Video Profile Monitors Development for the CTF3 Probe Beam Linac

The innovative CLIC concept is currently under study in the CLIC Test Facility (CTF3) at CERN where the acceleration of a probe beam will be demonstrated at an accelerating gradient of 100 MV/m. This probe beam, to be delivered by a linac called CALIFES, is composed of short electron bunches (0.75 ps, 0.6 nC) at 177 MeV with normalized emittance lower than 20 mm.mrad. Measurements of longitudinal charges distribution, transverse emittance and energy spectrum rely on video profile monitors (VPM) after appropriate manipulations of the beam (deflecting cavity, quad scan and analysis dipole). We report the design, development and test of these new VPM based on selectable YAG/OTR screens, optical line and CCD camera. Two selectable magnifications (1.75 and 0.33) are available via motorized lens mounts to comply both with resolution (20 μm) and field of view (10x10 mm). Study of optical line characteristics have been realised with Apilux code and Modulation Transfer Function (MTF) were measured. A grid pattern can be inserted at the screens position to check optical characteristics during operations. Tilt of the CCD plan in order to compensate the screen tilt of 15° has not proven to improve the depth of field and was not implemented on this device. However, this disposition is used on the third profiler used for energy spectrum measurement where the screen is larger and tilted by 45°. 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 future very large collider: 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 from a drive beam linac after appropriate bunch interlacing in a delay loop and a combiner ring. This RF power feeds accelerating structures that provide a high accelerating gradient to the main beam. For CTF3, an accelerator called CALIFES injects this beam, acting here as a probe beam, into the 12 GHz accelerating structures. This installation is now due to start its operation very soon (Aug. 08). PROBE BEAM SPECIFICATIONS The accelerator CALIFES is built with 3 former LEP injector linac (LIL) sections. 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 177 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 <20πmm.mrad Fit in 12 GHz structure acceptance Energy spread < ± 2% Measurement resolution Bunch charge 0.6 nC Bunch spacing 0.667 ns ∼ CLIC parameters Number of bunches 1 32 226 Measure 12 GHz structure transients rms bunchlength < 0.75 ps Acceleration with 12 GHz Most of these diagnostics are grouped in the diagnostic section (Fig. 1) just before the beam delivery to the test stand. Apart from 6 BPMs and a beam current transformer, all diagnostics (emittance, energy spectrum, charge distribution) rely on the acquisition of the transverse profile of the beam by 3 VPMs. Figure 1: Diagnostics section. Proceedings of EPAC08, Genoa, Italy TUPC024 06 Instrumentation, Controls, Feedback & Operational Aspects T03 Beam Diagnostics and Instrumentation