The injection process of electrons into the plasma cavity in laser-wakefield accelerators is a nonlinear process that strongly influences the property of the accelerated electrons. During the acceleration electrons perform transverse (betatron) oscillations around the axis. This results in the emission of hard x-ray radiation (betatron radiation) whose characteristics depend directly on the dynamic of the accelerated electrons. Thus, betatron radiation can be utilized as a powerful diagnostic tool to investigate the acceleration process inside the wakefield. Here we describe our recent LWFA experiments deploying ionization induced injection technique carried out with the Draco Ti:Sapphire laser. We focused 30 fs short pulses down to a FWHM spot size of 19 μm resulting in a normalized vacuum laser intensity a0 = 3.3 on a gas target. The target, which was a supersonic gas jet, provided a flat plasma profile of 3mm length. By varying the plasma density from 2x10^18 cm^-3 to 5x10^18 cm^-3 and the laser pulse energy from 1.6 J to 3.4 J we were able to tune the electron bunch and betatron parameters. Electron spectra were obtained by acquiring an energy resolved and charge calibrated electron profile after detection from the beam axis by a permanent magnetic dipole. Simultaneously, a back-illuminated and deep-depleted CCD placed on axis recorded the emitted x-ray photons with energies up to 20keV. Equipped with an 2D spectroscopy technique based on single pixel absorption events, we reconstructed the corresponding energy resolved x-ray spectrum for every shot and deduced the betatron source size at the plasma exit. Combining the data of the electron and betatron spectrum, we compare the characteristics of the betatron spectra for different electron bunches. In our experiments we recorded a total number of 25x10^4 photons per shot within a divergence angle of 1 mrad and betatron radii in the order of 1 μm. Finally, we compare our results with simulated spectra from the parallel classical radiation calculator Clara2 that is based on the Liénard-Wiechert potentials.
Alexander Koehler, Jurjen P. Couperus, Omid Zarini, Richard Pausch, Jakob M. Krämer, Alexander Debus, Michael Bussmann, Arie Irman, and Ulrich Schramm, "Investigation of electron dynamics in an ionization-injection laser-wakefield accelerator via betatron radiation (Conference Presentation)," Proc. SPIE 10234, Relativistic Plasma Waves and Particle Beams as Coherent and Incoherent Radiation Sources II, 1023403 (Presented at SPIE Optics + Optoelectronics: April 27, 2017; Published: 21 June 2017); https://doi.org/10.1117/12.2265029.5463398343001.
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