High-efficiency refuelling of ELMy H-mode tokamak discharges with solid deuterium pellets injected from the magnetic high-field side is demonstrated. Compared to standard low-field side injection, the fuelling efficiency was enhanced by a factor of 4, the pellet penetration more than 2 times. This experimental result can be qualitatively explained by the magnetic force pushing a diamagnetic plasma cloud towards lower magnetic field, causing rapid particle loss for shallow lowfield side injection, but enhancing fuelling efficiency and pellet penetration for high-field side injection.[S0031-9007 (97)03857-X] PACS numbers: 28.52.Cx, 52.55.FaNext generation fusion devices like ITER will have to operate at densities well beyond the Greenwald limit obtained in present day tokamaks with gas refuelling [1]. Though the detailed nature of this empirical limit is still under discussion, it was shown that it can easily be overcome by injection of frozen hydrogen isotope pellets penetrating much deeper than cold gas particles (e.g., Franck-Condon atoms from molecule disintegration) [2]. In discharges with high heating power and especially in type-I ELMy H-mode plasmas with high edge temperatures, a large fraction of the deposited material was rapidly expelled from the plasma column [3], resulting in significantly reduced fuelling efficiencies´f especially with shallow penetration [2,4]. In Ref. [4] it was shown that at least part of this mass was lost in the vicinity of the injection point along a trace aligned with the helical magnetic field (please note the correct sequence of Figs. 3(a) and 3(b) is reproduced in the corrigendum).In this and all previous experiments, pellets were injected from the magnetic low-field side (LFS), i.e., from the torus outside, which is easily accessible in a tokamak. It was therefore argued [4] that, because of the unfavorable toroidal curvature, part of the diamagnetic pellet plasma cloud could have been expelled before it was captured by the background plasma. If so, injection from the magnetic high-field side (HFS), i.e., the torus inside, should be much superior, since the same effect would help to transport the pellet mass deeper into the bulk plasma. In order to clarify this question, experiments have been conducted in ASDEX Upgrade where pellets were injected from both sides into H-mode plasmas and f as well as pellet penetration depths were compared. ASDEX Upgrade is a midsize divertor tokamak (tokamak radius R 0 1.65 m, plasma radius a 0.5 m, V plasma 13 m 3 , plasma elongation b͞a 1.6; singlenull divertor). Wall elements in contact with the plasma are covered by graphite tiles, the divertor target plates were tungsten coated. Calibrated valves mounted at the vessel midplane are used for gas puffing, and turbomolecular pumps with a pumping speed of 14 m 3 ͞s for D 2 (deuterium) gas to control particle exhaust.The experiments described here were carried out in D with plasma currents I p 0.8 1.2 MA, toroidal magnetic field jB t j 1.7 2.5 T, safety factor q 95 2.7 4.2, and additional bea...

Tearing mode formation is investigated for ohmic density limit discharges on the ASDEX Upgrade tokamak at medium and high safety factors (q95=3.8, 4.9, 6.0). Low electron temperatures inside magnetic islands and the observation of localized C III impurity radiation suggest that a thermal instability, as proposed by Rebut and Hugon, destabilizes the (m, n)=(3,1) and (4,1) islands, which grow during the current profile contraction phase. In contrast, the (2,1) islands appear to be thermally stable. Minor disruptions lead to step-wise loss of confinement, first localized at the 4 2 surface, then after a time delay comparable to the resistive time-scale, at the q=3 surface and after a second time delay, at the q=4 surface. It is found that (3,1) islands, unlike (2,1) islands, are quenched by the high heat flux during minor disruptions

The L mode and H mode density operational window in the vicinity of the density limit has been investigated with a combination of gas puff refuelling and improved fine tuning of neutral beam injection (NBI) heating power. In this way, a novel strategy is achieved by means of a parallel increase of density and heating power. As the density limit is approached, H modes degrade into L modes independently of heating power; this is in contrast to the generally accepted L to H mode threshold scaling PheatL-H varies as neB. Furthermore, contrary to the well known heating power independent Greenwald limit, the L mode density limit increases moderately with rising heating power, neDL varies as Pheat0.3+or-0.1, if a simple power law is assumed. The power dependence becomes more obvious when analysed in terms of edge densities and powers flowing across the separatrix into the scrape-off layer, nesep varies as Psep0.6+or-0.2. The corresponding H mode studies show that before an H mode quenches into an L mode the maximum achievable density (i.e. The H mode density limit) is practically independent of the heating power, as observed on many machines

