Abstract
Inverse Compton scattering (ICS) is a unique mechanism for producing fast pulses---picosecond and below---of bright photons, ranging from x to $\ensuremath{\gamma}$ rays. These nominally narrow spectral bandwidth electromagnetic radiation pulses are efficiently produced in the interaction between intense, well-focused electron and laser beams. The spectral characteristics of such sources are affected by many experimental parameters, with intense laser effects often dominant. A laser field capable of inducing relativistic oscillatory motion may give rise to harmonic generation and, importantly for the present work, nonlinear redshifting, both of which dilute the spectral brightness of the radiation. As the applications enabled by this source often depend sensitively on its spectra, it is critical to resolve the details of the wavelength and angular distribution obtained from ICS collisions. With this motivation, we present an experimental study that greatly improves on previous spectral measurement methods based on x-ray $K$-edge filters, by implementing a multilayer bent-crystal x-ray spectrometer. In tandem with a collimating slit, this method reveals a projection of the double differential angular-wavelength spectrum of the ICS radiation in a single shot. The measurements enabled by this diagnostic illustrate the combined off-axis and nonlinear-field-induced redshifting in the ICS emission process. The spectra obtained illustrate in detail the strength of the normalized laser vector potential, and provide a nondestructive measure of the temporal and spatial electron-laser beam overlap.
Highlights
Inverse Compton scattering (ICS) [1,2,3] is an emerging technique for obtaining narrow bandwidth, highly directional x rays from the collision of intense relativistic electron beams and lasers
We have developed and utilized in strong nonlinear-regime ICS experiments, again performed at the Brookhaven National Laboratory Accelerator Test Facility (BNL ATF), an x-ray spectrometer [25] that is capable of revealing the double differential radiation spectrum (DDS) obtained from the scattered photons
The shape and width of the ICS near-axis photon energy spectrum are seen to give quantitative insight, within experimental error, into the relative laser-electron beam spots κ, as well as the degree of nonlinearity parametrized by a0
Summary
Inverse Compton scattering (ICS) [1,2,3] is an emerging technique for obtaining narrow bandwidth, highly directional x rays from the collision of intense relativistic electron beams and lasers. The radiation characteristics of such ICS sources are similar to those obtained from undulators in storage ringbased light sources and the spontaneous radiation from freeelectron lasers (FELs), with a qualitative difference This is found in the scale of the periodic fields that provoke the radiation production. In contrast to the centimeterperiod λu associated with a magnetostatic undulator array that yields a resonant radiation wavelength ∝λu=2γ2, in ICS one employs a counterpropagating electromagnetic wave that gives a wavelength ∼ ∝ λL=4γ2, where λL is the (opticalto-infrared) laser wavelength.
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