The Lebt Chopper for the Spiral2 Project*

The Spiral2 driver uses a slow chopper situated in the common section of the low energy beam transport line to change the beam intensity, to cut off the beam in case of critical loss and to avoid hitting the wheel structure of rotating targets. The device has to work up to 10 kV, 1 kHz repetition frequency rate and its design is based on standard power circuits, standard vacuum feed-through and custom alarm board. The paper summarizes the design principles and describes the test results of the final device, which has been installed on the beam line test bench. THE CHOPPER IN THE LEBT LINE The low energy beam transport (LEBT) line carries continuous wave (CW), high intensity beams of protons (5mA), deuterons (5mA) and ions (1mA) with m/q=3 from the sources to the radiofrequency quadruple. The source voltage for the three different particles are 20, 40 and 60 kV respectively, to match the RFQ input energy of 20keV/A. The layout of the injector LEBT is shown in Fig. 1. The slow chopper [1] is placed just before the beam stop in the common section of the line. Figure 1: The injector low energy lines and the slow chopper position. The proton/deuteron line section and the common one are presently installed at CEA Saclay to be tested before being installed in the SPIRAL2 building. The slow chopper system was moved to Saclay in June 2011 and is going to be tested with the beam in the next few months. The device assembled on the line is shown in Fig. 2. The yellow arrow, in frame A, shows the beam direction. The beam stop and the micro channel plate are shown in frame B and the deflecting electrodes in frame C. REQUIREMENTS AND DESCRIPTION The chopper will be used to progressively increase the beam power during accelerator tuning, to avoid hitting the wheel spokes of rotating targets and to rapidly remove the beam in case of failure detection by the machine protection system (MPS). The tuning of the high power (200 kW) beam requires low repetition rates but a very large duty cycle range: from 10 (0.01%) to CW. Rapid transition times are required to avoid losing the beam not being perfectly deviated on the beam stop, and for fast response to beam stop commands from the MPS. Figure 2: The slow chopper installed in the beam line The applied voltage depends on the ion energy, on the geometry of the plates and on the beam-stop distance. The beam transversal section is quite large at the electrode position (76 mm), the equivalent hard-edge electrode length is of 160 mm and a total voltage of 17 kV has to be applied to deflect the beam onto the beam-stop. Electrode voltage of 10 kV with amplitude stability around 1-2% and rise/fall time less than 100 ns are requested. Geometry of the electrodes The geometry shown in Fig. 3 was chosen to obtain a flat transversal field. Field-maps were introduced in the beam dynamics simulation codes and two cases were simulated: a) one plate positively biased and one grounded and b) both plates biased with opposite voltages. No significant differences were observed in beam behaviour and the first geometry was implemented, requiring fewer electronics and vacuum components and then considered more reliable. Figure 3: plate dimensions, the bending angle is 20° ___________________________________________ *Work supported by EU commission 7 framework project n. 212692. [email protected] New affiliation: Associazione Euratom/ENEA sulla Fusione, CP 65-00044 Frascati, Rome, Italy Proceedings of IPAC2011, San Sebastián, Spain TUPS082 07 Accelerator Technology T30 Subsystems, Technology and Components, Other 1731 C op yr ig ht c ○ 20 11 by IP A C ’1 1/ E PS -A G — cc C re at iv e C om m on sA tt ri bu tio n 3. 0 (C C B Y 3. 0)