Low Harmonic Modes Research Articles

Influence of electron-electron collisions on the propagation of ion-acoustic space-charge waves in a warm plasma waveguide

The influence of electron–electron collisions on the propagation of the ion-acoustic space-charge wave is investigated in a cylindrical waveguide filled with warm collisional plasma by employing the normal mode analysis and the method of separation of variables. It is shown that the frequency of the ion-acoustic space-charge wave with higher-harmonic modes is always smaller than that with lower-harmonic modes, especially in intermediate wave number domains. It is also shown that the collisional damping rate of the ion-acoustic space-charge wave due to the electron–electron collision effect with higher-harmonic modes is smaller than that with lower-harmonic modes. In addition, it is found that the maximum position of the collisional damping rate shifts to large wave numbers with an increase of the harmonic mode. The variation of the wave frequency and the collisional damping rate of the ion-acoustic space-charge wave is also discussed.

Lorentzian characteristics of the Buneman space-charge wave in a kappa plasma-filled waveguide

The nonthermal and harmonic-mode effects on the Buneman space charge wave are investigated in a kappa dusty plasma-filled waveguide by employing the normal mode analysis and the method of separation of variables. The growth rate and the real frequency of the Buneman space charge wave are obtained as functions of the harmonic roots of the Bessel function and the spectral index of the kappa plasma. It is shown that the growth rate of Buneman instability and the real frequency with higher-harmonic modes are smaller than those with lower-harmonic modes. It is also shown that the nonthermal effect suppresses the growth rate as well as the real frequency of the Buneman space-charge wave. The variations of the growth rate and the real frequency of the space charge wave are also discussed.

Ion Bernstein instability dependence on the proton‐to‐electron mass ratio: Linear dispersion theory

AbstractFast magnetosonic waves, which have as their source ion Bernstein instabilities driven by tenuous ring‐like proton velocity distributions, are frequently observed in the inner magnetosphere. One major difficulty in the simulation of these waves is that they are excited in a wide frequency range with discrete harmonic nature and require time‐consuming computations. To overcome this difficulty, recent simulation studies assumed a reduced proton‐to‐electron mass ratio, mp/me, and a reduced light‐to‐Alfvén speed ratio, c/vA, to reduce the number of unstable modes and, therefore, computational costs. Although these studies argued that the physics of wave‐particle interactions would essentially remain the same, detailed investigation of the effect of this reduced system on the excited waves has not been done. In this study, we investigate how the complex frequency, ω = ωr+iγ, of the ion Bernstein modes varies with mp/me for a sufficiently large c/vA (such that ) using linear dispersion theory assuming two different types of energetic proton velocity distributions, namely, ring and shell. The results show that low‐ and high‐frequency harmonic modes respond differently to the change of mp/me. For the low harmonic modes (i.e., ωr∼Ωp), both ωr/Ωp and γ/Ωp are roughly independent of mp/me, where Ωp is the proton cyclotron frequency. For the high harmonic modes (i.e., , where ωlh is the lower hybrid frequency), γ/ωlh (at fixed ωr/ωlh) stays independent of mp/me when the parallel wave number, k∥, is sufficiently large and becomes inversely proportional to (mp/me)1/4 when k∥ goes to zero. On the other hand, the frequency range of the unstable modes normalized to ωlh remains independent of mp/me, regardless of k∥.

High-Efficiency Excitation of a Third-Harmonic Gyrotron

High-harmonic gyrotrons are challenging to generate high interaction efficiency and simultaneously suppress the mode competition. Investigation in this paper reveals that a high- $Q$ cavity is potential to realize high efficiency in a 94-GHz third-harmonic TE02 mode gyrotron. Unfortunately, the high- $Q$ cavity exhibits severe mode competition from the lower harmonic modes. A start-up scenario of active parameter control is employed to suppress the mode competition. The third-harmonic TE02 mode gyrotron finally achieves the steady single-mode operation with efficiency up to 20%. The physical mechanism during the mode formation process is theoretically investigated according to frequency-domain and time-domain nonlinear theories. The theoretical investigation in this paper is of guidance for future developing high-harmonic gyrotrons, especially toward terahertz applications.

Field-line resonances in a time-varying magnetosphere

Field line resonances, which represent shear Alfven standing modes of magnetohydrodynamic (MHD) waves are studied in space for the case in which magnetosphere is being perturbed in time by solarwind variations. A linearizedMHD wave model has been developed with time-varying Alfven speeds. When the magnetosphere becomes quiet, field line resonances consistently occur over many harmonics. A time-varying Alfven speed is found to cause a significant broadening of each spectral peak, which strongly depends on the frequency. While fundamental mode or lower harmonics become relatively stable with less broadening, higher harmonics appear with strong dispersion. Our results are consistent with the observed feature that fundamental or lower harmonic modes are dominant and higher harmonics are less frequently excited in space where the system is often perturbed in time.

Nonlinear Dynamics of Longitudinal Oscillations in a Homogeneous Collisionless Plasma with an External Magnetic Field

The effect of wave-wave scattering of longitudinal oscillations in a homogeneous collisionless plasma with an external magnetic field is derived. The nonlinear dynamics of the modes existing near multiples of the electron cyclotron frequency are discussed, and it is shown that resonant wave-wave scattering of unstable low harmonic modes is an important mechanism for transferring energy to the higher harmonics.