At a major presentation in Hamburg on 23 March, the DESY laboratory formally unveiled the future with a plan for a 33 km superconducting electron-positron collider. This TESLA (TeV-Energy Superconducting Linear Accelerator) will supply 500 GeV collision energy, extendable to 800 GeV. It will also have an integrated X-ray laser laboratory.
As well as catering for particle physics, the new machine will offer research facilities for condensed matter physics, chemistry, material science and structural biology.
A high-energy electron-positron collider such as the TESLA machine is widely seen as the natural next step for particle physics after CERN's LHC proton collider, now under construction and scheduled to begin operation for physics in 2006.
In addition, the machine could drive a free electron laser (FEL) to provide intense polarized X-rays in the 0.1 nm wavelength range to take snapshots of macromolecules, chemical reactions and bulk physical processes.
An impressive amount of technical research and development work by the worldwide TESLA collaboration has provided the groundwork for the Technical Design Report. This collaboration, involving research institutes in Armenia, China, Finland, France, Germany, Italy, Poland, Russia, Switzerland and the US, has built the TESLA Test Facility at DESY, which has been in operation for several years and provides a wealth of valuable information and experience. The FEL technique has also been convincingly demonstrated at DESY (see Towards the ultimate X-ray source;July 2000).
TESLA consists of two long linear accelerator (linac) cannons - one for electrons, the other for positrons - pointing directly at each other. Each cannon will have its own source, the positrons being generated (via electron-positron pairs) from the high-energy electron beam. Damping rings will groom the beams at 15 GeV prior to final acceleration. The design foresees 2820 bunches of 2 x 1010 particles per pulse, with a bunch spacing of 337 ns and a pulse length of 950 µs. The electron linac will also drive the X-ray laser.
After passing through a 1.6 km beam delivery system and final focus, the electron and positron bunches will be squeezed down to a tiny size of 5 nm height and 550 nm width, and will collide in a central region for high-energy physics experiments. The design luminosity is 3.4 x 1034 cm2/s.
Each linac will contain about 10 000 1 m superconducting cavities to power the beam, using 1.3 GHz radiofrequency fields supplied by 2 x 292 klystrons, each generating about 10 MW. The superconducting cavities have to provide 23.4 MV/m accelerating fields, and the development of the necessary technology is a major achievement for the collaboration.
The nine-cell cavities of solid niobium will be cooled by superfluid helium at 2 K. More than 60 such cavities have been made to date, and the 23.4 MV/m required for 500 GeV are now exceeded routinely by cavities built by industry.
Using a new surface treatment (electropolishing) single-cell test cavities have reached gradients as high as 42 MV/m, thus opening the way for operating TESLA at 800 GeV, which needs gradients of 35 MV/m. Two complete accelerator modules, with eight nine-cell cavities each, are part of a 300 MeV prototype accelerator of the TESLA Test Facility, which has been operated successfully for more than 8600 hours, providing valuable system experience and the first proof of the laser principle.
The baseline design can permit collision energies of up to 650 GeV, but 800 GeV would require substantial upgrades of cooling for cryogenics and of radiofrequency power.
The superfluid helium cryogenic system for the 33 km TESLA project would be comparable in size to that being built for CERN's LHC collider, which is 27 km in circumference.
On this basis, DESY has put forward a proposal to the international scientific community, the German Federal government and the North German state governments that TESLA should be built in the Hamburg region. One possible approach to such a collaboration would be a "Global Accelerator Network" (seeAccelerators to span the globe;June 2000). The Federal German Research Ministry has asked the National Science Council to review the project.
The cost of the 500 GeV collider is estimated at Ý3 136 million, the accelerator components for the X-ray FEL Ý241, equipment for FEL experiments (5 beamlines each with 3 experiments, plus 5 beamlines with 1 experiment) Ý290, and Ý210 for the particle physics detector. Building the accelerators would require a total of 7000 man-years, over a total of eight years.
A fuller description of the TESLA proposal will be published in the next issue of CERN Courier.