An interpretation of the collisionality dependence of β N , which has been observed in many tokamaks, is presented in terms of neoclassical tearing mode theory. Magnetic islands are predicted to be driven to large saturated widths by the perturbed bootstrap current, provided their width exceeds a certain threshold value. This width is of the order of the ion banana width for low collisionality plasmas, but is substantially larger in collisional plasmas (though still within the banana regime); below this threshold value the stabilizing ion polarization current causes an island to decay. Assuming that 'background' MHD events such as ELMs or sawteeth can provide an initial 'seed' magnetic island of width w 2 cm (through toroidal coupling, for example) one can develop a model equation which retains the essential properties of neoclassical tearing mode theory and is able to reproduce many of the experimental trends associated with the reduced β-limit.

Puffing of impurities (neon, argon) and deuterium gas in the main chamber is used to feedback control the total radiated power fraction and the divertor neutral particle density simultaneously in the ASDEX Upgrade tokamak. The variation of Psep=Pheat-Prad(core) by impurity radiation during H mode shows a similar effect on the ELM behaviour as that obtained by a change of the heating power. For radiated power fractions above 90%, the ELM amplitude becomes very small and detachment from the divertor plates occurs, whilst no degradation of the global energy confinement is observed (completely detached high confinement mode). Additional deuterium gas puffing is found to increase the radiated power per impurity ion in the plasma core owing to the combined effect of a higher particle recycling rate and a lower core penetration probability. The outer divertor chamber, which is closed for deuterium neutrals, builds up a high neutral pressure, the magnitude of which is determined by the balance of particle sources and pumping. For this particular situation, the effective pumping time of neon and argon is considerably reduced, to less than 0.3 s, mainly owing to an improved divertor retention capability. The radiation characteristics of discharges with a neon driven radiative mantle are modelled using a 1-D radial impurity transport code that has been coupled to a simple divertor model describing particle recycling and pumping. The results of simulations are in good agreement with experiment

The results of divertor studies on ASDEX Upgrade, at currents of up to 1.2 MA and heating powers up to 10 M W are described, with emphasis on the ELMy H-mode. The spatial and temporal characteristics of their heat load, and the simulation of ELMs by a time-dependent scrape-off layer code are described. High gas puff rata were found to lead to a large increase in divertor neutral pressure, at modest changes in %, and to a strong reduction in timeaveraged power flow and complete detachment from both target plates in between ELMs. Using pre-programmed puffs of neon and argon, the radiative power losses could be raised to 75% of the heating power, in H-regime discharges, and the regime of enhanced divertor neutral pressure was found also to lead to an improved pumping of recycling impurities. 1.Introduction:ASDEX Upgrade is a mid-size tokamak w i t h non-circular cross-section (major radius R, , = 1.625m, horizontal minor radius a = 0.5 m, elongation b/a = 1.6), purpose-designed as a poloidal divertor device (Figure 1). Further distinguishing features of it are the poloidal field coils placed outside the toroidal ones, and the presence of a saddle coil ("PSL" .. pssive Stabilising loop) inside the vacuum vessel for stabilising the vertical displacement instability. Together, these two features provide a relatively large space between the vacuum vessel and the X-point of the poloidal field lines, although the present divertor configuration, selected to optimise the heat load distribution, places the target plates relatively close to the x-point.

Feedback-controlled puffing of neon and deuterium has been applied to control the edge-localizedmode behavior and the target plate power deposition during high-power H-mode discharges in ASDEX Upgrade.A regime has been found in which more than 90% of the heating power is lost through radiation and divertor detachment occurs, without deterioration of the energy confinement. The plasma remains in the 0 mode, exhibiting small-amplitude, high-frequency ELM's, which do not penetrate to the target plates in the strike zone region. PACS numbers: 52.55.Fa Reduction of the energy Aux density to the target plates to below the values attainable by purely geometric spreading of the divertor fan is one of the most critical requirements for a fusion reactor [1]. Impurity radiation losses from the outer regions of the main plasma and scrape-off layer are, at present, considered the most viable option for attaining such a mode of operation [2]. The resulting state corresponds to a low-power Bow to the target plates (~0.1 of the total heating power), and low plasma densities and pressures in front of them, and has been termed detachment [3]. Experimentally, this aim has been pursued by raising the edge plasma density by strong gas puffing, and by the controlled introduction of light impurities. The most successful previous experiments, with strong additional heating, had involved feedback control of either the impurity puff (in TEXTOR [4]) or of the deuterium puff rate (in JET [5]). While continuous detachmentcould thereby be achieved in limiter and L-mode divertor discharges, experiments in H-mode resulted in either a relapse into the L regime or a reduction only of the time-averaged power Row, with heat pulses associated with edge localized modes (ELM's) still penetrating to the target plates [6,7].The experiments on the ASDEX Upgrade reported here, were employed for the first time in a divertor tokamak feedback control of the radiated power losses through impurity (neon) addition. Simultaneously, we applied deuterium gas puffing, feedback controlling, and also the divertor neutral density. In the most successful operating mode, we attained divertor detachment both in between and during ELM s, while maintaining standard H-regime energy confinement.Device, operating range, and diagnostics description -ASDEX Up.grade is a midsize tokamak (Ro = 1.65 m, a = 0.5 m, and plasma elongation b/a = 1.6) with a single null divertor (Fig. 1). All plasma-facing components are graphite-covered, the vessel is routinely boronized, and turbomolecular pumps allow control of the hydrogen and noble gas particle content of the vessel. The experiments described here were carried out in deuterium, with n,~1.2 X 10 m

1 See appendix. 2 See the author list of 'Overview of progress in European Medium Sized Tokamaks towards an integrated plasma-edge/wall solution' by Meyer [22].