Gun Play at SPEAR3
Last month a small team of SLAC physicists conquered another challenge in the quest to take the SPEAR3 synchrotron permanently up to its full operational capability of 500 milliamperes (mA). Using the existing electron gun and accelerator, the team demonstrated that a laser can be used to create a large pulse of electrons, which will aid rapid injection of beam into the ring.
As it now operates, SPEAR3's electron gun produces electron pulses as the gun's cathode is heated to nearly 2,000 degrees Fahrenheit (1,050 Celsius). This configuration allows only minimal control over the timing and the amount of power in each pulse because the hot cathode continually emits a pulsed stream of a few thousand electron bunches, of which only five or six are useable for injection into the SPEAR3 storage ring. Using a brief laser pulse to stimulate a burst of electrons eliminates limitations associated with a hot cathode, producing a single, precise electron microbunch with much higher charge.
"This is a first step," said physicist John Schmerge who worked on the project. "Basically what we've done is turn off the heater and put in a laser."
The Linac Coherent Light Source (LCLS) injector gun already in operation also uses a laser to produce electrons. In fact, the laser used by Schmerge and colleagues is the same one used to test the LCLS prototype electron gun.
SPEAR3 is designed to operate at a current of 500 mA, but at the moment the synchrotron is limited to 100 mA until a handful of systems are upgraded to accommodate the higher power (see a related SLAC Today story here). Running at 500 mA, the present scheme of refilling the ring three times a day would cause a large variation of beam power on delicate x-ray optics between fills, reducing beam stability and data quality. Upgrading the electron gun to operate by laser will enable SPEAR3's operators to inject new current into the ring, not only much more quickly, but more frequently and in shorter intervals to reduce the variation of stored current between injection cycles—a process called "top off injection." This rapid mode of injection will minimize the thermal strain on delicate optics and will even out disruption to the users as the storage ring current is maintained at a nearly constant level.
Brad Plummer, SLAC Today, July 12, 2007
Above photo: John Schmerge in the SPEAR3 injector vault with the laser setup and cathode gun used to test a new method of generating electrons. (Click on image for larger version.)