R. Alley

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.

A laser-based beam profile monitor for the SLC/SLD interaction region☆

Beam size estimates made using beam-beam deflections are used for optimization of the Stanford Linear Collider (SLC) electron-positron beam sizes. Typical beam sizes and intensities expected for 1996 operations are 2.1 × 0.6 μm (x, y) at 4.0 × 1010 particles per pulse. Conventional profile monitors, such as scanning wires, fail at charge densities well below this. The laser-based profile monitor uses a finely-focused 350-nm wavelength tripled YLF laser pulse that traverses the particle beam path about 29 cm away from the e+/e− IP. Compton scattered photons and degraded e+/e− are detected as the beam is steered across the laser pulse. The laser pulse has a transverse size of 380 nm and a Rayleigh range of about 5 μm.

The design for the LCLS RF photoinjector

Abstract We report on the design of the RF photoinjector of the Linac Coherent Light Source. The RF photoinjector is required to produce a single 150 MeV bunch of ∼1 nC and ∼100 A peak current at a repetition rate of 120 Hz with a normalized rms transverse emittance of ∼1 π mm-mrad. The design employs a 1.6-cell S-band RF gun with an optical spot size at the cathode of a radius of ∼1 mm and a pulse duration with an rms sigma of ∼3 ps. The peak RF field at the cathode is 150 MV/m with extraction 57° ahead of the RF peak. A solenoidal field near the cathode allows the compensation of the initial emittance growth by the end of the injection linac. Spatial and temporal shaping of the laser pulse striking the cathode will reduce the compensated emittance even further. Also, to minimize the contribution of the thermal emittance from the cathode surface, while at the same time optimizing the quantum efficiency, the laser wavelength for a Cu cathode should be tunable around 260 nm. Following the injection linac the geometric emittance simply damps linearly with energy growth. PARMELA simulations show that this design will produce the desired normalized emittance, which is about a factor of two lower than has been achieved to date in other systems. In addition to low emittance, we also aim for laser amplitude stability of 1% in the UV and a timing jitter in the electron beam of 0.5 ps rms, which will lead to less than 10% beam intensity fluctuation after the electron bunch is compressed in the main linac.

A High Performance Spot Size Monitor

Beam size estimates made using beam-beam deflections are used for optimization of the Stanford Linear Collider (SLC) electron-positron beam sizes. Beam size and intensity goals for 1996 were 2.1 x 0.6 μm (x,y) at 4.0x10 10 particles per pulse. Conventional profile monitors, such as scanning wires, fail at charge densities well below this. Since the beam-beam deflection does not provide single beam information, another method is needed for Interaction Region (IP) beam size optimization. The laser based profile monitor uses a finely focused 349 nm. wavelength , frequency-tripled YLF laser pulse that traverses the particle beam path about 29 cm away from the e+/e- IP. Compton scattered photons and energy degraded e+/e- are detected as the beam is steered across the laser pulse. The laser pulse has a transverse size, ( σ0, ), of 380 nm and a Rayleigh range of about 5 μm. This is adequate for present or planned SLC beams. Design and results are presented.

The high peak current polarized electron source of the Stanford Linear Collider

Abstract The Stanford Linear Collider injector requires two 2 ns pulses of 4.5–5.5 × 10 10 electrons, separated by 61 ns at 120 Hz, from its source. Since 1992, these currents have been provided by a polarized electron source based on GaAs photocathodes. A beam polarization of 76 ± 4% has been measured at the end of the 50 GeV linac. At low photocathode quantum efficiencies, and for excitation near threshold, the maximum current delivered by the source is constrained, not by the space charge limit of the gun, but by a “charge limit” of the photocathode. The charge limited current is proportional to the photocathode quantum efficiency, but the proportionality varies for different photocathode types. Experience with high polarization strained GaAs photocathodes on a test beamline and on the SLC is presented.

Performance of the SLC polarized electron source with high polarization

For the 1992 operating cycle of the SLAC Linear Collider (SLC), the polarized electron source (PES) during its maiden run successfully met the pulse intensity and overall efficiency requirements of the SLC. However, the polarization of the bulk GaAs cathode was low (/spl sim/27%) and the pulse-to-pulse stability was marginal. We have shown that adequate charge for the SLC can be extracted from a strained layer cathode having P/sub espl sim/80% even though the quantum efficiency (QE) is <1%. The recent addition of a separate chamber to the PES-which allows cathodes to be loaded into the gun after the vacuum bake and after high voltage (HV) processing without breaking vacuum-increases the reliability for achieving an adequate photoelectron yield. A new SLAC-built pulsed Ti:sapphire laser permits operation of the PES at the required wavelength with sufficient power to fully saturate the yield, and thus improve the e/sup -/ beam stability. The performance of the PES during the 1993 SLC operating cycle with these and other improvements is discussed.<<ETX>>

Operation of a Ti:sapphire laser for the SLAC polarized electron source

A new laser system has been developed as the light source for the SLAC polarized electron source for the 1993 SLD physics run. A Q-switched and cavity-dumped Ti:Sapphire laser, pumped by a doubled YAG laser is used. This laser delivers typically 50 /spl mu/J to the photocathode with the required 2 nanosecond, double pulse, 120 Hz time structure. The laser operates at wavelengths between 760 nm and 870 nm. The laser was installed on the SLAC linac in January 1993, and is currently in use.<<ETX>>

Study of nonlinear photoemission effects in III-V semiconductors

Our experience at SLAC with photoemission-based polarized electron sources has shown that the charge limit is an important phenomenon that may significantly limit the performance of a photocathode for applications requiring high intensity electron beams. In the process of developing high performance photocathodes for the ongoing and future SLC high energy physics programs, we have studied the various aspects of the charge limit phenomenon. We find that the charge limit effect arises as a result of the nonlinear response of a photocathode to high intensity light illumination. The size of the charge limit not only depends on the quantum efficiency of the cathode but also depends critically on the extraction electric field. In addition, we report the observation of charge oversaturation when the intensity of the incident light becomes too large.<<ETX>>

The SLAC polarized electron source

The SLAC PES, developed in the early 1990s for the SLC, has been in continuous use since 1992, during which time it has undergone numerous upgrades. The upgrades include improved cathodes with their matching laser systems, modified activation techniques and better diagnostics. The source itself and its performance with these upgrades will be described with special attention given to recent high-intensity long-pulse operation for the E-158 fixed-target parity-violating experiment.

Polarization studies of strained GaAs photocathodes at the SLAC Gun Test Laboratory

The SLAC Gun Test Laboratory apparatus, the first two meters of which is a replica of the SLAC injector, is used to study the production of intense, highly-polarized electron beams required for the Stanford Linear Collider and future linear colliders. The facility has been upgraded with a Mott polarimeter in order to characterize the electron polarization from photocathodes operating in a DC gun. In particular, SLAC utilizes p-type, biaxially-strained GaAs photocathodes which have produced longitudinal electron polarizations greater than 80% while yielding pulses of 5 A/cm/sup 2/ at an operating voltage of 120 kV. Among the experiments performed include studying the influences of the active layer thickness, temperature, quantum efficiency and cesiation on the polarization. The results might help to develop strained photocathodes with higher polarization.