J. Gernhardt

Parametric Decay in the Edge Plasma of ASDEX During Fast Wave Heating in the Ion Cyclotron Frequency Range

Two types of parametric decay instabilities were observed in the scrape-off layer of ASDEX during hydrogen second harmonic heating, in a single as well as in a two ion species plasma. The first type was identified as decay into an ion Bernstein wave and an ion cyclotron quasi-mode, and the second as decay into an ion Bernstein wave and a low frequency electron quasi-mode. The parametric decay processes are due to the high electric fields near the fast wave antennas. Theoretical growth rate calculations predict a threshold for the electric fields near the antennas. In the relevant experiments the electric fields, estimated from a full-wave code, indeed exceeded the thresholds. These parametric decay processes, as well as the harmonics of the generator frequency also seen in the scrape-off layer, provide a mechanism that might contribute to the observed direct energy deposition in the edge plasma and to ICRF induced impurity production.

Studies of edge localized modes on ASDEX

The MHD behaviour connected with edge localized modes (ELMs) is studied using fast sample rate fluctuation diagnostics. It is found that ELMs on ASDEX are generally preceded by a coherent MHD mode of medium poloidal mode number (m=10-15). At the onset of the ELM the mode behaviour changes to a turbulent one, leading to an outflux of heat and particles. These results are related to a model by Kerner and Jakoby, as described in Nucl. Fusion 29 (1989) 1959 (Section 7.2), which explains the ELM as a nonlinear coupling of free-surface modes of resistive ballooning character. This model is in good agreement with the experimental findings

MHD Mode Structure and Propagation in the ASDEX Tokamak

The Mirnov oscillations observed in the ASDEX tokamak are analysed with regard to the generally accepted interpretation scheme according to which (1) Mirnov oscillations are caused by currents flowing parallel to the magnetic field on rational magnetic surfaces and (2) the field perturbation is frozen within the plasma. If the second statement holds, the frequency is obtained from the profiles of the electron density, the ion temperature and, if applicable, the toroidal or poloidal rotation velocity. It is shown that there are modes which are consistent with the above interpretation. On the other hand, mode coupling is observed. Mode coupling is also invoked to reconcile the experimental findings with the predictions of a theory based on statement (1); according to this theory, there is a poloidal variation of both the phase velocity and the amplitude of the Mirnov oscillations. While the observed phase velocity fits well into this picture, the poloidal variation of the amplitude cannot be ascribed to only one mode in the majority of cases. In addition, the possibility of coherent MHD activity due to currents in the scrape-off layer is discussed.

Density Limit Investigations on ASDEX

Density limit investigations on ASDEX have been performed under a variety of conditions: ohmically heated and neutral injection heated plasmas in H2, D2 and He have been studied in different divertor configurations, after various wall coating procedures, with gas puff and pellet fuelling, and in different confinement regimes with their characteristically different density profiles. A detailed description of the parametric dependence of the density limit, which in all cases is a disruptive limit, is given. This limit is shown to be a limit to the density at the plasma edge. Therefore, the highest densities corresponding to neRqa/Bt>30*1019 m-2.T-1 are obtained with centrally peaked ne profiles. Radiation from the main plasma at the density limit is always significantly below the total input power. The plasma disruption is due to an m=2 instability which for medium and high qa is preceded by one or more minor disruptions. In this range of qa, the disruptive instability is initiated by the occurrence of a Marfe on the high field side as a consequence of strong plasma cooling in this region. The duration of the Marfe increases with increasing distance between the plasma edge and the q=2 surface. After penetrating onto closed flux surfaces the Marfe leads to a current contraction and a subsequent destabilization of the m = 2 mode. In helium plasmas a strongly radiating, poloidally symmetric shell is observed before the density limit instead of a Marfe. An instantaneous destabilization of this mode is observed at low qa. Detailed measurements of plasma edge and divertor parameters close to the density limit indicate the development of a cold, dense divertor plasma before the disruption. Models describing the scrape-off layer and the divertor region predict an upper limit to the edge density at low divertor temperatures according to power balance considerations. Their relations to the experimental findings, especially the low field side cooling, ar

MHD stability and disruption physics in ASDEX Upgrade

The MHD activity giving rise to the beta - and the density limit in ASDEX Upgrade is analyzed. A detailed description of the MHD phenomena occuring prior to and during disruptions is given. The MHD characteristics of the different ELM types occurring in ASDEX Upgrade are described.

