ABSTRACT
Magnetic turbulence is observed at the beginning of the current quench in intended TEXTOR disruptions. Runaway electron (RE) suppression has been experimentally found at magnetic turbulence larger than a certain threshold. Below this threshold, the generated RE current is inversely proportional to the level of magnetic turbulence. The magnetic turbulence originates from the background plasma and the amplitude depends strongly on the toroidal magnetic field and plasma electron density. These results explain the previously found toroidal field threshold for RE generation and have to be considered in predictions for RE generation in ITER.
ABSTRACT
A spectrometer has been developed to measure relativistic electrons produced in different types of plasmas, such as tokamak plasmas and laser produced plasmas. The spectrometer consists of nine Y2SiO5:Ce crystals, which are shielded by stainless steel filters. The absolute calibration of the spectrometer was performed at the superconducting electron linear accelerator Electron Linac for beams with high Brilliance and low Emittance. The spectrometer can provide information about energy distribution of electrons and their numbers for the energy range between 4 and 30 MeV. The spectrum is analyzed by means of the Monte Carlo three-dimensional GEANT4 code. An energy resolution of about 10% is achieved.
ABSTRACT
Energy spectral measurements of runaway electrons were performed by a scanning probe with high spatial and temporal resolution in the TEXTOR tokamak. The probe consists of ten YSO (Y(2)SiO(5):Ce) crystals, which are shielded by tungsten filters. The probe can resolve electrons with different energies between 4 and 30 MeV. An insertion of the probe to the plasma boundary several times during the discharge allowed spectral measurements of runaway electrons at different minor radial positions at the plasma edge as well as the study of runaway production in time. The high temporal resolution of the probe, 0.05 ms, enabled measurements of runaway electrons not only during low density discharges, but also during plasma disruptions.
ABSTRACT
The article presents a detailed investigation of the fast disruption mitigation valve developed at FZJ Juelich. The essence of this study is the novel direct observation of the piston motion by means of a fast framing camera. The piston stroke and the injection duration are shown to strongly depend on the operational pressure and the used gas. The same is true for the valve throughput. The analysis revealing the leading contribution of the injection duration in this modification is given. The knowledge of the injection duration is also used to reconstruct the characteristic pressure decay rates and the gas outflow rates. The means to increase the gas outflow are discussed. The main found valve characteristics are: (1) valve reaction time, i.e., the delay between the application of the trigger signal and the achievement of reliably observable opening 0.5 mm, is about 0.3 ms; (2) the maximum achieved throughput is 7.5 bar l for argon and 9.5 bar l for helium; (3) the maximum delivery rates are 500 bar l s(-1) for Ar and 1500 bar l s(-1) for He.
ABSTRACT
The ergodization of the magnetic field lines imposed by the dynamic ergodic diverter (DED) in TEXTOR can lead both to confinement improvement and to confinement deterioration. The cases of substantial improvement are in resonant ways related to particular conditions in which magnetic flux tubes starting at the X points of induced islands are connected with the wall. This opening process is connected with a characteristic modification of the heat deposition pattern at the divertor target plate and leads to a substantial increase and steepening of the core plasma density and pressure. The improvement is tentatively attributed to a modification of the electric potential in the plasma carried by the open field lines. The confinement improvement bases on a spontaneous density built up due to the application of the DED and is primarily a particle confinement improvement.
ABSTRACT
Systematic measurements on the edge turbulence and turbulent transport have been made by Langmuir probe arrays on TEXTOR under various static Dynamic Ergodic Divertor (DED) configurations. Common features are observed. With the DED, in the ergodic zone the local turbulent flux reverses sign from radially outwards to inwards. The turbulence properties are profoundly modified by energy redistribution in frequency spectra and suppression of large scale eddies. The fluctuation poloidal phase velocity changes direction from electron to ion diamagnetic drift, consistent with the observed reversal of the Er x B flow. In the laminar region, the turbulence is found to react to an observed reduced flow shear.
ABSTRACT
The magnetic field penetration process into a magnetized plasma is of basic interest both for plasma physics and astrophysics. In this context special measurements on the field penetration and field amplification are performed by a Hall probe on the dynamic ergodic divertor (DED) on the TEXTOR tokamak and the data are interpreted by a two-fluid plasma model. It is observed that the growth of the forced magnetic reconnection by the rotating DED field is accompanied by a change of the plasma fluid rotation. The differential rotation frequency between the DED field and the plasma plays an important role in the process of the excitation of tearing modes. The momentum input from the rotating DED field to the plasma is interpreted by both a ponderomotive force at the rational surface and a radial electric field modified by an edge ergodization.
ABSTRACT
The magnetic-field perturbation produced by the dynamic ergodic divertor in TEXTOR changes the topology of the magnetic field in the plasma edge, creating an open chaotic system. The perturbation spectrum contains only a few dominant harmonics and therefore it can be described by an analytical model. The modeling is performed in the vacuum approximation without assuming a backreaction of the plasma and does not rely on any experimentally obtained parameters. It is shown that this vacuum approximation predicts in many details the experimentally observed plasma structure. Several experiments have been performed to prove that the plasma edge behavior is defined mostly by the magnetic topology of the perturbed volume. The change in the transport can be explained with the knowledge of only the magnetic structures; i.e., the ergodic pattern dominates the plasma properties.
ABSTRACT
A significant influence of the dynamic ergodic divertor (DED) on the density limit in TEXTOR has been found. In Ohmic discharges, where without DED detachment normally arises at the density limit, a MARFE (multifaceted asymmetric radiation from the edge) develops when the DED is operated in a static regime. The threshold of the MARFE onset in the neutral beam heated plasmas is increased by applying 1 kHz ac DED at the high-field side. The theoretical predictions based on the parallel energy balance taking poloidal asymmetries into account agree well with the experimental observation.
ABSTRACT
The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating in the m/n=3/1 mode configuration, are presented. The deeply penetrating external magnetic field perturbation of this configuration increases the toroidal plasma rotation. Staying below the excitation threshold for the m/n=2/1 tearing mode, this toroidal rotation is always in the direction of the plasma current, even if the toroidal projection of the rotating magnetic field perturbation is in the opposite direction. The observed toroidal rotation direction is consistent with a radial electric field, generated by an enhanced electron transport in the ergodic layers near the resonances of the perturbation. This is an effect different from theoretical predictions, which assume a direct coupling between rotating perturbation and plasma to be the dominant effect of momentum transfer.
ABSTRACT
A stochastic magnetic boundary, produced by an applied edge resonant magnetic perturbation, is used to suppress most large edge-localized modes (ELMs) in high confinement (H-mode) plasmas. The resulting H mode displays rapid, small oscillations with a bursty character modulated by a coherent 130 Hz envelope. The H mode transport barrier and core confinement are unaffected by the stochastic boundary, despite a threefold drop in the toroidal rotation. These results demonstrate that stochastic boundaries are compatible with H modes and may be attractive for ELM control in next-step fusion tokamaks.
