Visualization of relativistic laser pulses in underdense plasma

Schwab, M. B. and Siminos, E. and Heinemann, T. and Ullmann, D. and Karbstein, F. and Kuschel, S. and Sävert, A. and Yeung, M. and Hollatz, D. and Seidel, A. and Cole, J. and Mangles, S. P.D. and Hidding, B. and Zepf, M. and Skupin, S. and Kaluza, M. C. (2020) Visualization of relativistic laser pulses in underdense plasma. Physical Review Accelerators and Beams, 23 (3). 032801. ISSN 1098-4402

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    We present experimental evidence of relativistic electron-cyclotron resonances (RECRs) in the vicinity of the relativistically intense pump laser of a laser wakefield accelerator (LWFA). The effects of the RECRs are visualized by imaging the driven plasma wave with a few-cycle, optical probe in transverse geometry. The probe experiences strong, spectrally dependent and relativistically modified birefringence in the vicinity of the pump that arises due to the plasma electrons' relativistic motion in the pump's electromagnetic fields. The spectral birefringence is strongly dependent on the local magnetic field distribution of the pump laser. Analysis and comparison to both 2D and 3D particle-in-cell simulations confirm the origin of the RECR effect and its appearance in experimental and simulated shadowgrams of the laser-plasma interaction. The RECR effect is relevant for any relativistic, magnetized plasma and in the case of LWFA could provide a nondestructive, in situ diagnostic for tracking the evolution of the pump's intensity distribution with propagation through tenuous plasma.