at a wavelength of 98 nm was achieved in September 2001, setting a world first and proving the feasibility of FELs using the principle of self-amplified spontaneous emission (SASE) in the VUV range. Subsequently, tunability between 80–120 nm was demonstrated, as well as a high degree of lateral coherence of the beam. The duration of the gigawatt radiation pulses was estimated to be between 30 and 100 fs, but a tool for the direct measurement of such ultrashort pulses was not available at the time. The peak brilliance of the FEL radiation was around 1029 photons/(s mrad2 mm2 0.1% bandwidth), i.e. about eight orders of magnitude higher than that available at the best synchrotron radiation storage rings.
Given the huge increase in brilliance from state-of-the-art third-generation synchrotron sources to FELs, it is crucial for all experiments performed at FELs to understand the interaction of the extremely intense, ultrashort X-ray pulses with matter on the atomic and molecular level. It was therefore natural to perform the first experiments at the TTF on atoms and clusters, the latter being on an intermediate scale between atoms and condensed matter.
Thomas Möller’s group studied xenon clusters using 98 nm FEL radiation at power densities up to 7 × 1013 W cm–2 (Wabnitz et al. 2002). The most striking result revealed in the single-shot time-of-flight mass spectra is the big difference between the spectra measured for xenon atoms and for large clusters. Whereas only single-charged ions are observed after irradiation of isolated atoms, atomic ions with charges up to 8+ are observed when irradiating clusters, although the photon energy of 12.7 eV is only slightly higher than xenon’s ionization potential of 12.1 eV. This effect is strongly dependent on the power density of the photon pulses. At a power density of 7 × 1013 W cm–2, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons during about 100 fs, the duration of the FEL pulse. Möller’s team suggests that the clusters are heated up very efficiently by the VUV radiation. Electrons are emitted after acquiring sufficient energy, before the clusters finally disintegrate completely by Coulomb explosion. These first results obtained at the TTF stimulated a broad discussion in the community and a wealth of further experimental and theoretical work.
The operation of the first stage of expansion of the TTF, where emphasis was on the development and test of the superconducting linac for the TESLA project, was concluded in December 2002. In the next phase it was extended to 260 m and converted to a soft X-ray FEL user facility, the commissioning of which started in autumn 2004. The first lasing at a wavelength of 32 nm – another world record – was observed in January 2005. After the official start of user operation in August 2005, the facility was renamed FLASH, for Free-Electron Laser in Hamburg.
The FLASH user facility
Compared with the first expansion stage, the most important changes were the increase in electron energy from about 240 MeV to 700 MeV by adding/replacing cryomodules, installing a second bunch-compressor, increasing the number of undulators from three to six and installing a collimator section to protect the undulator against bremsstrahlung and in case of electron-beam steering accidents. The electron bypass allowed accelerator studies without danger of damaging the undulators. Additionally, an experimental station was installed in the bypass inside the tunnel for damage and other studies using the electron beam. Later, a sixth cryomodule was added and the electron energy increased to around 1 GeV, which enabled lasing at 6.5 nm, first observed in October 2007. The photon beams are delivered to five experimental stations in the FLASH experiment hall.
In 2006/2007, the spectral range of FLASH was also extended into the far-infrared (IR) by installing a long-period undulator and a new IR beamline. The device produces intense, truly synchronized IR pulses tunable in a broad spectral range, e.g. for two-colour pump-probe experiments. The first experiments at the IR beamline aimed to determine the spatial and temporal properties of individual pulses from FLASH and to unveil their temporal substructure.
During a major upgrade from September 2009 to February 2010, key components of FLASH were replaced or improved. A third-harmonic accelerator module that is now installed creates a much more homogeneous electron distribution in the bunch. Also, the linac was fitted with a seventh cryomodule, boosting the electron energy to 1.2 GeV and the wavelength to 4.45 nm. At the end of September 2010 the electron energy was pushed to 1.25 GeV and FLASH achieved lasing at 4.12 nm, thus reaching the “water window” in the fundamental harmonic, which opens up exciting new research opportunities for biological studies in particular.
