D. Reiter

Supersonic gas injection on Tore Supra

Abstract A fueling system by supersonic pulsed gas injection has been installed on the high field side of Tore Supra. First results are encouraging, demonstrating a fueling efficiency four times higher than that of conventional gas puff. One-dimensional modeling shows that the increase of efficiency is linked to the short injection time and to the prompt cooling of the plasma edge consecutive to the massive injection of matter. Improvements of the system could lead to the formation of a high-β blob which could experience a drift down the magnetic field, analogously to pellet injection, thus further increasing the fueling efficiency of the method.

Plasma-wall interaction and plasma performance in textor — A review

Abstract TEXTOR, a tokamak primarily devoted to plasma-wall interaction studies, has been operated during the last 2–3 years under a wide variety of discharge conditions. Discharges with Ohmic heating only and with additional ICR heating in the MW-range were produced over an extended range of plasma parameters. Different limiter materials were used — Inconel, stainless steel, graphite, TiC coated graphite. The surface of the first wall (liner) consisted either of Inconel or of a carbon layer deposited upon the Inconel. Characteristic features of the discharge have been modified by making use of the modular pump limiter ALT-I and by applying a localized perturbation coil for “ergodization” of the magnetic field structure in the boundary. The characteristics of plasma-wall interaction under these conditions and their influence on the global performance of the TEXTOR plasma are described and discussed. Use has been made of novel diagnostics specifically developed for edge plasma investigation.

Comparing scrape-off layer and divertor physics in JET pure He and D discharges

Though helium plasmas are one option for the low activation phase of ITER, little effort has thus far been devoted to studying them in a large, diverted tokamak. A recent campaign on JET has therefore sought to address some of the important questions related to helium operation (He concentrations near 90%) in single null configuruations, particularly with regard to edge and divertor physics. This contribution compiles a selection of results from these experiments, in which, in each case, discharges have been chosen to match as closely as possible previous, well characterised D plasmas in both L and ELMing H-modes. These matched pulses are used to draw conclusions regarding the principle source and location of carbon production in D plasmas, to compare and contrast the mechanisms of the density limit and the detachment process in D and He, to investigate the nature of cross-field power transport in the SOL and to gain insight into the process by which ELM energy is transported to the divertor targets

Geometry effects on particle removal with the pump limiter ALT-I

Abstract In the course of the ALT-I pump limiter experiments on TEXTOR, limiter heads of different shapes, exhaust channels and materials have been applied and their performance has been compared. Particle removal rates up to 10 Torr l s have been observed in the case of TiC surfaces. Values with the graphite head at the same radial position are typically a factor of 2–3 lower. The removal rate for a “detached plasma” discharge is 30–50% reduced as compared to a normal one. The removal efficiency reaches values in excess of 50%. The results suggest a guiding rule for the design of pump limiters with high removal efficiency: The effective pumping speed at the neutralizer plate should be large compared to the conductance for neutral particles backstreaming from the neutralizer plate to the main plasma.

Plasma influence on throat conductance of the textor pump limiter ALT-I

Abstract On the TEXTOR pump limiter ALT-I conductance measurements for the backstreaming of gas from the pump limiter vessel through the pump limiter entrance have been performed. In these experiments neutral gas has been injected into the pump limiter plenum during a short pulse. The influence of the instreaming plasma results in a reduction of the conductance of the outstreaming gas. For helium the conductance is reduced to about 40% of the molecular conductance when a plasma flux of 0.8 A / cm 2 ( T e = T i = 11 eV ) is streaming into the pump limiter throat. The reduction of the conductance for backstreaming hydrogen and deuterium under the same plasma conditions is smaller; about 70% of the molecular conductance is obtained. This reduction can be explained by an increased recycling of ions which have been produced in the throat back to the neutralizer plate. The experimental results can be reproduced by Monte Carlo neutral transport code calculations if the recycling coefficient is about 0.85 for hydrogen and deuterium and about 0.95 for helium ions. Processes causing these high recycling coefficients are discussed and their influence is estimated.

Modelling of the advanced limiter test facility (ALT-I), a module for TEXTOR

Abstract The molecular and atomic neutral gas build up within the vacuum vessel of ALT I in general and particularly in the hood of ALT-I-FG and in the pump duct of ALT-I-VG are investigated by means of the 3-D Monte-Carlo code EIR-ENE. The “scrape-off”-module of the code SIMTOK is employed to describe the plasma in the throat of ALT-I-VG. Presupposing a neutralized flux of 100 A at the deflector plate(s), the maximum atomic and molecular pressures in case of ALT-I-FG are 1.5×10 −3 mbar and 10 −4 mbar, respectively. The analogous values for ALT-I-VG in the duct near the deflector plate are 1.9×10 −3 mbar and 10 −3 mbar. Because of the recycling processes within the throat the plasma flux neutralized at the deflector plate turns out to be about four times higher than the net flux entering the throat; this numerical factor indicates the effectiveness of the plasma plugging.

Transport code calculations on the particle and energy unloading through overlapping magnetic islands

Superposition of magnetic islands generated by currents in auxiliary windings leads in the plasma boundary to a magnetically “mixed” layer with destroyed magnetic flux surfaces. The magnetic field structure is globally described by an only radially dependent mean inclination angle of the field lines against the toroidal direction. The model is applied to the parameters of TEXTOR with 4.5 MW additional heating. The maximum mean inclination angle is 2 × 10−3. The temperatures at the separatrix and at the limiter edge are 450 eV and 54 eV. Without “mixing” the analogous values are 1040 eV and 112 eV. respectively.

An overview of JET edge modelling activities

A number of codes are in use at JET to model the edge plasma. The range of edge codes is described as is the range of physics issues being explored by these codes. The balance between focussed modelling (that looking at particular physics effects) and integrated modelling (attempting to combine codes or encapsulate the physics from some codes into other codes) is examined.

Screening efficiency of the magnetically mixed layer generated in the boundary region of the TEXTOR tokamak by a localized high multipolar conductor configuration

Abstract The impurity transport in, a magnetically mixed layer is investigated on the basis of the average ion model and a two-dimensional model for the neutral impurity atoms. The magnetic field structure is globally described by the mean inclination angle of the field lines against the toroidal direction. The radial dependence of this angle is computed from the data of the specific perturbation coil which is foreseen for the TEXTOR tokamak. The particle and energy flux densities in the magnetically mixed layer are essentially derived from the inclination angle, the partial pressures of the different species, and the friction between the hydrogen ions and the impurity ions. The calculations show that the impurity density profiles become hollow in the boundary layer. The central iron density is decreased by a factor of 4 and the neutral oxygen concentration is increased by a factor of 30.

Resonant conductor configurations for a magnetic limiter in TEXTOR

Abstract Conductor configurations generating a radially disturbing magnetic field resonant to the q = 4 flux surface of the TEXTOR plasma are considered. The Fourier spectrum of the the magnetic field of the “near plasma” configurations is characterized by one dominant Fourier component ( m = 4, n = 1). These configurations are based essentially on helical windings with the same pitch angle as the field lines of the resonant flux surface. The “far plasma” configurations, consisting of long or horseshoe-like solenoids, generate uni-directional magnetic fluxes at each end of the solenoid and thus avoid the unfavourable dipole fields of the near plasma configuration. Besides the dominant Fourier component ( m = 4, n = 1), the component m = 3 and n = l also appears, if the solenoids at the inner side of the torus are omitted for engineering reasons.