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Title: Development of a high average current polarized electron source with long cathode operational lifetime

Abstract

Substantially more than half of the electromagnetic nuclear physics experiments conducted at the Continuous Electron Beam Accelerator Facility of the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory) require highly polarized electron beams, often at high average current. Spin-polarized electrons are produced by photoemission from various GaAs-based semiconductor photocathodes, using circularly polarized laser light with photon energy slightly larger than the semiconductor band gap. The photocathodes are prepared by activation of the clean semiconductor surface to negative electron affinity using cesium and oxidation. Historically, in many laboratories worldwide, these photocathodes have had short operational lifetimes at high average current, and have often deteriorated fairly quickly in ultrahigh vacuum even without electron beam delivery. At Jefferson Lab, we have developed a polarized electron source in which the photocathodes degrade exceptionally slowly without electron emission, and in which ion back bombardment is the predominant mechanism limiting the operational lifetime of the cathodes during electron emission. We have reproducibly obtained cathode 1/e dark lifetimes over two years, and 1/e charge density and charge lifetimes during electron beam delivery of over 2?105???C/cm2 and 200 C, respectively. This source is able to support uninterrupted high average current polarized beam delivery to three experimental halls simultaneously formore » many months at a time. Many of the techniques we report here are directly applicable to the development of GaAs photoemission electron guns to deliver high average current, high brightness unpolarized beams.« less

Authors:
; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Thomas Jefferson National Accelerator Facility, Newport News, VA
Sponsoring Org.:
USDOE - Office of Energy Research (ER)
OSTI Identifier:
899851
Report Number(s):
JLAB-ACT-07-558; DOE/ER/40150-4222
Journal ID: ISSN 1098-4402; TRN: US0702107
DOE Contract Number:
AC05-84ER40150
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Special Topics. Accelerators and Beams; Journal Volume: 10; Journal Issue: 2
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; ACCELERATORS; AFFINITY; BRIGHTNESS; CATHODES; CEBAF ACCELERATOR; CESIUM; CHARGE DENSITY; ELECTRON BEAMS; ELECTRON EMISSION; ELECTRON GUNS; ELECTRON SOURCES; ELECTRONS; LASERS; LIFETIME; NUCLEAR PHYSICS; OXIDATION; PHOTOCATHODES; PHOTOEMISSION; PHOTONS; POLARIZED BEAMS

