J. Frisch

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.

Observation of a charge limit for semiconductor photocathodes

The Stanford Linear Accelerator Center is currently operating with a photocathode electron gun (PEG) to produce polarized electrons for its experimental program. Bunch intensities of up to 1011 electrons within 2 ns (8 A) are required from the electron gun. Operation of PEG has demonstrated a charge limit phenomenon, whereby the charge that can be extracted from the gun with an intense laser beam saturates at significantly less than 1011 electrons (the expected space‐charge‐limited charge) when the photocathode quantum efficiency is low. Studies of this charge limit phenomenon observed with a GaAs photocathode are reported.

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.

SLAC's polarized electron source laser system and minimization of electron beam helicity correlations for the E-158 parity violation experiment

SLAC E-158 is an experiment designed to make the first measurement of parity violation in Moller scattering. E-158 will measure the right-left cross-section asymmetry, ALRMoller, in the elastic scattering of a 45-GeV polarized electron beam from unpolarized electrons in a liquid hydrogen target. E-158 plans to measure the expected Standard Model asymmetry of ∼10−7 to an accuracy of better than 10−8. To make this measurement, the photoemission-based polarized electron source requires an intense circularly polarized laser beam and the ability to quickly switch between right- and left-helicity polarization states with minimal right-left helicity-correlated asymmetries in the resulting beam parameters (intensity, position, angle, spot size, and energy), beamALRs. This laser beam is produced by a unique SLAC-designed flashlamp-pumped Ti:Sapphire laser and is directed through a carefully designed set of polarization optics. We analyze the transport of nearly circularly polarized light through the optical system and identify several mechanisms that generate beamALRs. We show that the dominant effects depend linearly on particular polarization phase shifts in the optical system. We present the laser system design and a discussion of the suppression and control of beamALRs. We also present results on beam performance from engineering and physics runs for E-158.

Status of the SCA-FEL☆

Abstract The SCA-FEL can now provide FEL beams to users in a wavelength range extending from longer than 3.5 to shorter than 0.5 μm. Harmonic generation can extend the short-wavelength range. As an example of the operational characteristics, during a 3 week run in May–June 1989, the SCA-FEL provide 18 days of FEL beam at 16–18 hours/day. The FEL operated at 3.5 μm, and in a 20% band centered at 1.54 μm. 30 W of optical power was extracted in 3 ms macropulses with a 0.08% line width and a 3 ps micropulse.

Cavity beam position monitor system for the Accelerator Test Facility 2

The Accelerator Test Facility 2 (ATF2) is a scaled demonstrator system for final focus beam lines of linear high energy colliders. This paper describes the high resolution cavity beam position monitor (BPM) system, which is a part of the ATF2 diagnostics. Two types of cavity BPMs are used, C-band operating at 6.423 GHz, and S-band at 2.888 GHz with an increased beam aperture. The cavities, electronics, and digital processing are described. The resolution of the C-band system with attenuators was determined to be approximately 250 nm and 1 � m for the S-band system. Without attenuation the best recorded C-band cavity resolution was 27 nm.

Proposing a laser based beam size monitor for the future linear collider

Compton scattering techniques for the measurement of the transverse beam size of particle beams at future linear colliders (FLC) are proposed. At several locations of the beam delivery system (BDS) of the FLC, beam spot sizes ranging from several hundreds to a few micrometers have to be measured. This is necessary to verify beam optics, to obtain the transverse beam emittance, and to achieve the highest possible luminosity. The large demagnification of the beam in the BDS and the high beam power puts extreme conditions on any measuring device. With conventional techniques at their operational limit in FLC scenarios, new methods for the detection of the transverse beam size have to be developed. For this laser based techniques are proposed capable of measuring high power beams with sizes in the micrometer range. In this paper general aspects and critical issues of a generic device are outlined and specific solutions proposed. Plans to install a laser wire experiment at an accelerator test facility are presented.

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.

Initial commissioning experience with the LCLS injector

The linac coherent light source (LCLS) is a SASE X- ray free-electron laser (FEL) project presently under construction at SLAC [1]. The injector section, from drive-laser and RF photocathode gun through first bunch compressor chicane, was installed in fall 2006. Initial system commissioning with an electron beam is taking place during the spring and summer of 2007. The second phase of construction, including second bunch compressor and full linac, will begin later, in the fall of 2007. We report here on experience gained during the first phase of machine commissioning, including RF photocathode gun, linac booster section, S-band and X-band RF systems, first bunch compressor, and the various beam diagnostics.