DIAGNOSTICS FOR THE CTF3 PROBE BEAM LINAC CALIFES

CALIFES is the probe beam linac developed by the CEA/DAPNIA and CNRS/LAL in the frame of the CFT3 collaboration at CERN. Its objective is to "mimic" the main beam of CLIC in order to measure the performances of the 12 GHz CLIC accelerating structures. The requirements on the bunched electron beam in terms of emittance, energy spread and bunch-length are quite stringent and lead to use the most advanced techniques: laser triggered photo-injector, velocity bunching, RF pulse compression... In order to tune the machine and assess its performances before delivering the beam to the CLIC structure test stand a complete suit of diagnostics is foreseen including charge monitor, beam position and video profile monitors, deflecting cavity, RF pick-up and analysis dipole. All these diagnostics will be interfaced to the CERN control system network. A special effort has been done on the Video Profile Monitors that make use of both scintillation and OTR (Optical Transition Radiation) screens and are fitted with 2 optical magnifications to fulfill field of view and resolution performances (<20μm). Their performances can be checked via an integrated resolution pattern. SCOPE OF CTF3 CTF3 (CLIC Test Facility 3 phase) is a collaboration driven by the CERN aimed to demonstrate before 2010 the feasibility of the future multi-TeV linear collider CLIC (Compact Linear Collider). Its scope is to check the two main innovative concepts of this accelerator: the two beams acceleration scheme using 12 GHz RF power and copper structures offering a very high gradient of acceleration (100 MV/m) [1]. The RF power at 12 GHz is generated by decelerating an electron beam (150 MeV – 30 A peak current, 5 Hz repetition frequency) in PETS cavities (Power Extraction and Transfer Structure). This beam, called “drive beam”, is produced in a Linac followed by a delay loop and a combiner ring in order to “densify” the trains of bunches from 1.5 GHz to 12 GHz, and so to increase their intensity (fig. 1) This RF power feeds accelerating structures that provide a high accelerating gradient to the main beam reaching in the CLIC project the energy of 1.5 TeV after 15 km. For CTF3, an accelerator called CALIFES (Concept d’Accelerateur Lineaire pour Faisceau d’Electron Sonde) injects this beam, acting here as a probe beam, into the 12 GHz accelerating structures. Its installation in a new building is to be completed by the end of 2007. Figure 1: CTF3 Complex at CERN The TBTS (Two Beam Test Stand) will allow studying the beam quality after high gradient acceleration (emittance, energy spread...) and to test the accelerating structures in operation (breakdown rate, wake-fields...). PROBE BEAM SPECIFICATIONS The accelerator CALIFES is built, for cost saving purposes, with 3 former LEP injector linac (LIL) sections at 3 GHz. A laser driven photo-injector delivers trains of short bunches (6 ps) that are velocity compressed in the first section down to 0.75 ps and accelerated up to 200 MeV in the 2 following ones [2]. The stringent beam characteristics detailed in table 1 with their reasons, are checked by a set of diagnostics that allow the tuning of the accelerator (power and phase of the RF, laser pulse power...). Table 1: CALIFES beam parameters Parameters Motivation Energy ∼ 200 MeV Avoid beam disruption in high RF fields norm. rms emittance < 20 π mm.mrad Fit in 30 GHz structure acceptance Energy spread < ± 2% Measurement resolution Bunch charge 0.6 nC ∼ CLIC parameters Bunch spacing 0.667 ns Number of bunches 1 32 226 Measure 30 GHz structure transients rms bunchlength < 0.75 ps Acceleration with 30 GHz The diagnostic suit includes: 6 BPM (beam position monitors) to check the axis of the beam all along the accelerator and tune the steerers, a ICT (Integrated Current Transformer) to measure the bunches train charge just behind the photo-injector, 3 VPM (Video Profile Monitors) to measure the transverse profile of the beam and a RF pick-up for pulse length measurement and tuning of the velocity compression. Transverse emittance measurement is performed with scanning the gradients of the triplet of quadrupoles associated with transverse profile measurement. Energy spread measurement is performed with a spectrometer 8 m 2 m D F F D