Drift Cyclotron Loss Cone Mode Research Articles

Stability of drift-cyclotron loss-cone waves in H-mode plasmas

The drift-cyclotron loss-cone mode was first studied in mirror machines. In such devices, particles with small pitch angles are not confined, creating a hole in the velocity distribution function that is a source of free energy and leads to micro-instabilities in the cyclotron-range of frequencies. In the edge region of tokamak devices operating under H-mode conditions, ion loss also occurs. In this case, gradient drift carries ions moving opposite to the plasma current preferentially into the divertor, creating a one-sided loss cone. A simple analysis shows that for the quiescent H-mode plasmas in DIII-D the critical gradient for instability is exceeded within 2 cm of the separatrix, and the maximum growth rate at the separatrix is 3 × 107 s−1.

Studies of a Straight Field Line Mirror with Emphasis on Fusion-Fission Hybrids

The straight field line mirror (SFLM) field with magnetic expanders beyond the confinement region is proposed as a compact device for transmutation of nuclear waste and power production. A design with reactor safety and a large fission-to-fusion energy multiplication is analyzed. Power production is predicted with a fusion Q = 0.15 and an electron temperature of ~500 eV. A fusion power of 10 MW may be amplified to 1.5 GW of fission power in a compact hybrid mirror machine. In the SFLM proposal, quadrupolar coils provide stabilization of the interchange mode, radio-frequency heating is aimed to produce a hot sloshing ion plasma, and magnetic coils are computed with an emphasis on minimizing holes in the fission blanket through which fusion neutrons could escape. Neutron calculations for the fission mantle show that nearly all fusion neutrons penetrate into the fission mantle. A scenario to increase the electron temperature with a strong ambipolar potential suggests that an electron temperature exceeding 1 keV could be reached with a modest density depletion by two orders in the expander. Such a density depletion is consistent with stabilization of the drift cyclotron loss cone mode.

Effect of general loss-cone distribution function on electrostatic ion-cyclotron instability in an inhomogeneous plasma: particle aspect analysis

The electrostatic ion-cyclotron instability (EICI) in low β (ratio of plasma to magnetic pressure), anisotropic, inhomogeneous plasma is studied by investigating the trajectories of the particles using the general loss-cone distribution function (Dory-Guest-Harris type) for the plasma ions. In particular, the role of the loss-cone feature as determined by the loss-cone indices, in driving the drift-cyclotron loss-cone (DCLC) instability is analysed. It is found that for both long and short wavelength DCLC mode the loss-cone indices and the perpendicular thermal velocity affect the dispersion equation and the growth rate of the wave by virtue of their occurrence in the temperature anisotropy. The dispersion relation for the DCLC mode derived here using the particle aspect analysis approach and the general loss-cone distribution function considers the ion diamagnetic drift and also includes the effects of the parallel propagation and the ion temperature anisotropy. It is also found that the diamagnetic drift velocity due to the density gradient of the plasma ions in the presence of the general loss-cone distribution acts as a source of free energy for the wave and leads to the generation of the DCLC instability with enhanced growth rate. The particle aspect analysis approach used to study the EICI in inhomogeneous plasma gives a fairly good explanation for the particle energisation, wave emission by the wave–particle interaction and the results obtained using this particle aspect analysis approach are in agreement with the previous theoretical findings using the kinetic approach.

Amplitude Oscillation of an Instability in a Plasma in the Presence of a Damping Mechanism

The time evolution of an instability in a plasma in the presence of a damping mechanism is studied. The drift cyclotron loss-cone mode in a mirror plasma is adopted as an example. A quasilinear model and a simulation code taking into account the electron bounce resonance damping have been developed to describe the amplitude oscillation of this mode, which has been observed experimentally. It was found that this oscillatory behavior of the amplitude is caused by the temporal variation of the growth rate and the effect of electron bounce resonance damping on the amplitude of this mode.

Observations of the dependence of unstable drift cyclotron loss cone mode characteristics on plasma density

Results of precise measurements on the variation of drift cyclotron loss cone frequency and perpendicular wavenumber as a function of ion density are reported. Careful quantitative comparisons to theory are made. In particular, it was found that a local linear slab model accurately predicted the unstable mode frequencies, even under conditions where nonlocal effects should be significant—i.e., when characteristic gradient scale lengths are on the order of an unstable mode’s wavelength. Inaccurate predictions of unstable perpendicular wavelength, however, may be a consequence of nonlocal effects.

