The Compact Linear Collider, or CLIC, is the name of a project aimed at developing an electron-positron linear collider for very high-energy physics research (3 TeV center-of-mass energy). The machine will allow physicists to carry out experiments that are beyond the reach of the existing LHC facility. Covering a distance of 50 km, the facility will include the use of high-performance copper accelerating cavities. These will resonate at a frequency of 12 GHz and accelerate particles at a 100 MV/m gradient. As part of the experimental validation of the novel two-beam acceleration concept, called CTF3 (CLIC Test Facility 3rd phase), the CEA has built an accelerator called Califes at CERN. Califes is designed to inject a test beam into prototype accelerating structures at 12 GHz.
The Califes accelerator
Based on a photoinjector gun developed by the LAL in Orsay, and on three 3 GHz accelerating sections recovered from the LEP injector, the project required the collaboration of many teams from the Saclay site (SACM, SIS, SEDI, and the DEN for the laser part). Since December 2008, Califes has produced a beam over four periods, coming nearer to nominal performance requirements each time. Following three years of intense development work at Saclay, a new type of phase shifter has been installed at the RF entrance of the first structure. It will offer enhanced operating flexibility, with higher beam energy (up to 170 MeV) or shorter electron bunches (0.75 ps).
12 GHz accelerating structures
The inner surface of the cavities is, in some spots, subjected to electric fields of up to 200 MV/m and a 50°C rise in temperature over 200 ns pulse durations. These characteristics are near the breakdown threshold, so the main challenge is to reduce the risk of an arc occurring during operation.
Since 2008, SACM has been working with CERN on a program to develop these high-gradient structures. The program involves studying and designing cavities, manufacturing them under industrial conditions, and testing them using the Califes electron probe beam at the CTF3. One special feature of these structures is a detector, known as a wake-field monitor, or WFM, that is designed to align the cavity with the particle beam while the machine is in operation.
The chief technical achievements to be highlighted are:
· Simulations of wake fields generated in the accelerating structures by an off-axis beam and the implementation of instrumentation to detect them;
· Precision machining to obtain accelerating cells with a form tolerance of 5 µm, flatness of 1 µm, and roughness of 25 nm;
· Procurement of a solid-state modulator generating pulses with the following characteristics: 430 kV amplitude, 2.6 µs pulse width, and 50 Hz repetition frequency. Testing this modulator on a resistive load;
· Design and manufacture of overmoded X-band radiofrequency components.