G. Mank

Electron temperature and electron density profiles measured with a thermal He-beam in the plasma boundary of TEXTOR

The knowledge of the electron temperature profiles T e ( r ) in the boundary of a tokamak is important in several regards, e.g., for the calculation of impurity fluxes from line intensities, for an estimate of the radiation losses in the boundary, and for the modelling of the particle and power fluxes in the scrape-off layer. Since other diagnostic methods have not offered satisfactory solutions so far, we have developed a diagnostic technique which is based on the measurement of the radial intensity profiles of selected lines emitted by an injected thermal He-atom beam. For this purpose 4.8×10 18 atoms/s at 150°C are injected into the plasma through a collimating hole structure. Due to the high ionization potential of 24.59 eV, the He-atoms penetrate several centimeters beyond the limiter radius into the plasma ( n e × l =2×10 13 cm −2 ). The line intensities at two wavelengths (λ 1 =706.52 nm and λ 2 =728.13 nm) are recorded simultaneously with linear 128 elements array cameras, and their intensity ratio is compared with values calculated from a collisional radiative model in order to obtain T e ( r ). The accessible range is 10 eV≤ T e ≤150 eV, which nearly closes the gap between the outer channel of the core diagnostics and probe measurements in the plasma edge. The lower limit is determined by the relaxation length of the He-beam entering the plasma whereas the upper limit is given by the penetration depth of the He-atoms. In our case the spatial and the temporal resolution is 1.2 mm, resp. 10 ms. By selecting another line at λ 3 =667.82 nm and taking the line intensity ratio of λ 2 the electron density profiles can be obtained simultaneously allowing a self-consistent development of the data.

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

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.

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.

Deuterium-tritium concentration measurements in the divertor of a tokamak via a modified Penning gauge

Abstract The measurement of the relative concentrations of hydrogen, deuterium, tritium and helium is an important task in the nuclear fusion research area. Control of the deuterium-tritium (D-T) isotropic ratio and limiting the helium ash content in a fusion plasma are the key to optimizing the fuel burn in a fusion reactor such as ITER. A diagnostic technique has been developed to measure the D-T isotopic ratio in the divertor of a tokamak with a Penning vacuum gauge. The Penning discharge provides a source of electrons to excite the neutral deuterium and tritium in the pumping duct. Subsequently, the visible light from the hydrogen isotopes is collected in an optical fibre bundle, transferred away from the tokamak into a low radiation background area and detected in a high resolution Czerny-Turner spectrometer, equipped with a fast CCD (charge-coupled device) camera for optical detection. The intensity of the observed line emission (D α , 6561.03 A; T α , 6560.44 A) is directly proportional to the partial pressure of each gas found in the divertor. The line intensity of each isotope is calibrated as a function of pressure. The ratio of the line intensities thus provides a direct measurement of the D-T isotopic ratio. The lower limit for the determination of the D-T isotopic ratio is about 0.5%. This system is applicable for the pressure range from 10 −5 mbar to a few times 10 −2 mbar.

Numerical modelling of pump limiter biasing on TEXTOR-94 and Tore Supra

The two-dimensional multifluid code TECXY has been used to model the biasing (with respect to the first wall) of the toroidal belt limiter ALT-II on the tokamak TEXTOR-94 and of the new toroidal pump limiter being installed on Tore Supra tokamak in the framework of the CIEL project. It is well known that the edge flow pattern can be influenced by the poloidal electric drifts from imposing radial electric fields. The modelling with TECXY introduces imprinted bias currents in the scrape-off layer (SOL) for the case of negative (limiter) biasing, and imprinted bias potentials for the case of positive biasing. This allowed us to simulate sufficiently well the experimental I-V characteristics for either biasing of ALT-II and also reproduced the essential features and trends of the observed plasma profiles in the SOL of TEXTOR-94. For negative biasing a moderate improvement of the pumping exhaust efficiency can be achieved in the case of TEXTOR. For Tore Supra, however, only a negligible improvement of the limiter performance with biasing can be predicted, which is explained by the relatively weak drift flows in Tore Supra.

