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Ultrashort pulse filamentation and monoenergetic electron beam production in lwfas

Thomas, A.G.R. and Mangles, S.P.D. and Murphy, C.D. and Dangor, A.E. and Foster, P.S. and Gallacher, J.G. and Jaroszynski, D.A. and Kamperidis, C. (2009) Ultrashort pulse filamentation and monoenergetic electron beam production in lwfas. Plasma Physics and Controlled Fusion, 51 (2). ISSN 0741-3335

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Abstract

In the experiments reported here, the filamentation of ultrashort laser pulses, due to non-optimal choice of focusing geometry and/or electron number density, has a severely deleterious effect on monoenergetic electron beam production in laser wakefield accelerators. Interactions with relatively small focal spots, w0 < λp/2, and with pulse length cτ λp, incur fragmentation into a large number of low power filaments. These filaments are modulated with a density dependent size of, on average, close to λp. The break-up of the driving pulse results in shorter interaction lengths, compared with larger focal spots, and broad energy-spread electron beams, which are not useful for applications. Filamentation of the pulse occurs because the strongly dynamic focusing (small f-number) of the laser prevents pulse length compression before reaching its minimum spot-size, which results in non-spherical focusing gradients.