Photoinjector improvements at CEBAF in support of parity violation experiments

Three photoinjector modifications were undertaken at CEBAF to help ensure successful completion of the PREx and Qweak parity violation experiments: the development of a pockels cell high voltage switch that provides stable voltages at 960 Hz helicity flip rate with 60 μs rise/fall time, the installation of a two-Wien-filter spin flipper for slow spin reversal, and the installation of a new photogun with inverted insulator geometry that operates at higher bias voltage

Evaluation of Niobium as Candidate Electrode Material for DC High Voltage Photoelectron Guns

The field emission characteristics of niobium electrodes were compared to those of stainless steel electrodes using a DC high voltage field emission test apparatus. A total of eight electrodes were evaluated: two 304 stainless steel electrodes polished to mirror-like finish with diamond grit and six niobium electrodes (two single-crystal, two large-grain, and two fine-grain) that were chemically polished using a buffered-chemical acid solution. Upon the first application of high voltage, the best large-grain and single-crystal niobium electrodes performed better than the best stainless steel electrodes, exhibiting less field emission at comparable voltage and field strength. In all cases, field emission from electrodes (stainless steel and/or niobium) could be significantly reduced and sometimes completely eliminated, by introducing krypton gas into the vacuum chamber while the electrode was biased at high voltage. Of all the electrodes tested, a large-grain niobium electrode performed the best, exhibiting no measurable field emission (< 10 pA) at 225 kV with 20 mm cathode/anode gap, corresponding to a field strength of 18:7 MV/m

Load-locked dc high voltage GaAs photogun with an inverted-geometry ceramic insulator

A new dc high voltage spin-polarized photoelectron gun has been constructed that employs a compact inverted-geometry ceramic insulator. Photogun performance at 100 kV bias voltage is summarized. I. INTRODUCTION All of the nuclear physics experiments conducted at the Continuous Electron Beam Accelerator Facility (CEBAF) at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) receive electron beams from a dc high voltage spin-polarized GaAs photoemission gun. Since 1995, the year the first polarized electron source was installed at CEBAF [1], there have been four different photogun designs with each new gun an improvement over its predecessor. The most recent design employs a compact, tapered ceramic insulator that extends into the vacuum chamber. This gun geometry is commonly referred to as an ‘‘inverted’’ gun design, a reference to the first such implementation by Breidenbach et al., at SLAC [2]. The main reason for pursuing the inverted ceramic design at Jefferson Lab was to help overcome field emission problems of the previous gun design that used a conventional large-bore cylindrical ceramic insulator common to most dc high voltage spin-polarized GaAs photoguns worldwide [3–7]. The inverted insulator design helped to eliminate field emission because it provided a means to increase the distance between biased and grounded parts of the photogun. This helped reduce the field gradient at some locations not related to beam delivery. Perhaps more importantly, the design significantly reduced the amount of metal biased at high voltage, so there is less metal to generate field emission. Another appealing feature of the design is that the insulator is a common element of medical x-ray sources, and therefore relatively inexpensive compared to cylindrical insulators purchased solely for accelerator electron gun applications. This paper describes the CEBAF invertedgun design, construction, and performance