Abstract.Progress, since the ITER Physics Basis publication, in understanding the processes that will determine the properties of the plasma edge and its interaction with material elements in ITER is described. Experimental areas where significant progress has taken place are : energy transport in the SOL in particular of the anomalous transport scaling, particle transport in the SOL that plays a major role in the interaction of diverted plasmas with the main chamber material elements, ELM energy deposition on material elements and the transport mechanism for the ELM energy from the main plasma to the plasma facing components, the physics of plasma detachment and neutral dynamics including the edge density profile structure and the control of plasma particle content and He removal, the erosion of low and high Z materials in fusion devices, their transport to the core plasma and their migration at the plasma edge including the formation of mixed materials, the processes determining the size and location of the retention of tritium in fusion devices and methods to remove it and the processes determining the efficiency of the various fuelling methods as well as their development towards the ITER requirements. This experimental progress has been accompanied by the development of modelling tools for the physical processes at the edge plasma and plasma-materials interaction and the further validation of these models by comparing their predictions with the new experimental results. Progress in the modelling development and validation has been mostly concentrated in the following areas : refinement of the predictions for ITER with plasma edge modelling codes by inclusion of detailed geometrical features of the divertor and the introduction of physical effects, which can play a 2 major role in determining the divertor parameters at the divertor for ITER conditions such as hydrogen radiation transport and neutral-neutral collisions, modelling of the ion orbits at the plasma edge, which can play a role in determining power deposition at the divertor target, models for plasma-materials and plasma dynamics interaction during ELMs and disruptions, models for the transport of impurities at the plasma edge to describe the core contamination by impurities and the migration of eroded materials at the edge plasma and its associated tritium retention and models for the turbulent processes that determine the anomalous transport of energy and particles across the SOL. The implications for the expected performance of the reference regimes in ITER, the operation of the ITER device and the lifetime of the plasma facing materials are discussed. Introduction.This chapter outlines the significant progress achieved since the ITER Physics Basis in understanding basic scrape-off layer (SOL) and divertor processes in a tokamak. The interaction of plasma with first-wall surfaces will have considerable impact on the performance of fusion plasmas, the lifetime of plasma facing components, and the retention of tritium in next step Burning Plasma E...

A tokamak, which is the most successful device now on the road to controlled fusion, has the major disadvantage of pulsed operation because of a need to induce a toroidal current in the plasma. The application of rf to drive the current in steady-state tokamak reactors has been considered by a number of authors. 1 " 5 A method of producing continuous current carried by electrons in the tail of distribution function via quasilinear Landau damping of high-phase-velocity rf waves near the lower hybrid (LH) frequency has been proposed. 4,5 The linear and quasilinear Landau damping of slow electrostatic waves near LH frequency has been confirmed in a linear test device 6 and in the LH electron heating experiment on the tokamak (Doublet IL4). 7 These experiments provide a physical base for understanding the quasilinear Landau damping in the toroidal plasma with a relatively high electron temperature. Recently, the current generated by the unidirectional electron plasma waves has been observed in linear devices 8 * 9 and a toroidal device. 10 These experiments have been carried out in a plasma with a lower electron temperature, in which a transfer of momentum from LH waves to electrons via collisional absorption is significant.In order to make effective coupling between the LH waves and electrons, it is necessary to avoid the deposition of the rf energy into ions resulting ^. Mandelbrot, Fractals: Form, Chance, and Dimension (Freeman, San Francisco, 1977). e from the linear mode conversion and the excitation of parametric instabilities. The previous experiments on the rf ion heating indicated that i-for w Q /w lh {$) ^ l c 6 the ions did not interact with i the rf waves and the parametric decay instabilities almost disappeared, 11,12 where ou 0 is the frequency of the applied rf field and oo lh (0) is the LH QS frequency at the center of the plasma column. In this Letter, we report the experimental study on s the coupling between the rf waves and electrons under the conditions of oo 0 /uo lh (0) < 2 and the relatively high electron temperature in a tokamak. J-The experiment, with a 750-MHz rf source, e 6 was performed in the J FT-2 (JAERI Fusion Torus) i tokamak, which was a conventional tokamak with a major radius of JR 0 = 90 cm and a minor radius of a = 25 cm. The experimental setup and the discharges were reported in detail, 13 and hence will be described only briefly here. In the present l-experiment, the following discharge was used as a magnetohydrodynamically stable operation; toroidal magnetic field B t = 14 kG, plasma current I p = 3Q kA, mean line-of-sight electron density n ^3xl0 12 cm -3 , central electron temperature T^ -(250 eV)/k and effective ionic charge Z e ff of 2-5. The working gas was deuterium. The wave-3 guide array employed here consists of four indei pendently driven waveguides mounted 1.5 cm 5 away from the plasma edge, which is defined by It is observed that the waves launched from a phased array antenna of four waveguides couple effectively with electrons under the condition of oo 0 /oo lh (...

