K. Behringer

H mode discharges with feedback controlled radiative boundary in the ASDEX Upgrade tokamak

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

Plasma Boundary Layer in Limiter and Divertor Tokamaks

Experimental results and theoretical models for the boundary layer of toroidal confinement devices are described. Emphasis is put on the scrape-off layer of poloidal divertor tokamaks which have demonstrated a significant reduction in impurity content as compared to limiter operation. The competition of transport processes parallel and perpendicular to the field lines, determining the profiles of plasma density, temperatures and impurity concentrations, is described. The role of hydrogen recycling in the divertor chamber is underlined, which decides the flow Mach number in the scrape off, the plasma parameters in the target plate vicinity and the regime of energy transport along the field lines. Limiter and divertor operation in ASDEX are compared and the favourable results of the latter explained by the reduced impurity production of the target plates and their distance from closed field lines, which impurity ions would have to overcome by diffusion against the plasma flow into the divertor chamber.

Long-living plasmoids from an atmospheric water discharge

Ball-like plasmoids were generated from discharging a capacitor bank via a water surface. In the autonomous stage after current zero they have diameters up to 0.2?m and lifetimes of some hundreds of milliseconds, thus resembling ball lightning in some way. They were studied by applying high speed cameras, electric probes, calorimetric measurements, and spectroscopy. The plasmoids are found to consist of a true plasma surrounded by a cold envelope. Decreasing electron densities in the order of 1020?1022?m?3 were measured from Stark broadening in the initial (formation) phase. The electron temperature is estimated to be 2000?5000?K during most of the plasmoids lifetime. The temperature of the neutral particles can exceed 1300?K. Calcium hydroxide molecular band emission is the major source of visible radiation in the autonomous phase. Chemiluminescence reactions between dissociation products of water and dissolved calcium are proposed as a source for this emission. The plasmoids colder boundary layer consists of electric double layers that may be attributed to the characteristic shape of the balls.

The influence of opacity on hydrogen excited-state population and applications to low-temperature plasmas

Atomic hydrogen lines and line ratios are being used for diagnostics of technical plasmas in hydrogen or of edge plasmas in fusion research. In the presence of hydrogen molecules, dissociative excitation also contributes to this radiation. The H Lyman lines become optically thick quite easily, which modifies the excited-state population and ionization balance. Line ratios are then a function of electron temperature and density, but also of molecular densities and opacity. To quantify these effects, collisional-radiative population calculations were carried out for the conditions of technical low-pressure plasmas using the most recent hydrogen cross sections and population escape factors. The model for computing opacity is described and results are shown as a function of optical depth. Various spatial emission profiles and spectral line profiles can be included. These results allow the analysis of hydrogen lines from low-temperature plasmas. Measurements are presented, which were carried out in microwave discharges in mixtures of hydrogen or of deuterium and helium. Atom densities and dissociation degrees were determined from absolute Balmer line intensities and from line ratios. The effects of non-Maxwellian electron energy distributions are briefly discussed. The results demonstrate the influence of dissociative excitation and opacity. Taking into account these processes, very consistent results were obtained within the experimental error limits, thus confirming the analysis methods and the rate coefficients used. Dissociation degrees of 0.1-10% were measured depending on pressure and hydrogen concentration. For standard diagnostics, a suitable method can be chosen according to the experimental conditions.

Volume recombination in divertor I of ASDEX Upgrade

High density operation in the ASDEX Upgrade divertor I with horizontal target plates is reported. Density rampup experiments were carried out to characterize detached plasma conditions in the divertor. During the detached phases, hydrogen continua and spectral line emission from high-n shells were observed in the divertor due to the volume recombination. The spectroscopic measurements provide a consistent picture of the evolution of the divertor plasma parameters during the density ramp. By means of the ADAS atomic physics program package, the rate of volume recombination was evaluated, including the effect of opacity. The relative importance of volume recombination in comparison with the target plasma sink is discussed. Observations indicating differences in volume recombination between the two divertor legs are presented, and the connection of volume recombination to divertor detachment is addressed.

Modelling of the Impurity Pumping by a Tokamak Scrape-Off Layer

Abstract The impurity flow along magnetic field lines in a collisional tokamak scrape-off layer is numerically investigated using a testfluid approach. Transport perpendicular to the magnetic field is approximately included by a local loss time constant τ⊥. Results for various impurities and hydrogen background plasma parameters show the significance of thermal forces in the high recycling, subsonic flow regime. In this case, impurity flow reversal on “hot” field lines may cause impurity circulation in the scrape-off layer and, as a consequence, a stronger coupling between the main plasma and a separate pumping chamber than previously assumed.

