Showing papers by "Liu Chen published in 2003"

Bounce precession fishbones in the national spherical torus experiment

Abstract: Bursting modes, which are identified as bounce precession frequency fishbone modes, are observed on a low aspect ratio tokamak, the national spherical torus experiment (Ono M. et al 2000 Nucl. Fusion 40 557). They are predicted to be important in high current, low shear discharges with a significant population of trapped particles with a large mean bounce angle, such as produced by near tangential beam injection into a small aspect ratio device. Such a distribution is often stable to the usual precession-resonance fishbone mode. These modes could be important in ignited plasmas, driven by the trapped alpha particle population.

Kinetic theory of geomagnetic pulsations: 4. Hybrid gyrokinetic simulation of drift‐bounce resonant excitation of shear Alfvén waves

Abstract: [1] A one-dimensional linear hybrid gyrokinetic-magnetohydrodynamic δf particle in cell simulation code is developed to study the detailed mechanisms of energetic particle drift-bounce resonant destabilization of Alfven modes in the ring-current region of the magnetosphere. The model plasma is composed of a cold (10–100eV) component which provides inertia plus a tenuous energetic (∼10 keV) “ring-current” component which provides both resonant destabilization and compressional stabilization of MHD modes. Full kinetic effects such as finite Larmor radii and particle magnetic bounce and precessional drift motions are retained nonperturbatively. A simple finite β dipolar equilibrium model is assumed (β is the ratio between plasma and magnetic pressures). Simulations show excellent agreement with earlier perturbative analyses. Results show that when the energetic ion thermal velocity is super-Alfvenic, the ions destabilize both odd and even parity shear Alfven MHD modes via the drift-bounce resonances. The growth rates of the resulting modes scale linearly with plasma β. The most unstable of these modes are found to be drift-bounce resonance destabilized modes with odd parity, with wave numbers such that k⟂ρ ≈ 0.5 at the equator (ρ is the energetic ion Larmor radius). The destabilization typically occurs at a critical wave number k⟂ρ ≈ 0.3. When the wave number is close to this critical value and the plasma β is close to the ideal MHD critical value, the mode frequency is determined by the energetic particle dynamics, similar to the energetic particle modes (EPMs) observed in laboratory fusion plasma experiments. When the energetic particles have Alfvenic or sub-Alfvenic thermal velocity, they contribute to damping of the MHD modes via the bounce resonance.

Nonlinear saturation of high‐m Alfvén‐ballooning modes in magnetospheric plasmas

Abstract: [1] A theoretical model is proposed for the nonlinear saturation of high-m Alfven-ballooning instabilities in magnetospheric plasmas. Here, m is the azimuthal wave number. In the present model, a broad spectrum of Alfven waves nonlinearly generate ion-sound density perturbations; which, in turn, scatter the Alfven turbulence toward lower frequencies. Balancing the linear instability growth rate with the nonlinear scattering (Landau damping) rate then yield the corresponding saturated spectrum, which tends to peak near the bottom of the eigenmode frequencies. The theory also gives estimates of wave amplitudes at saturation in reasonable agreement with satellite observations.

Zonal Flow Dynamics and Size-scaling of Anomalous Transport

Abstract: Nonlinear equations for the slow space-time evolution of the radial drift wave envelope and zonal flow amplitude have been self-consistently derived for a model nonuniform tokamak equilibrium within the coherent 4-wave drift wave-zonal flow modulation interaction model of Chen, Lin, and White [Phys. Plasmas 7 (2000) 3129]. Solutions clearly demonstrate turbulence spreading due to nonlinearly enhanced dispersiveness and, consequently, the device-size dependence of the saturated wave intensities and transport coefficients.