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As a result of an experiment at JT-60U on H mode transition power threshold scaling, which is one of the urgent ITER Physics R&D issues, it has been found that: (a) the derived scaling law of Pth(MW)=0.18ne0.5 (1019 m-3) BT1.0 (T) R1.5 (m) with presumed non-dimensional constraints predicts a threshold heating power of ⩽ 100 MW at 5 × 1019 m-3 for the ITER EDA design, and (b) an increase of the neutral particle density at the plasma edge results in a reduction of the edge ion collisionality just before the transition below unity and thereby an increase of the threshold heating power. It has also been found that both the low density boundary, below which the transition cannot occur, and the apparent density dependence of the threshold power scaling are related to the edge neutrals. It is suggested that a charge exchange process might well be the potential mechanism of the neutral effect

Experiments of suppressing fast-ion-driven MHD instabilities such as energetic particle modes (EPMs) and global Alfvén eigenmodes (GAEs) have been made by using a second harmonic X-mode electron cyclotron heating (ECH) and current drive (ECCD) in the helical-axis heliotron device, Heliotron J. ECCD experiments show that the GAE destabilized by fast ions of neutral beam injection (NBI) with the observed frequency around 140 kHz are fully stabilized, and the EPMs with the observed frequency around 90 kHz are suppressed when the EC-driven plasma current flowing in the counter direction reaches approximately 0.7 kA. The low magnetic shear under the vacuum condition is modified into positive magnetic shear when counter-ECCD is applied, and the amplitude of GAEs and EPMs decreases with an increase of the EC-driven plasma current. These results indicate that magnetic shear is a key role in controlling GAEs as well as EPMs. The comparison of the calculation of shear Alfvén spectra with experimental results shows that the increasing continuum damping rate with an increase in local magnetic shear by EC-driven current is important for both EPMs and GAEs. Moreover, the increase in plasma current lead to the inward movement of GAEs. This effect would also contribute to suppression of GAEs because the continuum damping rate increases more and more toward core. Steady ECH is also found experimentally to be effective to control the amplitude of both GAEs and EPMs. The amplitude of EPMs, and especially for GAEs decreases with an increase in the ECH power under fixed density conditions.

In order to obtain improved confinement plasmas with high radiation at high density, Ar gas was injected into ELMy H mode plasmas in JT-60U. A confinement improvement of HH 98(y,2) ≈ 1 was obtained with a high radiation loss power fraction (∼80%) at an electron density of ∼0.65nGW . The HH factor was about 50% higher than that in plasmas without Ar injection.

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Volume recombination of C4+ and e− into C3+ is observed for the first time in detached divertor plasmas with an X-point MARFE. The recombination zone is located around the X-point, and the electron temperature and density are evaluated to be 6.3 eV and 7.8 × 1020 m−3, respectively. In this zone, the volume recombination flux is larger by two orders of magnitude than the ionization flux of C3+. However, the radiation power due to the recombination process is only 2% of the total radiation power, measured by a bolometer. In contrast, the radiation power due to the excitation process from the ground state of C3+ by electron collision dominates the total radiation power.