We present recent experiments on the characterization of Betatron radiation in the blowout regime of laser-wakefield acceleration. We observed Betatron x-rays up to 80 keV, and the characterization of the angular dependence of the x-ray spectrum suggests anisotropic electron trajectories in the plasma. The characterization of the source opens up new possibilities for application experiments.
Betatron x-rays with multi-keV photon energies have been observed from a GeV-class laser-plasma accelerator. The experiment was performed using the 200 TW Callisto laser system at LLNL to produce and simultaneously observe GeV-class electron beams and keV Betatron x-rays. The laser was focused with two different optics (f/8 and f/20), and into various gas cells with sizes ranging from 3 to 10 mm, and containing mixed gases (He, N, CO2, Ar, Ne) to accelerate large amounts of charge in the ionization induced trapping regime. KeV betatron x-rays were observed for various concentrations of gases. Electron spectra were measured on large image plates with the two-screen method after being deflected by a large 0.42 Tesla magnet spectrometer. Betatron oscillations observed on the electron spectra can be benchmarked against a simple analytical model (Runge-Kutta algorithm solving the equation of motion of an electron in the wakefield), in order to retrieve the electron injection conditions into the wake.
C. Clayton, S. Martins, J. Martins, D. K. Johnson, S. Wang, K. Marsh, P. Muggli, M. J. Hogan, D. Walz, R. Fonseca, E. Oz, C. D. Barnes, C. L. O'Connell, I. Blumenfeld, N. Kirby, R. Ischebeck, C. Huang, M. Zhou, W. Lu, S. Deng, T. Katsouleas, W. Mori, R. H. Siemann, L. Silva, C. Joshi
When a highly relativistic electron is injected off-axis into an ion channel, the restoring force of the radial field of the
ions will cause the electron to accelerate towards the axis, overshoot, and begin to undergo oscillations about the ioncolumn
axis at a characteristic frequency; the betatron frequency. This so-called betatron motion will cause the electron
to radiate hard x-rays in the forward direction. In two recent experiments at the Stanford Linear Accelerator Center
(SLAC), betatron x-rays in the 1-20kV range and in the 1-50MV range were produced with an electron beam with an
energy of 28.5 GeV for ion densities of about 1 x 1014 cm-3 and 1 x 1017cm-3, respectively. To make such an x-ray source
more compact, the 3km long SLAC linac would be replaced by a source of electrons from a Laser Wakefield accelerator
(LWFA). To increase the efficiency of converting laser into photons at high photon energies, we propose adding a
second stage where the LWFA electrons radiate via a second ion channel, independent of the accelerating process. This
two stage concept allows one to control the critical frequency of the emitted radiation as well as the efficiency of the
process.
David Neely, Colin Danson, Ric Allott, F. Amiranoff, E. Clark, Chris Clayton, J. Collier, A. Dangor, A. Djaoui, Christopher Edwards, P. Flintoff, Daniel Gordon, P. Hatton, Mark Harman, M. Hutchinson, K. Krushelnick, G. Malka, Victor Malka, A. Modena, Z. Najmudin, David Pepler, Ian Ross, M. Salvati, M. Santala, M. Tatarakis, M. Trentelman, T. Winstone
Frequency doubling a large aperture sub ps, chirped pulse amplified (CPA) 1053 nm beam for laser matter interaction studies was investigated at the Central Laser Facility. Efficiencies > 50 percent were achieved using a 4 mm thick KDP crystal to convert a 140 X 89 mm 700 fs beam. Measurements of the 527 nm beam's focal spot quality when the doubling crystal was driven at high intensities 200 GWcm-2 are presented. Using data from 2 and 4 m thick 25 mm diameter test crystals, the optimum crystal thickness in terms of conversion efficiency is reviewed for 1053 nm CPA systems in the 0.3-3 ps region and options for fourth harmonic production discussed.
A two-frequency CO2 laser beam was used to beat-excite a large amplitude electron plasma wave in a resonant density plasma. The accelerating fields of the relativistic plasma wave were probed with collinear injected 2.1 MeV electrons from an electron linac. Some electrons gained at least 7 MeV in traversing the approximately 1 cm length of the beat wave accelerator, with the measurement limited by the 9.1 MeV high energy cut-off of the detection system. The corresponding average acceleration gradient is > 0.7 GeV/m and the average wave amplitude n1/n0 is > 8%. Estimates based on collective Thomson scattering indicate that peak wave amplitudes of 15 - 30% may have been achieved.
KEYWORDS: Plasma, Free electron lasers, Electron beams, Picosecond phenomena, Magnetism, Photons, Pulsed laser operation, Wave propagation, 3D modeling, Information operations
The feasibility of using relativistically moving plasma waves as short wavelength undulators for possible free-electron laser (FEL) and Compton scattering applications is considered. Focus is placed on the spontaneous emission emitted by a single electron bunch as it traverses a plasma wave wiggler. The basic characteristics of the radiation from such a wiggler are discussed, and attention is given to the wave-particle interaction physics. The electron trajectories in the plasma wave wiggler are simulated using a three-dimensional model which includes emittance effects, and the resulting radiation pattern produced by the electrons is calculated. The scattered-electron energies and spatial distributions are analyzed.
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