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The outward acceleration of enhanced- $\ensuremath{\beta}$ plasmoids formed when a frozen hydrogen pellet is injected into a hot tokamak plasma has been directly observed. The plasmoid characteristics, acceleration, and velocities measured agree with theoretical expectations. The plasmoid motion is always in the positive major radius direction, consistent with high fueling efficiency for high-field side, and rapid mass loss for low-field side injection. In parallel a weak acceleration of the ablating pellet itself in the same direction was observed, probably caused by a net radial rocket force component.

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.

Detailed experiments have been carried out in ASDEX-Upgrade to characterize detached plasma conditions in divertor II. This paper presents measurements of the hydrogen emission in the visible and VUV spectral range that distinguishes between the ionization front and the recombining divertor plasma. Plasma parameters in the recombination dominated region were obtained from the Balmer continuum and line emission. By means of the ADAS programme package the rate of volume recombination is evaluated including the effect of opacity. The optical thickness was experimentally determined from the line ratio of L β and H α . In the recombining plasma we have observed an inverse response of H α light during the spatial broadening of ELMs. Inverse ELMs show signatures of convective transport.

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

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 low-field side injection, but enhancing fuelling efficiency and pellet penetration for high-field side injection.

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 influence of impurity radiation on the and transitions is investigated for highly radiative divertor discharges in the ASDEX Upgrade tokamak. The transitions between H- and L-mode depend on the net heat flow across the separatrix, , calculated from the heating power and bolometric radiation profiles. For typical radiative boundary conditions in ASDEX Upgrade, the radiation distribution is dominated by line emission in the vacuum-ultraviolet region and peaks near the separatrix. For the case of neon used as seed impurity, about 2/3 of the main chamber radiation is emitted inside, but close to the separatrix. Argon seed results in a higher fraction of core radiation, while the line emission is shifted further outside for nitrogen. The radiation-corrected threshold is not affected by gas puffing and is described by [MW, , T, amu]. The threshold power, which is typically lower by a factor of two without strong deuterium puffing, is increased by heavy gas puffing leading to . In the vicinity of the radiation-induced transition, a general alignment of H and L mode is observed with regard to global energy confinement time and edge density and temperature profiles. The transition itself exhibits a smooth evolution in time. Reduction of target plate power load down to about 10% of the total heating power is easily achieved by edge radiation in the CDH-mode for low conditions. However, this reduction is attributed mainly to radiation from inside the separatrix and is connected to relatively high values of the core . These results emphasize the importance of the development of more closed divertor concepts, allowing for higher divertor radiation levels in connection with lower core .

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

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

Characteristics of the various types of edge localized modes (ELMs) observed during the H-mode of the ASDEX Upgrade tokamak are presented. Magnetic high-mode-number ELM precursor activity is found for type I ELMs and type III ELMs, the former only during neutral beam injection in the counter direction. The effect of ELMs on electron temperature and density profiles is discussed.

The completely detached high confinement (CDH) regime established recently in high power ASDEX-Upgrade single null divertor discharges is described. The standard CDH mode scenario with feedback controlled external deuterium and neon puffing is analysed in detail with respect to its divertor detachment and confinement properties. Impurity transport and pumping effects and the density operation windows of the CDH mode are outlined. First results with other impurity radiators (N,Ar) are given. Finally the CDH mode operation space is discussed in terms of main chamber and scrape-off layer plus divertor radiation and possible consequences for future fusion experiments are outlined.

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 so-called 'dithering cycles', i.e. periodic L-H-L transitions, are studied in the ASDEX Upgrade tokamak. The cycle repetition frequency is approximately 2 kHz; high-resolution ECE measurements show that Tc changes during the cycle consistent with the assumption of the periodic build-up and destruction of the edge transport barrier characteristic of the H-mode. A study of the conditions under which the cycles occur reveals that with the unfavourable Del B drift direction, the cycles are suppressed. These observations are interpreted in a schematic bifurcation diagram; we find that the width of the bifurcation varies with the reversal of the Del B drift.

Detailed experimental divertor data are presented on the profiles of density and temperature in the inner and outer divertor fans, the radiated power distribution, the gas pressure and the spectroscopically derived particle fluxes, all as a function of the discharge density. At low and medium density, the inner divertor is cold and dense compared to the outer divertor. At high density, strong X-point MARFE and separatrix radiation partially detaches the inner divertor. Probe measurements which penetrate into the X-point MARFE at the outer divertor are presented.

