Asdex Team

The H-Mode of ASDEX

The paper is a review of investigations of the H-mode on ASDEX performed since its discovery in 1982. The topics discussed are: (1) the development of the plasma profiles, with steep gradients in the edge region and flat profiles in the bulk plasma, (2) the MHD properties resulting from the profile changes, including an extensive stability analysis, (3) the impurity development, with special emphasis on the MHD aspects and on neoclassical impurity transport effects in quiescent H-phases, and (4) the properties of the edge plasma, including the evidence of threedimensional distortions at the edge. The part on confinement includes scaling studies and the results of transport analysis. The power threshold of the H-mode is found to depend weakly on the density, but there is probably no dependence on the toroidal field or the current. For the operational range of the H-mode, new results for the limiter H-mode on ASDEX and the development of the H-mode under beam current drive conditions are included. A number of experiments are described which demonstrate the crucial role of the edge electron temperature in the L-H transition. New results of magnetic and density fluctuation studies at the plasma edge within the edge transport barrier are presented. Finally, the findings on ASDEX are compared with results obtained on other machines and are used to test various H-mode theories.

The Isotope Effect in ASDEX

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

Measurements on the particle balance in diverted ASDEX discharges

Abstract In the ASDEX divertor we have measured the flux of neutral particles to the wall, the plasma flux to the neutralizer plates and the electron temperature and density. After a description of our apparatus we give some results for ohmically heated discharges and discuss them in connection with the particle and power balance and the dominant fueling mechanism in diverted ungettered discharges. We estimate from our results ionization rate and power fluxes in the divertor. The ionization rate is in reasonable agreement with the plasma flux to the neutralizer plates and the power fluxes transferred by neutrals and photons to the wall are not in disagreement with bolometer measurements.

A study of runaway electron confinement in the ASDEX tokamak

The results of runaway electron confinement experiments from ASDEX are analysed to elucidate the structure of electromagnetic turbulence which may cause anomalous electron heat transport in the L-mode confinement regime. From a simple model, the radial correlation length (W) of the magnetic turbulence is determined to be about 1 mm. Using this value and that of the experimentally deduced electron thermal diffusivity, the authors determine the radial magnetic fluctuation level at the plasma edge in the L-mode to be (r/B0) ~ 2 × 10−4. Scalings of W and r/B0 are deduced from parameter scans. From a comparison of these results with the predictions of various theoretical models, it is concluded that skin depth turbulence, electromagnetic drift wave turbulence, rippling modes, and microtearing modes are inferior candidates and that resistive ballooning modes offer the best possibility for a consistent interpretation of the data.

Impurity transport and neoclassical predictions

The authors present a brief review of collisional (classical and neoclassical) and anomalous transport. Particular emphasis is devoted to the question of charge independence of the anomalous transport coefficients and the combined action of anomalous and collisional transport. In the light of these results the experimental facts are analysed and interpreted. It is found that impurity accumulation-characterized by peaked zeff-profiles-is caused by the combined effects of improved confinement (i.e. reduction of anomalous transport) and peaking of the electron density profile. For the cases of pellet refuelled plasmas and counter neutral injection heating quantitative comparisons are performed which show good agreement between the experimental measurements and simulations based upon neoclassical theory.

The ASDEX divertor tokamak

ASDEX is a large tokamak (R = 1.65 m, a = 0.4 m, Ip ≤ 500 kA) that started operation in 1980. Its distinctive features are a double- (or single-) null poloidal divertor, the capability for long-pulse operation (up to 10 s) and high-power neutral-beam (4.3 MW), ICRH (3 MW) and LH (2.4 MW) heating systems. Several highly significant experimental findings have been obtained, facilitated by the large flexibility of the machine. The high-recycling divertor regime is considered the most viable solution for handling the large power fluxes envisaged for the next generation of tokamak devices. Studies of impurity transport in the bulk plasma led to the postulation of an additional inward particle drift, while gas puffing experiments revealed the importance of thermoelectric forces along field lines for impurity retainment by the divertor. The high-confinement regime (H-mode) of neutral-beam-heated plasmas, discovered on ASDEX, is also intimately connected with the divertor configuration. The H-mode.confinement, in turn, enabled the investigation of β-limits resulting in the experimental scaling 2.8 Ip/(a BT) [%; MA, m, T], in excellent agreement with theoretical predictions. The most important results from recent experiments with RF heating and current drive include the recharging of the OH transformer at constant plasma current, and the observed high heating efficiency obtained with combined second-harmonic ICRH and neutral-injection heating.

Low energy neutral particle fluxes to the walls of ASDEX during He and D2 discharges

Neutral particle fluxes onto the walls of ASDEX have been investigated using a time-of-flight (TOF) method. The energy distributions of the neutrals could be determined in the range of 10–1000 eV/amu. Ohmic divertor and limiter discharges with equal plasma currents and densities have been compared for He and D2. The He0 outflux at ∼2000 eV from He discharges is 110 of the corresponding D0 flux in D2 discharges. At lower energies this difference is much smaller. In all cases many more He neutrals were observed than was anticipated from the CX rate-coefficients for He2+. The impurity fluxes due to sputtering by the CX-neutrals show no significant difference for He and D2 discharges. For divertor discharges CX-sputtering can fully account for the Fe impurity content determined spectroscopically.

Turbulence in the SOL of ASDEX and W7-AS

Electrostatic fluctuations have been measured in the scrape-off layer of ASDEX and W7-AS by Langmuir probes and by observation of H? light and light emitted from a fast Li atom beam with high spatial and temporal resolution. It was demonstrated that these fluctuations contribute a significant, if not dominant, fraction of the anomalous radial particle transport. The basic properties of the fluctuations are the same in both experiments. A model for an instability mechanism specific to the SOL is presented including density, temperature and electric potential fluctuations. From this model mixing length estimates for the radial transport and resulting density and pressure gradients in the SOL are derived and compared to measurements in the mid-plane and in the divertor of ASDEX. In spite of several simplifications in the model a quantitative agreement up to factors of 1-3 and a qualitative agreement for variations of discharge parameters is achieved between the model predictions and the measurements. Data from poloidal multi-pin probe arrays are decomposed into a sum of spatial-temporal events by means of a fitting procedure. Centres of selected events serve as reference points for the analysis of the dynamics in their surroundings. Averaging shows that positive and negative potential events appear mostly in pairs with the E x B drift in between directed radially outward.

Determination of impurity transport coefficients by harmonic analysis

Impurity transport coefficients were determined by means of sinusoidally modulated gas puffing. The principles of the method are outlined, and results for impurity diffusion coefficients, evaluated with respect to a simplified transport model, are presented for measurements on ASDEX during stationary phases. The dependence of the diffusion coefficient on various plasma parameters is found to be in striking contrast to neoclassical theory.

Studies on Impurity Retainment in the ASDEX Divertor

Abstract The retainment and exhaust capability of the ASDEX divertor for neon and argon is investigated during ohmic heating and neutral injection heating. No pronounced influence of the scrape-off layer on the divertor particle outflux is observed, though ionization of neutrals in the divertor throats is most likely. The divertor outfluxes behave as if they are determined by the molecular flow conductances of the divertor throats. The plasma impurity outfluxes show strong top-bottom asymmetries which reverse with the direction of the toroidal field. The exhaust capability can be demonstrated by comparison with limiter discharges. The exhaust efficiency of the double-null configuration is about twice that of a single-null configuration.