Abstract
A general approach to the modifications of the spectrum of a laser pulse interacting with matter is elaborated and used for spectral diagnostics of laser wakefield generation in guiding structures. Analytical predictions of the laser frequency red shift due to the wakefield excited in a capillary waveguide are confirmed by self-consistent modeling results. The role of ionization blue shift, and nonlinear laser pulse and wakefield dynamics on the spectrum modification, is analyzed for recent experiments on plasma wave excitation by an intense laser pulse guided in hydrogen-filled glass capillary tubes up to 8 cm long. The dependence of the spectral frequency shift, measured as a function of filling pressure, capillary tube length and incident laser energy, is in excellent agreement with the simulation results, and the associated longitudinal accelerating field is in the range 1–10 GV m−1.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. The interaction of short, intense laser pulses with plasmas produces large amplitude wake waves. The high field amplitude associated with these wake plasma waves can be used to accelerate particles to high energies over very short lengths compared to conventional accelerator technology. In linear or moderately nonlinear regimes, these fields are of the order of 1 to 10 GV per meter, and relativistic electrons injected into the wave can acquire an energy of the order of one GeV over a length of the order of a few centimetres. Controlling the characteristics of the electron beam as it is accelerated is crucial for achieving a usable laser-plasma accelerator unit.
Main results. Analytical and self-consistent modelling of the excitation of a plasma wave by short intense laser pulse inside capillary tubes is compared to experimental results. The optical diagnostic used to determine the amplitude of the plasma wave is based on the modification of the laser spectrum associated to the changes of the index of refraction of the plasma produced by the plasma wave. A detailed comparison of numerical and experimental results shows that they are in excellent agreement, and that a plasma wave with an accelerating field in the range 1–10 GV is created over 8 cm.
Wider implications. The excitation and characterization of a plasma wave over a distance comparable to the dephasing length of electrons accelerated in low-density plasmas is a first step towards the design of a linear laser wakefield accelerator.
Figure. Laser wavelength shift measured in the experiment (squares with error bars) and obtained in the modeling (line with circles) as functions of the laser pulse energy for 70 mm long capillary filled with hydrogen gas at a pressure of 40 mbar. Dashed line is the analytical prediction.