The H-mode obtained in ASDEX Upgrade by three heating methods (Ohmic, NBI, ICRF) is analysed. The power threshold is shown to be relatively low compared with other tokamaks and to be independent of the heating method. The operational window for the different types of H-phases, (dithering, ELMing with type-III and I ELMs, ELM-free) is given and discussed. The features of the H-mode discharges, in particular quasi-stationary ELMing phases with type I ELMs, are described and discussed. Finally, it is shown that the power flux through the plasma edge is a key parameter for H-mode operation and control.

Arc traces and luminous phenomena at the divertor tiles of ASDEX-Upgrade were recorded, and experimentally modelled by artificially ignited arcs. At the graphite surfaces the arcs produced relatively immobile spots that were uniformly distributed in toroidal direction. To explain the latter effect, the behavior of flux tubes emanating from the arc spots is discussed. It is concluded that arcing should not be neglected in theoretical models that describe processes in high power machines.

ASDEX Upgrade is a poloidal divertor experiment very similar to ITER with respect to magnetic field properties and especially to the plasma boundary geometry. A large part of the programme on ASDEX Upgrade is therefore dedicated to investigating and optimizing reactor-relevant plasma boundary issues. This contribution is concerned with H-mode studies and edge physics in general. The L/H-power threshold, H-mode confinement, details of ELM dynamic and 'dithering' L/H-transitions will be discussed. The edge physics investigations are concerned with model calculations and validations, the characteristic 'approach' to the density limit (DL), target plate sputtering and the influence of hydrocarbons, asymmetries and drifts depending on the direction of the toroidal magnetic field, and finally first results of high-Z material experiments.

An apparatus is described which assesses the focus of a laser on a surface. An image of the focus is scanned by fiber optics. A rotating slit together with a photodetector behind the fibers detects whether a fiber is illuminated or not. When the focus coincides with the surface of the target, the number of illuminated fibers is a minimum. The apparatus is especially advantageous for adjustment when the radiation is not visible, for example, with neodymium lasers or with frequency doubled radiation from ruby lasers.

As part of a USDOE/ASDEX collaboration, a detailed examination of pellet ablation in ASDEX with a variety of diagnostics has allowed a better understanding of a number of features of hydrogen ice pellet ablation in a plasma. In particular, fast‐gated photos with an intensified Xybion CCD video camera allow in situ velocity measurements of the pellet as it penetrates the plasma. With time resolution of typically 100 ns and exposures every 50 μs, the evolution of each pellet in a multipellet ASDEX tokamak plasma discharge can be followed. When the pellet cloud track has striations, the light intensity profile through the cloud is hollow (dark near the pellet), whereas at the beginning or near the end of the pellet trajectory the track is typically smooth (without striations) and has a gaussian‐peaked light emission profile. New, single pellet Stark broadened Dα, Dβ, and Dγ spectra, obtained with a tangentially viewing scanning mirror/spectrometer with Reticon array readout, are consistent with cloud densiti...

Feedback-controlled puffing of neon and deuterium has been applied to control the edge-localized-mode 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 H mode, exhibiting small-amplitude, high-frequency ELM's, which do not penetrate to the target plates in the strike zone region.

ASDEX Upgrade is equipped with an ICRH system consisting of 4 generators of 2 MW power each and 4 double loop antennas. The generators, tuneable in frequency from 30 to 120 MHz, cover several heating scenarios over a wide range of magnetic fields (1 T

As part of a USDOE/ASDEX collaboration, a detailed examination of pellet ablation in ASDEX with a variety of diagnostics has allowed a better understanding of a number of features of hydrogen ice pellet ablation in a plasma. In particular, fast‐gated photos with an intensified Xybion CCD video camera allow in situ velocity measurements of the pellet as it penetrates the plasma. With time resolution of typically 100 ns and exposures every 50 μs, the evolution of each pellet in a multipellet ASDEX tokamak plasma discharge can be followed. When the pellet cloud track has striations, the light intensity profile through the cloud is hollow (dark near the pellet), whereas at the beginning or near the end of the pellet trajectory the track is typically smooth (without striations) and has a gaussian‐peaked light emission profile. New, single pellet Stark broadened Dα, Dβ, and Dγ spectra, obtained with a tangentially viewing scanning mirror/spectrometer with Reticon array readout, are consistent with cloud densiti...

Penetration depths for frozen D2 pellets into ASDEX ohmically and auxiliary heated discharges are reported for a wide range of plasma and pellet parameters. It is found that the data are well correlated by a single parameter (pellet diameter, pellet velocity, central electron density and temperature), which emerges from the neutral shielding model. The pellet depths, however, are systematically smaller than is expected from the model, implying a higher ablation rate in the outer plasma layers.

