J. C. Sheppard

Models and simulations

On-line mathematical models have been used successfully for computer controlled operation of SPEAR and PEP. The same model control concept is being implemented for the operation of the LINAC and for the Damping Ring, which will be part of the Stanford Linear Collider (SLC). The purpose of this paper is to describe the general relationships between models, simulations and the control system for any machine at SLAC. The work we have done on the development of the empirical model for the Damping Ring will be presented as an example.

High Resolution Beam Profile Monitors in the SLC

In the SLC linac, low emittance beams with typical transverse dimensions less than 0.2 mm must be accelerated without effective emittance growth. In order to monitor this we have installed a high resolution beam profile monitor system which consists of an aluminum target covered with a fine-grained phosphor, a magnifying optical system, a television camera and video signal recording electronics. The image formed when the beam strikes the phosphor screen is viewed on a CRT monitor at the console and selected horizontal and vertical slices of the beam spot intensity are recorded. A 20 MHz transient waveform recorder is used to sample and digitize the raw video signal along the selected slice. The beam width is determined by fitting the background subtracted data to a Gaussian. Beam spots less than 6 × 3 mm can be viewed. Beam spot sizes ¿x,y < 80¿m have been measured.

Update on the High-Current Injector for the Stanford Linear Collider

The high current injector has become operational. There are two crucial areas where improvements must be made to meet collider specifications: 1. While the injector can produce up to 1011 e-in a single S-band bucket, initially much of this charge was captured in a low energy tail and was thus not suitable for transport through the accelerator and injection into the damping ring. 2. Pulse to pulse position jitter has been observed, resulting in transverse wake fields which increase beam emittance. The problems described above contribute to substantial current loss during transport from the injector (40 MeV) to the SLC damping ring (1.2 GeV). Experimental studies are continuing with the aim of understanding and improving beam characteristics including bunch length, pulse to pulse stability and emittance. The present status of these studies is reported.

RF Beam Deflection Measurements and Corrections in the SLC Linac

The requirements of RF acceleration in the SLC Linac to produce high energy beams are complicated by the presence of small transverse RF beam deflections which arise from several sources. These RF deflections place stringent tolerances on the phase and amplitude stability of the klystrons. They also force the use of special magnetic bumps to correct the trajectories of oppositely charged beams that will pass down the linac. If left unabated, RF deflections can limit the performance of the SLC. There are several methods to reduce the deflections. Many measurements of RF deflections have been made in the low energy part of the linac where the beams are most sensitive.

Real Time Bunch Length Measurements in the SLC Linac

The longitudinal charge distribution of bunches accelerated in the Stanford Linear Collider (SLC) linac will strongly affect the performance of the Collider. Bunch lengths are chosen in a balance between the deleterious effects of longitudinal and transverse wakefields. The former impacts on the beam energy spread whereas the latter is important to the transverse emittance. Two bunch length measurement ports have been installed in the SLC linac: one in the injector region and one after the emittance damping ring to linac reinjection point. These ports utilize a fused quartz Cerenkov radiator in conjunction with an electrooptic streak camera to permit real time monitoring of single s-band buckets with a resolution of several picoseconds. The design of the radiators and light collection optics is discussed with an emphasis on those issues important to high resolution. Experimental results are presented.

Emittance Calculations for the Stanford Linear Collider Injector

A computer code has been implemented for on-line acquisition and analysis of data for emittance measurements of the SLC injector beam. The beam emittances have been determined experimentally using this code; measured beam emittance values have been found to be within expectations. When the system operates in the automatic mode, an emittance measurement takes less than two minutes. This emittance measurement method has been thoroughly tested and has been extended to other regions of the SLC system. Resultant beam sigma matrices are used in the lattice design models to calculate the strengths of quadrupoles required for the optical matching of the CID beam into the SLC Linac.

Linac design for the LCLS project at SLAC

The Linac Coherent Light Source (LCLS) at SLAC is being designed to produce intense, coherent 0.15-nm X-rays. These X-rays will be produced by a single pass of a 15 GeV bunched electron beam through a long undulator. Nominally, the bunches have a charge of 1 nC, normalized transverse emittances of less than 1.5/spl pi/ mm-mr and an rms bunch length of 20 /spl mu/m. The electron beam will be produced using the last third of the SLAC 3-km linac in a manner compatible with simultaneous operation of the remainder of the linac for PEP-II. The linac design necessary to produce an electron beam with the required brightness for LCLS is discussed, and the specific linac modifications are described.

On-Line Control Models for the Stanford Linear Collider

Models for computer control of the SLAC three-kilometer linear accelerator and damping rings have been developed as part of the control system for the Stanford Linear Collider. Some of these models have been tested experimentally and implemented in the control program for routine linac operations. This paper will describe the development and implementation of these models, as well as some of the operational results.

Accelerator Physics Measurements at the Damping Ring

Besides the optics measurements described elsewhere, machine experiments were done at the SLC damping ring to determine some of its parameters. The synchrotron radiation energy loss which gives the damping rates was measured by observing the RF-voltage dependence of the synchronous phase angle. The emittance was obtained from the synchrotron light monitor, scraper measurements and by extracting the beam through a doublet and measuring its size for different quadrupole settings. Current dependent effects such as parasitic mode losses, head tail instabilities, synchrotron and betatron frequency shifts were measured to estimate the impedance. RF-cavity beam loading and its compensation were also studied and ion collection was investigated. All results agree reasonably well with expectations and indicate no limitations to the design performance.

Generation and Acceleration of High Intensity Beams in the SLC Injector

A new gun pulser and substantially increased focusing have been added to the first 100 m of the SLAC linac in order to provide a pair of intense electron bunches to the SLC damping ring. Each bunch from this injector must have 5 x 1010 electrons, an invariant emittance ¿¿ ¿ 1.8 x 10-3 m-rad and the pair must have an energy spread of less than 2% . Wakefield instabilities present in earlier versions of this injector 1 have been controlled by reducing the transverse beam dimension by a factor of 3.