Lower hybrid experiments in the ASDEX tokamak

Interaction of lower hybrid waves at 1.3 GHz with ions and electrons was studied in the density range 0.2-5*1013 cm-3 in the ASDEX tokamak. At high densities, ne>or approximately=4*1013 cm-3, fast ions with mainly perpendicular velocities are produced by the RF power at the plasma periphery. They are not well confined and do not lead to any bulk plasma heating. At lower densities, 2*1013<or approximately=ne<or approximately=4*1013 cm-3, electron and ion heating is observed. The heating is better in deuterium than in hydrogen plasmas. At very low densities, ne<or approximately=2*1013 cm-3, the discharge becomes suprathermal as soon as the RF power is switched on. Launching an asymmetric spectrum of waves in a low density plasma leads to the generation of an RF-driven DC-plasma current.

Heating and Confinement in the Ion Cyclotron Range of Frequencies on the Divertor Tokamak ASDEX

The paper summarizes the experiments performed with ion cyclotron resonance heating (ICRH) on ASDEX, from November 1984 until March 1986; the most interesting results are reported and discussed in detail. Heating and confinement studies using the hydrogen second harmonic scheme and the hydrogen minority scheme (PIC < 2.6 MW, tIC < 1.5 s) show a typical L-mode behaviour, i.e. a power dependent confinement degradation, which is rather similar to that found with neutral beam injection (NBI) heating. ICRH is accompanied by a slightly improved particle and energy confinement compared with that of NBI; this is also true for a combined ICRH + NBI scheme, up to Ptot ≈ 4.5 MW, absorbed in the plasma. Particular efforts have been devoted to investigations of the second harmonic regime in H/D plasmas with nH/ne ≈ 0.1 - 1, with a view to heating mixtures in reactor relevant plasmas. The achievement of H-mode transitions with ICRH alone in the hydrogen minority scheme at an absorbed RF power of about 1.1 MW supports the assumption of common confinement properties in auxiliary heated tokamaks, since they appear to be widely independent of the additional heating method. ICRH specific impurity problems, such as the strong release of iron from the vessel walls, have been overcome by applying extensive in situ wall carbonization. The mechanisms responsible for impurity generation have partly been identified and analysed; however, the problem still remains to be solved. Impurities preferentially released from the ICRH antenna do not pose problems.

Design of the magnetic diagnostic for ITER

The detailed design of the magnetic diagnostic for ITER is presented. The system consists of groups of pickup coils and flux loops on the vessel wall, the back plate, and the divertor. These sensors provide the measurements for the equilibrium reconstruction and the fluctuation analysis. The system is supplemented by Rogowski coils for halo current measurements and by a diamagnetic loop. The complete system meets the measurement requirements, matching those of contemporary large divertor tokamaks. The maximum radiation exposure is such that the sensors will survive for the lifetime of the ITER.

Magnetohydrodynamic Activity During Edge Localized Modes on ASDEX

Magnetohydrodynamic (MHD) activity during edge localized modes (ELMs) has been monitored in the ASDEX tokamak. Besides a fast inward shift of the plasma column, a helical MHD perturbation has been found to accompany ELMs. For the discharges investigated, the helical instability has a poloidal mode number m of 3 or 4 and a toroidal mode number n of 1, and it propagates in the direction of the electron diamagnetic drift. It seems to be identical with the Toi mode, which has been observed to disappear during L-H transitions.

Confinement regime transitions in ASDEX

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.