Investigations of impurity accumulation phenomena in ASDEX are reviewed. There are four different operating regimes where pronounced accumulation is observed and these regimes are also characterized by improved energy confinement. In particular, medium-Z metallic ions are involved in accumulation processes whereas low-Z ions appear almost unaffected. The rapid accumulation observed in the case of metallic ions may be explained by neoclassical inward drifts if we assume that the anomalous diffusion is sufficiently suppressed, some indication of this being found from laser blow-off studies. The present results, however, can only be partly explained by neoclassical theory, according to which accumulation of low-Z impurities should also occur. The temporal behaviour of accumulation and the retarding effect of proton dilution for collision dominated transport are also discussed. Finally, we conclude that the full benefits of improved energy confinement can be achieved only if the impurity influxes are kept to a sufficiently low level. Expressed in terms of concentrations under low confinement conditions we have to postulate, for ASDEX, concentrations ≲ 10−4 for metals and ≲ 2% for all light impurities.

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 e = 0.8 × 1020 m-3 without pellets to e = 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.

Discharges fuelled by stationary pellet injection (PI), gas puffing (GP) or a combination of the two methods are compared with respect to recycling in the divertor and particle confinement. Fuelling by PI yields much better global particle confinement than by GP. This has been found for both low and high recycling. In the low-recycling case this improvement is due to the deeper particle deposition for PI than for GP since the transport in the inner plasma is not reduced. For high recycling the improvement results from both the deeper deposition and a reduction in the transport. The best global particle confinement was found for phases with low or no GP. This, however, can be reached for short times only. Since with PI alone it is impossible to keep the recycling on a high level, GP is unavoidable for sustaining the favourable high-recycling condition.

The Divertor Tokamak ASDEX, its neutral injection system and its ICRH system have been modified to permit additional heating with a power of 6 MW for pulse lengths up to 10 s. The paper summarizes the arguments for long-pulse heating, describes the technical modifications of the divertor performed, their effect on the operational behaviour of the tokamak and presents a few typical results of recent experiments exploiting the long-pulse heating facilities.

The first pellet refuelling of a reversed field pinch has been successfully demonstrated in ZT-40M. By injecting a series of pellets, density increases of 600% have been observed, with no disruptions. Because of a coincidence of pellet transit time, Ohmic reheat time and particle confinement times (about 0.5 ms) a significant fraction of the plasma inventory 'leaks out' before pellet ablation is complete. Strong on-axis peaking of the density profile due to pellet fuelling is not typically observed; this is due to large poloidal and toroidal deflections observed in the pellet trajectory, beginning in the plasma edge. Asymmetric ablation of the pellet by a suprathermal electron population is suspected and simple modelling can account for the observed trajectories.

Strongly peaked electron density profiles have been obtained in ASDEX by different refuelling methods: pellet fuelling (ohmic and co-injection heating), NBI counter-injection and recently by reduced gas puff fuelling scenarios. These discharges show in common increased density limits, a canonical electron temperature profile independent of the density profile and an improvement of the particle and energy confinement. Whereas the changes in particle transport are not fully understood, transport analyses point out that the improved energy transport can be explained by reduced ion conduction losses coming close to the neoclassical ones. The different results for the ion transport with flat and peaked density profiles are quantitatively consistent with that expected from eta i-driven modes. The analyses cannot yet explain the anomalous electron energy transport, apart from identified continuous trends such as inverse scaling with the isotope mass and enhancement with heating power.

The four barrel pneumatic pellet injector originally used on the Alcator C tokamak at MIT has been reconfigured for use on the ZT‐40M reversed field pinch (RFP) at Los Alamos National Laboratory. The lower temperature and density in this device require operation at velocities significantly lower than on Alcator. Allowing a 2‐s warmup prior to firing the injector has enabled operation with pressures as low as 40 psig and velocities ranging as low as 200 m/s with H2 pellets, without major cryostat modification. Initial injection experiments in the ZT‐40M plasma have verified that pellet refueling is effective for density control, but the net particle inventory increase is less than might be expected based on simple models. Indications of nonoptimal penetration and curvature of the pellet trajectory, in addition to comparable time scales for particle confinement and pellet transit, may account for this.

A status report of November 1970 on experimental and theoretical studies of laserproduced plasmas at the Institut fur Plasmaphysik is given. Experimental investigations of plasma production by nanosecond laser pulses from targets of solid hydrogen and deuterium, of Raman scattering from these materials, and of plasma production by picosecond laser pulses are described. The theoretical part comprises numerical studies of the expansion of a two-temperature plasma (Te≠Ti) and of a multicomponent plasma and also calculations of the fusion rate of an inertially confined plasma.

