Temporal evolution of longitudinal bunch profile in a laser wakefield accelerator

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Temporal evolution of longitudinal bunch profile in a laser wakefield accelerator

M. Heigoldt, A. Popp, K. Khrennikov, J. Wenz, S. W. Chou, S. Karsch, S. I. Bajlekov, S. M. Hooker, and B. Schmidt

Phys. Rev. ST Accel. Beams 18, 121302 – Published 16 December 2015

Abstract

We present single-shot measurements of the longitudinal bunch profile from a laser-wakefield accelerator with sub-fs resolution, based on detection of coherent transition radiation in a broad spectral range. A previously developed phase retrieval algorithm enables reconstruction of the bunch profile without prior assumptions about its shape. In this study, a variable-length gas target is used to explore the dynamics of bunch evolution. Our results show that once the laser energy is depleted the time structure of the generated electron beam changes from a single bunch to a double bunch structure, well suited for driver-witness type experiments.

Received 4 August 2014

DOI:https://doi.org/10.1103/PhysRevSTAB.18.121302

This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Authors & Affiliations

M. Heigoldt, A. Popp, K. Khrennikov, J. Wenz, S. W. Chou, and S. Karsch^*

Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany and Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany

S. I. Bajlekov and S. M. Hooker

Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom

B. Schmidt

Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany

^*Corresponding author. stefan.karsch@mpq.mpg.de

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Issue

Vol. 18, Iss. 12 — December 2015

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Images

Figure 1
Experimental setup: Accelerated electrons (dark blue) leaving the plasma generate coherent transition radiation (light blue) when they traverse the steel tapes. CTR from the 2nd tape is separated from the copropagating electron bunch, collimated by an off-axis paraboloid ( $f_{eff} = 190.5 mm$ ), split by silicon wafers and directed into 3 optical spectrometers covering a spectral range from $0.4 - 7.1 μ m$ . The electron beam energy and charge is analyzed by a pair of dipole magnets and a scintillating screen. Additionally a second scintillating screen (not shown) located in front of the magnets can be inserted to determine the beam divergence and transverse profile.
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Figure 2
Measured and reconstructed longitudinal form factor $| F_{∥} (ω) |$ and reconstructed bunch profiles $I (z)$ for 2 representative shots for gas cell lengths of 5 and 13 mm, respectively: (a,b) Measured (black) and reconstructed (red) CTR spectrum, and reconstructed spectral phase (grey); Error bars show the rms error of the mid-infrared spectrometer (due to readout noise, uncertainty in spectral response of individual pyro-electric detectors and transmission of the silicon wafer). (c,d) Reconstructed longitudinal bunch profiles.
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Figure 3
Influence of a longitudinal phase-space correlation. Longitudinal phase-space distribution for (a) the case of an uncorrelated bunch and (b) for the case of a correlated bunch. (c) Simulated CTR spectra corresponding to the distributions shown in (a) and (b) for different FWHM durations.
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Figure 4
(a) Bunch profile evolution at a plasma electron density of $n_{e} = 3.9 \times 10^{18} {cm}^{- 3}$ : Retrieved bunch profiles averaged over 30 consecutive shots for different gas-cell lengths. The grey band shows $\pm 1 σ$ , where $σ$ is the shot-to-shot standard deviation for each 30-shot data set. (b) Electron energy evolution at $n_{e} = 3.9 \times 10^{18} {cm}^{- 3}$ : Measured electron spectra for gas cell lengths between 3 and 14 mm, recorded in 1 mm steps. For each length setting, 30 consecutive electron spectra are plotted as thin vertical lines. Red dots show the average cutoff energy (set to 10% of maximum spectral charge density) at each length; white dots show the average charge. Error bars show the shot-to-shot standard deviation. (c) Measured bunch separation as a function of the plasma electron density at a fixed gas cell length of $L = 12 mm$ : Error bars on the x-axis represent the uncertainty in electron density. Markers show the mean value of the bunch separation obtained in ten individual runs of the Bubblewrap algorithm, each started from a random seed. Error bars on the y-axis represent one standard deviation. The black line shows the plasma wavelength as given by 1D linear theory.
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