EPAC’06 showcases the latest in accelerators

EPAC’06: une vitrine pour les accélérateurs

La belle ville historique d’Edimbourg, en Écosse, a constitué le cadre de la dixième conférence européenne sur les accélérateurs de particules (EPAC’06), qui s’est tenue en juin et a rassemblé plus de 1000 participants venant de 33 pays. Le programme scientifique couvrait tous les aspects des accélérateurs et des technologies annexes, depuis les très hautes énergies du Grand collisionneur de hadrons et du projet de Collisionneur linéaire international jusqu’aux moindres détails de la commande des faisceaux et des contraintes techniques de l’hadrothérapie. Cette manifestation a montré en particulier combien l’avenir de la physique des hautes énergies est prometteur, grâce aux physiciens qui proposent une multitude d’idées innovantes, lesquelles sont à l’étude dans des collaborations internationales.

The 10th European Particle Accelerator Conference, EPAC’06, took place in Edinburgh on 26–29 June. Attended by more than 1000 participants from 33 countries on six continents, it offered a wide scientific programme, covering all aspects of accelerators and their technology. In particular, the meeting showed that the future of high-energy physics looks bright, thanks to a community that is generating innovative ideas, which are being studied in worldwide collaborations. All-in-all, EPAC’06 was a bumper edition, and this article reports only briefly on the highlights.

The packed programme included some special sessions, during which the three European Physical Society Accelerator Prizes for 2006 were awarded. These went to Axel Winter of DESY and Hamburg University, to Lutz Lilje of DESY, and to Vladimir Teplyakov of IHEP, Protvino, who was unable to attend. The conference also featured a talk by Roger Penrose from Oxford, with the intriguing title “Big Bang: An Outrageous New Perspective, and its Implications for Particle Physics”, and Stefano Chiocchio from Garching, a leading member of the ITER fusion project team, gave an inspiring closing presentation on “ITER and International Scientific Collaboration”.

The high-energy frontier

The imminent operation of the Large Hadron Collider (LHC) at CERN was one of the major issues at the conference and was covered during the sessions devoted to circular colliders. Speakers described the challenges of construction, installation, the first test beams and the future upgrades, which are already foreseen. A good proportion of the talks and posters about the LHC were the work of an enlarged world community, reflecting the trend of increased co-operation among the particle-physics laboratories.

Existing colliders, the Tevatron at Fermilab and HERA at DESY, will pass the baton for the high-energy frontier to the LHC. Talks described the recent successful runs at the Tevatron and the technical challenges during operation of HERA, in both cases emphasizing possible lessons for the LHC.

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven is currently in a very active phase. In six years of operation, there have been collisions between various ion species; now a long future is envisaged, with runs with enhanced luminosity, new ion species and e-RHIC, which will become HERA heir with a programme of electron–proton collisions. The high-luminosity frontier at colliders featured in presentations on the success of the programmes at the B and φ factories, for which there are new and innovative ideas for increasing luminosity.

With the LHC on the horizon, the accelerator community is already preparing the next steps to push the high-energy frontier even further in work on linear colliders, lepton acceleration and new acceleration techniques. Innovative ideas and novel techniques are being developed, which require technologies beyond the state of the art, and ambitious and challenging R&D programmes are being pursued often by fruitful worldwide collaborations.

The importance of these developments was highlighted in the opening presentation at EPAC’06, by Barry Barish, director of the Global Design Effort (GDE) for an International Linear Collider (ILC). He emphasized the worldwide consensus that has emerged for an electron–positron linear collider as the favoured next high-energy physics facility, to complement the LHC and to study the properties of the particles that the LHC should discover. Barish presented the technological challenges of the ILC based on superconducting accelerating cavities, and described the GDE by a worldwide collaboration to optimize the design, do the R&D and prepare the technology transfer to industry. The aim is to have a 500 GeV linear collider ready to build from 2010, possibly extendable to 1 TeV.

Ambitious test facilities are planned in the US and Japan to develop superconducting radio-frequency (RF) technology beyond the state of the art as developed by the DESY-based TESLA collaboration. The Compact Linear Collider (CLIC) study is pushing linear-collider technology even further into the collision energy range of multi-tera-electron-volts by using a novel scheme of two-beam acceleration at high frequency. The ambitious CLIC Test Facility is being developed at CERN to demonstrate the feasibility of the concept in a multi-lateral collaboration.

The key to high luminosity in linear colliders is very small beam dimensions (a few nanometres) at the collision point. This requires beams that are cooled to extremely small emittances in damping rings and very strong focusing in beam delivery systems. Large collaborations are developing these techniques and setting up test facilities.

