Analytic expressions for mode conversion in a plasma with a linear density profile

Abstract

The transformation of electromagnetic waves into Langmuir oscillations (and vice versa) is explicitly examined in the vicinity where the wave frequency matches the electron plasma frequency in an inhomogeneous plasma. For an unmagnetized plasma with a linear density profile of scale length L, closed-form, analytic expressions are derived, in terms of Airy functions, for the reflection and mode conversion coefficients of Langmuir and electromagnetic waves utilizing a source approximation that is valid when the electromagnetic field scale length is large compared to that of the electrostatic field. The technique developed to determine the energy flux coefficients and the fields is general enough to apply to a plasma with a profile other than a linear one, and should prove useful in other problems where a scale length separation is valid. The reflection coefficient for the ‘‘direct’’ problem (incident electromagnetic wave) is equal in magnitude to that of the ‘‘inverse’’ problem (incident electrostatic wave) and the corresponding mode conversion coefficients satisfy energy conservation. The mode conversion coefficient of the warm, collisionless problem is independent of electron temperature, T, in the limit of small T, and is equal in magnitude to the mode conversion coefficient of the cold, collisional problem, which is likewise independent of the collision frequency. It also depends on the angle of incidence θ and the vacuum scale length k0L, and for T/mc2≪1, agrees with earlier, numerical calculations.

Broadband wide-angle absorption enhancement due to mode conversion in cold unmagnetized plasmas with periodic density variations

“…In the case where the mode conversion in cold unmagnetized plasmas occurs at only one value of z in space, it has been well-known that the maximum value of A is about 0.5 [18,19]. Therefore, the present result confirms that mode conversion is greatly enhanced due to the fact that it occurs within a transmission band of the PPC.…”

“…The irreversible transfer of electromagnetic wave energy to the resonance region associated with the mode conversion occurring in unmagnetized or magnetized inhomogeneous plasmas plays a central role in a wide range of phenomena in plasma physics [17][18][19][20][21]. We are mainly interested in the mode conversion of transverse electromagnetic waves into longitudinal plasma oscillations in cold unmagnetized plasmas, which occurs at the regions where the local dielectric permittivity vanishes, or equivalently, the wave frequency matches the local plasma frequency [18,19,[22][23][24].…”

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

“…This is a more restrictive region of parameter space than previously calculated (e.g. Hinkel-Lipsker et al 1992;Yin & Ashour-Abdalla 1999).…”

Attosecond control of collective electron motion in plasmas

“…In the steep plasma density gradient, these plasma oscillations, which are initially longitudinal electrostatic modes, can couple to transverse electromagnetic modes and thus radiate light at the plasma frequency ωp(x) [21]. This can be seen as the time-reverse of resonant absorption [22], where laser light is partially converted into collective electron oscillations in a gradient of plasma density (both effects correspond to linear mode conversion mechanisms [22,23]). This emission, known as coherent wake emission or CWE [24], consists of a sub-femtosecond burst of coherent radiation, superimposed with the laser light reflected at the plasma surface [25], with a spectrum extending into the extreme ultraviolet (XUV) up to ωmax = N solid /Nc ωL, emitted from the region of solid plasma density N solid (Fig.…”