Near-infrared noise in intense electron bunches

This article investigates electron bunch density fluctuations in the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mn>1</a:mn> <a:mo>−</a:mo> <a:mn>10</a:mn> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mrow> <a:mi mathvariant="normal">μ</a:mi> <a:mi mathvariant="normal">m</a:mi> </a:mrow> </a:math> wavelength range, focusing on their impact on coherent electron cooling (CEC) in hadron storage rings. In this study, we compare the shot-noise model with experimental observations using bandwidth-filtered near-infrared optical transition radiation (OTR) photodiode signals, where the transverse bunch size is much larger than the OTR wavelength of interest. The relativistic electron bunch ( <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>γ</e:mi> <e:mo>≈</e:mo> <e:mn>50</e:mn> </e:math> ) parameters are close to those proposed for the coherent electron cooler in the electron-ion collider (EIC) project. Preliminary feasibility experiments were conducted, and the noise factors are presented, supported by particle tracking. No major density fluctuations or effective cooling rate decrease were revealed for EIC CEC design parameters. Additionally, longitudinal-space-charge-induced microbunching for the chicane-compressed bunch was observed with coherent OTR enhancements up to 100 times, providing further calibration of the measurement method.