More exotic technologies could extend the high-energy frontier even further in the future. The conference heard about the concept of acceleration using plasma wake-fields induced by lasers able to produce accelerating fields of several tera-electron-volts a metre. A recent demonstration showed quasi-mono-energetic beam acceleration, thus opening the door to multi-giga-electron-volt applications in a variety of domains by taking advantage of progress in lasers. The meeting also reported impressive post-acceleration in plasma excited by beam.

Another avenue for continuing the quest for higher energies could involve muons, which have all the advantages of electrons without the intrinsic limitations owing to synchrotron radiation. They could be ideal for future high-energy research if successful cooling can be achieved. A Muon Ionisation Cooling Experiment is being built at the Rutherford Appleton Laboratory (RAL) in the UK to demonstrate the feasibility of the novel technique of ionization cooling, with a first beam expected in October 2007. Research with neutrinos, which are produced naturally in the decay of muons, could also benefit from this technique. The conference heard of various methods being studied for a possible neutrino factory. They range from using muon decay in storage rings to the innovative techniques of “beta beams”, involving the decay of unstable ions produced by a high-power proton beam hitting a target or ionization in a dedicated ring.

Light sources and hadron rings

Covering storage rings, linacs and energy-recovery linac-based sources, the presentations on synchrotron light sources and free-electron lasers (FELs) began with a flash rather than a bang. FLASH, the FEL facility at DESY, was reported to be delivering 13 nm and has achieved a peak brilliance that exceeds all other sources by orders of magnitude. There were several reports of R&D programmes in the US, the UK and Japan into injector and superconducting RF systems that would meet the challenges of future advanced energy-recovery sources such as the fourth-generation light source (4GLS) proposed for the UK.

Turning to new third-generation storage rings, the old adage that you wait for one and then three come along together has proved true. The Australian Light Source has just announced first beam in the storage ring, and the SOLEIL and Diamond projects in France and the UK, respectively, have both seen beam commissioning activities this year. Talks on the two European projects illustrated the power of modern digitally based diagnostic systems in beam commissioning and, coincidentally, reported delays caused by cooling water – not a glamorous system but a vital component of any large accelerator.

The growing role for conventional laser systems within single-pass FEL facilities was reported in a talk that reviewed a diversity of applications. One such application, highlighted separately, was synchronization, a critical aspect in achieving and exploiting short-pulse sources. Other highlights included the successful lasing of the SPring-8 Compact SASE Source test accelerator in Japan, a report on injector systems for FELs, and a review of single-pass FELs, which reported that these devices are now relatively mature drivers of user facilities and also discussed the challenges of extending this technique to shorter wavelengths.

The session on hadron accelerators featured several commissioning reports, as well as new projects and developments. The Spallation Neutron Source (SNS) at Oak Ridge has recently gone through its initial commissioning stage with the first high-energy beam pulses on the target producing spallation neutrons. Norbert Holtkamp, the SNS director of the Accelerator Systems, who is soon to take up a position as principal deputy director-general of ITER, presented some of the highlights from the commissioning.

The very large Facility for Antiproton and Ion Research being prepared at GSI will provide antiprotons and ions of all charge states with intensities that are orders of magnitude higher than are available today. Presentations on other high-intensity machines and their operation included status reports on the accelerators of the Japan Proton Accelerator Research Complex, the upgrade to 1.8 MW of the proton facility at PSI, and the upgrades to the ISIS pulsed neutron source at RAL.

More novel topics included the ideas of an Energy Recovery Linac to be used as a high-energy electron cooler for RHIC; the use of crystals and channelling of beams in accelerator extraction, deflection and collimation; and the ideas and first tests with a circular RF quadrupole. At CERN, the Low Energy Antiproton Ring – a cooler storage ring – has recently been rejuvenated as the Low Energy Ion Ring for use as a lead-ion cooler and accumulator ring for the LHC.

Beam dynamics and control

As at previous conferences, beam dynamics and electromagnetic fields received the largest number of contributions. Almost 300 papers were presented in this category, underlining the extremely high level of activity and continuous interest that the accelerator community has in this area. Eleven talks and the poster sessions covered a broad spectrum, ranging from beam optics and single-particle dynamics, through collective effects and instabilities, to developments in computer code and simulation studies.

