Ryan Roger Lindberg

Self-consistent Langmuir waves in resonantly driven thermal plasmas

The longitudinal dynamics of a resonantly driven Langmuir wave are analyzed in the limit that the growth of the electrostatic wave is slow compared to the bounce frequency. Using simple physical arguments, the nonlinear distribution function is shown to be nearly invariant in the canonical particle action, provided both a spatially uniform term and higher-order spatial harmonics are included along with the fundamental in the longitudinal electric field. Requirements of self-consistency with the electrostatic potential yield the basic properties of the nonlinear distribution function, including a frequency shift that agrees closely with driven, electrostatic particle simulations over a range of temperatures. This extends earlier work on nonlinear Langmuir waves by Morales and O’Neil [G. J. Morales and T. M. O’Neil, Phys. Rev. Lett. 28, 417 (1972)] and Dewar [R. L. Dewar, Phys. Plasmas 15, 712 (1972)], and could form the basis of a reduced kinetic treatment of plasma dynamics for accelerator applications or...

Slowly varying envelope kinetic simulations of pulse amplification by Raman backscattering

A numerical code based on an eikonal formalism has been developed to simulate laser-plasma interactions, specifically Raman backscatter (RBS). In this code, the dominant laser modes are described by their wave envelopes, avoiding the need to resolve the laser frequency; appropriately time-averaged equations describe particle motion. The code is fully kinetic, and thus includes critical physics such as particle trapping and Landau damping which are beyond the scope of the commonly used fluid three-wave equations. The dominant forces on the particles are included: the ponderomotive force resulting from the beat wave of the forward and backscattered laser fields and the self-consistent plasma electric field. The code agrees well, in the appropriate regimes, with the results from three-wave equations and particle-in-cell simulations. The effects of plasma temperature on RBS amplification are studied. It is found that increasing the plasma temperature results in modification to particle trapping and the satura...

Reduced kinetic description of weakly-driven plasma wavesa ...

A model of kinetic effects in Langmuir wave dynamics is presented using a nonlinear distribution function that includes particle separatrix crossing and self-consistent electrostatic evolution. This model is based on the adiabatic motion of electrons in the wave to describe Bernstein–Greene–Kruskal-like Langmuir waves over a wide range of temperatures (0.1⩽kλD⩽0.4). The asymptotic distribution function yields a nonlinear frequency shift of the Langmuir wave that agrees well with Vlasov simulations, and can furthermore be used to determine the electrostatic energy required to develop the phase-mixed, asymptotic state. From this incoherent energy, energy conservation is employed to determine a simplified model of nonlinear Landau damping. The resulting nonlinear, dynamic frequency shift and damping are then used in an extended three-wave-type model of driven Langmuir waves and compared to Vlasov simulations in the context of backward Raman scattering.A model of kinetic effects in Langmuir wave dynamics is presented using a nonlinear distribution function that includes particle separatrix crossing and self-consistent electrostatic evolution. This model is based on the adiabatic motion of electrons in the wave to describe Bernstein–Greene–Kruskal-like Langmuir waves over a wide range of temperatures (0.1⩽kλD⩽0.4). The asymptotic distribution function yields a nonlinear frequency shift of the Langmuir wave that agrees well with Vlasov simulations, and can furthermore be used to determine the electrostatic energy required to develop the phase-mixed, asymptotic state. From this incoherent energy, energy conservation is employed to determine a simplified model of nonlinear Landau damping. The resulting nonlinear, dynamic frequency shift and damping are then used in an extended three-wave-type model of driven Langmuir waves and compared to Vlasov simulations in the context of backward Raman scattering.

Autoresonant beat-wave generation

Autoresonance offers an efficient and robust means for the ponderomotive excitation of nonlinear Langmuir waves by phase-locking of the plasma wave to the slowly chirped beat frequency of the driving lasers via adiabatic passage through resonance. This mechanism is analyzed for the case of a cold, relativistic, underdense electron plasma, and its suitability for particle acceleration is discussed. Compared to traditional approaches, this new autoresonant scheme achieves larger accelerating electric fields for given laser intensity; the plasma wave excitation is much more robust to variations in plasma density; it is largely insensitive to the precise choice of chirp rate, provided only that it is sufficiently slow; and the suitability of the resulting plasma wave for accelerator applications is, in some respects, superior. As in previous schemes, modulational instabilities of the ionic background ultimately limit the useful interaction time, but nevertheless peak electric fields approaching the wave-break...

Ultraintense and Ultrashort Laser Pulses from Raman Amplification in Plasma for Laser-Plasma Accelerators

We present analysis and simulations of electron trapping effect in the Raman pulse amplification in plasma. An ultraintense and ultrashort laser pulse is a very essential part in an advanced acceleration scheme using laser and plasma. Recently many prominent experimental observations and simulations were reported, where laser pulses of a few terawatt in power and less than 100 fs in the duration were used. To make such strong pulses, a novel scheme of using Raman backscatter in plasma was proposed and has been studied intensively. The Raman amplification in plasma does not have a restriction in material damage threshold. However, for the new amplifier to be a promising alternative of the CPA technique, more extensive studies on various issues are required. One of the fundamental issues is the electron kinetic effect such as particle trapping or wavebreaking. We describe averaged-particle-in-cell (aPIC) scheme to simulate the Raman amplification system and discuss the particle trapping effect using the aPIC model.