In addition, an external “seeding” experiment called sFLASH has been installed. With sFLASH, the FEL process is seeded by pulses of 38 nm wavelength and 20 fs (FWHM) in duration, which are produced via high-harmonics generation. These pulses are overlaid with the FLASH electron bunches and amplified by being passed through 10 m of undulators. The photon beam is reflected out to an experimental hutch where time-resolved, pump-probe experiments will be pursued. The goal is to run the seeded FEL parasitically to normal FLASH operation by taking only one bunch out of each bunch train.
Recently, the decision was taken within the Helmholtz Association of German Research Centres to construct an extension of FLASH by adding a second undulator beamline and a second experimental hall with up to six experimental stations. The extension will have tunable gap undulators, use various seeding schemes and in a later stage provide polarized FEL radiation.
FLASH is the world’s first FEL user facility in the VUV and soft X-ray spectral range, and many pioneering experiments in a large variety of fields have been performed to date. These include work on atoms, molecules and clusters; matter in extreme conditions, warm dense matter, radiation damage; single-shot, lensless imaging of cells and diffraction from nanoscale crystals; condensed matter spectroscopy and scattering; and photon-beam diagnostics. Today the FLASH website lists 126 publications in refereed journals.
One of the most attractive scientific drivers for X-ray FELs is the single-shot coherent imaging of nanoscale objects. Obtaining enough signal in a single-shot image requires about 1012 photons focused on a spot a few micrometres in diameter. However, such high-power densities destroy the sample and the question is whether an experiment can obtain the structural information before the target explodes. Model calculations show that to achieve this goal, the duration of the FEL pulses should be of the order of 10 fs. Henry Chapman and colleagues have performed the proof-of-principle experiment at FLASH using intense (4 × 1013 W cm–2) pulses of around 25 fs duration that contain some 1012 photons at 32 nm wavelength (Chapman et al. 2006). Figure 1 shows the experimental set-up. The graded multilayer mirror also acts as a filter for the 32 nm FEL radiation and thus improves the signal-to-noise ratio on the CCD detector. Figure 2 shows the single-shot scattering picture, together with the reconstructed image. The agreement with the transmission electron microscopy picture of the target shown in figure 1 is excellent. After transmission of the FLASH pulse, the target heats up to about 60,000 K on picosecond timescales. As a result, the scattering picture taken later with a second FLASH pulse corresponds essentially to a hole in the target.
Figure 3 shows the spectral peak brilliance calculated for FLASH, the Linac Coherent Light Source (LCLS) at SLAC (Making X-rays: bright times ahead for FELs) and the future European XFEL, in comparison with the performance of third-generation synchrotron radiation facilities. Overall, the gain with FELs amounts to approximately nine orders of magnitude. It is interesting to note that the experimental results achieved at FLASH for the fundamental harmonic at 13.7 nm, as well as for the third and fifth harmonics, nicely reproduce the earlier theoretical predictions. The peak brilliance exceeds 1030 photons/(s mrad2 mm2 0.1% bandwidth) and for an average pulse energy of 40 μJ of the fundamental at 13.7 nm, a power of 0.25 ± 0.1 μJ was measured for the third (4.6 nm) and 10 ± 4 nJ for the fifth (2.75 nm) harmonic. By going to higher-order radiation, the number of photons per pulse is reduced by two to four orders of magnitude. These weaker beams allow, for example, the spectroscopic studies of condensed matter where the full beam intensity would create space charges that mask the ground-state properties of the system under investigation. For example, in combination with a synchronized optical laser, femtosecond time-resolved, core-level photoelectron spectroscopy experiments were performed at FLASH using 118.5 ± 0.2 eV photons from the third harmonic.
FELs such as FLASH have already opened up new vistas in time-resolved science. With the advent of X-ray lasers of even shorter wavelengths with shorter pulses and higher peak powers, such as the LCLS in the US, the SPring-8 XFEL in Japan and the European XFEL in Germany, the photon-science community will finally accomplish the transition from the study of static systems to time-resolved investigations of the dynamics of physical, chemical and biological processes on atomic length and timescales.
