A. Krämer-Flecken

Tearing mode stabilization by electron cyclotron resonance heating demonstrated in the TEXTOR tokamak and the implication for ITER

Controlled experiments on the suppression of the m/n = 2/1 tearing mode with electron cyclotron heating and current drive in TEXTOR are reported. The mode was produced reproducibly by an externally applied rotating perturbation field, allowing a systematic study of its suppression. Heating inside the island of the mode is shown to be the dominant suppression mechanism in these experiments. An extrapolation of these findings to ITER indicates that the projected system for suppression of the tearing mode could be significantly more effective than present estimates indicate, which only consider the effect of the current drive but not of the heating inside the island.

Dynamics of fast ions during sawtooth oscillations in the TEXTOR tokamak measured by collective Thomson scattering

Experimental investigations of sawteeth interaction with fast ions measured by collective Thomson scattering on TEXTOR are presented. Time-resolved measurements of localized 1D fast-ion distribution functions allow us to study fast-ion dynamics during several sawtooth cycles. Sawtooth oscillations interact strongly with the fast-ion population in a wide range of plasma parameters. Part of the ion phase space density oscillates out of phase with the sawtooth oscillation during hydrogen neutral beam injection (NBI). These oscillations most likely originate from fast hydrogen ions with energies close to the full injection energy. At lower energies passing fast ions in the plasma centre are strongly redistributed at the time of sawtooth collapse but no redistribution of trapped fast ions is observed. The redistribution of fast ions from deuterium NBI in the plasma centre is found to vary throughout velocity space. The reduction is most pronounced for passing ions. We find no evidence of inverted sawteeth outside the sawtooth inversion surface in the fast-ion distribution function.

Overview of radiative improved mode results on TEXTOR-94

The radiative improved (RI) mode is a tokamak regime offering many attractive reactor features. In the article, the RI mode of TEXTOR-94 is shown to follow the same scaling as the linear ohmic confinement regime and is thus identified as one of the most fundamental tokamak operational regimes. The current understanding derived from experiments and modelling of the conditions necessary for sustaining the mode is reviewed, as are the mechanisms leading to L-RI mode transition. The article discusses the compatibility of high impurity seeding with the low central power density of a burning reactor, as well as RI mode properties at and beyond the Greenwald density.

Complete multi-field characterization of the geodesic acoustic mode in the TCV tokamak

The geodesic acoustic mode (GAM) is a coherently oscillating zonal flow that may regulate turbulence in toroidal plasmas. Uniquely, the complete poloidal and toroidal structure of the magnetic component of the turbulence-driven GAM has been mapped in the TCV tokamak. Radially localized measurements of the fluctuating density, ECE radiative temperature and poloidal flow show that the GAM is a fully coherent, radially propagating wave. These observations are consistent with electrostatic, gyrokinetic simulations.

Mitigation of disruptions by fast helium gas puffs

In order to mitigate the effect of disruptions in tokamaks, it is proposed to inject quickly a relatively large amount of helium; first experiments on this topic have been performed on TEXTOR. For this purpose, a fast valve has been developed which releases 10 mbar L of helium gas within 1 ms; the valve is located at a vessel flange such that a fast response is guaranteed even if it is triggered at the onset of the disruption. The amount of gas is sufficient to exceed the density limit even with low density discharges. The intention of the proposal is to shorten the plasma current decay phase, to reduce halo currents, to suppress runaway electrons and to provide good conditions for the start of the following discharge. In particular, for achieving the last goal, helium is the optimum choice of all the elements. The experiments performed on TEXTOR have proven various of these mitigation aspects: the current decay time is shortened, runaway electrons are expelled by the gas puff and the conditions for the start of the next discharge have neither deteriorated with respect to gas release from wall components nor with respect to excessive impurity production.

