Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

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Measuring sub-femtosecond temporal structures in multi-ten kiloampere electron beams
Zarini, O.ORC
In laser wakefield acceleration, an ultra-short high-intensity laser pulse excites a plasma wave, which can sustain accelerating electric fields of several hundred GV/m.
This scheme advances a novel concept for compact and less expensive electron accelerators, which can be hosted in a typical university size laboratory. Furthermore, laser wakefield accelerators (LWFA) feature unique electron bunch characteristics, namely micrometer size with duration ranging from several fs to tens of fs. Precise knowledge of the longitudinal profile of such ultra-short electron bunches is essential for the design of future table-top X-ray light-sources and remains a big challenge due to the resolution limit of existing diagnostic techniques.

Spectral measurement of broadband coherent and incoherent transition radiation (TR) produced when electron bunches passing through a metal foil is a promising way to analyze longitudinal characteristics of these bunches. Due to the limited reproducibility of the electron source this measurement highly requires single-shot capability.
An ultra-broadband spectrometer combines the TR spectrum in UV/NIR (200-1000 nm), NIR (0.9-1.7 µm) and mid-IR (1.6-12 µm). A high spectral sensitivity, dynamic bandwidth and spectral resolution are realized by three optimized dispersion and detection systems integrated into a single-shot spectrometer.
A complete characterization and calibration of the spectrometer have been done concerning wavelengths, relative spectral sensitivities, and absolute photometric sensitivities, also taking into account for the light polarization.
The TR spectrometer is able to characterize electron bunches with charges as low as 1pC and can resolve time-scales of 0.4 fs. Electron bunches up to 16 fs (rms width) can be reconstructed from their TR spectrum.

In the presented work, the self-truncated ionization induced injection (STII) scheme has been explored to study the relevant beam parameters especially its longitudinal bunch profile and the resulting peak current.
Proper focusing of a high power laser pulse into a supersonic gas-jet target and tailoring the conditional laser and plasma density and taking advantage of the relativistic self-focusing effects are investigated in this PhD thesis in order to study the final beam parameters as well as the consequent beam loading effects by producing nC-class mono-energetic electron beams.

In the experiment at HZDR, the DRACO 100TW Ti:Sa based laser system is used in conjunction with a He-N₂ mixed, supersonic gas-jet target. Under optimized conditions, mono-energetic electron bunches are accelerated, which are massively loaded up to several 100 pC at 300 MeV peak energy with a narrow energy spread of a few 10 MeV. Reconstruction results of TR spectra, measured by TR spectrometer, show that the shortest electron bunch duration is at about 13 fs FWHM corresponding to a peak current as high as 20 kA.
Such peak current is about one order of magnitude higher than those generated by conventional RF linear accelerator. This landmarks a significant finding of this thesis.
Keywords: Laser wakefield acceleration, laser plasma accelerator, self-truncated ionization injection, high peak current, high bunch charge, beam loading, bunch duration measurement, coherent transition radiation, broadband spectrometer, infrared spectrometer, prism spectrometer, echelle spectrometer, phase reconstruction algorithm, Foldwrap reconstruction algorithm, PIConGPU
  • Open Access LogoWissenschaftlich-Technische Berichte / Helmholtz-Zentrum Dresden-Rossendorf; HZDR-100 2019

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Publ.-Id: 29222 - Permalink