Radiation Losses and Global Energy Balance for Ohmically Heated Discharges in ASDEX

Global energy balance, radiation profiles and dominant impurity radiation sources are compared for Ohmically heated limiter and divertor discharges in the ASDEX tokarnak. In discharges with a poloidal stainless-steel limiter, total radiation from the plasma is the dominant energy loss channel. The axisymmetric divertor reduces this volume-integrated radiation to 30–35% of the heating power and additional Ti-gettering halves it again to 10–15%. Local radiation losses in the plasma centre, which are mainly due to the presence of iron impurity ions, are reduced by about one order of magnitude. In high-current (Ip = 400 kA) and high-density (e = 6 × 1013 cm−3) ungettered divertor discharges, up to 55% of the heating power is dumped into a cold-gas target inside the divertor chambers. The bolometrically detected volume power losses in the chambers can mainly be attributed to neutral hydrogen atoms with kinetic energies of a few eV. In this parameter range, the divertor plasma is dominated by inelastic molecular and atomic processes, the main process being Franck-Condon dissociation of H2 molecules.

Divertor efficiency in ASDEX

Abstract The divertor efficiency in ASDEX is discussed for ohmically heated plasmas. The parameters of the boundary layer both in the torus midplane and the divertor chamber have been measured. The results are reasonably well understood in terms of parallel and perpendicular transport. A high pressure of neutral hydrogen builds up in the divertor chamber and Franck-Condon particles recycle back through the divertor throat. Due to dissociation processes the boundary plasma is effectively cooled before it reaches the neutralizer plates. The shielding property of the boundary layer against impurity influx is comparable to that of a limiter plasma. The transport of iron is numerically simulated for an iron influx produced by sputtering of charge exchange neutrals at the wall. The results are consistent with the measured iron concentration. First results from a comparison of the poloidal divertor with toroidally closed limiters (stainless steel, carbon) are given. Diverted discharges are considerably cleaner and easier to create.

Atomic data for modelling fusion and astrophysical plasmas

Trends and focii of interest in atomic modelling and data are identified in connection with recent observations and experiments in fusion and astrophysics. In the fusion domain, spectral observations are included of core, beam penetrated and divertor plasma. The helium beam experiments at JET and the studies with very heavy species at ASDEX and JET are noted. In the astrophysics domain, illustrations are given from the SOHO and CHANDRA spacecraft which span from the solar upper atmosphere, through soft x-rays from comets to supernovae remnants. It is shown that non-Maxwellian, dynamic and possibly optically thick regimes must be considered. The generalized collisional-radiative model properly describes the collisional regime of most astrophysical and laboratory fusion plasmas and yields self-consistent derived data for spectral emission, power balance and ionization state studies. The tuning of this method to routine analysis of the spectral observations is described. A forward look is taken as to how such atomic modelling, and the atomic data which underpin it, ought to evolve to deal with the extended conditions and novel environments of the illustrations. It is noted that atomic physics influences most aspects of fusion and astrophysical plasma behaviour but the effectiveness of analysis depends on the quality of the bi-directional pathway from fundamental data production through atomic/plasma model development to the confrontation with experiment. The principal atomic data capability at JET, and other fusion and astrophysical laboratories, is supplied via the Atomic Data and Analysis Structure (ADAS) Project. The close ties between the various experiments and ADAS have helped in this path of communication.

First experimental determination of ion flow velocities and temperatures in the ASDEX Upgrade divertor

For the analysis of ion dynamics (i.e. ion temperature and ion flow velocity distributions) in the divertor I of ASDEX Upgrade an optical spectroscopic system with high spectral, temporal and spatial resolution was brought into operation. Integrated along up to 74 lines of sight with different orientations to the magnetic field lines, emission spectra of atoms and ions were recorded with high resolution by an Echelle spectrometer and a 2D CCD camera. With this setup, ion flow velocities and temperatures in a divertor plasma could be determined experimentally for the first time. In the attached high-recycling divertor regimes analysed, neutrals such as helium do not flow preferentially parallel to any of the installed lines of sight. In contrast, all the ions investigated always showed pronounced Doppler shifts. The corresponding ion flow is directed towards the target plates, in the outer as well as in the inner divertor. For typical L- and H-mode discharges (attached high-recycling divertor conditions) the ions flow at speeds of about in the L-mode and in the ELMy H-mode phases. The temperatures of the ions near the divertor plates range from about 5 eV (L-mode) to up to 20 eV (ELMy H-mode phases). The comparison of these results with computer code calculations showed good agreement.