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

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 $2\times {10}^5\mathrm{C}/{\mathrm{cm}}^2$ 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.

Figure 1

The first CEBAF polarized electron source, developed in collaboration with the University of Illinois. The heater is shown inserted into the stalk cathode holder, and is removed during high-voltage operation.

Figure 2

The Z-style spin manipulator used with the first polarized electron source.

Figure 3

The second polarized electron source, originally oriented in the vertical plane and later in the horizontal plane. Nonevaporable getter modules surround the cathode/anode gap. Other improvements are described in the text.

Figure 4

The Wien filter spin manipulator used with CEBAF’s second and third polarized electron sources. The magnet is not shown in the cutaway view.

Figure 5

The three CEBAF photoinjector designs. The photoguns used for the first and second polarized injectors extend out of the plane of (a) and (b) and are not shown. Today’s photoinjector, (c), is so reliable the thermionic gun (T-gun) has been removed.

Figure 6

The MOPA gain-switched diode seed laser and diode amplifier system: SM, steering mirror; PM, picomotor; ISO, optical isolator; AMP, single pass diode amplifier; SF, spatial filter; CL, cylindrical lens.

Figure 7

Electron bunch length plots at the chopper slits at four different beam currents, obtained from a bulk GaAs cathode in the first polarized electron source. The observed bunch lengthening at higher current led to the installation of a 1497 MHz prebuncher between the gun and the injector chopper.

Figure 8

Comparison of the optical pulse width and the electron bunch length at low current. The green curve is a 52 ps FWHM Gaussian fit to the optical pulse as measured by autocorrelation.

Figure 9

The harmonic mode-locked Ti-sapphire laser: L, lens; OC/IC, output/input coupler; FR, Faraday rotator; SM, steering mirror; SEED, gain-switched diode seed laser. The Ti-sapphire laser emits rf-pulsed light (exiting after the Faraday rotator) when the seed laser pulse repetition rate is set to a multiple of the Ti-sapphire laser cavity free spectral range.

Figure 10

A typical laser system configuration. The harmonic-mode-locked Ti-sapphire laser provides high current, high-polarization beam to one experimental hall. MOPA lasers provide low current, high polarization, and high current unpolarized beams to the other two halls. Numerous optical components, many remotely controlled, are required for routine operation and are described in the text: T, tune-mode diode laser; SD, gain-switched diode seed laser; SH, shutter; DM, dichroic mirror; TS, telescope; IHWP, insertable half-wave plate; PC, Pockels cell; C, camera; ATTEN, laser attenuator; PM, power meter; RHWP, rotating half-wave plate; X/Y, focusing lens mounted to translation stages; PS, periscope mirrors.

Figure 11

Residual gas spectra for the nominally identical photoguns of the third polarized source. The spectra are virtually identical with hydrogen the dominant gas species. Trace amounts of methane (masses 12 through 16) and carbon monoxide are observed. The mass 19 peak is a result of electron stimulated desorption associated with our use of ${\mathrm{NF}}_3$ for cathode activation. The labels Gun2 and Gun3 are historical designations, with Gun1 corresponding to the original CEBAF thermionic gun, now removed.

Figure 12

The atomic hydrogen/deuterium source used to clean photocathodes prior to installation in the gun.

Figure 13

QE scans of a photocathode obtained over many weeks of continuous operation. Beam was extracted from one location at a time, corresponding to each of the diagrams (a) through (c). The laser was moved to a new location on the photocathode after the QE had degraded sufficiently. The pattern of QE degradation indicates damage from ion back bombardment.

Figure 14

The CEBAF load-locked gun: (a) top view; (b) side view.