The resistive wall mode (RWM) and neoclassical tearing mode (NTM) have been simultaneously suppressed in the DIII-D for durations over 2 seconds at beta values 20% above the no-wall limit with modest electron cyclotron current drive (ECCD) and low plasma rotation. The critical plasma rotation was significantly lower than reported at the IAEA FEC in 2006. However, even in this stabilized regime, stable steady-state operation is not unconditionally guaranteed. Various localized MHD activities such as edge localized modes (ELMs) and fishbones begin to couple to the RWM branch near the no-wall limit. Feedback is useful to improve the stability. Simultaneous operation of slow dynamic error field correction and fast feedback suppressed the ELM-induced RWM at high normalized beta. The result implies that successful feedback operation requires careful control of residual RWMs. The effectiveness of feedback operation was demonstrated using a reproducible current-driven RWM. The present findings are extremely useful in the challenge of control of RWM and NTM in the unexplored physics territory of burning plasmas in ITER.

The requirements of the DIII-D physics program have led to the development of many operational control results with direct relevance to ITER. These include new algorithms for robust and sustained stabilization of neoclassical tearing modes (NTM) with electron cyclotron current drive (ECCD), model-based controllers for stabilization of the resistive wall mode (RWM) in the presence of ELMs, coupled linear-nonlinear algorithms to provide good dynamic axisymmetric control while avoiding coil current limits, and adaptation of the DIII-D Plasma Control System (PCS) to operate next-generation superconducting tokamaks. Development of integrated plasma control, a systematic approach to model-based design and controller verification, has enabled successful experimental application of high reliability control algorithms requiring a minimum of machine operations time for testing and tuning. The DIII-D PCS hardware and software and its versions adapted for other devices can be connected to integrated plasma control simulations to confirm control function prior to experimental use. This capability has been important in control system implementation for tokamaks under construction and is expected to be critical for ITER.

Plasma Phys. Control. Fusion 52 (2010) 104004 Y In et al are based on a single mode assumption, so the investigation of the second least stable RWM is of high interest.

Active feedback control in the DIII-D tokamak has fully stabilized the current-driven ideal kink resistive-wall mode (RWM). While complete stabilization is known to require both low frequency error field correction (EFC) and high frequency feedback, unambiguous identification has been made about the distinctive role of each in a fully feedback-stabilized discharge. Specifically, the role of direct RWM feedback, which nullifies the RWM perturbation in a time scale faster than the mode growth time, cannot be replaced by low frequency EFC, which minimizes the lack of axisymmetry of external magnetic fields.

A method to control the diocotron instability of a hollow electron beam with periodic dipole magnetic fields has been investigated by a two-dimensional particle-in-cell simulation. At first, relations between the diocotron instability and several physical parameters such as the electron number density, the current and shape of the electron beam, and the solenoidal field strength are theoretically analyzed without periodic dipole magnetic fields. Then, we study the effects of the periodic dipole magnetic fields on the diocotron instability using the two-dimensional particle-in-cell simulation. In the simulation, we considered the periodic dipole magnetic field applied along the propagation direction of the beam, as a temporally varying magnetic field in the beam frame. A stabilizing effect is observed when the oscillating frequency of the dipole magnetic field is optimally chosen, which increases with the increasing amplitude of the dipole magnetic field.

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Hot plasma dielectric response models, which are now used in most linear full wave codes, are formulated in Fourier space assuming that particle's Larmor radius is much smaller than the scale of spatial nonuniformity of magnetic field. Such approximation assumes that the spatial scale of plasma dielectric response to the RF field is limited to a few Larmor radii, which is accurate for a limited range of wave frequencies ω. The scale of plasma dielectric response along the magnetic field line could be comparable to the scale of the magnetic field nonuniformity when ω is close to the particle's cyclotron frequency ωc or when ω is much smaller than ωc, which requires the use of a more accurate model. In the present approach, the hot plasma dielectric response is formulated in configuration space without limiting approximations by numerically calculating the plasma conductivity kernel based on the solution of the linearized Vlasov equation in nonuniform magnetic field. Results of the conductivity kernel calculation in hot collisionless plasma are presented for 1-D mirror and 2-D tokamak magnetic field configurations for ω∼ωc. Self-consistent simulation of RF fields using the calculated conductivity kernel of 1-D mirror magnetic field is made. A new parallel full wave RF code, based on the presented approach of accurate self-consistent modeling of plasma dielectric response in configuration space, is under development.

An updated and more accurate database for single- and double-ionization cross sections for almost all argon ions has been developed for the modeling of the charge state distribution (CSD) within an electron cyclotron resonance ion source. When the highly non-Maxwellian anisotropic electron-distribution function, is modeled by a Fokker–Planck code, one has to use the ionization cross sections instead of the Maxwellian rate coefficients. Most of the fitting coefficients used within the well-established semi-empirical formulas for direct ionization and double ionization have been recalculated using more accurate crossed-beam experimental data available. The shift of the CSD to higher-charge states due to the contribution of excitation autoionization and double ionization is presented by comparing the GEM code modeling using the Lotz formula and the cross sections with updated fitting coefficients.