In the 20 years since the first International Tau Workshop there has been remarkable progress, through work by experiments such as CLEO, BaBar, Belle, those at the Large Electron-Positron collider and the Tevatron, as well as several neutrino experiments, not to mention the work by theorists. The early efforts saw the acceptance of the τ as a standard lepton, a heavy copy of the electron and the muon. But the τ is massive enough to decay into hadrons as well as leptons, so its decays provide a rich laboratory for studies of a large range of physics topics. More recently, the τ has come into use as a tool to search for physics beyond the Standard Model, for example through lepton-flavour violation, charge-parity violation or the production of the τ in decays of possible new particles produced at the Tevatron and the LHC.
Tau 2010, the 11th International Workshop on Tau Lepton Physics, took place at the University of Manchester on 13–19 September. Some 80 delegates from 20 countries participated in a lively meeting that covered many topics in τ physics, including lepton-flavour violation, QCD, the muon-anomalous magnetic moment, τ neutrino physics, τ physics at the Tevatron and the LHC, as well as the outlook for the field. The workshop had a programme of more than 60 talks.
New physics signatures
The τ lepton is important as a potential way to observe the violation of lepton flavour (LFV). Apostolos Pilaftsis of Manchester stressed in his introduction to the LFV session that any observation of it would be an unambiguous signature for physics beyond the Standard Model. While the τ cannot be produced in such copious quantities as the muon, a significant advantage comes from its relatively large mass. The session contained a number of theory contributions and updates on searches for LFV from BaBar and Belle. In his summary of work by the Heavy Flavour Averaging Group, Swagato Banerjee of Victoria showed the recent enormous progress that these B-factory experiments have made in this area (figure 1). Huge improvements on the limits for LFV in τ decays to many possible final states have been made in comparison with those from the CLEO experiment, which had previously been the leader in this area. Further progress is to be expected at Belle II and at the proposed SuperB facility. For LFV in muon decays, the MEG collaboration – searching for muon decays to an electron plus a photon – reported an analysis of its first data, with tantalizing evidence for some possible candidates. Plans for the proposed Mu2e, COMET and PRISM/Prime experiments were also outlined at the workshop.
New physics might also be found in the τ sector through unexpected violations of charge-parity (CPV). This well known phenomenon in K and B physics reflects subtle differences in nature between the behaviour of matter and antimatter. Both the BaBar and Belle experiments have new, complementary results from the decay τ → πK0ν. A small amount of CPV is expected in this process from the properties of the K0 system, but neither experiment has found any excess and each has set limits on the strength of any possible CP-violating contributions.
Measurements of the decays of B→ τν were presented from both BaBar and Belle, each reporting an excess above the rate expected in the Standard Model. Such an excess could come from mediation of the decay by a virtual charged Higgs particle. However, the excess is small, and more data are needed to confirm its existence.
The mass of the τ is a fundamental parameter in the Standard Model of particle physics, and therefore important to measure in its own right. Also, a precise value for the mass is needed for testing lepton universality, by relating the lepton electroweak couplings and the muon and τ lifetimes. The most precise measurement to date, from the KEDR experiment at the VEPP-4M electron–positron collider in Novosibirsk, was reported at the workshop. The measurement comes from a threshold scan of the τ-pair cross-section using the technique of resonant depolarization to obtain a precise measurement of the beam energy. The new result for the τ mass is 1776.69 GeV with a precision of 0.013%. Plans are in place to improve this further at the BES III experiment in Beijing.
In his introductory talk for the QCD session, Antonio Pich of Valencia stressed the great value of the hadronic decays of the τ as a laboratory for studying QCD. With naive counting of possible fermionic final states, the τ would decay about 60% of the time via qqν. The hadronic decays make up about 65% of the total – the small difference from 60% arising mainly from QCD effects. It turns out that the non-perturbative contribution to the QCD corrections is small despite the low mass of the τ.
The workshop saw some lively discussion of the various approaches to the calculation of the perturbative terms, with recent developments in contour-improved perturbation theory challenging the approaches based on fixed-order perturbation theory. Despite the theoretical uncertainties, the value of the strong coupling constant, αs, obtained from the τ decay data remains the most precise experimental measurement and provides a low-energy measurement with a small uncertainty that helps to confirm the running of αs expected in QCD.
