E. Ihloff

Transmission of Megawatt Relativistic Electron Beams through Millimeter Apertures

High-power, relativistic electron beams from energy-recovering linacs have great potential to realize new experimental paradigms for pioneering innovation in fundamental and applied research. A major design consideration for this new generation of experimental capabilities is the understanding of the halo associated with these bright, intense beams. In this Letter, we report on measurements performed using the 100 MeV, 430 kW cw electron beam from the energy-recovering linac at the Jefferson Laboratorys Free Electron Laser facility as it traversed a set of small apertures in a 127 mm long aluminum block. Thermal measurements of the block together with neutron measurements near the beam-target interaction point yielded a consistent understanding of the beam losses. These were determined to be 3 ppm through a 2 mm diameter aperture and were maintained during a 7 h continuous run.

Transmission Polarimetry at MIT Bates

The polarization dependence of Compton scattering in magnetized iron can be used to determine the polarization of an incident photon beam. This can in turn be related to the polarization of the electron beam which radiated the photons. It is difficult to calculate the analyzing power of these devices absolutely, however they are of great utility for rapid, relative measurements of electron beam polarization. These devices have been used at Bates as relative electron polarization monitors at 20 and 200 MeV. Efforts are now being made to use the device at 850 MeV as an online measure of the beam polarization in the South Hall Ring. A technique to calibrate these devices and build an affordable, absolute polarimeter is also being explored.

MIT‐Bates Polarized Source

During the fall of 2001, the polarized source at MIT delivered over 140 Coulombs of high quality polarized beams to the SAMPLE‐III 125 MeV parity violating experiment. Prior to the experiment, the source was reconfigured to deliver highly polarized beam using the new high power diode array laser system with large aperture beam optics, and a strained layer GaAsP photocathode from St. Petersburg. The results of these tests will be presented. The production run for SAMPLE‐III was then carried out with a bulk GaAs and the Ar‐Ti:Sapphire laser system. Since April of this year, the source has been delivering high polarization beams to the South Hall Ring for commissioning of the BLAST spectrometer. The stored current in the ring exceeds 100 mA. This is accomplished using the high power diode laser system and high‐gradient‐doped strained GaAsP photocathodes from Bandwidth Semiconductor Inc. tuned for 810 nm. The operational lifetime of the photocathode is excellent. A status report of the Bates polarized source ...

New results from the MIT-Bates polarized source and the test beam setup

During the summers of 1998 and 1999, the polarized source at MIT delivered over 300 Coulombs of high quality polarized pulsed beam to the SAMPLE parity violation experiment. This success in beam delivery signified achievements of long lifetime for the Bates polarized source in both high average and high peak current regimes. The injection requirements of high polarization beams for the South Hall Ring will be discussed. They are relatively modest for the storage mode but very demanding in peak current and repetition rate for the extraction mode. Because of these requirements, a 60 keV test beam setup with a Mott polarimeter was constructed at Bates and will be described here. This setup is independent of the main accelerator and has provided a platform for R&D in photocathodes and high power lasers. New results will be presented on high peak current photoemission from strained GaAsP samples using fiber coupled diode array laser systems on this test setup. A brief report will be presented on the photoemiss...

Polarized electrons at MIT-Bates

A description of the MIT-Bates polarized electron source will be presented. Improvements to the polarized injector in recent years, including implementation of a multiple gun system, have made delivery of pulsed polarized beams for medium energy experiments routine and trouble-free. The spin orientation from the source is controlled by a Wien filter which has the added benefit of sweeping backstreaming positive ions away from the photocathode surface. The present challenge for the Bates polarized source is an unprecedented set of requirements on the stability and quality of the beam for the SAMPLE parity violation experiment now in progress, including limits on the source lifetime and helicity correlated beam position differences. The current status of the source lifetime is discussed. Recent results in laser beam position differences are presented as is a description of the diagnostics implemented on the laser transport line for fast and accurate measurements of these differences. Finally, we will discuss the plans for using a mode-locked laser system to achieve the higher laser power necessary for the South Hall Ring.

Polarized electrons at Bates: Source to storage ring

The MIT Bates 1 GeV electron scattering facility has recently completed a demanding set of parity violating experiments on the proton and the deuteron. These experiments required a polarized electron beam of unprecedented quality and high average currents. The facility has invested heavily in infrastructure for producing polarized beams and measuring beam polarization. This infrastructure includes a test beam set-up, a transmission polarimeter at 20 MeV, a laser back-scattering Compton polarimeter and a Siberian Snake. The polarized source group is also actively pursuing laser systems and photocathodes that could deliver high polarization, high peak current and moderate average current beams to meet the needs of the physics programs at Bates in the coming years.

Electron scattering with polarized beams at MIT-Bates accelerator center

A brief introduction to electron scattering with polarized beams as a precise tool for studying electromagnetic structure of nucleons and nuclei will be presented. The status of a parity violating experiment SAMPLE [1] at Bates using a polarized beam to probe the strange quark content of proton will be reported. The polarized source at Bates as used for the SAMPLE experiment is described.