KCGセミナー Two Accelerator Projects at SLAC:Luminosity Dither Feedback for PEP-2 and Beam-Loss Protection for LCLS-2

概要

This talk discusses two SLAC diagnostics projects, over 10 years apart. The first was a feedback that optimized the beam overlap at the IP of PEP-2. Changes in luminosity were tracked as the positron beam in the low-energy ring was shifted by small amounts relative to the electrons in the high-energy ring. The previous feedback used corrector magnets to move x, y, and y′ sequentially. The settling time of the correctors and the sequential pattern slowed this “dither” feedback. We developed a faster approach in which the beams were dithered in all three directions simultaneously and sinusoidally, but with a different frequency for each plane. A lock-in amplifier at each frequency detected the effect on luminosity and determined periodic adjustments made with correctors. This method is being applied to SuperKEKB.

After PEP-2 closed in 2008, SLAC focused on the LCLS x-ray free-electron laser, which used the third km of the copper linac. The first km is now being replaced with a superconducting 4-GeV linac driven by continuous 1.3-GHz RF. Electron bunches will be delivered at rates of up to 1 MHz, with a beam power of 120 kW. An upgrade to 8 GeV is planned, with correspondingly higher power. Careful attention must be paid to beam loss, to protect people outside the tunnel and to prevent damage to the machine. SLAC’s customary detectors are ionization chambers, but with high losses, the slow ion transit can lead to space-charge pile-up and nonlinearity. New detectors based on quartz optical fibers and diamonds have been tested extensively on LCLS and are being installed on LCLS-2.