F.-J. Decker

Simulation of a 50GeV PWFA Stage

The plasma afterburner has been proposed as a possible advanced acceleration scheme for a future linear collider. In this concept, a high energy electron(or positron) drive beam from an existing linac such as the SLC will propagate in a plasma section of density about one order of magnitude lower than the peak beam density. The particle beam generates a strong plasma wave wakefield which has a phase velocity equal to the velocity of the beam and this wakefield can be used to accelerate part of the drive beam or a trailing beam. Several issues such as the efficient transfer of energy and the stable propagation of the particle beam in the plasma are critical to the afterburner concept. We investigate the nonlinear beam‐plasma interactions in such scenario using a new 3D particle‐in‐cell code called QuickPIC. Preliminary simulation results for electron acceleration, beam‐loading and hosing instability will be presented.

High-intensity single bunch instability behavior in the new SLC damping ring vacuum chamber

New low-impedance vacuum chambers were installed in the SLC damping rings for the 1994 run after finding a single bunch instability with the old chamber. Although the threshold is lower with the new vacuum chamber, the instability is less severe, and we are now routinely operating at intensities of 4.5/spl times/10/sup 10/ particles per bunch (ppb) compared to 3/spl times/10/sup 10/ ppb in 1993. The vacuum chamber upgrade is described, and measurements of the bunch length, energy spread, and frequency and time domain signatures of the instability are presented.

USING A FAST-GATED CAMERA FOR MEASUREMENTS OF TRANSVERSE BEAM DISTRIBUTIONS AND DAMPING TIMES*

With a fast‐gated camera, synchrotron light was used for studying the transverse beam distributions and damping times in the Stanford Linear Collider (SLC) damping rings. By digitizing the image in the camera signal, the turn‐by‐turn time evolution of the transverse beam distribution was monitored and analyzed. The projections of the digitized image were fit with Gaussian functions to determine the moments of the distribution. Practical applications include the determination of injection matching parameters and the transverse damping times. In this report we describe a synchrotron light monitor and present experimental data obtained in the SLC damping rings.

Commissioning of the SPPS linac bunch compressor

First results and beam measurements are presented for the recently installed linac bunch compressor chicane. The new bunch compressor produces ultra-short electron bunches for the Sub-Picosecond Photon Source (SPPS) and for test beams such as the E164 Plasma Wakefield experiment. This paper will give an overview of the first experiences with tuning and optimizing the compressor together with a description of the beam diagnostics and beam measurements. These measurements form the basis for further detailed study of emittance growth effects such as CSR and wakefields in a previously unmeasured regime of ultra-short bunch lengths.

Vibration studies of the Stanford Linear Accelerator

Vibration measurements of the linear accelerator structures in the SLC linac show a 1 micron RMS vertical motion. This motion reduces to 0.2 micron RMS motion when the cooling water to the accelerator structures is turned off. The quadrupoles have 250 nanometer RMS vertical motion with the accelerator structure cooling water on and 60 nanometer motion with it off. These results together with measurements of the correlations as a function of frequency between the motions of various components are presented.

Flat beam studies in the SLC Linac

The Stanford Linear Collider (SLC) was recently converted to flat beam operation (/spl gammaspl epsivsub x/=10 /spl gammaspl epsivsub y/), producing a factor of two increase in luminosity. In this paper we review the results of flat beam studies in the SLC Linac. In summary, the injected beams from the damping rings had invariant horizontal emittances as low as 30 mm-mrad and invariant vertical emittances as low as 2 mm-mrad. The emittances measured at the end of the linac after tuning for 3/spl times/10/sup 10/ particles are about 5 to 8 mm-mrad vertically and 40 to 50 mm-mrad horizontally. Flat beam operation began 3/17/93.<<ETX>>

Dispersion and betatron matching into the linac

In high-energy linear colliders, the low-emittance beam from a damping ring has to be preserved all the way to the linear accelerator (LINAC), in the LINAC and to the interaction point. In particular, the ring-to-LINAC (RTL) section of the SLAC Linear Collider (SLC) should provide an exact betatron and dispersion match from the damping ring to the LINAC. A beam with a nonzero dispersion shows up immediately as an increased emittance, while with a betatron mismatch the beam forms filaments in the LINAC. Experimental tests and tuning procedures have shown that the linearized beta matching algorithms are insufficient if the actual transport line has some unknown errors not included in the model. Also, adjusting quadrupole strengths steers the beam if it is offset in the quadrupole magnets. These and other effects have led to a lengthy tuning process, which in the end improves the matching, but is not optimal. Different ideas are discussed to improve this matching procedure and make it a more reliable, faster, and simpler process.<<ETX>>

Beam-based monitoring of the SLC linac optics with a diagnostic pulse

The beam optics in a linear accelerator may be changed significantly by variations in the energy and energy spread profile along the linac. In particular, diurnal temperature swings in the SLC klystron gallery perturb the phase and amplitude of the accelerating RF fields. If such changes are not correctly characterized, the resulting errors will cause phase advance differences in the beam optics. In addition RF phase errors also affect the amplitude growth of betatron oscillations. We present an automated, simple procedure to monitor the beam optics in the SLC linac routinely and non-invasively. The measured phase advance and oscillation amplitude is shown as a function of time and is compared to the nominal optics.

Effects of temperature variation on the SLC linac RF system

The RF system of the Stanford Linear Collider in California is subjected to daily temperature cycles of up to 15/spl deg/C. This can result in phase variations of 15/spl deg/ at 3 GHz over the 3 km length of the main drive line system. Subsystems show local changes of the order of 3/spl deg/ over 100 meters. When operating with flat beams and normalized emittances of 0.3*10/sup -5/ m-rad in the vertical plane, changes as small as 0.5/spl deg/ perturb the wakefield tail compensation and make continuous tuning necessary. Different approaches to stabilization of the RF phases and amplitudes are discussed.

High current, long beam pulse with SLED

A proposed, high charge, fixed target experiment (E-158) is planned to run with the highest possible energies available at the Stanford Linear Accelerator Center (SLAC), at 45 and 48 Gev. The charge is up to 6/spl middot/10/sup 11/ particles in a 370 ns long beam pulse. The SLAC Energy Development (SLED) RF system generates an increasing no-load beam energy, with a linearly decreasing slope. We show how to obtain a current variation that tracks the no-load voltage, resulting in zero energy spread. We discuss the results of a lower energy experiment that verifies the predicted charge and current at the energies required for E-158.