J. E. Clendenin

The Stanford linear accelerator polarized electron source

The Stanford 3-km linear accelerator at SLAC has operated exclusively since early 1992 using a polarized electron beam for its high-energy physics programs. The polarized electron source now consists of a diode-type gun with a strained-lattice GaAs photocathode DC biased at high voltage and excited with circularly polarized photons generated by a pulsed, Ti:sapphire laser system. The electron polarization at the source is > 80%. To date the source has met all the beam requirements of the SLC and fixed target programs with < 5% downtime.

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.

Prospects for generating polarized electron beams for a linear collider using an rf gun

Abstract The next generation of linear colliders — represented by the Japanese Linear Collider (JLC) and the Next Linear Collider (NLC) — will probably utilize polarized electrons generated by a photocathode gun. A photocathode gun with high polarization ( P e ) photocathodes (up to P e ∼ 80% achieved to date) is currently providing polarized electrons for the SLC. The SLC source requires subharmonic bunching at low energy to reduce the bunch length prior to S-band bunching and a damping ring at high energy to reduce the transverse emittance. The use of an rf gun can eliminate the former and possibly simplify the latter. However, rf guns as presently developed have serious problems with vacuum contamination, which would quickly lower the quantum efficiency (QE) of a semiconductor photocathode. In addition, the “charge limit” previously reported for high peak current pulses puts a limit on the laser power usable for photoexciting a low QE cathode near the bandgap threshold. These problems have so far precluded any serious attempt to design an rf gun for polarized electrons. Several technical advances that now improve the prospects for a practical polarized electron rf gun are described. Finally, new ideas for high polarization photocathodes that permit operation in a relatively poor vacuum and techniques being explored to mitigate the low QE “charge limit” are discussed.

High-yield positron systems for linear colliders

It is noted that the SLC (Standford Linear Collider), having electron energies up to 50 GeV, presents the possibility of generating positron bunches with a useful charge even exceeding that of the initial electron bunch. The exact positron yield to be obtained depends on the particular capture, transport, and damping system used. Using 31-GeV electrons impinging on a W-type converter target, and with adequate matching of the positron beam phase-space at the target to the acceptance of the capture RF section, the SLC source is capable of producing, for every electron, up to two positrons within the acceptance of the positron damping ring. The design of this source and the performance of the positron system as built are described. Also, future prospects and limitations for high-yield positron systems are discussed.<<ETX>>

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.

Single Bunch Beam Measurements for the Proposed SLAC Linear Collider

Single S-band bunches of ~ 109 electrons have been used to study the characteristics of the SLAC linac in anticipation of its operation as a linear collider. Emittance measurements have been made, the longitudinal charge distribution within single bunches has been determined and transverse emittance growth has been produced by deliberately missteering the beam. New equipment is being installed and checked out, and the sensitivity of new traveling-wave beam position monitors has been measured.

A Solid State High Power Amplifier for Driving the SLC Injector Klystron

The SLC injector klystron RF drive is now provided by a recently developed solid-state amplifier. The high gain of the amplifier permits the use of a fast low-power electronic phase shifter. Thus the SLC computer control system can be used to shift the phase of the high-power RF rapidly during the fill time of the injector accelerator section. These rapid phase shifts are used to introduce a phase-energy relationship in the accelerated electron pulse in conjunction with the operation of the injector bunch compressor. The amplifier, the method of controlling the RF phase, and the operational characteristics of the system are described.

A Multi-Channel Pulser for the SLC Thermionic Electron Source

A new pulser developed for the SLC thermionic gun has been operational since September 1984. It consists of two planar triode amplifiers with a common output triode driving the gun cathode to produce two independent pulses of up to 9 A with a 3 nsec FWHM pulse width. Three long-pulse amplifiers are also connected to the cathode to produce pulses with widths controllable between 100 nsec and 1.6 ¿sec. Each amplifier has independent timing and amplitude control through a fiber optic link to the high voltage plane of the gun cathode-grid structure. The pulser and its operating characteristics are described.

Polarized RF guns

RF guns employing photocathodes are now well established as viable electron sources for accelerator applications. For high-energy accelerators, the desirable properties of such sources include the relative ease of pulse formation (using the source laser system to establish the pulse shape and number) and a low transverse beam emittance, eliminating the need for rf chopping and bunching systems and reducing the demands on emittance-reducing damping rings. However, most high-energy accelerators now require polarized electrons. Polarized electron beams can in principle be generated by substituting a III–V semiconductor, such as GaAs, for the traditional photocathode in a conventional rf gun structure. In the past, the principal criterion for selection of photocathodes for rf guns has been the ability to maintain a reasonable quantum yield from the cathode over a relatively long operating period. It is well known that GaAs is significantly more sensitive to the vacuum environment than any of the cathode types...