Citation Formats

C. K. Sinclair, P. A. Adderley, B. M. Dunham, J. C. Hansknecht, P. Hartmann, M. Poelker, J. S. Price, P. M. Rutt, W. J. Schneider, and M. Steigerwald. Development of a high average current polarized electron source with long cathode operational lifetime. United States: N. p., 2007. Web. doi:10.1103/PhysRevSTAB.10.023501.
C. K. Sinclair, P. A. Adderley, B. M. Dunham, J. C. Hansknecht, P. Hartmann, M. Poelker, J. S. Price, P. M. Rutt, W. J. Schneider, & M. Steigerwald. Development of a high average current polarized electron source with long cathode operational lifetime. United States. doi:10.1103/PhysRevSTAB.10.023501.
C. K. Sinclair, P. A. Adderley, B. M. Dunham, J. C. Hansknecht, P. Hartmann, M. Poelker, J. S. Price, P. M. Rutt, W. J. Schneider, and M. Steigerwald. Thu . "Development of a high average current polarized electron source with long cathode operational lifetime". United States. doi:10.1103/PhysRevSTAB.10.023501. https://www.osti.gov/servlets/purl/899851.
@article{osti_899851,
title = {Development of a high average current polarized electron source with long cathode operational lifetime},
author = {C. K. Sinclair and P. A. Adderley and B. M. Dunham and J. C. Hansknecht and P. Hartmann and M. Poelker and J. S. Price and P. M. Rutt and W. J. Schneider and M. Steigerwald},
abstractNote = {Substantially more than half of the electromagnetic nuclear physics experiments conducted at the Continuous Electron Beam Accelerator Facility of the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory) require highly polarized electron beams, often at high average current. Spin-polarized electrons are produced by photoemission from various GaAs-based semiconductor photocathodes, using circularly polarized laser light with photon energy slightly larger than the semiconductor band gap. The photocathodes are prepared by activation of the clean semiconductor surface to negative electron affinity using cesium and oxidation. Historically, in many laboratories worldwide, these photocathodes have had short operational lifetimes at high average current, and have often deteriorated fairly quickly in ultrahigh vacuum even without electron beam delivery. At Jefferson Lab, we have developed a polarized electron source in which the photocathodes degrade exceptionally slowly without electron emission, and in which ion back bombardment is the predominant mechanism limiting the operational lifetime of the cathodes during electron emission. We have reproducibly obtained cathode 1/e dark lifetimes over two years, and 1/e charge density and charge lifetimes during electron beam delivery of over 2?105???C/cm2 and 200 C, respectively. This source is able to support uninterrupted high average current polarized beam delivery to three experimental halls simultaneously for many months at a time. Many of the techniques we report here are directly applicable to the development of GaAs photoemission electron guns to deliver high average current, high brightness unpolarized beams.},
doi = {10.1103/PhysRevSTAB.10.023501},
journal = {Physical Review Special Topics. Accelerators and Beams},
number = 2,
volume = 10,
place = {United States},
year = {Thu Feb 01 00:00:00 EST 2007},
month = {Thu Feb 01 00:00:00 EST 2007}
}
  • In mass spectrometers the gas to be studied is ionized by an electron current which is produced by a glowing cathode and which, for the obtention of accurate measurements, must produce a very high constancy with deviation of less than 1 per thousand. This constant electric current can only be obtained by maintenance of the heating current or temperature of the cathode, since the emission frequency of the cathode surface is permanently changed by the surrounding gas. The electron current is therefore measured and the heat current of the cathode controlled with a discharge voltage proportional to the electron current.more » By the application of an alternating current of more than 100 Hz, preferably in the range between 10 and several hundred kHz, to the heating of the cathode, an especially high constancy of the electron current is obtained.« less
  • The results of reproducibly generating an electron beam with a current density of up to 5 kA/cm{sup 2}, without the cathode-anode gap being shorted by the plasma formed inside the cathode carbon-epoxy capillaries, in a ∼350 kV, ∼600 ns diode, with and without an external guiding magnetic field, are presented. The cathode sustained hundreds of pulses without degradation of its emission properties. Time- and space-resolved emissions of the plasma and spectroscopy analyses were used to determine the cathode plasma's density, temperature, and expansion velocity.
  • An electron beam with current density greater than 30 A/cm/sup 2/ and total current of 92 A has been generated in 5 ..mu..s pulses by accelerating the electrons from a glow discharge in a narrow gap between two grids. The ratio of the extracted electron beam current to discharge current is approximately 1. The gun also operates in a dc mode.
  • GaAs-based dc high voltage photoguns used at accelerators with extensive user programs must exhibit long photocathode operating lifetime. Achieving this goal represents a significant challenge for proposed high average current facilities that must operate at tens of milliamperes or more. This paper describes techniques to maintain good vacuum while delivering beam, and techniques that minimize the ill effects of ion bombardment, the dominant mechanism that reduces photocathode yield of a GaAs-based dc high voltage photogun. Experimental results presented here demonstrate enhanced lifetime at high beam currents by: (a) operating with the drive laser beam positioned away from the electrostatic centermore » of the photocathode, (b) limiting the photocathode active area to eliminate photoemission from regions of the photocathode that do not support efficient beam delivery, (c) using a large drive laser beam to distribute ion damage over a larger area, and (d) by applying a relatively low bias voltage to the anode to repel ions created within the downstream beam line. A combination of these techniques provided the best total charge extracted lifetimes in excess of 1000 C at dc beam currents up to 9.5 mA, using green light illumination of bulk GaAs inside a 100 kV photogun.« less
  • Two K{sub 2}CsSb photocathodes were manufactured at Brookhaven National Lab and delivered to Jefferson Lab within a compact vacuum apparatus at pressure ~ 10{sup -11} Torr. These photocathodes were evaluated using a dc high voltage photogun biased at voltages up to 200 kV, and illuminated with laser light at wavelengths 440 or 532 nm, to generate dc electron beams at currents up to 20 mA. Some conditions produced exceptionally large photocathode charge lifetimes, without measurable quantum efficiency (QE) decay, even from the center of the photocathode where operation using GaAs photocathodes is precluded due to ion bombardment. Under other conditionsmore » the charge lifetime was poor, suggesting a complex QE decay mechanism likely related to chemistry and localized heating via the laser beam. Following beam delivery, the photocathodes were evaluated using a scanning electron microscope with energy dispersive x-ray spectroscopy capability, to determine surface morphology and chemical composition.« less