The penetration depths of different impurity pellets, such as carbon and neon, injected into different thermonuclear devices were reproduced by means of a single numerical code with the same set of assumptions, only the atom physical data being changed. All major characteristics of the ablation process were calculated: the spatial variation of the ablation rate, the depositon of ablated particles at a succession of magnetic flux surfaces, the expansion of deposited particles in the directions both parallel and perpendicular to the magnetic field lines, and the temporal and spatial variations of the radiant power emitted by the expanding impurity cloud. The calculations were done by means of a time dependent quasi-three-dimensional code consisting of three modules accounting for the B⊥ and B|| expansions of the cloud and the traversing motion of the pellet, operated interactively and, when needed, iteratively. The radiation characteristics were computed by a collisional-radiative loss model, developed for low temperature light impurities, without the usual equilibrium assumptions. With some modifications, the code is adaptable to predictive pre-disruptive `killer pellet' scenario calculations for future large scale machines, such as ITER.

Efforts have been made to develop scenarios allowing more flexible plasma density control by injecting cryogenic solid hydrogen pellets. While the injection of pellets during ohmic discharges was found to be most efficient and also improves the plasma performance, an increase in the auxiliary heating power causes a deterioration of the pellet fuelling efficiency. A further strong reduction of the pellet fuelling efficiency by an additional process was observed for the more reactor relevant conditions of shallow particle deposition during H mode phases. With injection during type I ELMy H mode phases, each pellet was found to trigger the release of an ELM and therefore cause particle losses mainly from the edge region. In the type I ELMy H mode, only sufficient pellet penetration allowed noticeable, persistent particle deposition in the plasma by the pellets. Applying adequate pellet injection conditions and favourable scenarios using combined pellet/gas puff refuelling, significant density ramp-up to densities exceeding the empirical Greenwald limit by up to a factor of two was achieved even for strongly heated H mode plasmas.

The so-called 'dithering cycles', i.e. periodic GH-L transitions, are Studied in the ASDEX Upgrade tokamak. The cycle repetition frequency Is .. 2 !dk; high-resolution ECE measuremenk show that T, changes during the cycle consistent with the assumption of the periodic build-up and destruction of the edge transport barrier characteristic of the Bmode. A study of the conditions under which the cycles occur reveals that with the unfavourable V B drift direction, the cycles are suppressed. These observations are interpreted in a schematic bifurcation diag", we find that the width of the bifurcation varies with the revenal of the V B drift.

Tokamak discharges with repetitive pellet fuelling were investigated in the ASDEX divertor device. The importance of sufficiently high divertor recycling for a high density at the separatrix and for successful density buildup in the bulk plasma was demonstrated. In contrast to low recycling discharges where no permanent improvement of the energy confinement time was achieved, in OH-heated discharges with high recycling an energy confinement time of 160 ms was reached, the normal value being 80 ms in the rollover region. The peaked density profiles in this case were accompanied by reduced or suppressed sawtooth activity and finally ended in a phase of strong central impurity accumulation. The particle transport was characterized by strong, non-classical inward drift, while the improved energy transport can be explained by the following alternatives: (la) a local model which assumes neo-Alcator χe for the electrons and χi= 3χneocl for the ions in the gas puff cases, reducing to χi= χneocl for the optimum pellet cases; (1b) the assumption χi= χneocl under all conditions and an electron energy confinement worse than neo-Alcator in the rollover region in gas-puff-discharges; (2) a profile consistency picture where Te(a) determines the energy confinement. Low power, NI heated discharges with pellet fuelling behave like Ohmic discharges, while for high power in the L-mode no successful density buildup was possible, and τE was not improved. The H-regime was extended from a density maximum n̄e = 0.8 × 1020 m-3 without pellets to n̄e = 1.2 × 1020 m-3 by the injection of pellets. In this case a density buildup takes place, but further density profile peaking could not be observed.

Injection of impurity pellets has been shown to be a successful method of reducing thermal and mechanical loads during disruptions. The evolution of the quenching plasma after pellet injection in ASDEX Upgrade is described and the requirements of such a method for mitigating disruptions in future devices are discussed.