Theory and simulation of resonant absorption in a hot plasma

Abstract

A detailed theoretical and simulation study of resonant absorption in a hot plasma is presented which isolates the behavior of the plasma for times short compared to an ion response time. The extent to which an electron fluid model can describe the absorption process in the kinetic regime is discussed. At high intensities the absorbed energy is observed to be deposited in a suprathermal tail of electrons whose energy varies approximately as the square root of the incident power. The density profile modification due to the ion response to the ponderomotive force is also discussed.When an electromagnetic wave is obliquely incident on an inhomogeneous plasma and polarized in the plane of incidence, it is well known that it can be absorbed resonantly by linear mode conversion into an electron plasma wave. ' ' This process, known as resonant absorption, has important implications for laser target experiments and microwave laboratory experiments. ' Most theoretical work has been done for a cold plasma, '" ' while warm-plasma calculations have been either incomplete' or incorrect. 'For gradient lengths long compared to the wavelength of light or koL»1 (where ko is the incident free-space wave number and L is the density scale length), computer simulations in a hot plasma, with fixed ions show that the absorption coefficient is virtually unmodified from the cold-electron case. Theoretical calculations based on a fluid description which agree with these computer simulations indicate that the absorption coefficient is virtually unmodified for temperatures up to 100 keV. At low' intensities these theoretical calculations predict the field structures seen in simulations, while at high intensities a nonlinear dissipation must be added to obtain agreement. This nonlinear dissipation is required at high intensities to account for the acceleration towards the low-density region of the plasma of a small number of electrons to very high energies. To describe resonant absorption in a hot plasma, we combine the linearized electron-momentum equation with Maxwell's equations. An adiabatic pressure law is assumed for the high-frequency electron motion, ion motion is neglected, and the fields are assumed to vary as e' '. V~E = 4~en"V x E = i (~/c) B-, VxB =(4n/c)7 -(i~/c) E, e'in, E ieT, Vn" J= . + yVn -n m(tvvv'v) m(tvviv ) ' ' v, )'where n, is the background plasma density; T, , m, and e are the electron's temperature, mass, and charge, respectively; c is the velocity of light; and y is the usual ratio of specific heats and is chosen equal to 3. In factoring the damping in the electron-momentum equation a different damping rate appears in the electric field term than in the electron pressure term. The significance of this phenomenological damping is discussed below. Combining these equations we obtain the general steady-state wave equation for E:where~~=47)noe'/m. ln particular, we consider the case of a slab of plasma with no =no(x) only and the electromagnetic wave obliquely incident on this slab, with the electric field polarized i...

Mode Conversion and Reflection of Langmuir Waves in an Inhomogeneous Solar Wind

“…In Sections 3 and 4, these equations are solved numerically with appropriate boundary conditions to study the physics of Langmuir wave reflection and mode conversion in an inhomogeneous medium. This derivation closely follows similar derivations in Forslund et al (1975) and Means et al (1981). In order to simplify the analysis, the following assumptions are made:…”

“…Many earlier analyses focused solely on calculating the mode conversion efficiency, which may be determined without resorting to a full wave field solution to the wave equations. One exception in the early literature is Forslund et al (1975), who numerically solved the wave equations for the 'direct' problem in a warm unmagnetised plasma, where a transverse wave incident from a vacuum encounters a linear density gradient. This approach was generalised by Means et al (1981) for incident waves from a homogeneous plasma (not necessarily a vacuum).…”

“…However, these analyses rely on an approximation which only allows an accurate estimation of the wave fields very close to the mode conversion point. For this reason, we follow the approach of Forslund et al (1975) and Means et al (1981), which yields exact solutions for the wave fields and does not involve any approximations beyond the assumptions that form the basis of the model. This paper is structured as follows: in Section 2, the derivation of the mode conversion equations is outlined.…”

Collisional/resonance absorption in cold/warm magnetized plasmas of the F‐region high‐latitude ionosphere

“…This partition of the conductivity tensor is due to the use of n L in the electron pressure term in the electron momentum equation. The use of the phenomenological damping rate permits large damping of the ES component of the wave without altering the EM component [Forslund et al, 1975]. The linear theory of weak collisionless damping can be used to approximate the damping rate of the propagating short plasma waves.…”

“…The study of LMC in an inhomogeneous magnetized warm plasma is a complex problem whose analytical treatment becomes cumbersome, and the solution of the full wave equations by numerical simulations is required. The resonant absorption by conversion of EM waves into ES waves has been simulated for a hot unmagnetized laboratory plasma by Forslund et al [1975]. Most recently, the LMC process has been numerically studied in magnetized cold ionospheric plasmas by Gondarenko et al [2003].…”