Speakers reviewed the well developed art of electromagnetic field computation, stressing the benefits of an interdisciplinary approach involving computer science and applied mathematics along with accelerator physics. One impressive example concerned a complex 3D model of a complete superconducting accelerator module of the type that was developed by the TESLA collaboration at DESY. Another overview focused on modelling the effects of space charge and coherent synchrotron radiation in bunch compressor systems – a topic of the highest relevance for FEL projects operating with high peak-current electron bunches. The electron cloud instability, a potential performance limitation in both positron and proton storage rings, retains high activity in both experimental and simulation studies. A great deal of effort has gone into refining computer modelling of the effect, for example, to include aspects of surface science and the magnetic fields of quadrupole and wiggler magnets.

Further presentations concerned space-charge driven resonances and halo formation in high-intensity hadron beams; improvement of collimation systems by non-linear optics insertions; single-particle dynamics near the half-integer resonance in the KEKB facility in Japan and in the presence of betatron coupling in RHIC; suppression of longitudinal coupled bunch instabilities by phase modulation; local bunch shortening by strong RF focusing in the DAFNE machine at Frascati; and analysis of a fast beam-ion instability occurring in a small gap undulator at the Pohang Light Source.

The session on beam instrumentation and feedback featured numerous contributions on R&D for the ILC and FEL facilities. For the ILC these included new developments in cavity and re-entrant-cavity beam-position monitors with sub-micrometre resolution, high-resolution beam-size monitoring systems based on laser wires and Fresnel zone plates, and fast beam-based feedback. Higher-order-mode (HOM) signals induced in superconducting cavities have been used to measure the position of the cavity centres, the beam phase relative to the phase of the accelerating frequency, the beam position, and in a HOM-based feedback to minimize the HOM power in a module.

Talks related to FELs presented new developments and challenges in measuring ultrashort longitudinal bunch profiles, including the intrabunch structure, together with recent experimental data. New applications with ultrafast laser diagnostics, which achieve resolutions approaching 10 fs, were also described, as well as a bunch arrival-time monitor system that has yielded a precision of about 30 fs and could be applied to measure the beam position with an error of only about 3 µm. The future developments discussed included, for example, the combination of ultrafast lasers and light emitted in FELs.

Other highlights in this session included recent measurements of the transverse profiles of the counter-rotating beams at the Tevatron, which have been achieved using ionization profile monitors with new fast electronics. There was also a report on progress on developments for a test of the CPT theorem, whereby the depolarization frequency of two electron bunches was measured with record accuracy.

Technology and applications

The session on accelerator technology covered three aspects at the forefront of developments in different areas of accelerators: RF systems for linear and circular machines, insertion devices for synchrotron radiation facilities, and gantry designs for the latest accelerators for proton and carbon cancer therapy. In addition, there was a report on the unique experience of building a 27 km cryogenic installation at CERN for the LHC.

Superconducting cavities are now the preferred choice for RF for new accelerators, with various designs and applications. There are different options for producing the RF power to feed the cavities: klystrons, inductive output tubes and solid-state amplifiers all have their place, depending on the final power that is needed. To control and regulate these systems digital electronics is the choice everywhere.

Synchrotron-radiation facilities depend on the quality of the synchrotron light that they produce; better quality comes from installing insertion devices. The conference introduced the latest developments in this area: superconducting and in-vacuum undulators and wigglers, cryocooled magnetic structures, and so on.

As usual, a good proportion of the posters and talks on the applications of accelerators was devoted to medicine, in particular protons and light-ion therapy. Hadron accelerators have been used experimentally for cancer treatment for more than a decade, and new projects are now being built to provide general and regular treatment. One of the most demanding aspects in terms of fulfilling the strict safety regulations is the stability and precision of the gantry that delivers the particle beam. There are now designs for gantries weighing 650 tonnes, with a precision of 0.5 mm.

While more than a dozen hospital-based facilities for proton therapy are in operation or under construction around the world, Japan has the only two dedicated centres for cancer therapy with light ions, namely, carbon ions. However, two new centres are being built in Europe, and these will offer beams of both protons and ions. One is located in Heidelberg and the other at the Centro Nazionale Adroterapia Oncologica in Italy. Several others are in advanced stages of planning, and in Japan, the construction of a third carbon-therapy facility has started at Gunma University.

This session also heard about fixed-field alternating-gradient accelerators and their potential use, for example, in accelerator-driven systems (subcritical nuclear reactors), muon acceleration or the production of neutrons for boron neutron capture therapy. In the latter case, neutrons would be produced by 10 MeV protons in an internal beryllium target, and the energy loss and emittance increase of the stored beam caused by scattering in the target would be counteracted by a process similar to the one that is used in ionization cooling.

• The next conference will take place in Genoa, Italy, in June 2008.

Further reading

The proceedings, published less than a month after the conference, are available open-access from the Joint Accelerator Conferences Website (JACoW) at www.jacow.org.