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Ion-ripple laser as an advanced coherent radiation source

We describe the Ion-Ripple Laser as an advanced scheme for generating coherent radiation. A relativistic electron beam obligately propagating through an ion ripple excites electromagnetic radiation which is coupled to slow electrostatic waves with peak growth rate at the resonance frequency ω≈ 2λ2Okwc via backward Raman scattering. This new scheme may provide novel tunable sources of coherent high-power radiation. By proper choice of device parameters, sources of microwaves, optical, and perhaps even x rays can be made. By employing fluid theory the dispersion relation for wave coupling is derived and used to calculate the radiation frequency and linear growth rate. The nonlinear saturation mechanism is due to trapping of the beam electrons by the ponderomotive potential. For an energetic electron beam, the peak growth rate is ωi = ω5/2peεisinθ/λO3/4(2kωc)3/2 and the efficiency is η= ωpe/(2kωcλO3/2). A 1 2/2 D-PIC simulation code was developed to verify the ideas, scaling laws, and nonlinear mechanism. From the observed power spectrum, backward Raman scattering is show to be responsible for the radiation. The growth rates and efficiencies given by the simulation match the ones of theory for different wiggler wave lengths and beam λ. Both of them show a slow decrease with momentum spread. Momentum spread also broadens the radiation spectrum.