Influence of the dynamic ergodic divertor on transport properties in TEXTOR

Experiments to investigate transport properties under the influence of the dynamic ergodic divertor (DED) on TEXTOR are discussed. Relativistic runaway electrons are applied for studying transport properties of ergodization such as enhanced runaway loss. The ergodization causes an enhanced loss rate; this loss is higher for low relativistic electrons than for highly relativistic ones, in good agreement with particle orbit mapping. Edge transport can be controlled by the DED perturbation: in limiter H-mode plasmas ELM-like particle and heat bursts associated with the formation of enhanced edge pressure gradients are mitigated in the 6/2 configuration on the expense of a reduced pedestal height. Finally, the plasma is driven back to L-mode under the influence of the magnetic perturbation. In the 3/1 configuration the onset of tearing modes limits the possibility to affect edge transport. A mode of spontaneous density built-up has been found for the TEXTOR-DED as well. This mode is in particular strong for an inward shifted plasma; the built-up has a resonant character with respect to q(a). Langmuir probe measurements with two probe arrays show a strong influence of the magnetic ergodization on both the edge plasma equilibrium and fluctuation parameters. In particular, in the ergodic zone the turbulence properties and turbulence-driven flux are profoundly modified.

Dependence of the threshold for perturbation field generated m/n = 2/1 tearing modes on the plasma fluid rotation

The dynamic ergodic divertor (DED) on the TEXTOR tokamak allows for the creation of static and rotating helical magnetic perturbation fields. In the 3/1 configuration the strong m/n = 2/1 sideband excites a locked 2/1 tearing mode above a critical perturbation field strength. The mode onset threshold depends strongly on the plasma fluid rotation with respect to the mode. Rotation in plasma current direction destabilizes the mode in a certain range of rotation frequencies, whereas counter-rotation has a stabilizing influence. The threshold shows a minimum when the frequency of the external perturbation equals the MHD frequency of the mode.

Overview of experiments with radiation cooling at high confinement and high density in limited and diverted discharges

An overview is presented of recent experiments with radiating mantles on limiter and divertor machines, realizing simultaneously high confinement and high density at high-radiation levels. A variety of operational regimes has been observed and the characteristics of each are documented. High-performance plasmas (i.e. edge localized mode (ELM)-free H-mode confinement quality and normalized beta values simultaneously) with radiating mantles have been demonstrated under quasistationary conditions during the maximum flattop time of the machine (equal to tens of confinement times) on DIII-D and TEXTOR-94. Maximum values for up to 4 and for the advanced tokamak confinement-stability product up to 13, have been obtained in very high confinement mode (VH-mode) like discharges with radiating mantles in DIII-D. There is a striking similarity between improved ohmic confinement discharges (with or without Ne seeding) and radiating mantle discharges, indicating a possible common origin for the confinement improvement observed. Possible scenarios for the application of radiating mantles on larger machines such as JET and JT-60U are indicated.

Observation of geodesic acoustic modes (GAMs) and their radial propagation at the edge of the TEXTOR tokamak

The electrostatic potential and density fluctuations have been measured at the edge of the TEXTOR tokamak by two toroidally distant Langmuir probe arrays. The geodesic acoustic mode (GAM) zonal flows (ZFs) are observed in potential fluctuations with a toroidal and poloidal symmetric structure. The GAM frequency, fGAM, changes monotonically with the local temperature and is close to the frequency-dispersion predicted by theories. Bispectral analysis shows clear nonlinear coupling between the GAM and broadband ambient turbulence. The GAM packet has a narrow radial extent with kr 0.5–0.7 cm−1 and exhibits explicitly a radially outward propagation. Furthermore, the radial correlation structure of GAMs and their radial propagation have been investigated in a wide range of parameters by varying plasma density and edge safety factor (5.0 ≤ q(a) ≤ 5.9). It is found that the magnitude of the GAM correlations reduces remarkably with the increase in the plasma density approaching the density limit, while the radial wavelength of GAMs only decreases slightly in higher density and larger q(a) discharges. With increasing plasma density, the radial propagating phase speed of GAMs is strongly reduced along with the drop in the local temperature. The results provide new evidence on the propagation properties of GAM ZFs.

Electron cyclotron resonance heating on TEXTOR

The 110 GHz and the new 140 GHz gyrotron systems for electron cyclotron resonance heating (ECRH) and ECCD on TEXTOR are described and results of ECRH experiments with the 110 GHz system are reported. Central ECRH on Ohmic plasmas shows the presence of an internal electron transport barrier near q = 1. This is confirmed by modulated ECRH experiments. A central barrier is also indicated by ECRH in radiatively improved (RI) mode discharges and up to two barriers are seen with ECRH during the current ramp phase. ECRH control of sawteeth is reported for both Ohmic and RI mode target plasmas.