U.S. Department of Energy


Date of this Version



Physical Review Special Topics - Accelerators And Beams 13, 010101 (2010); DOI: 10.1103/PhysRevSTAB.13.010101


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.


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.