An investigation of sparks produced in a number of gases by means of a transversely excited atmospheric‐pressure (TEA) CO2 laser has revealed the existence of a clearly visible plasma filament which propagates at an almost constant velocity in the same direction as the laser beam.

Cylindrical D 2 -pellets of different sizes (diameter and length respectively 0.6, 1.5 and 2.5 mm) were launched with velocities of 400 to 1000 m/s through straight and curved guide tubes at room temperature up to a length of 12 m. Velocity losses of 12%, mass losses of 10 to 30% and divergence angles of the pellet trajectory up to 2° were measured depending on tube diameter and length. No difference could be observed for straight and curved guide tubes of curvature radius larger than 1 m.

Plasma production from solid hydrogen targets with laser radiation at 0.69 μ and 1.06 μ has been well investigated in past years and good agreement has been found between a model based on hydrodynamic calculations and experiments. The model was developed by Afanasyev, Krokhin and Sklizkov1, Caruso and Gratton2, Mulser3 and others. It assumes that the absorption of laser radiation is due to inverse bremsstrahlung at the surface of the solid target. A hot plasma is produced and expands towards the laser into the vacuum. Then the radiation will be absorbed in the plasma near the critical density ne, which is given by $${n_c} = \frac{{\omega _L^2\,\,{m_e}}}{{4\;\pi \;{e^2}}}$$ (1) where ωL is the laser frequency.

With the improvement of high power lasers it has become possible to produce plasmas of very high density and temperatures which possibly are of interest for thermonuclear research. Recently substantial neutron emission from such plasmas has been reported1–6. In order to investigate the future possibilities in this field of plasma physics detailed understanding of the properties of these plasmas is necessary. In this paper we report results obtained by using a variety of diagnostic techniques applied to plasmas produced within a wider range of laser power and show the evidence of non-thermal effects occurring in laser produced plasmas at moderate energies.

Densities achievable in ASDEX Upgrade discharges are restricted by a disruptive limit in the L-mode caused by an edge-power imbalance which is linking divertor detachment, Marfe formation and the separatrix density. The attainable average densities depend then on the internal particle sources and the core transport and can exceed the empirical Greenwald density. In H-mode an upper density limit is found which represents a non-disruptive H - L back transition, which is preceded by the occurrence of type-III ELMs. Close to the Greenwald limit this H - L transition cannot be avoided at any power flux across the separatrix and - at high external neutral gas fluxes - confinement compared with ITER H-92P scaling degrades even before the back transition. The H-mode operational window is determined by local edge-barrier parameters and their gradients, respectively. The boundaries are represented by the L - H transition-temperature threshold, the ideal ballooning edge-pressure gradient limit, the upper temperature limit for type-III ELMs and an upper H-mode barrier density limitation. The cause for the last limitation is not yet identified; it may be due to resistive ballooning modes or the separatrix density limit. Despite the limited edge densities the Greenwald density could be surpassed by a factor of three with pellet refuelling from the low magnetic-field side. Pellet injection from the high-field side gains from the strong increase of fuelling efficiency due to the assisting toroidal outward drift of the formed high- ablatant. Higher densities are achievable in H-mode compared with low-field side injection and diminished convective losses avoid confinement degradation up to the Greenwald density. In gas-puffed type-I ELMy H-modes the plasma thermal energy and the edge-pressure gradients, which are limited by ballooning stability, are linked via a robust temperature-profile stiffness and the flat density profiles resulting from dominant edge refuelling at high densities. Their confinement does not improve with increasing density (and neutral gas fluxes) and may even slightly degrade. Therefore, the superior confinement of type-I ELMy H-modes compared with type-III ELMy ones at medium densities is actually offset at densities close to the Greenwald density. In contrast to the temperature-profile resilience density profiles can be changed both by deep refuelling (with pellets) and intrinsic transport improvements connected with density peaking (observed in CDH-modes), which offers the combination of high confinement and high density operation. The possible alliance with radiation cooling, divertor detachment and divertor compatible type-III ELMs could solve the power exhaust problem.

The paper contains recent results in the field of pellet diagnostics obtained on the T-10 and ASDEX-Upgrade tokamaks (collaboration between STU and IPP) and a proposal to inject high Z impurity pellets for control of the disruption and ramp-down phase in ITER.

This paper investigates the propagation of a spherical blast wave from a laser produced gas discharge and compares it with a theory of Sakurai. The energy of the blast wave was determined for various gases and pressures.