Development of a Beam Loss Detection System for the CLIC Test Facility 3

The CLIC test facility 3 (CTF3) provides a 3.5A, 1.6μs electron beam pulse of 150MeV at the end of the linac. The average beam power is 4 kW. Beam losses will be monitored all along the linac in order to keep the radiation level as low as possible. The heavy beam loading of the linac can lead to time transients of beam position, size and energy along the pulse. To compensate for these transients effectively, the beam loss monitor (BLM) technology must have a time response faster than a few nanoseconds. Preliminary tests have been performed in 2003 on the already existing part of the accelerator with the aim of studying the requirements for the system to be built in the future. The experimental data are compared to the results of Geant3 simulations. Based on these results, a complete beam loss detection system is currently designed for the observation of the beam transient loss and its minimization. Development of a Beam Loss Detection System for the CLIC Test Facility 3 T. Lefèvre, M. Velasco, M. Wood Northwestern University, Evanston, USA H.H. Braun, R. Corsini, M. Gasior, F. Tecker CERN, Geneva, Switzerland Abstract. The CLIC test facility 3 (CTF3) provides a 3.5A, 1.6μs electron beam pulse of 150MeV at the end of the linac. The average beam power is 4 kW. Beam losses will be monitored all along the linac in order to keep the radiation level as low as possible. The heavy beam loading of the linac can lead to time transients of beam position, size and energy along the pulse. To compensate for these transients effectively, the beam loss monitor (BLM) technology must have a time response faster than a few nanoseconds. Preliminary tests have been performed in 2003 on the already existing part of the accelerator with the aim of studying the requirements for the system to be built in the future. The experimental data are compared to the results of Geant3 simulations. Based on these results, a complete beam loss detection system is currently designed for the observation of the beam transient loss and its minimization. The CLIC test facility 3 (CTF3) provides a 3.5A, 1.6μs electron beam pulse of 150MeV at the end of the linac. The average beam power is 4 kW. Beam losses will be monitored all along the linac in order to keep the radiation level as low as possible. The heavy beam loading of the linac can lead to time transients of beam position, size and energy along the pulse. To compensate for these transients effectively, the beam loss monitor (BLM) technology must have a time response faster than a few nanoseconds. Preliminary tests have been performed in 2003 on the already existing part of the accelerator with the aim of studying the requirements for the system to be built in the future. The experimental data are compared to the results of Geant3 simulations. Based on these results, a complete beam loss detection system is currently designed for the observation of the beam transient loss and its minimization. INTRODUCTION A new test facility, named CTF3 [1], is under construction at CERN with the aim of demonstrating the feasibility of the Compact LInear Collider (CLIC) [2], as a high luminosity, 3TeV center of mass energy e-e Collider. The CTF3 linac is composed of 18 consecutive fully loaded 3GHz cavities which will accelerate a 3.5A, 1.6μs long electron beam pulse up to a final energy of 150MeV. Housed in the LEP injector tunnel, the linac is scheduled for completion by the end of 2004. The installation has started in January 2003. The injector, including the 3GHz bunching system and two accelerating cavities, has already delivered 20MeV electrons in July 2003. Two extra cavities have been installed during the rest of the summer and commissioned toward the end of the year. Northwestern university is currently developing the beam loss detection system for the CTF3 linac. Thermal calculations have shown that losses are dangerous only if they exceed 10% of the total beam current. This means that the machine protection system can be based on wall current monitors [3]. Beam losses are measured and minimized in order to keep the radiation level and the activation as low as possible all along the linac. It is however important to remind that for the CLIC Drive beam linac beam losses would have to be measured with an accuracy which is not achievable with wall current monitors. And the development of beam loss monitor on CTF3 must be pursued with the aim of acquiring the required technology relevant for CLIC. Moreover, even on CTF3, the control of beam losses will become much more important in the future, in particular for the CLIC Experimental areas (CLEX) [4]. Quantitative beam loss measurements will be necessary to ensure the good operation of the machine. As a first step, simulations of beam losses and the corresponding e/e showers have been performed using Geant3 [5] in order to define the requirements for the BLM system to be built. In this paper we present the results of a preliminary test performed in November 2003 on the already existing part of the linac. First the layout of the beam line is described, showing the main components of the accelerator. The technical characteristics of the detector chosen for this test are then described. Finally measurements are compared to Geant3 predictions and the perspectives for the final design of the system are presented. EXPERIMENTAL SETUP ON CTF3 The machine layout is shown in Figure 1. The beam line is composed of three main parts: the injector, the magnetic chicane and a first accelerating module. A 1.6μs long high current beam is emitted from a thermionic DC gun. The electrons enter then a 3GHz bunching and accelerating section, which brings their energy up to 20MeV. The heavy beam loading in the accelerating cavities induces strong time transients and generates energy dispersion. The beam head experiences a higher accelerating field than the rest of the beam. The energy gain per accelerating cavity for the leading edge of the beam pulse can be as high as twice the nominal value (7MeV). On top of that any variation of the beam current along the pulse duration will induce a corresponding energy variation. A cleaning chicane consisting of a set of four dipole magnets and a tungsten slit-collimator is used to remove the undesired part of the beam energy spectrum. Dipole magnets create a dispersive region, where the horizontal position of the particles is correlated to their energy. By displacing the slit and varying its width, a well defined part of the beam energy spectrum can be selected. The beam is then sent to the linac modules. Every module is composed of quadrupoles, which ensure the transverse focusing, then two 3GHz accelerating structures and one monitor (BPM) measuring the beam position and current [6]. The beam misalignment is corrected using vertical and horizontal steerers installed in each module just upstream the first cavity. In November 2003, only the first module was installed.