Laser-Plasma Accelerator Workshop 2019

Multi Spectral Coherent Transition Radiation Imaging and Interferometry of Laser Wakefield Accelerated Electron Bunches

08.05.2019, 18:00

15m

Main Hall (MedILS)

Oral Contribution Laser-driven electron acceleration

Sprecher

Prof. Michael Downer (University of Texas at Austin)

Beschreibung

The low transverse emittance of electron bunches from laser wakefield accelerators (LWFAs) makes these advanced accelerators attractive for compact FELs and colliders. Single-shot, direct, non-intercepting diagnostics of this emittance outside the LWFA are, however, needed. Here we present single-shot coherent transition radiation (CTR) imaging and interferometry data from electron bunches only ~1mm after emerging from a 300 MeV LWFA. We combine near field (NF) and far field (FF) interferometric imaging of CTR emitted from a foil placed directly after our gas jet and a laser-rejection foil. We also employ a multi-octave CTR spectrometer to diagnose the longitudinal structure of the beam (see presentation by A. Debus). The NF system directly images the foil with high resolution for each of several narrow visible bandwidths. Through point spread function (PSF) analysis, the transverse profile of the beam component responsible for each CTR wavelength is deduced from this measurement. We find typical transverse rms radii 2 ≤ $\sigma$ perp ≤ 3 µm. The FF system collects transition radiation from the same foil and from a foil ~2 cm downstream. The sum of this radiation is then focused onto a camera with a narrow bandpass filter. This double foil Wartski interferometer [1] diagnoses divergence of the portion of the bunch that is micro-bunched at each observed wavelength. We find typical rms divergence ${\sigma }_{\theta }$ ≈ 0.5 mrad, several times smaller than the ensemble divergence (${\sigma }_{\theta }$ ≈ 2.5 mrad ) measured at a downstream electron spectrometer screen. From beam size and divergence, we determine emittance at the first foil location on a single shot for the fraction of the beam that is coherent at the selected bandwidth. Since our CTR imaging/interferometry system measures multiple 10 nm bandwidth spectral regions simultaneously on each shot, and our CTR spectrometer measures spectra over > 5 octave bandwidth, the potential exists to reconstruct a 3D charge distribution at the foil. We will present data and analysis on bunch size and divergence from a combination of FF and NF imaging systems as well as bunch substructure revealed by these diagnostics.
[1] L. Wartski et al, IEEE Trans. Nucl. Sci. NS-20 (1973)544