Cherenkov Instability Research Articles

Electromagnetic wave amplification of cyclotron Cherenkov maser

We use the Vlasov-Maxwell equations to investigate the kinetic instability caused by the interaction of the electron beam with the electromagnetic wave in small signal limit. The orthogonality condition of the electromagnetic modes has been used to derive the dispersion relation for the dielectric-loaded cylindrical waveguide driven by a relativistic electron beam when a constant guiding magnetic field is applied. When the guiding field is not so strong, there arises cyclotron Cherenkov instability as well as Cherenkov instability. We show that the cyclotron Cherenkov instability has a growth rate comparable to that of the Cherenkov instability, and the amplification of an electromagnetic wave is possible in the range of millimeter to sub-millimeter arising from the anomalous Doppler effect.

Cyclotron-Cherenkov and Cherenkov instabilities

The effect of the conventional Cherenkov instability on the cyclotron-Cherenkov maser is investigated for the case of a relativistic electron beam propagating along a background magnetic field in a dielectric using particle simulations. Both Cherenkov and cyclotron-Cherenkov instabilities are excited when the phase velocity of light in the dielectric is less than the beam velocity. It is demonstrated in the high-power regime, where the cyclotron-Cherenkov mode has the higher growth rate, that the Cherenkov mode has little effect on the nonlinear efficiency of the cyclotron-Cherenkov mode. High-efficiency is possible, confirming previous predictions based on single mode calculations. The effect of beam momentum spread is studied.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electrostatic plasma waves and HF auroral hiss generated at low altitude

Large‐amplitude (10 to 100 μV m−1 Hz−½) natural radio emissions in a wide frequency range (100 kHz up to 2 MHz) are frequently observed on board the AUREOL/ARCAD 3 satellite at high latitude and at altitudes between 400 and 2000 km. The simultaneous measurement of the local cold plasma density allows the identification of cutoff and resonance frequencies. Three different kinds of wave are observed: (1) electrostatic emissions near the local value of the plasma frequency (fp), (2) electromagnetic whistler mode emissions, sometimes associated with type (1) emissions, and (3) electromagnetic Z mode emissions, also associated with type 1 emissions, but occurring more rarely than the whistler mode emissions and then only when fp is greater than the electron cyclotron frequency (fce). These emissions are always associated with high levels of ELF electrostatic turbulence and a high flux of low‐energy precipitating electrons, extending in energy down to the lower limit of the detectors (∼100 eV). The statistical distribution of the emissions in geomagnetic coordinates shows an occurrence greater than 80% in the polar cusp region and between 25% and 60% in the nightside auroral zone. A generation mechanism for such emissions is proposed, based on the calculation of the growth rate of the kinetic Cherenkov instability, associated with a beamlike suprathermal tail in the parallel distribution of the bulk electron population. In particular, suprathermal, downward electron beams of about 100–200 eV energy, with a thermal spread of the same order, are found to be responsible for the generation of whistler mode and Z mode HF emissions in a source region extending down to 1000 km of altitude. It is suggested that such intense radio emissions should be considered as one of the energy dissipation processes resulting from magnetosphere‐ionosphere coupling, through interdependent electrodynamic mechanisms such as current systems, parallel anomalous resistivity, plasma turbulence, energy diffusion, and heating.

On theory of Cherenkov instability of ultrarelativistic beams with one-dimensional distribution of momenta of particles

It is shown that in a beam-plasma system with one-dimensional ultrarelativistic distribution of momenta of particles two regimes of Cherenkov instability-the monoenergetic one and that with the high thermal spread in momenta-are possible. It is shown that the growth rate of instabilities of the beam with the high thermal spread can be greater than that of the monoenergetic beam if the thermal spread is not too high. The physical reason for this faster growth rate is discussed and the applicability region of this result is obtained. It is suggested that the effect being discussed should be important in the theory of pulsar plasma.

Canonical momenta and numerical instabilities in particle codes

A set of warm plasma dispersion relations appropriate to a large class of electromagnetic plasma simulation codes is derived. The numerical Cherenkov instability is shown by analytic and numerical analysis of these dispersion relations to be the most significant nonphysical effect involving transverse electromagnetic waves. The instability arises due to a spurious phase shift between resonant particles and light waves, caused by a basic incompatibility between the Lagrangian treatment of particle positions and the Eulerian treatment of particle velocities characteristic of most PIC-CIC algorithms. It is demonstrated that, through the use of canonical momentum, this mismatch is alleviated sufficiently to completely eliminate the Cherenkov instability. Collateral effects on simulation accuracy and on other numerical instabilities appear to be minor.