New theories suggest that, at very high energies - such as existed shortly after the Big Bang - all four forces merge to become one single force. The discovery of supersymmetry and the measurements of the properties of supersymmetric particles could provide a glimpse of the underlying fundamental theory.
Also, astronomical evidence suggests that more than 90% of the mass in the universe is invisible and of a type that is totally different from ordinary matter. The nature of this "dark matter" is completely unknown, but supersymmetric particles might provide an explanation. If supersymmetry is indeed realized in nature, these particles will surely be found and investigated in detail at TESLA.
While most of the particle physics programme will be using TESLA as an electron-positron collider, the facility can also be operated to generate photon-photon, photon-electron and electron-electron collisions. The electron beam of TESLA could also be used for other studies in particle and nuclear physics, such as the analysis of the inner structure of the nucleon and the properties of the strong force.
X-ray free electron laser
The X-ray free electron laser laboratory proposed as part of the TESLA project is conceived as a multi-user facility following the experience of existing large synchrotron radiation facilities like HASYLAB at DESY and ESRF in Grenoble. The X-ray laser hall will comprise 20 experimental stations, which could be increased to 30. It is located near the collider interaction point on the TESLA research campus in the middle of the facility. The TESLA X-ray lasers will make use of the electron beam accelerated in the first part of the linear collider, which will then generate intense beams of X-ray laser radiation via the self-amplified spontaneous emission (SASE) process inside a series of long undulators (see Towards the ultimate X-ray source;July 2000).
X-rays play a crucial role when the structural and electronic properties of matter are to be studied on an atomic scale – particularly when looking at atoms in molecules, in large biomolecular complexes and in condensed matter. They are one of the most important tools in basic science and medical diagnostics, as well as in industrial R&D. The TESLA free electron lasers (FEL) will open up a whole range of new possibilities for X-ray research: they will provide lateral, fully coherent polarized X-rays of wavelengths between 1 and 0.1 nm, and with peak brilliances more than a 100 million times as high as are available today from the best synchrotron radiation sources.
In addition, the X-rays will be delivered in flashes with a
duration of 100 fs or less, allowing the observation of fast chemical processes with atomic-scale
spatial resolution. Scientists will be able to address challenging questions such as: Can we take
pictures of single macromolecules? Can we see the dynamical behaviour of the electrons as they
form chemical bonds? Can we make a movie of a chemical reaction or of fast switching in
magnetic storage devices? Can we make real-time studies of the formation of condensed matter?
Can we follow, for instance, a viral infection in a cell at high resolution?
In condensed matter physics, traditional techniques such as neutron scattering or spectroscopy taking place at today's synchrotron light sources, face their limits of applicability for many questions related to the study of novel materials, especially when trying to understand ultrafast processes on a nanometre scale. The X-rays from the TESLA FEL probe the dynamic state of matter and can thus be used to study non-equilibrium states and very fast transitions between the different states of matter. These non-equilibrium states are of eminent importance for the tailoring of material properties in nanoscale devices.
Superconducting technology
The TESLA collider is composed of two linear accelerators pointing towards one another, one for electrons - which will be used in parallel to drive the X-ray FEL - and one for positrons. The TESLA approach differs from other linear collider concepts in its choice of superconducting accelerating cavities as its basic technology. Both the Next Linear Collider and the Japan Linear Collider are based on normal-conducting copper cavities, whereas TESLA uses a total of 21 024 niobium cavities operating at -271 °C, which are fed with pulsed radiofrequency (RF) electromagnetic fields of 1.3 GHz to accelerate the particles.
Superconducting technology provides important advantages for a linear collider. As the power dissipation in the cavity walls is extremely small, the power transfer efficiency from the RF source to the particles is very high, thus keeping the electrical power consumption within acceptable limits (around 100 MW), even for a high average beam power. The high beam power is the first essential requirement for obtaining a high rate of electron-positron collisions, the second being extremely small electron and positron beams at the interaction point.
The low RF frequency of the TESLA linear accelerators is ideally suited to conserving the ultrasmall size of the beams during acceleration, since the interfering wakefields generated by the particle beams are much weaker in the larger cavities of accelerators working at low RF frequencies than in smaller cavities operating at higher frequencies. For the same reasons the superconducting linear accelerator of TESLA is also extremely well suited to driving the X-ray FEL, which also requires an electron beam with large average power, high bunch charge, small energy spread and small beam size.
The benefits of superconducting cavities have been known since the beginning of linear collider research and development. However, the accelerating fields achieved in the early 1990s were too low and the projected costs based on the then existing superconducting installations too high for a collider facility. The main challenge for TESLA was therefore a reduction in the cost per unit accelerating voltage by a factor of 20.
Building on experience
Building on existing experience with superconducting cavities from CERN, CEBAF (Jefferson), Cornell, DESY, KEK, Saclay and Wuppertal, the TESLA collaboration met the challenge: by continued improvements of the base material (niobium), the cavity treatment, and the welding/ assembly procedures, accelerating gradients exceeding 25 MV/m have been reliably achieved. Recently, further progress in cavity performance has been obtained by applying a new surface treatment - electropolishing - to the niobium surface. One-cell test cavities have reached gradients of as high as 42 MV/m, thus paving the way for the operation of TESLA at 800 GeV, which requires gradients of 35 MV/m.
Through numerous design optimizations the costs per unit length of the superconducting structures and the cryostats were reduced by a factor of four for a large-scale production. These achievements - including the successful operation of the TESLA test facility with its test accelerator and FEL for more than 8600 h - now provide the basis for a realistic superconducting linear collider, with all of its advantages.
In preparation for the German position concerning the approval of TESLA, the German Research Ministry has asked the German Science Council (Wissenschaftsrat), which advises the German government in matters of science, to review TESLA together with other large-scale projects.
In parallel, a number of international reviews are taking place on a European and worldwide scale, addressing the long-term road map of particle physics, and the scientific potential and the technologies of electron-positron colliders and X-ray lasers. The German Federal Government, the Senate of the City of Hamburg and the Federal State Government of Schleswig-Holstein will then have to come to a decision on the project. Construction is expected to take around eight years, so the TESLA facility could be in operation by 2011.