D. W. Forslund

Theory of stimulated scattering processes in laser‐irradiated plasmas

Analytic theory of the linear and nonlinear behavior of the one‐dimensional Brillouin and Raman scattering instabilities is given. Results are presented for the problems of an infinite homogeneous plasma and of a finite inhomogeneous plasma. Nonlinear fluid equations can predict backscatter energies the order of the incident laser energy; however, the size of the interaction region and nonlinear effects on the excited electrostatic wave are very important in determining the amount of backscatter. In many cases of contemporary interest for a high power laser incident on a target plasma, the latter effects can play a crucial role in reducing backscatter to a tolerable level.

Electromagnetic ion beam instabilities

The linear theory of electromagnetic instabilities driven by an energetic ion beam streaming parallel to a magnetic field in a homogeneous Vlasov plasma is considered. Numerical solutions of the full dispersion equation are presented. At propagation parallel to the magnetic field, there are four distinct instabilities. A sufficiently energetic beam gives rise to two unstable modes with right‐hand polarization, one resonant with the beam, the other nonresonant. A beam with sufficiently large T⊥/T∥ gives rise to the left‐hand ion cyclotron anisotropy instability at relatively small beam velocities, and a sufficiently hot beam drives unstable a left‐hand beam resonant mode. The parametric dependences of the growth rates for the three high beam velocity instabilities are presented here. In addition, some properties at oblique propagation are examined. It is demonstrated that, as the beam drift velocity is increased, relative maxima in growth rates can arise at harmonics of the ion cyclotron resonance for both...

An implicit method for electromagnetic plasma simulation in two dimensions

Abstract A new method for modeling low-frequency plasma phenomena is presented. The method uses an implicit formulation of the Vlasov-Maxwell equations to relax restrictions on the time-step and mesh spacing so that larger values which correspond to the frequencies and wavelengths of interest can be used. As a result, the range of length and time scales accessible to plasma simulation is increased by orders of magnitude. The algorithm, as embodied in a new code VENUS for electromagnetic plasmas in two dimensions, is described, its stability and accuracy analyzed through linear and nonlinear analysis, and its properties, including suppression of the finite grid instability, illustrated through its application to the Weibel instability.

Existence of rarefaction shocks in a laser‐plasma corona

General conditions under which rarefaction shocks can exist in the expanding corona of a plasma heated by a laser are derived. In particular, for the case of a two‐electron temperature isothermal plasma with temperatures Th and Tc, such a shock is shown to occur if Th/Tc≳5+√24. The case of rarefaction shocks induced by the ponderomotive force is also briefly discussed.

Nonlinear evolution of the lower‐hybrid drift instability

The results of simulations of the lower‐hybrid drift instability in a neutral sheet configuration are described. The simulations use an implicit formulation to relax the usual time step limitations and thus extend previous explicit calculations to weaker gradients, larger mass ratios, and long times compared with the linear growth time. The numerical results give the scaling of the saturation level, heating rates, resistivity, and cross‐field diffusion and a demonstration by comparison with a fluid electron model that dissipation in the lower‐hybrid drift instability is caused by electron kinetic effects.

Plasma simulation studies of stimulated scattering processes in laser- irradiated plasmas

The behavior of stimulated Brillouin and Raman scattering is studied by a number of periodic and aperiodic plasma simulations. The one‐dimensional studies at low power, roughly defined by the oscillating velocity of an electron being less than its thermal velocity, tend to support the spatial theory. Typically, in the aperiodic simulations trapping nonlinearities of the electrostatic wave and electron heating can saturate the amount of backscatter at a reasonable level. At high powers in both periodic and aperiodic simulations an x‐type breaking or wave collapse phenomenon occurs after saturation which leads to an equilibration between pondermotive force, electron pressure, and ion pressure. Simulations of a random frequency modulated laser are discussed. Two‐dimensional simulations and their relationship to the one‐dimensional results are briefly mentioned.

Theoretical derivation of laser induced plasma profiles

The hydrodynamic modification of an expanding plasma by coherent radiation for polarization out of the plane of incidence is investigated theoretically in the absence of parametric and modulational instabilities. The pondermotive force of the incident wave modifies the plasma flow from that of a rarefaction wave into a subsonic flow in the overdense region with a transition to supersonic flow at less than critical density. A constraint imposed at the sonic transition point allows a complete determination of the equilibrium as observed in simulations. The theory predicts that the average flow in the subcritical density region scales as the square root of the incident power.

The detuning of relativistic Langmuir waves in the beat‐wave accelerator

In the beat‐wave accelerator, a large‐amplitude Langmuir wave is produced by the beating of two laser beams whose frequencies differ by approximately the plasma frequency. The growth of this Langmuir wave saturates because of a nonlinear shift in its natural frequency. At present, there are three different formulas for the nonlinear frequency shift in the literature. By taking all relevant nonlinearities into account, the original result of Akhiezer and Polovin [Dokl. Akad. Nauk SSSR 102, 919 (1955)] is shown to be correct. The maximum amplitude of the Langmuir wave depends on the incident laser intensity and the frequency mismatch, which is the difference between the beat frequency of the incident waves and the plasma frequency. Two different studies have produced contradictory conclusions on the ‘‘optimum’’ frequency mismatch. The reasons for this contradiction are discussed and the result of Tang, Sprangle, and Sudan [Phys. Fluids 28, 1974 (1985)] is shown to be essentially correct. However, the requir...

Electromagnetic current instabilities

Linear electromagnetic waves in an infinite, homogenous current‐carrying Vlasov plasma with ion β≈1 are considered. Three instabilities emerge: a long wavelength, ’’kink‐like’’ mode, and two instabilities with wavenumbers greater than the reciprocal ion Larmor radius, the ’’electromagnetic ion acoustic instability’’ and the ’’whistler current instability.’’ The linear properties of these instabilities are investigated in detail; they have substantially lower thresholds than the electrostatic ion‐acoustic instability, and are generally favored by increasing ion β. In the regime of linear growth, the whistler current instability can give rise to an anomalous resistivity, and preferentially heats the ions.

An implicit moment electromagnetic plasma simulation in cylindrical coordinates

An electromagnetic PIC plasma simulation code incorporating the implicit moment method and a two-dimensional cylindrical mesh, with r- and z-coordinate dependence, has been developed. The code is an extension of the VENUS code from the original two-dimensional Cartesian mesh. The physical model employed in the code will be discussed, with emphasis on aspects unique to cylindrical geometry. An application to self-generated magnetic fields and electron transport in a laser-irradiated disk is presented that highlights the usefulness of cylindrical coordinates.