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LCLS Injector Gun: The One-minute Overhaul

(Image)The injector for the Linac Coherent Light Source has a shiny new electron source, or cathode, thanks to a recent repair project aimed at correcting a problem that began earlier this summer. After days of detailed planning, replacing the LCLS cathode took only about one minute, according to Axel Brachmann, the injector physicist who led the effort.

"You just have to plan it well, then you do it," Brachmann said of the pit-crew-style changeout.

The injector cathode is a highly polished copper plate used to generate the initial pulse of electrons for the LCLS. The process starts when a short burst of light from a drive laser hits the cathode's shiny surface. Normally, the cathode would produce one electron for every 100,000 photons. But earlier this year during injector commissioning, physicists noticed a drop in the number of electrons coming off the cathode. Attempts to clean the cathode using the drive laser showed promise at first, but after several such attempts it became clear the cathode would have to be replaced. The LCLS collaboration keeps a few spare cathodes on hand for just such an occasion, under vacuum in specially designed transport chambers.

The repair team faced several challenges—the injector gun where the cathode resides stays under vacuum, and breaking the vacuum to access the cathode could expose delicate components to contamination. To get around this, the gun was first filled with nitrogen, and a curtain of filtered air was directed across it to deflect ambient air from entering the gun and spoiling the clean inner surfaces.

The switch took place a few weeks before the annual shut-down. The entire project, from take-down to having the gun back up and running, took only two shifts.

—Brad Plummer
SLAC Today, August 26, 2008

Above image: Physicist Axel Brachmann with the transport chamber that houses the defective cathode from the LCLS injector, removed earlier this month. (Photo by Brad Plummer. Click for larger image.)