Slc Beam Dynamics Issues

The Stanford Linear Collider (SLC)1*2 accelerates ingle bunches of electrons and positrons to 47 GeV per beam and collides them with small beam sizes and at high currents. The beam emittances and intensities required for present operation have significantly extended traditional beam quality limits. The electron source produces over lo1 1 ein each of two bunches. The damping rings provide coupled invariant emittances of 1.8 x 10m5 r-m at 4.5 x lOlo particles. The 50 GeV linac has successfully accelerated over 3 x lOlo particles with design invariant emittances of 3 x 10s5 r-m. The collider arcs are now sufficiently decoupled and matched in betatron space, so that the final focus can be chromatically corrected, routinely producing spot sizes (ax, cry) of 2.5 pm at the interaction point. Spot sizes below 2 pm have been made during tests. Instrumentation and feedback systems are well advanced, providing continuous beam monitoring and considerable pulse-by-pulse control. The luminosity reliability is about 60%. Overviews of the recent accelerator physics achievements used to obtain these parameters and the present limiting phenomena are described for each accelerator subsystem. 1.0 Introduction The present operating parameters of the SLC during colliding beams for high energy physics are shown in Table 1. The best parameters in each category obtained during tests are also shown (not taken under simultaneous conditions). The goals for 1992 with the SLD physics detector are shown to indicate which parameters are under active improvement3. 2.0 Electron source The recent main task for the electron injector has been to develop a method to transport high charge, multiple electron bunches from the gun and buncher to the damping ring while maintaining reasonable mittances, good energy spreads, and equal energies4. The stability of the injector has been significantly improved through reproducible accelerator parameter configurations developed to maintain high current levels. These configurations often remain appropriate for weeks. The emittance of the high charge electron beam at the entrance to the damping ring has often been too large, causing particle loss and enhanced emittance on extraction. This enlarged emittance has been traced to transverse wakelields in the early accelerating structure in the injector (30 to 200 MeV). An addition of a betatron oscillation to the beam at the appropriate phase over the later two thirds of the injector (80 m) produces additional wakefield effects which cancel the earlier wakefield effect$. More information on these cancelling oscillations is given in Section 5. With these corrections 5.6 x lOlo ein one bunch and 4.7 x lOlo ein each of two bunches (4 x lOlo is routine) have been extracted from the ering. Loading and transverse wakefields in the subharmonic buncher and s-band structure make the two bunches have different trajectories and RF phases and, thus, impact their respective intensities. The positron system, after extensive tuning, achieved a yield of 1.25, meaning the ratio of the number of positrons captured in the damping ring to the number of incident 30 GeV electrons on the conversion target6. The main gains came from aligning the injector linac (90 m), beta matching the po@ons into the linac, and fixing aperture restrictions just upstream of the damping ring. A positron charge of 3.8 x lOlo in a single bunch has been injected into the main linac.