H. Verbeek’s research while affiliated with Max Planck Institute for Plasma Physics and other places

A review of particle-solid processes pertinent to modelling plasma-wall interactions is presented, and sets of recommended data are given. Analytic formulas are used where possible; otherwise, data are presented in the form of tables and graphs. The incident particles considered are e−, H, D, T, He, C, O, and selfions. The materials include the metals aluminum, beryllium, copper, molybdenum, stainless steel, titanium, and tungsten and the nonmetals carbon and TiC. The processes covered are light ion reflection, hydrogen and helium trapping and detrapping, desorption, evaporation, sputtering, chemical effects in sputtering, blistering caused by implantation of helium and hydrogen, secondary electron emission by electrons and particles, and arcing.

Measurements of the negative charge-state fraction of the reflected particles resulting from D2+ bombardment of a thick Na target are presented. Incident energies of 6 keV and 12 keV produced particles with exit energies between 400 eV and 5000 eV. The incident angle was 0° from the surface normal and the exit angle was 85°, giving a scattering angle of 95°. The negative charge-state fraction was 0.20 at an exit energy of 400 eV and decreased monotonically to 0.05 at an exit energy of 5000 eV. The negative charge-state fraction was also independent of the incident energy. The present results are compared to measurements made at an incident angle of 85° with the same exit angle as above, where there is a remarkable difference in the negative charge-state fractions. The differences in the results for the different incident angles are discussed in terms of a trajectory-dependent formation probability of the negative ions.

The paper describes the effect of the isotopic mass on plasma parameters as observed in the ASDEX tokamak. The paper comprises Ohmic as well as L mode, H mode and H* mode scenarios. The measurements reveal that the ion mass is a substantial and robust parameter, which affects all the confinement times (energy, particle and momentum) in the whole operational window. Both core properties such as the sawtooth repetition time and edge properties such as the separatrix density change with the isotopic mass. Specific emphasis is given to the edge parameters and changes of the edge plasma due to different types of wall conditioning, such as carbonization and boronization. The pronounced isotope dependences of the edge and divertor parameters are explained by the secondary effect of different power fluxes into the scrape-off layer plasma and onto the divertor plates. Finally, the observations serve to test different transport theories. With respect to the ion temperature gradient driven turbulence, the isotope effect is also studied in pellet refuelled discharges with peaked density profiles. The results from ASDEX are compared with the results from other experiments

The closed ASDEX Upgrade Divertor II, `LYRA', is capable of handling heating powers of up to 20 MW or P/R of 12 MW/m, owing to a reduction of the maximum heat flux to the target plates by more than a factor of 2 compared with the open Divertor I. This reduction is caused by high radiative losses from carbon and hydrogen inside the divertor region and is in agreement with B2-EIRENE modelling predictions. At medium densities in the H mode, the type I ELM behaviour shows no dependence on the heating method (NBI, ICRH). ASDEX Upgrade-JET dimensionless identity experiments showed compatibility of the L-H transition with core physics constraints, while in the H mode confinement, inconsistencies with the invariance principle were established. At high densities close to the Greenwald density, the MHD limited edge pressures, the influence of divertor detachment on separatrix parameters and increasing edge transport lead to limited edge densities and finally to temperatures below the critical edge temperatures for H mode. This results in a drastic increase of the H mode threshold power and an upper H mode density limit with gas puff refuelling. The H mode confinement degradation approaching this density limit is caused by the ballooning mode limited edge pressures and `stiff' temperature profiles relating core and edge temperatures. Repetitive high field side pellet injection allows for H mode operation well above the Greenwald density; moreover, higher confinement than with gas fuelling is found up to the highest densities. Neoclassical tearing modes limit the achievable β depending on the collisionality at the resonant surface. In agreement with the polarization current model, the onset β is found to be proportional to the ion gyroradius in the collisionless regime, while higher collisionalities are stabilizing. The fractional energy loss connected with saturated modes at high pressures is about 25%. A reduction of neoclassical mode amplitude and an increase of β have been demonstrated by using phased ECRH and ECCD in the O point of islands. Advanced tokamak operation with internal transport barriers for both ions and electrons has been achieved with flat shear profiles and q0 > 1 or with reversed shear and qmin > 2. With flat shear a stationary H mode scenario was maintained for 40 confinement times and several internal skin times with βN = 2 and HITERL-89P = 2.4, where fishbones keep q0 at 1. βN is limited by either neoclassical tearing modes in the case of flat shear or kink modes with reversed shear.

