The CEBAF Beam Scraping Monitor

Abstract

The Beam Scraping Monitor (BSM) is used to detect small amounts of beam loss in Jefferson Lab's Continuous Electron Beam Accelerator Facility (CEBAF). The BSM acts as an early warning system that alerts CEBAF operators when the beam loss exceeds 10 to 20 nA. This allows the CEBAF operators to adjust the beam orbit before beam loss reaches the Beam Loss Accounting (BLA) system threshold (presently 5000 µA-µ sec) which shuts down the accelerator. The reference oscillators of four digital signal processing (DSP) lock-in amplifiers are connected to two vertical and two horizontal corrector magnets in the CEBAF injector. These magnets excite transverse oscillations on the electron beam that are significantly smaller than the beam size, producing a negligible effect on beam delivery. To cover the entire accelerator enclosure, the two correctors of each plane are separated by π/2 betatron phase advance. When the electron beam approaches the beam enclosure some electrons get scraped off, causing an amplitude modulation of the beam current at the reference oscillator frequencies. This is picked up by the BCM cavities and relayed to the lock-in amplifiers for synchronous integrated detection. The lock-in amplifiers then transmit the scraping information serially to a LINUX computer running EPICS Portable Channel Access Server software.

Beam Diagnostics Based on AC Modulation of System Parameters

“…In addition to the setup mode diagnostics, there are several related systems which are designed for constant monitoring of the accelerator during beam delivery. The Beam Scraping Monitor [10] uses low-level modulation of beam position (tens of microns) to search for chronic beam scraping on an obstacle. This could occur from an insertable beam line element failing to retract properly or from drifts in beam position allowing the beam to be too close to a material object like a septum.…”

Development of a BPM lock-in diagnostic system

“…The BSM system [2] applies a minute perturbation to the transverse beam orbit and detects resultant current fluctuations due to beam scraping. The kick is applied with air-core magnets in the machine's injector at four distinct frequencies.…”