Impurity release and deposition processes close to limiter surfaces in TEXTOR-94

Abstract Measurements on the formation of hydrocarbons on plasma exposed surfaces performed by mass- and optical emission spectroscopy in TEXTOR is reported. The temperature dependence of hydrocarbon formation and the contribution of the hydrocarbon source to the CII ion densities near the limiter has been observed using a graphite limiter which is externally heatable up to 1400 K. It has been found that hydrocarbon formation occurs in a broad temperature region decreasing only for target temperatures above 1300 K and that hydrocarbons contribute to about 40% to the CII light. Strong methane release has been observed on copper and stainless steel limiters positioned at the LCFS while it is negligible on molybdenum and tungsten limiters under similar plasma edge conditions. Local transport and redeposition of molecules have been studied by gas injection of methane and silane through holes in the limiter surfaces and by local Monte Carlo calculations. Local deposition efficiencies between 4 and 7% have been measured for injected methane and silane. Monte Carlo calculations show, in general, a larger redeposition probability depending only little on local plasma parameters but significantly on the assumptions of the sticking and release properties of redeposited ions and radicals on the surface. For higher surface temperatures possible carbon release by radiation enhanced sublimation (RES) has been investigated. No increase of carbon release could be observed demonstrating that carbon release from RES is negligible under these conditions. Possible reasons for this behavior are discussed.

The influence of plasma-edge properties on high confinement discharges with a radiating plasma mantle at the tokamak TEXTOR-94

The radiative improved mode obtained on the limiter tokamak TEXTOR-94 combines the possibility of power exhaust by a radiating plasma boundary (with a fraction of the radiated power with respect to the total input power up to 90% with neon or argon cooling) with improved energy confinement (as good as in the ELM-free H-mode in divertor tokamaks) at high plasma densities (line-averaged central-electron density equal to or even above the Greenwald density limit nGW) in quasi-stationary discharges. An overview is given of the substantial changes in plasma-edge properties occurring at high radiated power levels . These changes are characterized by a reduction of the plasma-edge density and temperature, a reduction of particle transport out of the confined plasma volume and an increase of the penetration depth of deuterium and impurity atoms. As a consequence, the particle confinement time increases and the electron-density profiles steepen. The transition to improved confinement takes place as soon as the density peaking reaches a critical threshold. An internal transport barrier is observed in the bulk of RI-mode plasmas (at r=a6 0:6) characterized by an increase of the pressure gradient and of the shear of the toroidal velocity compared to discharges without additional impurity seeding. The dilution at the plasma boundary is strongly increased by the seeded impurities whereas the central dilution is only weakly affected.

A scheme for a dynamic ergodic divertor in TEXTOR

A helical multipolar coil system for dynamic edge ergodization of the TEXTOR plasma is proposed being energized by a three phase ac-current. This would permit a sweeping of the perturbation pattern with the aim (I) to establish a uniform distribution of the heat load at the wall components, (II) to increase the thickness of the scrape-off layer, (III) to improve the helium exhaust and to enhance impurity shielding for controlled edge radiation cooling, and (IV) to possibly influence plasma rotation. Resonant interaction with internal modes may be foreseen in a later phase.

Improved confinement in TEXTOR

A new regime of enhanced confinement (I-mode) is found in plasmas with circular cross-sections in the pump limiter tokamak TEXTOR with boronized walls. This regime is obtained with three types of auxiliary heating, namely NBI co-injection, NBI counter-injection+ICRH and NBI co-injection+NBI counter-injection, and has many similarities with the H-mode regime obtained in divertor tokamaks. The energy confinement times obtained in these discharges scale as favourably as those in stationary H-mode discharges with edge localized modes. A detailed analysis of the scaling of the confinement time with plasma current, heating power and plasma density is presented. Characteristic electron density and temperature profiles are observed, with large central values and well developed edge pedestals. They are compared with those found in H-mode discharges and supershots. A poloidal beta limit of 1.6 is found in the I-mode discharges of TEXTOR. The maximum toroidal beta values obtained reach nearly 1%, i.e. 0.7 times the Troyon limit in TEXTOR. I-mode confinement is always linked with low recycling and absence of MHD activity. If these conditions are not met. L-mode scaling is retrieved. MHD activity, which is more likely to occur at low plasma densities and currents, can cause a sudden drop to L-mode scaling. So far, no transition from the L-mode scaling to the I-mode scaling has been observed