Mode-Coupling Analysis of Parametric Decay Instability in Magnetized Plasmas

In this paper, derived from Maxwell and fluid equations of plasmas, unified nonlinear wave equations are used to describe the parametric decay instability （PDI） in magnetized plasmas, and in view of mode-coupling, we can obtain all the possible PDI channels. By solving the nonlinear equations with a mode-coupling method, we obtain the growth rate of the PDI, of which all of the three waves are ordinary mode （O-mode） or extraordinary mode （X-mode） wave. Under the dipole approximation, an explicit formula of the growth rate of the X-mode and the condition of the equilibrium density scale are obtained. According to the existence conditions of three X-mode waves, this kind of instability might exist in ECRH with the second harmonic X-mode wave.

Numerical study on matching conditions of Langmuir parametric instability and the formation of Langmuir turbulence in ionospheric heating

Parametric decay instability(PDI)is an important process in ionospheric heating.This paper focuses on the frequency and wavevector matching condition in the initial PDI process,the subsequent cascade stage,and the generation of strong Langmuir turbulence.A more general numerical model is established based on Maxwell equations and plasma dynamic equations by coupling highfrequency electromagnetic waves to low-frequency waves via ponderomotive force.The primary PDI,cascade process,and strong Langmuir turbulence are excited in the simulation.The matching condition in the initial PDI stage and cascade process is verified.The result indicates that the cascade ion acoustic wave may induce or accelerate the formation of cavitons and lead to the wavenumber spectrum being more enhanced at 2k_(L)(where k_(L) is the primary Langmuir wavenumber).The wavenumber spectra develop from discrete to continuous spectra,which is attributed to the caviton collapse and strong Langmuir turbulence.

Spectral Broadening of Ion Bernstein Wave Due to Parametric Decay Instabilities

The parametric decay instabilities （PDIs） of ion Bernstein wave with different input power levels are investigated via particle-in-cell simulation. It is found that the number of decay channels increases with the input power. Resonant mode-mode couplings dominate for a low input power. With increasing the input power, the nonresonant PDIs appear to dissipate the energy of the injected wave and give rise to edge ion heating. The generated child waves couple with each other as well as the injected wave and /or act as a pump wave to excite new decay channels. As a result, the frequency spectrum is broadened with the increase of the input power.

Overshoot phenomena:Observation and simulation

High-power O-mode radio waves can excite artificial instabilities in the F region,according to experiments conducted at the European Incoherent Scatter Science Association(EISCAT)heating facility.The main instabilities include the parametric decay instability(PDI),oscillating two-stream instability(OTSI),and thermal parametric instability(TPI).The PDI and OTSI not only compete with each other,but also compete with the TPI,leading to a two-stage overshoot phenomenon:a miniovershoot occurs on a millisecond time scale after pump-on,followed by the main overshoot.We gain insight into the miniovershoot via a generalized Zakharov model,whereas the main overshoot can be observed as an enhanced plasma line overshoot phenomenon in incoherent scatter radar spectra.We can also observe that the zero-frequency ion line exists only in the initial heating period after a cold start and that the upshifted and downshifted ion lines behave irregularly in the spectra.The simulation results show that competition between the PDI and OTSI leads to an initial peak,which we named the pre-miniovershoot.The following processes,namely ion density caviton generation,and collapse and cascade in the development of the PDI,contribute to the miniovershoot phenomenon.

Evidence of X-mode heating suppressing O-mode heating

In this study,we present three experiments carried out at the EISCAT(European Incoherent Scatter Scientific Association)heating facility on October 29 and 30,2015.The results from the first experiment showed overshoot during the O-mode heating period.The second experiment,which used cold-start X-mode heating,showed the generation of parametric decay instability,whereas overshoot was not observed.The third experiment used power-stepped X-mode heating with noticeable O-mode wave leakage.Parametric decay instability and oscillating two-stream instability were generated at the O-mode reflection height without the overshoot effect,which implies suppression of the thermal parametric instability with X-mode heating.We propose that the electron temperature increased because X-mode heating below the upper hybrid height decreased the growth rate of the thermal parametric instability.

Electron acceleration by Langmuir turbulence in ionospheric heating

In this study,we investigate the generation of parametric decay instability,Langmuir turbulence formation,and electron acceleration in ionospheric heating via a two-fluid model using the Fokker-Planck equation and Vlasov-Poisson system simulations.The simulation results of both the magnetofluid model and the kinetic model demonstrate the dynamics of electron acceleration.Further,the results of the Vlasov-Poisson simulations suggest the formation of electron holes in phase space at the same spatial scale as the Langmuir wave,which are shown to be related to electron acceleration.In addition,electron acceleration is enhanced through the extension of the wavenumber spectrum caused by strong Langmuir turbulence,leading to more electron holes in phase space.