Photo-cathodes for the CERN CLIC Test Facility

Since 1993 the CLIC Test Facility (CTF) has used laser-illuminated Tellurium Alkali photo-cathodes as intense electron sources (up to 50 nC in 10 ps), for the Drive Beam of a two-beam accelerator. These cathodes have been produced and tested in our photo­emission laboratory and transported under vacuum to the CTF. They are placed in a 3 GHz RF gun with a 100 MV/m electric field. This RF gun produces a train of 48 pulses, each of 13.4 nC charge and 10 ps length. The CTF Probe Beam has used air­transportable cesium iodide + germanium photo­cathodes in another RF gun, which produces a single pulse of the same duration but with only 1 nC charge. The optical damage threshold in the laser is the main limitation of energy available on the photo­cathode. From an operational point of view, the photo­cathode lifetime is defined to be the time during which the cathode is able to produce the nominal charge with the nominal laser energy. After having recalled the main characteristics of the photo-cathodes tested, this note describes in more detail the performa nce obtained in operation. The possibility of photo-cathode production at the RF gun in a simplified evaporation chamber will also be discussed

The photo-injector option for CLIC: past experiments and future developments

The Compact Linear Collider (CLIC) drive beam requires a long bunch train (92 us) consisting of 42880 bunches with a bunch charge of 17.5 nC in a bunch length of less than 20 ps. This train might be produced by an RF-photo-injector equipped with a Cs-Te cathode. After a short review of experience with such cathodes in the present CLIC Test Facility (CTF2), mainly focused on the production of short trains of high-charge bunches, we will present the scheme foreseen for CLIC and CTF3. The laser will be a diode-pumped MOPA (Master Oscillator Power Amplifier), operating at a repetition rate of 469 MHz for CLIC and 1.5 GHz for CTF3. The specific requirements of an RF-gun for this high-current operation are discussed. New experimental results on the photocathode lifetime at high average current are summarized

Demonstration of two-beam acceleration in CTF II

The second phase of the Compact LInear Collider (CLIC) Test Facility (CTF II) at CERN has demon-strated the feasibility of two-beam acceleration at 30 GHz using a high-charge drive beam, running paral lel to the main beam, as the RF power source. To date accelerating gradients of 59 MV/m at 30 GHz have been achieved. In CTF II, the two beams are generated by 3 GHz RF photo-injectors and are acceler ated in 3 GHz linacs, before injection into the 30 GHz modules. The drive beam linac has to accelerate a 16 ns long train of 48 bunches, each with a nominal charge of 13.4 nC. To cope with the very su bstantial beam-loading special accelerating structures are used (running slightly off the bunch repetition frequency). A magnetic chicane compresses the bunches to less than 5 ps fwhm, this is needed for efficient 30 GHz power generation. The 30 GHz modules are fully-engineered representative sections of CLIC, they include a 30 GHz decelerator for the drive beam, a 30 GHz accelerator for the main beam, high resolution BPM's and a wire-based active align-ment system. The performance achieved so far, as well as the operational experience with the first accelerator of this type, are reported

The PHIN photoinjector for the CTF3 Drive beam

A new photoinjector for the CTF3 drive beam has been designed and is now being constructed by a collaboration among LAL, CCLRC and CERN within PHIN, the second Joint Research Activity of CARE. The photoinjector will provide a train of 2332 pulses at 1.5 GHz with a complex timing structure (sub-trains of 212 pulses spaced from one another by 333 ps or 999 ps) to allow the frequency multiplication scheme, which is one of the features of CLIC, to be tested in CTF3. Each pulse of 2.33 nC will be emitted by a Cs2Te photocathode deposited by a co-evaporation process to allow high quantum efficiency in operation (>3% for a minimum of 40 h). The 3 GHz, 2 1/2 cell RF gun has a 2 port coupler to minimize emittance growth due to asymmetric fields, racetrack profile of the irises and two solenoids to keep the emittance at the output below 20 p.mm.mrad. The laser has to survive very high average powers both within the pulse train (15 kW) and overall (200 W before pulse slicing). Challenging targets are also for amplitude stability (<0.25% rms) and time jitter from pulse to pulse (<1ps rms). An offline test in a dedicated line is foreseen at CERN in 2007

Results from the CLIC Test Facility

In order to study the principle of the Compact Linear Collider (CLIC) based on the Two Beam Acceleration (TBA) scheme at high frequency, a CLIC Test Facility (CTF) has been set-up at CERN. After four years of successful running, the experimental programme is now fully completed and all its objectives reached, particularly the generation of a high intensity drive beam with short bunches by a photo-injector, the production of 30 GHz RF power and the acceleration of a probe beam by 30 GHz structures. A summary of the CTF results and their impact on linear collider design is given. This covers 30 GHz high power testing, study of intense, short single bunches; as well as RF-Gun, photocathode and beam diagnostic developments. A second phase of the test facility (CTF2) is presently being installed to demonstrate the feasibility of the TBA scheme by constructing a fully engineered, 10 m long, test section very similar to the CLIC drive and main linacs, producing up to 480 MW of peak RF power at 30 GHz and accelerating the beam up to 320 MeV. The present status of CTF2 is reported