Resonant enhancement of mode conversion in unmagnetized plasmas due to a periodic density modulation superimposed on a linear electron density profile

Abstract

Mode conversion of p-polarized electromagnetic waves into longitudinal plasma oscillations at resonance points in cold, unmagnetized, and stratified plasmas, where a small periodic density modulation is superimposed on a linear electron density profile, is theoretically studied. The mode conversion coefficient and the magnetic field distribution are calculated in a numerically exact manner using the invariant imbedding theory of mode conversion. It is found that resonant enhancement of mode conversion, which is sometimes as high as 100%, can be achieved by tuning the incident angle, the modulation period, or the wave frequency. This phenomenon is explained as due to the formation of a standing wave near the resonance point.

Temperature dependence of mode conversion in warm, unmagnetized plasmas with a linear density profile

“…33 In this paper, we restudy the temperature effect on the linear mode conversion between electromagnetic and Langmuir waves and calculate the mode conversion coefficients for the forward and backward mode conversion processes in a numerically accurate manner, using the invariant imbedding method developed previously by two of us. 14,15,28,[36][37][38][39][40][41][42][43][44] We verify that the principle of reciprocity for the forward and backward mode conversion coefficients holds precisely regardless of temperature and that the mode conversion peak shifts as the temperature increases. The detailed dependence of the mode conversion coefficient on temperature is different from those of previous results and is substantially stronger than the results of other studies.…”

“…[29][30][31][32][33][34][35] Most studies have been focused on the direct problem, namely the conversion of electromagnetic waves into Langmuir waves. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] It has been found that in the cold plasma limit, the mode conversion coefficient for the case with a linear or parabolic density profile has a universal dependence on some scaling parameter Q, which is a combination of the wave frequency and the incident angle, with a maximum value of about 0.5. In warm plasmas, it has been claimed that the mode conversion coefficient is approximately unchanged for temperatures up to 100 keV.…”

Effects of a random spatial variation of the plasma density on the mode conversion in cold, unmagnetized, and stratified plasmas

“…In a recent work, we have shown that the mode conversion efficiency can be greatly enhanced when a small periodic density modulation is superimposed on a linear profile. 22 In an earlier related work, 25 Willes and Cairns have shown that the conversion efficiency is greatly increased when an integer number of electrostatic wavelengths fit within a cavity superposed on a density profile. Another important case is that of a random density variation added to a slowly varying density profile.…”

“…Due to the symmetry between the forward mode conversion from electromagnetic waves into electrostatic oscillations and the corresponding inverse process, 12 our results are directly relevant to the inverse case, too. Using the invariant imbedding method, [17][18][19][20][21][22][23][37][38][39] we calculate the mode conversion coefficient, which is defined to be the fraction of the incident wave power converted into electrostatic oscillations, and the electromagnetic field distribution in a numerically exact manner for configurations in which a numerically generated random a) Electronic address: khkim@ajou.ac.kr 1070-664X/2013/20(12)/122104/6/$30.00 V C 2013 AIP Publishing LLC 20, 122104-1 density variation is added to the background linear density profile. We repeat similar calculations for a large number of independent random configurations and take an average of the results.…”

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

“…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]. In this paper, we consider plasma density configurations where this resonance condition is satisfied periodically along one direction in space.…”