Linac as Used in the SLC Collider * ( A )

The linac of the SLAC Linear Collider (SLC) must accelerate three high intensity bunches on each linac pulse from 1.2 GeV to 50 GeV with minimal increase of the small transverse emittance [l]. The procedures and adjustments used to obtain this goal are outlined. Some of the accelerator ... -parameters and components which interact are the beam energy, transverse position , component alignment, RF manipulation, feedback systems, quadrupole lattice, BNS damping, energy spectra, phase space matching, collimation, instrumentation, and modelling. The method to bring these interdependent parameters collectively into specification has evolved over several years. This review is ordered in the sequence which is used to turn on the linac from a cold start and produce acceptable beams for the final focus and collisions, Approximate time estimates for the various activities are given. Initial Checks and Tune-up Before any of the beams can be accelerated, there are -. a. host of mechanical, electrical, and control conditions which must be satisfied. They are listed here for completeness b.ut without much detail. The personnel protection system (PPS) circuits must be tested, peoTpL; cleared from the tunnel, and the tunnel locked. _ machine protection system (MPS) which . protects accelerator components from high power beams must be checked and activated. The vacuum pumps must be on and the gate va]ves open. _. -The water cooling systems for the magnets, klystrons, accelerator structures, and RF waveguides must be operating at the proper temperatures and the flow indicators checked. The RF high voltage, modulators, klystrons and subboosters are turned on and adjusted to bring them within tolerances. The modulator DQing circuits and the SLED cavities must be tuned. The computer system [2] must be functional including the micro computers (about 35) and associated CAMAC crates, the communication links, and the VAX mainframe. The magnet, timing, and klystron controls are exercised. The dipole and quadrupole magnets are calibrated, magnetically standardized, set to the proper values, and tr immed to within their respective tolerances. The profile monitors (about 20) are checked including target in/out, illumination, and iris control. The electronic modules for the beam position monitors (about 290) are calibrated. Finally, the toroids for monitoring beam intensity aze checked, * Work supported by Department of Energy contracts DE-AC03-81ER40050, DE-AC03-76F00515,DE-AC03-76SF00098, and DE-AC03-76SFOOOlO. Establish RF Conditions The goal for setting the RF parameters is to specify the energy and energy spectrum for both the electron and positron beams at the end of the linac. The bunch length compressors [3] in the ring-to-linac transport lines are checked to have a setting of about 33 MeV where maximum compression occurs. Modest beams of electrons and positrons ( < 1 X IO**10 per pulse ) are established to the energy measurement region at the end of the linac where the beams are separated horizontally to enter their respective Arcs. A momentum dispersion of about 70 m m is produced in that region. First, the feedback loop on the overall phase of the two mile long main drive line is turned on [4]. Next, the phase of each subbooster (which drives eight klystrons) is adjusted to maximize the energy given to the electron beam. The phase measurements involve changing the phase over about 90 degrees and fitting the energy change with a sine wave. The best phase settings ar.: recorded. Next, the optimum phases of the individual klystrons are determined in the same way using 180 degree shifts and rec6rded [5]. Phasing the ent’., linac requires about 32 hours. The energy spectrum of the electron beam is measured on a synchrotron radiation Xray monitor [6] or on a profile monitor in the dispersive region at the end of the linac. The spectrum is set to below 0.3 % using the RF phase. of the electron damping ring coupled with the RF phase of the electron bunch length compressor in the ring-to-linac _ transport line. This variable adjusts the time of arrival of the electron bunch in the linac. At low beam intensities the phase of maximum energy and the phase of min imum energy spectrum are almost the same but at high currents the phases can be up to ten degrees different [7]. The energy spectrum of the electrons which are extracted at Sector 19 to go to the positron production target is checked to be small. This spectrum can be set without affecting the spectrum at the end of the linac by separating the subbooster phases upstream and downstream of the extraction line. The electron overall phase can be set in twenty minutes. The number of klystrons accelerating electrons is adjusted to get the energy close to the desired value of 47.0 GeV. The remaining klystrons are maintained hot in standby mode. The slow energy feedback is turned on which fixes the energy to about 50 MeV once per minute. Several extra klystrons are added to the beam to produce an overhead of about I GeV for the feedback loop. The slow feedback adjusts the phase of two subboosters in opposite directions to change the overall RF amplitude and not phase. The feedback is established in a few minutes. The spectrum of the positron beam is set to below 0.3 % at 47 GeV using the positron damping ring (south) phase ramp which adjusts the ring RF phase and the positron bunch length compressor phase. The positron energy is adjusted by moving the RF timing of all the klystrons (about 500 timing numbers) so that the electron and positron bunches move along the SLED energy gain curve. The electron energy changes with this procedure as well as that of the positrons but the energy Presented at the IEEE Particle Accelerator Conference, Chicago, IL, March 20-23, 1989