Abstract
The role of plasma channels as waveguides for laser-wakefield accelerators is discussed in terms of the results of experiments performed with the Astra-Gemini laser, numerical simulations using the code WAKE, and the theory of self-focusing and self-guiding of intense laser beams. It is found that at a given electron density, electron beams can be accelerated using lower laser powers in a waveguide structure than in a gas-jet or cell. The transition between relativistically self-guided and channel-assisted guiding is seen in the simulations and in the behaviour of the production of electron beams. We also show that by improving the quality of the driving laser beam the threshold laser energy required to produce electron beams can be reduced by a factor of almost 2. The use of an aperture allows the production of a quasi-monoenergetic electron beam of energy 520 MeV with an input laser power of only 30 TW.
Highlights
Experimental set-upIn the work described in this paper, the plasma-channel was created using the hydrogen-filled capillary discharge waveguide [13, 14]
Is given by η = (1 −)1/2, where ωp is the plasma frequency and ω0 is the driving laser frequency
Quasi-monoenergetic electron beams with energies above 0.5 GeV were generated with input laser pulses of only 2.7 J and peak power of 30 TW, using plasma channels with an axial density as low as 1.8 × 1018 cm−3
Summary
In the work described in this paper, the plasma-channel was created using the hydrogen-filled capillary discharge waveguide [13, 14]. The gas was allowed to flow for 2.5 s so that a steady state within the capillary was reached before a 1.7 nF capacitor, New Journal of Physics 12 (2010) 045008 (http://www.njp.org/) For these calculations, the plasma channel was taken to be parabolic, with an axial electron density of ne = 1.8 × 1018 cm−3 and lowest-order mode of 41 μm FWHM; the gas in the cell was fully ionized, with a uniform electron density of ne(0); the temporal and transverse spatial profiles of the input laser pulses were Gaussian, with initial FWHM duration of 90 fs and focal spots of 35 μm. It is clear that the current measured for td 50 ns is due to charging of the cables, and does not correspond to current flow through the capillary
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