Nonlocal energetic particle mode in a JT-60U plasma

2014

Magnetohydrodynamic (MHD) instabilities driven by energetic alpha particles and beam deuterium particles are investigated for ITER operation scenarios using a hybrid simulation code for energetic particles interacting with an MHD fluid. The particle simulation method with finite Larmor radius effects is applied to both alpha and beam deuterium particles. For the steady-state scenario with 9 MA plasma current, beta-induced Alfvén eigenmodes (BAE modes) with low toroidal mode number (n = 3, 5) were found to become dominant in the nonlinear phase although many toroidal Alfvén eigenmodes (TAE modes) with n ∼ 15 are most unstable in the linear phase. The redistribution of energetic particles with δβ α ∼ δβ beam ∼ 0.07%, which respectively correspond to 6% and 8% of the central values, occurs in the nonlinear phase. When the toroidal mode number of the fluctuations is restricted to n ≤ 8, the redistribution is substantially reduced, thus, suggesting that the resonance overlap between the n ∼ 15 TAE and low-n BAE modes enhances the energetic particle transport in the run with full toroidal mode numbers. For the ITER scenario with 15 MA plasma current, an MHD instability with n = 3 that peaks around the q = 1 (q is the safety factor) magnetic surfaces is driven by bulk plasma current and bulk pressure, and results in significant redistribution of alpha particles with δβ α ∼ 0.3%. For the equilibrium profile with the safety factor profile uniformly raised by 0.1 to remove the q = 1 surfaces, only a benign MHD instability occurs and the energetic particle transport is negligible.

Section: Simulation Modelmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

Large-Scale Simulation of Energetic Particle Driven Magnetohydrodynamic Instabilities in ITER Plasmas

2014

“…[3]. In the present simulations, we choose to freeze the bulk density and pressure, ρ and P, since their evolution cannot be adequately described by MHD for the frequency range at hand.…”

Section: Modelmentioning

confidence: 99%

“…We use the hybrid code MEGA [3,4] in combination with the ideal MHD equilibrium code MEUDAS [5], which provides the equilibrium magnetic field geometry and bulk plasma pressure profile. The scenario considered is based on discharge E039672 in Japan Atomic Energy Research Institute Tokamak Upgrade, JT-60U, where the redistribution of energetic ions injected via negative neutral beams was studied during so-called Abrupt Large Events (ALE) and fast frequency sweeping (fast FS) modes [6].…”

Section: Introductionmentioning

confidence: 99%

Nonlinear Hybrid Simulations of Energetic Particle Modes in Realistic Tokamak Flux Surface Geometry

Aiba

et al. 2011

First results from nonlinear simulations of energetic particle modes and the resulting transport of energetic ions using realistic tokamak geometry are presented and compared with results obtained with a shifted-circle model equilibrium and otherwise equivalent parameters. The modes excited in both cases have similar frequencies and mode structures and cause a similar amount of energetic ion transport during the first few hundred Alfvén times of the nonlinear evolution. The similarity in transport is interesting since it stands in contrast to the reduced linear growth rate and saturation level in the non-circular case: for the parameters chosen, both are reduced by a factor of 2 compared to the circular case. These results motivate further studies, including a verification of our results with other codes, a clarification of the mechanisms underlying the linear stabilization, and a detailed analysis of the mode activity and particle redistribution during the nonlinear evolution.

“…Thus, in the following, the term "incompressible" is used in the thermodynamic sense (Γ = 1), which is different from incompressible flow (Γ → ∞) and from the case where linear stability is independent of compressibility (Γ = 0). The results are used to verify high-beta effects in the code used (MEGA [8,9]) and obtain directions for further research.…”

Section: Introductionmentioning

confidence: 99%

Nonlinear Simulation of Energetic Particle Modes in High-Beta Tokamak Plasma

Aiba

et al. 2013

The effect of bulk pressure on the nonlinear dynamics of energetic particle modes (EPM) in a neutral-beamdriven JT-60U discharge is examined through nonlinear hybrid (MHD + particle) simulations with realistic values of the plasma beta and in realistic flux surface geometry. In the scenario studied, the linear EPM growth rate is 20% higher in the finite-beta compared to the zero-beta case, but compressibility stabilizes the mode by roughly the same amount. In the finite-beta case, the nonlinear frequency shift and the radial spreading/propagation of the mode are affected by the beta-induced gap in the shear Alfvén continuum. Although the evolution of the modes is affected by bulk pressure, compressibility and geometry, the overall energetic ion transport during the first few 100 Alfvén times following the saturation of the EPM is comparable in all cases studied.