Progress in the area of MHD stability and disruptions, since the publication of the 1999 ITER Physics Basis document Nucl. Fusion 39 2137-2664, is reviewed. Recent theoretical and experimental research has made important advances in both understanding and control of MHD stability in tokamak plasmas. Sawteeth are anticipated in the ITER baseline ELMy H-mode scenario, but the tools exist to avoid or control them through localized current drive or fast ion generation. Active control of other MHD instabilities will most likely be also required in ITER. Extrapolation from existing experiments indicates that stabilization of neoclassical tearing modes by highly localized feedback-controlled current drive should be possible in ITER. Resistive wall modes are a key issue for S128 Chapter 3: MHD stability, operational limits and disruptions advanced scenarios, but again, existing experiments indicate that these modes can be stabilized by a combination of plasma rotation and direct feedback control with non-axisymmetric coils. Reduction of error fields is a requirement for avoiding non-rotating magnetic island formation and for maintaining plasma rotation to help stabilize resistive wall modes. Recent experiments have shown the feasibility of reducing error fields to an acceptable level by means of non-axisymmetric coils, possibly controlled by feedback. The MHD stability limits associated with advanced scenarios are becoming well understood theoretically, and can be extended by tailoring of the pressure and current density profiles as well as by other techniques mentioned here. There have been significant advances also in the control of disruptions, most notably by injection of massive quantities of gas, leading to reduced halo current fractions and a larger fraction of the total thermal and magnetic energy dissipated by radiation. These advances in disruption control are supported by the development of means to predict impending disruption, most notably using neural networks. In addition to these advances in means to control or ameliorate the consequences of MHD instabilities, there has been significant progress in improving physics understanding and modelling. This progress has been in areas including the mechanisms governing NTM growth and seeding, in understanding the damping controlling RWM stability and in modelling RWM feedback schemes. For disruptions there has been continued progress on the instability mechanisms that underlie various classes of disruption, on the detailed modelling of halo currents and forces and in refining predictions of quench rates and disruption power loads. Overall the studies reviewed in this chapter demonstrate that MHD instabilities can be controlled, avoided or ameliorated to the extent that they should not compromise ITER operation, though they will necessarily impose a range of constraints.

A stochastic magnetic boundary, produced by an applied edge resonant magnetic perturbation, is used to suppress most large edge-localized modes (ELMs) in high confinement (H-mode) plasmas. The resulting H mode displays rapid, small oscillations with a bursty character modulated by a coherent 130 Hz envelope. The H mode transport barrier and core confinement are unaffected by the stochastic boundary, despite a threefold drop in the toroidal rotation. These results demonstrate that stochastic boundaries are compatible with H modes and may be attractive for ELM control in next-step fusion tokamaks.

A critical issue for fusion plasma research is the erosion of the first wall of the experimental device due to impulsive heating from repetitive edge magneto-hydrodynamic (MHD) instabilities known as "edge-localized modes" (ELMs). Here, we show that the addition of small resonant magnetic field perturbations completely eliminates ELMs while maintaining a steady-state highconfinement (H-mode) plasma. These perturbations induce a chaotic behaviour in the magnetic field lines, which reduces the edge pressure gradient below the ELM instability threshold. The pressure gradient reduction results from a reduction in particle content of the plasma, rather than an increase in the electron thermal transport. This is inconsistent with the predictions of stochastic electron heat transport theory. These results provide a first experimental test of stochastic transport theory in a highly rotating, hot, collisionless plasma and demonstrate a promising solution to the critical issue of controlling edge instabilities in fusion plasma devices. Nature Physics. 3Maximizing the fusion power production in toroidally symmetric magnetic confinement devices (tokamaks 1,2 ) requires high-confinement (H-mode) plasma conditions that have high edge plasma pressures. A ubiquitous feature of these high edge pressure, steady state, H-mode tokamak plasmas is repetitive instabilities known as "edge-localized modes" (ELMs) which release a significant fraction of the thermal energy of the plasma to the first wall of the device.These instabilities are a class of ideal magneto-hydrodynamic (MHD) modes produced in a high pressure gradient region at the plasma edge (called the "pedestal") where pressure gradient driven "ballooning" modes can couple to current density driven "peeling" modes 3 . While ELMs provide a natural transport process that controls the core plasma density and edge impurity ion penetration, they also represent a significant concern for burning plasma devices such as the ! n e ped ) to achieve significant fusion power gain factors, Q ≥ 10, they must operate below ! " e * = 0.1. In this case each ELM is expected to expel up to 20% of the pedestal energy over a time interval of a few hundred µs. If allowed to reach plasma-facing wall components, energy impulses of this magnitude will cause increased erosion of plasma facing components and significantly reduce their lifetime 5,6 . Thus, controlling ELMs by replacing the energy impulses with an equivalent but more continuous transport process is a high priority issue for tokamak fusion research.A particularly appealing ELM control approach in low the RMP field causes a larger change in the edge particle balance (i.e., changes in the balance between outward particle transport and edge particle sources and sinks) rather than in the thermal transport across the pedestal is both surprising and theoretically challenging.As in previous high is satisfied, these small ELMs disappear, leaving the plasma in a very quiet state (Fig. 3a), and the pedestal density ! n e ped begins to fall w...

Large Type-I Edge Localized Modes (ELMs) are completely eliminated with small n = 3 resonant magnetic perturbations (RMP) in low average triangularity, " = 0.26, plasmas and in ITER Similar Shaped (ISS) plasmas, " = 0.53, with ITER relevant collisionalities v e " # 0.2. Significant differences in the RMP requirements and in the properties of the ELM suppressed plasmas are found when comparing the two triangularities. In ISS plasmas, the current required to suppress ELMs is approximately 25% higher than in low average triangularity plasmas. It is also found that the width of the resonant q 95 window required for ELM suppression is smaller in ISS plasmas than in low average triangularity plasmas. An analysis of the positions and widths of resonant magnetic islands across the pedestal region, in the absence of resonant field screening or a self-consistent plasma response, indicates that differences in the shape of the q profile may explain the need for higher RMP coil currents during ELM suppression in ISS plasmas. Changes in the pedestal profiles are compared for each plasma shape as well as with changes in the injected neutral beam power and the RMP amplitude. Implications of these results are discussed in terms of requirements for optimal ELM control coil designs and for establishing the physics basis needed in order to scale this approach to future burning plasma devices such as ITER.

Intermittent plasma objects ͑IPOs͒ featuring higher pressure than the surrounding plasma, and responsible for ϳ50% of the EϫB T radial transport, are observed in the scrape off layer ͑SOL͒ and edge of the DIII-D tokamak ͓J. Watkins et al., Rev. Sci. Instrum. 63, 4728 ͑1992͔͒. Conditional averaging reveals that the IPOs, produced at a rate of ϳ3ϫ10 3 s Ϫ1 , are positively charged and also polarized, featuring poloidal electric fields of up to 4000 V/m. The IPOs move poloidally at speeds of up to 5000 m/s and radially with EϫB T /B 2 velocities of ϳ2600 m/s near the last closed flux surface ͑LCFS͒, and ϳ330 m/s near the wall. The IPOs slow down as they shrink in radial size from 4 cm at the LCFS to 0.5 cm near the wall. The IPOs appear in the SOL of both L and H mode discharges and are responsible for nearly 50% of the SOL radial EϫB transport at all radii; however, they are highly reduced in absolute amplitude in H-mode conditions.