Two electron guns fire at Jefferson

25 January 2000


Spin-oriented (polarized) electron beams are high on the agenda at the Jefferson Laboratory, Newport News, Virginia. To ensure uninterrupted delivery of these beams, a second polarized electron gun has been added.

About 50% of all the Laboratory’s experiments require polarized beams and an even larger fraction of the major experiments use them. With the Continuous Electron Beam Accelerator Facility (CEBAF) delivering beams to two or three halls simultaneously, in practice the polarized source has to run 100% of the time. The Laboratory has been running polarized beams since April 1998 and plans to continue into the spring of 2001.

Several other improvements were made to the polarized source. For example, in the past, temperature fluctuations near the lasers that illuminate the photocathodes affected beam stability. An air-conditioned housing has now been built around the three lasers. Improved laser controls and electronics hardware were also installed. Work is in progress to reconfigure the laser systems so that switching beam delivery between the polarized guns can be done with the push of a button.

A key component of both electron guns is their dime-sized gallium arsenide photocathode. These photocathodes gradually lose their emitting properties over time. Although the laser can be refocused on different sections of the photocathode, eventually, the material’s effectiveness decreases and the entire crystal must be replaced.

During replacement, Injector Group personnel must open the ultra-high-vacuum chamber within the injector. Although this process lasts only minutes, re-establishing the ultra-high vacuum involves bakeout and can take up to 50 hours.

With two polarized guns, one can be taken out of operation as necessary. With planned upgrades to the laser system and continued investigations into more efficient and durable photocathode materials, researchers should be able to take full advantage of their allotted beam time in the coming months and years.

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