An energy analyser for neutral hydrogen and noble gas atoms is described. The neutral particles are ionized by electron stripping in a gas cell and the resulting ions are energy-analysed in an electrostatic spectrometer. The instrument is absolutely calibrated with nitrogen in the gas cell. The lowest energies which can be detected are 200 eV for hydrogen, 800 eV for helium and 1 keV for neon atoms.

The authors give an overview of the different confinement regimes observed on ASDEX and compare the changes during the transition phases with qualitative tendencies suggested by theoretical models. The transitions discussed are those between purely Ohmic heating and additional heating in the L-regime between the L- and the H-regime and between discharges with flat and peaked electron density profiles.

To compare different Ohmic confinement regimes in ASDEX, the edge conditions are analyzed in detail. The results show that the improved Ohmic confinement coincides with a drop of the separatrix density. This drop allows the density profile to peak and seems to be the trigger of a change in the transport. A universal scaling between the electron temperature and the electron density at the separatrix prevails for all Ohmic scenarios. In addition, the total particle flux across the separatrix is evaluated and found to be strongly correlated to the separatrix density. Thus, the convective energy loss contributes less to the total energy losses when the confinement is improved. Since the correlations between the edge parameters do not change in different Ohmic confinement regimes of ASDEX, the edge physics appears to remain the same. Improved Ohmic confinement is characterized by an optimum separatrix density which provides a sufficiently high edge temperature together with low particle fluxes. These optimum conditions yield the maximum particle confinement.

In the regime of Improved Ohmic Confinement (IOC) in ASDEX the energy confinement time tau E increases linearly with increasing line-averaged density ne up to the density limit. The establishment of the IOC is accompanied by a substantial reduction of the external gas feed, concomitant with large decreases of all plasma edge fluxes. However, the data do not supply conclusive evidence that the IOC is primarily connected with the recycling conditions. More recent observations with very clean machine conditions seem to indicate that the impurity radiation plays a significant role.

A survey of edge modeling and its application to running experiments is given with emphasis on poloidal divertors. The basic edge structure in axisymmetric and weakly perturbed tokamaks is first discussed. The ongoing modeling activities and the status of model validation are outlined. ASDEX data are mostly used for comparison, since sufficiently detailed and coherent edge measurements are not available in the literature for most experiments. Edge physics issues discussed in more detail are the basic model equations, parallel and perpendicular transport coefficients, thermoelectric effects, edge density limit and three-dimensional perturbations including magnetic field ergodization.

Tungsten-coated tiles, manufactured by plasma spray on graphite, were mounted in the divertor of the ASDEX Upgrade tokamak and cover almost 90% of the surface facing the plasma in the strike zone. Over 600 plasma discharges have been performed to date, around 300 of which were auxiliary heated with heating powers up to 10 MW. The production of tungsten in the divertor was monitored by a W I line at 400.8 nm. In the plasma centre an array of spectral lines at 5 nm emitted by ionization states around W XXX was measured. From the intensity of these lines the W content was derived. Under normal discharge conditions W-concentrations around or even lower were found. The influence on the main plasma parameters was found to be negligible. The maximum concentrations observed decrease with increasing heating power. In several low power discharges accumulation of tungsten occurred and the temperature profile was flattened. The concentrations of the intrinsic impurities carbon and oxygen were comparable to the discharges with the graphite divertor. Furthermore, the density and the limits remained unchanged and no negative influence on the energy confinement or on the H-mode threshold was found. Discharges with neon radiative cooling showed the same behaviour as in the graphite divertor case.