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The Sound of Science

(Photo - Klystron) Heard the buzz in the klystron gallery? The sound stems from synchronized pulses of electrons accelerating through 240 klystrons. Unlike the more famous electrons down below in the linac, these electrons will never see the beamline. Instead they spend their short career generating pulses of microwave power at 2856 MHz, tens of thousands times higher levels than in your microwave oven.

To accelerate electrons traveling through the linac to nearly the speed of light, accelerator physicists need specialized devices to produce this high frequency power. "To get to 2856 MHz, you can't rotate your typical motor-generator at the incredible speed that would be required," says accelerator physicist Greg Loew. "You have to build a different gadget."

Enter the klystron, which transforms electrical energy from the lab's power lines into microwave energy. This microwave energy pushes the electron beam along, with each klystron adding more energy to the beam.

Within each klystron, an electron gun beams electrons through six copper cavities. Each of these six cavities resonates to support an electric field that progressively shortens the bunches. The last cavity transforms the bunches' kinetic energy by resonating at the highest microwave energy level. All together, the six cavities amplify the input power from 240 to 64,000,000 watts. This power then travels through a waveguide—a rectangular copper pipe—to the accelerator where it boosts the energy of the electron or positron beams.

To make all of this happen, the linac's klystrons are driven by high voltage modulators 30 times per second. These modulators contain transformers with laminations which consequently also vibrate 30 times per second. As they push against air, the laminations generate sound waves just above the lowest note on the piano—hence the rather monotone music in the gallery.

"It's not Mozart—not even rock," admits Loew.

—Krista Zala
    SLAC Today, July 19, 2006

Above image: This klystron can be viewed in SLAC's Visitor Center. (Click on image for larger version.)