Observation of Shot Noise Suppression at Optical Wavelengths in a Relativistic Electron Beam

Abstract

Control of collective properties of relativistic particles is increasingly important in modern accelerators. In particular, shot noise affects accelerator performance by driving instabilities or by competing with coherent processes. We present experimental observations of shot noise suppression in a relativistic beam at the Linac Coherent Light Source. By adjusting the dispersive strength of a chicane, we observe a decrease in the optical transition radiation emitted from a downstream foil. We show agreement between the experimental results, theoretical models, and 3D particle simulations.

Time-resolved imaging of the microbunching instability and energy spread at the Linac Coherent Light Source

“…Though typically small enough not to affect LCLS lasing, SES limits more advanced schemes such as high gain harmonic generation [26], harmonic lasing [27,28], longitudinal space charge amplifiers [29,30], and dispersive noise suppression [31,32]. Here we report LCLS's first measurements of SES of the accelerated beam, as well as SES dependence on the laser heater and MBI.…”

Subradiant spontaneous undulator emission through collective suppression of shot noise

“…First, because different parts of the beam have different values of n 0 , there is no single value of R 56 that suppresses the entire beam. (See e.g., [12,35] for a discussion of a Gaussian distribution in a short interaction.) The beam is also expected to have strong peak current dependence along the longitudinal axis.…”

“…(1) [12]; at large angles, θ > θ max ¼ λ r =ð2πσ u Þ (for beam size in the undulator σ u ), the radiation is no longer transversely coherent and suppression disappears (see e.g., [33]). The experimental setup of [12] could not isolate the on-axis radiation, which limited the observed level of suppression in that study.…”

“…Previous observations of shot noise suppression in accelerators were only proof-of-principle experiments, using optical transition radiation (OTR) as a diagnostic [12,13], and only demonstrated weak suppression. Moreover, OTR has limited applications and its emission pattern is null on axis.…”

“…In the opposite regime, homogenization of the beam on the scale of the radiation wavelength results in destructive interference and consequently subradiance, i.e., lower intensity than expected from spontaneous radiation. In practice, beam homogenization corresponds to suppression of random shot-noise current fluctuations and recent theoretical proposals [8][9][10][11] and experimental demonstrations [12,13] have investigated various means of shot-noise suppression.…”