Also in the quark sector, τ decays to strange final states allow for determination of the Cabibbo-Kobayashi-Maskawa matrix element Vus. The measurement reported from BaBar, based on the ratio of the rate for τ → Kν to that for τ → πν, agrees well with results from other methods, while a more inclusive method based on the use of all strange decay modes gives a lower result. This may be a result of missing decay modes and/or problems with the underlying theory. More progress is expected.
It has been known for some years that the measured value of the muon’s anomalous magnetic moment, g-2, deviates from theory by a few standard deviations. The measurement, made by the E821 experiment at Brookhaven, remains one of the few hints for new physics. While electroweak theory and perturbative QCD allow for precise calculations of the principal contributions to the muon g-2, the non-perturbative QCD part from vacuum polarization effects (when a virtual photon fluctuates to a hadronic system) has to be based on experiment.
Andreas Hoeker of CERN introduced the session on this topic, showing how data on τ decays can be used to help with the calculations of the non-perturbative part via the use of conserved vector current to relate the isovector, spin-1 component of the τ decays to the hadronic systems produced in low-energy electron–positron annihilation. A great deal of recent experimental and theoretical progress was reported and while some discrepancies remain to be resolved, the difference between theory and experiment in the value of the muon’s anomalous magnetic moment remains at over 3σ. The workshop heard about plans for improved measurements of g-2 at both Fermilab and the Japan Proton Accelerator Research Complex.
William Marciano from Brookhaven introduced the session on neutrino oscillations, noting that there is great potential for major discoveries and surprises in the present and future neutrino experiments. There were reports from the OPERA experiment, including strong evidence for ντ appearance, searches for atmospheric ντ in SuperKamiokande, the status of the T2K experiment, searches for astrophysical ντ in the IceCube detector and latest results from the MINOS experiment.
The session on τ physics at the Tevatron and the LHC produced a particularly lively discussion. Among the highlights were limits on Higgs production from the DØ experiment at the Tevatron, a reconstructed candidate for the decay W → τν in ATLAS and a signal of some 20 events in CMS for the decay Z → ττ (figure 2). These were seen as encouraging indications of the thorough work done to develop suitable triggers and algorithms for τ selection at the hadron colliders. Clearly a rich harvest of τ-related physics is yet to come from the Tevatron and, in particular, from the LHC. The last session at the workshop pointed to exciting future potential for much new τ physics from Belle II and the proposed SuperB facility.
Michel Davier of the Laboratoire de l’Accélérateur Linéaire, Orsay, gallantly gave up a visit to Chatsworth House and the Derbyshire Peak District, as well as dinner in the Manchester Museum of Science and Industry, to prepare what was an excellent summary talk of the workshop. The series of Tau Workshops, which Davier in fact initiated in 1990, will now continue into its third decade, with Tau 2012 scheduled to take place in Nagoya, late in 2012.
The first Global Particle Physics Photowalk brought more than 200 photographers together at five particle physics laboratories: CERN in Switzerland; DESY in Germany; Fermilab in the US; KEK in Japan; and TRIUMF in Canada. They glimpsed the state of the art in particle and nuclear physics via visits to accelerators, detectors, computing centres and isotope facilities; witnessed scientists at work in control rooms; and saw test facilities for future projects.
Following the event on 7 August, which was organized by the InterAction collaboration of particle-physics laboratories, participants submitted thousands of photographs for local and global competitions. Each laboratory selected the top photographs by jury or by staff vote; the local winners will be exhibited at the laboratories in 2011. The photographs shown here were the finalists for two global competitions: a “people’s choice” online vote and a selection chosen by international jury.
8Pi experiment Photographer: Mikey Enriquez. Laboratory: TRIUMF. This image of the 8Pi nuclear-physics experiment won third place in TRIUMF’s local competition. The muted black and white image of the 8Pi experiment’s inner detectors captures the beauty and symmetry of physics.
