K.H. Finken

Experimental investigation of runaway electron generation in TEXTOR

An experimental study of the generation of runaway electrons in TEXTOR has been performed. From the infrared synchrotron radiation emitted by relativistic electrons, the number of runaway electrons can be obtained as a function of time. In low density discharges (ne < 1 × 1019 cm-3) runaways are created throughout the discharge and not predominantly in-the startup phase, From the exponential increase in the runaway population and the ongoing runaway production after the density is increased, it is concluded that the secondary generation, i.e. the creation of runaways through close collisions of already existing runaways with thermal electrons, provides an essential contribution to the runaway production. The effective avalanche time of this secondary process is determined to be teff = 0.9 ± 0.2 s

Identification and analysis of transport domains in the stochastic boundary of TEXTOR-DED for different mode spectra

At the TEXTOR tokamak an external resonant magnetic perturbation is applied with the dynamic ergodic divertor to control the edge transport properties. The approaches to analyse the impact of such a kind of edge stochastization on transport apply mostly a shell-like picture which includes a dependence of transport from magnetic field topology in the radial direction only. In this paper multiple experimental evidence is presented in contrast to these approaches the perturbation applied forms a poloidally heterogenous edge layer in which the transport characteristics are determined by the poloidally alternating field line behaviour. A thorough analysis of density and temperature profiles and their gradients for base mode spectra with poloidal/toroidal mode numbers of m/n = 12/4 and m/n = 6/2 is worked out in comparison with the modelled magnetic field topology and results from three dimensional transport modelling with EMC3/EIRENE. Hereby two poloidally adjacent transport domains are identified for the first time in such detail. A domain representing a helical scrape off layer is formed by field lines with short connection and therefore prevailing parallel transport to the wall elements. Here, the field lines are clustered into extended flux tubes embedded into a long connection length ergodic domain with diffusive transport characteristics and enhanced radial transport.

Disruption generated runaway electrons in TEXTOR and ITER

Runaway generation during a major disruption has been observed in TEXTOR. Measurements of the synchrotron radiation yielded number, energy and pitch angle of the runaways. A simple model, which assumes that the runaways take over the current density in the centre of the discharge, successfully describes these measurements. This model is applied to JET and ITER. One interesting result of the model is that it could be favourable for ITER to have a high runaway production. This leads to a lower runaway energy and less runaway damage. Quantitative predictions are sensitive to the value of the runaway parameter = E/Ecrit, which is determined by the post-disruption temperature. The present estimate for ITER gives = 0.02, which results in a maximum runaway energy of 300 MeV in a runaway beam with a total energy of 500 MJ. However, if is enhanced, these values will be reduced. An increase to = 0.04 is sufficient to decrease the maximum runaway energy to 55 MeV and the total beam energy to 130 MJ. Secondary generation plays an important role in these predictions

Observation of infrared synchrotron radiation from tokamak runaway electrons in TEXTOR

During runaway discharges in TEXTOR, intense infrared (IR) radiation is-emitted in the electron flow direction. This can only be explained by synchrotron radiation of fast electrons. The observed spectral dependence is consistent with electrons of 25-30 MeV energy; the intensity corresponds to about 1016 electrons or to an electrical current of 40 kA. From the spatial structure of the observed IR pattern, new insight into the spatial distribution of the runaway electrons and their perpendicular momentum can be gained. The runaway electrons populate a torus with a diameter of 0.5-0.6 m, which is slightly larger than the plasma radius; the perpendicular momentum is determined from the vertical extent of the IR pattern and amounts to about 5 m0c. The transformation rate of electrons to runaways can be estimated from the time delay of the IR signal as 2 × 10−4 s−1; this agrees with theoretical expectations derived from the ratio of the electrical field strength to the critical field strength. In TEXTOR, runaways are confined up to energies of 50 MeV, which is just below the limit where a phase should exist in which runaways radiate as much energy as they gain per turn.

Modelling of the magnetic field structures and first measurements of heat fluxes for TEXTOR-DED operation

The dynamic ergodic divertor (DED) was recently installed at the TEXTOR tokamak. One of the aims of the DED is to control and study heat and particle deposition on a plasma wall via modification of the plasma edge by external perturbation coils. Sixteen perturbation coils are mounted on the high-field side of the torus. The external magnetic perturbation creates a zone of chaotic field lines at the plasma edge by destroying several resonant surfaces. These structures have the properties of an open chaotic system while the field lines intersect the tokamak vessel. In order to study the topology of the field lines in different regimes, a set of tools called Atlas was created. Atlas uses a symplectic mapping technique to trace the magnetic field lines. The thermographic set-up is used to measure characteristic temperature patterns on the divertor target plates. The first measurements show the relation of the observed patterns with the results from the modelling with the Atlas codes.

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.

Effect of resonant helical magnetic fields on plasma rotation

The effect of a resonant helical magnetic field on plasma rotation is investigated numerically based on the two fluid equations. It is found that depending on the frequency and the direction of the original plasma rotation, a static helical field of a small amplitude can either increase or decrease the rotation speed. With increasing the field amplitude, the plasma rotation frequency approaches the electron diamagnetic drift frequency but rotates in the ion drift direction. These results provide a new understanding of the recent experimental observations of TEXTOR [K. H. Finken et al., Phys. Rev. Lett. 94, 015003 (2005)].

Tritium detection in plasma facing component by imaging plate technique

Tritium imaging plate technique (TIPT) has been successfully applied to measure the tritium areal distribution on various graphite tiles used as limiters in TEXTOR. It is observed that tritium distribution on the ALT-II tile is quite homogeneous and different from deuterium distribution and the tritium in redeposited layer is rather small. Such tritium distribution on the graphite tiles in TEXTOR behaves different compared to those in JET and TFTR where tritium was used as fueling gas or NBI injection. In JET and TFTR the tritium is part of the fuel and is co-deposited and retained in a similar manner as the deuterium. In a device like TEXTOR, the high-energy tritons are decoupled from the thermalized deuterons and show different behavior of retention; the main retention mechanism is deep implantation rather than co-deposition with eroded carbon on redeposition-dominated areas. It is also found that the tritium distribution measurements give useful new information on plasma-wall interactions.

Main characteristics of the fast disruption mitigation valve.

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.

Overview of magnetic structure induced by the TEXTOR-DED and the related transport

The dynamic ergodic divertor (DED), a new concept of the ergodic divertor, is presently installed for the TEXTOR tokamak. Beside the conventional ergodic divertor operation the DED also permits the operation with a rotating magnetic field which allows, in particular, to broaden the heat deposition pattern on the divertor plates. Since its first proposal of the DED in 1996 the structure of magnetic field, especially, the onset of ergodic zone of field lines and related transport in the DED-operation has been extensively studied using different theoretical and numerical methods. New methods to study the magnetic field, in particular, the field line mapping have been developed. The presentation gives the overview of the studies on the structure of magnetic field in the DED, the formation of the ergodic and laminar zones of field lines at the plasma edge. It also includes studies on the modelling efforts of the transport of heat and particles in the ergodic and laminar zones.