Stimulated-Raman-scatter behavior in a relativistically hot plasma slab and an electromagnetic low-order pseudocavity

International audienceParticle simulations on a flat-topped somewhat underdense typically n 0 / n c = 0.6 plasma slab by Nikolic et al. Phys. Rev. E 66, 036404 2002 were seen to give transient stimulated scattering behavior with frequency shift 0 − s p considerably less than the plasma frequency p. This has been linked to the electron acoustic wave EAW and the scattering was thus seen as another example of stimulated electron acoustic scattering inferred by Montgomery et al. Phys. Rev. Lett. 87, 155001 2001 from experiments on low-density plasmas. Montgomery et al. had noted the difficulty of how one could have a very narrow observed scattering from a wave whose damping was at least initially very high. Our Vlasov-Maxwell simulations for such somewhat underdense n 0 / n c 0.25 plasmas show that the simulation resonance was in fact determined by the beating of the pump with a new " radiating pseudocavity " electromagnetic mode for the slab at a frequency close to p with relatively low loss. This allows the initial narrow-band excitation of the kinetic electrostatic electron nonlinear KEEN waves the nonlinear " cousins " of EAWs at a well-defined frequency K 0 − p p which is not necessarily the value given by the EAW dispersion relation. The KEEN wave characteristics have been discussed by Afeyan et al. 33rd AAAC 2003, #238, IFSA 2003. The consideration of such a mechanism is relevant to moderately underdense hot plasmas

Nonlinear kinetic effects induced by electrostatic weak turbulence in Vlasov-Maxwell simulations of stimulated Raman Back-Scattering in optical mixing

We report here semi-lagrangian Vlasov-Maxwell simulations of laser-plasma instabilities in an underdense plasma in which an essential role is played not only by the conventional instabilities as Stimulated Raman Back-Scattering (SRBS) (involving the scattering off of high-frequency electron plasma waves) but also by organized trapped electron modes resulting from such instabilities. Electron hole merging in phase space modifies the nonlinear frequency shift of the electron plasma wave. A Raman-like parametric instability is generated with shifted scattered modes, matching conditions being conserved in time in spite of the shift

Transport barrier in 5D gyrokinetic flux-driven simulations

International audienceAbstract Two ways for producing a transport barrier through strong shear of the E × B poloidal flow have been investigated using GYSELA gyrokinetic simulations in a flux-driven regime. The first one uses an external poloidal momentum (i.e. vorticity) source that locally polarizes the plasma, and the second one enforces a locally steep density profile that also stabilizes the ion temperature gradient (ITG) instability modes linearly. Both cases show a very low local turbulent heat diffusivity coefficient χ T turb and a slight increase in core pressure when a threshold of ω E × B ≈ γ ¯ lin (respectively the E × B shear rate and average linear growth rate of ITG) is reached, validating previous numerical results. This pressure increase and χ T turb quench are the signs of a transport barrier formation. This behaviour is the result of a reduced turbulence intensity which strongly correlates with the shearing of turbulent structures as evidenced by a reduction of the auto-correlation length of potential fluctuations as well as an intensity reduction of the k θ spectrum. Moreover, a small shift towards smaller poloidal wavenumber is observed in the vorticity source region which could be linked to a tilt of the turbulent structures in the poloidal direction

Global gyrokinetic simulations of trapped-electron mode and trapped-ion mode microturbulence

Équipe 107 : Physique des plasmas chaudsInternational audienceThis paper presents a reduced kinetic model, which describes simultaneously trapped-ion (TIM) and trapped-electron (TEM) driven modes. Interestingly, the model enables the study of a full f problem for ion and electron trapped particles at very low numerical cost. The linear growth rate obtained with the full f nonlinear code Trapped Element REduction in Semi Lagrangian Approach is successfully compared with analytical predictions. Moreover, nonlinear results show some basic properties of collisionless TEM and TIM turbulence in tokamaks. A competition between streamer-like structures and zonal flows is observed for TEM and TIM turbulence. Zonal flows are shown to play an important role in suppressing the nonlinear transport and strongly depend on the temperature ratio T-e/T-i

Particle dynamics in a turbulent electric field

Charged particle velocity-space diffusion in a prescribed one-dimensional turbulent electric field is investigated through numerical trajectories in phase-space and compared against quasi-linear theory (QL), including resonance broadening (RB). A Gaussian spectrum electric field of variable amplitude E is studied in conjunction with two plasma dispersion relations, namely, the Langmuir and ion-acoustic dispersion. A first parameter scan shows that RB effects become significant for a Kubo number K of a few percent. A Kubo number scan shows that diffusion increases as a power law of D∝K3∝E3/2 for large Kubo numbers. Moreover, at large Kubo numbers, transport processes include significant diffusion measured at velocities much higher than the resonant region, where QL and RB predict negligible diffusion. For times much larger than the trapped particle flight time τb and the autocorrelation time τ0, the velocity distribution departs from a Gaussian. Nevertheless, measurements show that the variance increases linearly in time, with a Hurst parameter of H∼0.5, where the diffusion scales as K5/2∝E5/4 and K3/2∝E3/4 for small and large Kubo number, respectively

Particle dynamics in a turbulent electric field

International audienceCharged particle velocity-space diffusion in a prescribed one-dimensional turbulent electric field is investigated through numerical trajectories in phase-space and compared against quasi-linear theory (QL), including resonance broadening (RB). A Gaussian spectrum electric field of variable amplitude E is studied in conjunction with two plasma dispersion relations, namely, the Langmuir and ion-acoustic dispersion. A first parameter scan shows that RB effects become significant for a Kubo number K of a few percent. A Kubo number scan shows that diffusion increases as a power law of D ∝ K$^3$ ∝ E$^{3/2}$ for large Kubo numbers. Moreover, at large Kubo numbers, transport processes include significant diffusion measured at velocities much higher than the resonant region, where QL and RB predict negligible diffusion. For times much larger than the trapped particle flight time $\tau_b$ and the autocorrelation time $\tau_0$, the velocity distribution departs from a Gaussian. Nevertheless, measurements show that the variance increases linearly in time, with a Hurst parameter of $H∼0.5$, where the diffusion scales as K$^{5/2}$ ∝ E$^{5/4}$ and K$^{3/2}$ ∝ E$^{3/4}$ for small and large Kubo number, respectively

Impurity transport through two types of transport barrier in 5D gyrokinetic simulations

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