☆ 1st International jury.
DESY wire chamber Photographer: Hans-Peter Hildebrandt. Laboratory: DESY. This portrait of a wire chamber won first place in DESY’s local competition. This highly symmetrical image of a particle detector fascinated every member of the local DESY jury immediately. The rays leading from the centre, ending in a dark rim, separating the chamber’s sectors, and large hole in the middle that allows a blurry view of the things behind, evoke the image of a large eye. The local jury called it “technically flawless and simply fascinating”.
☆ 1st People’s choice, ☆ 2nd International jury.
The accelerator operator Photographer: Tony Reynes. Laboratory: Fermilab. This image of an accelerator operator on shift in Fermilab’s Main Control Room captured third place in Fermilab’s local competition. The Main Control Room is a mission control centre where scientists monitor the laboratory’s accelerator complex 24 hours a day, seven days a week.
☆ 2nd People’s choice
Quadrupole magnets Photographer: Heiko Roemisch. Laboratory: DESY. This image of two quadrupole magnets won second place in DESY’s local competition. The global jury noted the photo’s sense of humour and the DESY jury’s association with this image was “monstrous force”.
☆ 3rd International jury.
Electric cable at CERN Photographer: Christian Stephani. Laboratory: CERN. This image, placed third in CERN’s local competition, shows an electric cable connected to a valve that is designed to avoid pressure damage in a magnet.
KEK’s Accelerator Test Facility Photographer: Yuki Hayashi. Laboratory: KEK. This photograph of researchers working through the weekend in the Accelerator Test Facility won first place in KEK’s local jury and web competition.
Paperclips atop the world’s largest cyclotron Photographer: Ali Lambert. Laboratory: TRIUMF. This image won first place in TRIUMF’s local competition. Above the world’s largest cyclotron at TRIUMF, paperclips experience some fringe magnetic field and stand upright, appearing to dance on the table’s surface. High-school student Ali Lambert artfully captured this iconic experience of all visitors to TRIUMF.
Broken Symmetry Photographer: Ken Duszynski. Laboratory: Fermilab. This photograph of the Broken Symmetry sculpture at Fermilab’s main entrance won first place in the laboratory’s local competition. The arch straddles the road and appears perfectly symmetric when viewed directly from below, but has carefully calculated asymmetry from its other views.
Test beamline for CERN’s Linac4 project Photographer: Diego Giol. Laboratory: CERN. This won first place in CERN’s local competition. Linac4, when completed, will be CERN’s newest linear accelerator and the first link in the proton acceleration chain for the LHC.
HERA accelerator tunnel Photographer: Matthias Teschke. Laboratory: DESY. This classic image of HERA’s accelerator tunnel captured third place in DESY’s local competition. The photographer manages to guide the view around the corner and make the viewer curious about what’s behind the bend. The image plays with light and shadow, conveys a sense of space, almost infinity, while at the same time incorporating technicality.
☆ 3rd People’s choice.
Roof of the Meson Laboratory Photographer: Charles Peterson. Laboratory: Fermilab. This view of the roof inside the Meson Laboratory, one of the buildings in Fermilab’s fixed target experimental area, won second place in Fermilab’s local competition. Each scalloped section of the roof was intentionally built to be approximately the same size as the tunnel inside the Tevatron.
TRIUMF’s material-science facility Photographer: Mikey Enriquez. Laboratory: TRIUMF. This photograph of TRIUMF’s material-science facility won second place in TRIUMF’s local competition. The seemingly industrial and technical landscape of the facility is softened here by a digitally applied texturing technique.
Connection pipe for LHC magnet Photographer: Diego Giol. Laboratory: CERN. This photograph of a connection pipe from a spare quadrupole magnet for the LHC at CERN won second place in the laboratory’s local competition.
KEK collage Photographer: Akira Ominato. Laboratory: KEK. This collage of the KEK particle physics laboratory in Tsukuba, Japan, won second place in KEK’s local competition.
Belle Detector Photographer: Keisuke Mori. Laboratory: KEK. This photograph of the Belle Detector won second place in KEK’s local jury and web competition.
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