[en] It has been proposed that the Raman backscatter interaction in a plasma can be used to amplify ultra-intense laser pulses. To accomplish this, energy is transferred from a long drive pulse at frequency ω_pump to an intense seed pulse at frequency ω_seed, with a Langmuir plasma wave at frequency wp mediating the transfer; the frequencies are chosen to satisfy the resonant condition ω_p=ω_pump-ω_seed. Diffraction of the pulses limits the interaction length in a uniform plasma, and hence the energy transfer between the pulses. However in a parabolic plasma density channel it is shown, through two-dimensional particle-in-cell simulations, that such a plasma channel can be used to guide both the amplified and drive pulses over an interaction distance much greater than a diffraction length. The seed pulse is amplified by a factor of more than 200 in energy for pulses whose widths are matched to the channel size, and achieve a peak intensity of more than 6x10¹⁷ W/cm². Unmatched pump pulses are seen to generate much smaller gain

[en] Non-charge conserving current collection algorithms for relativistic PIC plasma simulations can cause errors in Gauss' law. These errors arise from violations of the continuity equation. Two techniques for removing these errors are examined and compared, the Marder correction, a method which corrects electric fields locally and primarily affects short wavelengths, and a divergence correction, which uses a Poisson solve to correct the electric fields so that Gauss' law is enforced. The effect of each method on the spectrum of the error (short wavelengths vs. long) are examined. Computational efficiency and accuracy of the two techniques is compared

[en] The coupling of lower hybrid waves to the plasma is a crucial issue for efficient current drive in tokamaks. In this work, the coupling problem is attacked with a new tool in this context: an electromagnetic particle-in-cell (PIC) code, XOOPIC (Verboncoeur J P et al 1995 Comp. Phys. Comm. 87 199). A model for a grill with 32 waveguides is constructed using perfectly conducting walls. The wave propagation in the waveguides and the coupling to plasma is followed. The wave-plasma interaction is studied and the evolution of the launched spectrum is resolved in the near-field of the grill. The reflection coefficients in the individual waveguides are determined. (author)

[en] Both relativistic and nonrelativistic magnetrons are under experimental and theoretical investigation at U of M. Relativistic (Titan-6-vane) magnetron experiments (300-400 kV, 1-10 kA, 0.5 microsecond) investigate mode control with various output coupling geometries. Mode competition between the pi mode and the 2/3 pi mode has been characterized for two-versus-three output extractors for comparison with particle in cell simulations. Phase measurements and time-frequency-analysis are performed for mode identification. Peak microwave output power on the order 0.5 GW has been measured, assuming equal output from 3 waveguides. Nonrelativistic (4 kV, <1A, kW microwave power) magnetron experiments are performed on commercial oven magnetrons for an in-depth investigation of crossed-field injection-locking and noise. Injection-locking is demonstrated by utilizing an oven magnetron as a reflection amplifier. Noise generation is explored as a function of injected signal and cathode conditions