Effects of the fast wave on the drift-cyclotron loss-cone mode

Particle orbits in the presence of the fast wave fields are calculated. It is found that the resonant interaction of ions with the wave can be neglected at ω0 = 2ωci, provided that the wave electric field strength is less than a critical value Ec( = 4k∥T/e). The dominant effect on the dispersion relation of the drift‐cyclotron loss‐cone mode is thus found to be due to the nonresonant effects of the wave on electrons and ions. It is found that a fast wave with ω0 = 2ωci destabilizes the mode. For ω0<2ωci, fast waves can stabilize the mode for certain values of the plasma β, which is calculated numerically.

Warm plasma effects on drift cyclotron loss cone mode

Although plasma density gradients in mirror machines are larger than the theoretical threshold for the drift cyclotron loss cone instability, turbulence is experimentally observed only at low harmonics of the ion cyclotron frequency. The evolution of the drift cyclotron loss cone instability as the loss cone velocity distribution is progressively filled with a warm Maxwellian component is studied. The addition of the warm plasma results in both a lowering of the growth rate and a distortion and narrowing of the frequency spectrum. Under conditions typical of an almost filled loss cone, the numerical result shows that the growth rate is largest for the lower ion cyclotron harmonics and is quite consistent with experimental observations.

Magnetic mirror confinement of laser-produced LiH plasmas

The results of measurement and analysis of the decay of mirror-confined lithium hydride plasmas are reported. The plasmas are produced by laser irradiation of a solid LiH particle suspended at the center of a mimimum-B magnetic field. In the initial state, the ratio of the Li3+ energy to H+ energy is 7 to 1, the mean energy of lithium ions is about 2000 eV, and the electrons are cold. In the subsequent decay of the mirror-confined plasmas from 3×1013 to 1012 cm−3, the H+ lifetime is sufficiently shorter than the lifetime of the Li3+ ions so that the plasma evolves to a lithium plasma. The density decay is faster than classical but quiescent for the first 250 μsec. After that time, there are sudden increases in the plasma decay rate and in the rf emission at the central cyclotron frequency of lithium ions, interpreted as evidence of the onset of the drift cyclotron loss cone instability. The quiescent behavior is correlated with the observation of optical radiation from the plasma between 70 and 250 μsec. The plasma luminosity is interpreted as evidence for the creation of cold plasma by ionization of cold neutral reflux from the walls of the baseball coil. A quasi-linear Fokker–Planck calculation of the ion distributions and the growth and damping of the drift cyclotron loss cone mode yielded results consistent with an explanation of the quiescent decay as the suppression of the instability by the cold plasma.

Nonlinear Landau damping in mirror plasmas

The well-known drift cyclotron loss cone mode is shown to couple nonlinearly to a low frequency stable Rayleigh–Taylor mode through the process of nonlinear Landau damping. When the drift cyclotron loss cone mode is linearly marginally stable, this coupling leads to an explosive instability, the time scale for which is in some agreement with experimental time scales.

Stabilization of Drift-Cyclotron Loss-Cone Mode by Low-Frequency Density Fluctuations

Low-frequency ($\ensuremath{\omega}\ensuremath{\ll}{\ensuremath{\omega}}_{\mathrm{ci}}$) density fluctuations (which may be produced by the drift-wave instability of the cold-ion species) can stabilize the drift-cyclotron loss-cone (DCLC) mode by scattering electrons and thereby providing anomalous high-frequency resistivity to the DCLC mode.

Warm plasma stabilization of drift-cone mode at finite beta

The fraction of warm plasma needed to stabilize the drift cyclotron loss-cone mode in a mirror machine is computed. With warm plasma present, a remnant short wavelength instability persists.

Turbulent Lifetimes in Mirror Machines

A quasilinear model is developed describing the time evolution of a mirror-confined plasma which is unstable to the drift-cyclotron loss-cone mode. Good correlation is obtained with experiments with and without an external stream of cold plasma.