Mitigation Plans for the Microbunching-Instability-Related COTR at ASTA/FNAL

TL;DR

This paper discusses mitigation strategies for coherent optical transition radiation (COTR) caused by microbunching instability at Fermilab's ASTA, using spectral filtering, scintillator selection, and timing adjustments to improve beam diagnostics.

Contribution

The paper introduces specific mitigation techniques for COTR effects at ASTA, including spectral filtering, scintillator choice, and timing control, to enhance beam profile measurements.

Findings

Spectral filtering at 420 nm reduces COTR effects.

Timing delay of CCD trigger suppresses prompt OTR signals.

Mitigation can decrease COTR enhancement by 100-1000 times.

Abstract

At the Advanced Superconducting Test Accelerator (ASTA) now under construction at Fermilab, we anticipate the appearance of the microbunching instability related to the longitudinal space charge (LSC) impedances. With a photoinjector source and up to two chicane compressors planned, the conditions should result in the shift of some microbunched features into the visible light regime. The presence of longitudinal microstructures (microbunching) in the electron beam or the leading edge spikes can result in strong, spatially localized coherent enhancements of optical transition radiation (COTR) that mask the actual beam profile. Several efforts on mitigation of the effects in the diagnostics task have been identified. At ASTA we have designed the beam profiling stations to have mitigation features based on spectral filtering, scintillator choice, and the timing of the trigger to the digital camera's CCD chip. Since the COTR is more intense in the NIR than UV we have selectable bandpass filters centered at 420 nm which also overlap the spectral emissions of the LYSO:Ce scintillators. By delaying the CCD trigger timing of the integration window by 40-50 ns, we can reject the prompt OTR signal and integrate on the delayed scintillator light predominately. This combination of options should allow mitigation of COTR enhancements of order 100-1000 in the distribution.