Anomalous Doppler Resonance Research Articles

Dynamics of runaway tails with time-dependent sub-Dreicer dc fields in magnetized plasmas

The evolution of runaway tails driven by sub-Dreicer time-dependent dc fields in a magnetized plasma are studied numerically using a quasilinear code based on the Ritz–Galerkin method and finite elements. It is found that the runaway tail maintained a negative slope during the dc field increase. Depending on the values of the dc electric field at t=0 and the electron gyrofrequency to the plasma frequency ratio the runaway tail became unstable to the anomalous Doppler resonance or remained stable before the saturation of the dc field at some maximum value. The systems that remained stable during this stage became unstable to the anomalous Doppler or the Čerenkov resonances when the dc field was kept at the saturation level or decreased. Once the instability is triggered, the runaway tail is isotropized.

Classical single-particle dynamics of the anomalous Doppler resonance

A gyroaveraged Lagrangian is used to describe the motion of a single electron undergoing anomalous Doppler resonance with a wave that has both electrostatic and electromagnetic components, propagating at arbitrary inclination to the magnetic field. It is shown that the flows of parallel and perpendicular energy are oppositely directed, and have magnitudes in the ratio 1+ω/(Ω/γ):1. The change in perpendicular kinetic energy, or equivalently Larmor radius, is related very simply to the E×B drift occurring at Landau resonance. The classical single-particle description of the anomalous Doppler resonance underlies the existing classical collective descriptions, and is the classical counterpart to the well-known quantum single-particle description.

The single-particle and collective descriptions of the anomalous Doppler resonance and the role of ion dynamics

The connection between three aspects—single-particle, beam, and continuous velocity distribution—of the anomalous Doppler effect are investigated. The key quantity is the power Re(j ⋅ E*) dissipated by electrostatic waves interacting with a non-Maxwellian electron velocity distribution. Its spatial components describe energy flows that are the counterparts in classical electrodynamics to the quantum properties of the single-particle anomalous Doppler effect. A complete plasma physics treatment requires the inclusion of ion dynamics. Examination of the large-k region of wavenumber space—in contrast to the well-known small-k region—shows that ion Landau damping is important in stabilizing plasmas for which the electron velocity distribution considered alone is destabilizing. Finally, by choosing simple models for the superthermal electron velocity distribution, general features of the instability, and those specific to particular tail models, are identified analytically and numerically.

Particle propagation effects on wave growth in a solar flux tube

view Abstract Citations (25) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Particle Propagation Effects on Wave Growth in a Solar Flux Tube White, S. M. ; Melrose, D. B. ; Dulk, G. A. Abstract The evolution of a distribution of electrons is followed after they are injected impulsively at the top of a coronal magnetic loop, with the objective of studying the plasma instabilities which result. At early times the downgoing electrons have beamlike distributions and amplify electrostatic waves via the Cerenkov resonance; the anomalous Doppler resonance is found to be less important. Slightly later, while the electrons are still predominantly downgoing, they are unstable to cyclotron maser generation of z-mode waves with omega(p) much less than Omega, or to second harmonic x-mode waves. The energetics of these instabilities, including saturation effects and heating of the ambient plasma, are discussed. It is suggested that coalescence of two z-mode waves generated by cyclotron maser emission of the downgoing electrons may produce the observed microwave spike bursts. Publication: The Astrophysical Journal Pub Date: September 1986 DOI: 10.1086/164512 Bibcode: 1986ApJ...308..424W Keywords: Magnetohydrodynamic Stability; Particle Acceleration; Plasma Diagnostics; Solar Flares; Solar Magnetic Field; Cerenkov Radiation; Cyclotron Radiation; Electron Distribution; Solar Physics; PARTICLE ACCELERATION; SUN: FLARES; SUN: MAGNETIC FIELDS full text sources ADS |

Runaway tails in magnetized plasmas

The evolution of a runaway tail driven by a dc electric field in a magnetized plasma is analyzed. Depending on the strength of the electric field and the ratio of plasma to gyrofrequency, there are three different regimes in the evolution of the tail. The tail can be (a) stable with electrons accelerated to large parallel velocities, (b) unstable to Čerenkov resonance because of the depletion of the bulk and the formation of a positive slope, (c) unstable to the anomalous Doppler resonance instability driven by the large velocity anisotropy in the tail. Once an instability is triggered (Čerenkov or anomalous Doppler resonance) the tail relaxes into an isotropic distribution. The role of a convection type loss term is also discussed.

Collisionless Effects on Beam-Return Current Systems in Solar Flares

A large fraction of the electrons which are accelerated during the impulsive phase of solar flares stream towards the chromosphere and are unstable to the growth of plasma waves. The linear and nonlinear evolution of plasma waves as a function of time is analyzed with the use of a set of rate equations that follow in time the non-linearly coupled system of plasma waves-ion fluctuations. The nonthermal tail formed during the stabilization of the precipitated electrons can stabilize the Anomalous Doppler Resonance instability and prevent the isotropization of the energetic electrons. The precipitating electrons modify the way the return current is carried by the background plasma. In particular, the return current is not carried by the bulk of the electrons but by a small number of high velocity electrons. For beam/plasma densities ≳ 10−3, this can reduce the effects of collisions and heating by the return current. For higher density beams where the return current could be unstable to current driven instabilities, the effects of strong turbulence anomalous resistivity is shown to prevent the appearance of such instabilities. Our main conclusion is that the beam-return current system is interconnected and how the return current is carried is determined by the beam generated strong turbulence.

Anomalous Doppler Resonance of Relativistic Electrons with Lower Hybrid Waves Launched in the Frascati Tokamak

Relativistic runaway electrons are detected by hard-x-ray and (photo)neutron emissions in low-density experiments with lower hybrid waves. During the rf pulse these signals can be explained in terms of the anomalous Doppler resonance on the primary lower hybrid waves which is effective on electrons having energy > 10 MeV. This effect is present independently of the density limit which determines the efficiency of the other resonance $\ensuremath{\omega}={k}_{\ensuremath{\parallel}}{v}_{\ensuremath{\parallel}}$; the latter is responsible for electron tail enhancement (up to 300 keV) and for wave absorption, heating, and current-drive effects.

Anomalous resistivity due to low‐frequency turbulence

Large amplitude ion cyclotron waves have been observed on auroral field lines. In the presence of an electric field parallel to the ambient magnetic field these waves prevent the acceleration of the bulk of the plasma electrons leading to the formation of a runaway tail. It is shown that low‐frequency turbulence can also limit the acceleration of high‐velocity runaway electrons via pitch angle scattering at the anomalous Doppler resonance.

Observation of lower-hybrid-current drive in the JIPP T-II torus

Current driven by injecting lower hybrid waves has been observed in low-density plasmas in the JIPP T-II. It is confirmed that RF-driven current is generated by momentum transfer from lower hybrid waves to suprathermal electrons with an energy of 8–25 keV. The driving efficiencies in tokamak and stellarator configurations are 1 kA·kW−1 and 0.3 kA·kW−1, respectively, at a power level of 40 kW. Enhanced electron cyclotron emission due to pitch-angle scattering of RF-driven suprathermal electrons is observed, and spikes in loop voltage and X-ray bursts appear coincidentally. In a long RF-pulse, these rapid changes advance to relaxation oscillations. It is concluded that the pulsating changes originate in the instantaneous scattering of RF-driven suprathermal electrons by the unstable waves excited at an anomalous Doppler resonance.