Production And Studies Of Photocathodes For High Intensity Electron Beams

For short, high-intensity electron bunches, alkali-tellurides have proved to be a reliable photo-cathode material. Measurements of lifetimes in an RF gun of the CLIC Test Facility II at field strengths greater than 100 MV/m are presented. Before and after using them in this gun, the spectral response of the Cs-Te and Rb-Te cathodes were determined with the help of an optical parametric oscillator. The behaviour of both materials can be described by Spicer's 3-step model. Whereas during the use the threshold for photo-emission in Cs-Te was shifted to higher photon energies, that of Rb-Te did not change. Our latest investigations on the stoichiometric ratio of the components are shown. The preparation of the photo-cathodes was monitored with 320 nm wavelength light, with the aim of improving the measurement sensitivity. The latest results on the protection of Cs-Te cathode surfaces with CsBr against pollution are summarized. New investigations on high mean current production are presented.Comment: Submission to LINAC2000 conference, Paper number MOB08, 3 pages, 6 figure

OTR FROM NON-RELATIVISTIC ELECTRONS

Abstract OTR EMISSION FROM NON RELATIVISTIC ELECTRONS The CLIC Test Facility 3 (CTF3) injector will provide pulsed beams of high average current; 5A over 1.56µs at 140keV. For transverse beam sizes of the order of 1mm, as foreseen, this implies serious damage to the commonly used scintillating screens. Optical Transition Radiation from thermally resistant radiators represents a possible alternative. In this context, the backward OTR radiation emitted from an aluminium screen by a 80keV, 60nC, 4ns electron pulse has been investigated. The experimental results are in good agreement with the theoretical expectations, indicating a feeble light intensity distributed over a large solid angle. Our conclusions for the design of the CTF3 injector profile monitor are also given. We consider the transition between the vacuum and a material with a relative permittivity ε. The screen is tilted with respect to the beam trajectory ( z r ) by an angle ψ, as shown in figure 1. The OTR emission results from the contribution of the direct ( n r ), the reflected ( ' n r ) and the refracted ( ' n' r ) radiations emitted by the particle. Usin

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

CLIC: a Two-Beam Multi-TeV e±e\pm Linear Collider

The CLIC study of a high-energy (0.5 - 5 TeV), high-luminosity (1034 - 1035 cm-2 sec-1) e+e- linear collider is presented. Beam acceleration using high frequency (30 GHz) normal-conducting structures operating at high accelerating fields (150 MV/m) significantly reduces the length and, in consequence, the cost of the linac. Using parameters derived from general scaling laws for linear colliders, the beam stability is shown to be similar to lower frequency designs in spite of the strong wake-field dependency on frequency. A new cost-effective and efficient drive beam generation scheme for RF power production by the so-called "Two-Beam Acceleration" method is described. It uses a thermionic gun and a fully-loaded normal-conducting linac operating at low frequency (937 MHz) to generate and accelerate the drive beam bunches, and RF multiplication by funnelling in compressor rings to produce the desired bunch structure. Recent 30 GHz hardware developments and CLIC Test Facility (CTF) results are described