U. Samm

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.

Analysis of tungsten melt-layer motion and splashing under tokamak conditions at TEXTOR

Behaviour and characteristics of W plasma-facing components under impinging high heat fluxes are investigated in view of the material choices for the divertor in future devices such as ITER and DEMO. Experiments have been carried out in the plasma edge of the TEXTOR tokamak to study melt-layer motion, macroscopic tungsten erosion from the melt layer as well as the changes in material properties such as grain size and abundance of voids or bubbles. The parallel heat flux at the radial position of the plasma-facing components (PFCs) in the plasma ranges around q|| ~ 45?MW?m?2 allowing samples to be exposed at an impact angle of 35? to 20?30?MW?m?2. Melt-layer motion perpendicular to the magnetic field is observed following a Lorentz force originating from thermoelectric emission of the hot sample. Up to 3?g of molten W are redistributed forming mountain-like structures at the edge of the sample. The typical melt-layer thickness is 1?1.5?mm. Those hills are, due to the changes in the local geometry, particularly susceptible to even higher heat fluxes of up to the full q||. Locally the temperature can reach up to 6000?K, high levels of evaporation are causing significant erosion in the form of continuous fine-spray (~1 ? 1024?atoms?m?2?s?1). Strong evaporation cooling is observed hindering the further heating of the samples. In addition, the formation of ligaments and splashes occurs several times during the melt phase ejecting droplets in the order of several 10??m up to 100??m probably caused by an instability evolving in the melt. In terms of material degradation several aspects are considered: formation of leading edges by redistributed melt, bubble formation and recrystallization. Bubbles are occurring in sizes between 1 and 200??m while recrystallization increases the grain size up to 1.5?mm. The power-handling capabilities are thus severely degraded. Melting of tungsten (W) in future devices is highly unfavourable and needs to be avoided especially in light of uncontrolled transients and possible unshaped PFCs

Characterization of the deuterium recycling flux in front of a graphite surface in the TEXTOR tokamak

In the TEXTOR tokamak, experiments were performed to simultaneously determine the molecular, atomic and total particle flux of deuterium in front of a graphite limiter, the temperature of which can be controlled independently of the plasma conditions. With rising limiter temperatures, TTL, but constant plasma conditions an increase in Balmer emission and a decrease in Fulcher-band emission were observed. This variation is associated with a change in the type of released species: molecules dominate at low temperatures (550 K < TTL < 1100 K), whereas at temperatures TTL ≥ 1100 K the direct atomic release starts to become important. The total number of deuterons remains constant for all temperatures. Since not all molecules dissociate into two potentially radiating atoms, it is necessary to take into account the ratio of atoms to molecules when deducing the total particle flux from the Balmer emission. We present a spectroscopic method which allows the determination of the atomic, molecular and total deuterium particle flux and which also gives effective conversion factors, (S/XB)eff, to deduce the total deuterium flux from Balmer-α emission alone.Analysis of the spectroscopic data of both species can be performed to determine the rotational and vibrational populations for the molecules by means of Fulcher-α spectroscopy, and the penetration depth and energy for the atoms using Balmer spectroscopy. This further analysis gives additional information about the release mechanism, showing that both species, atoms and molecules, are released predominantly as thermalized particles.

Plasma edge physics with siliconization in TEXTOR

Abstract By using silicon as a material for wall coatings significant improvements in the tokamak performance have been obtained. These include the lowest oxygen level ever achieved in TEXTOR, an improved density limit and an enhanced energy confinement also at high central electron densities. The present paper gives a survey on the plasma properties under siliconized wall conditions with special emphasis on plasma edge physics. The impurity fluxes released from the limiter have been determined as a function various parameters. An outstanding property of silicon as an impurity in the plasma is its strong influence on plasma edge parameters caused by line radiation. A high radiation level of up to 70% of the input power (1.5 MW) has been achieved, limited by the mechanisms which are responsible for the release of silicon. To describe and understand these processes at the plasma edge Od- and 1d-models are used to simulate the highly nonlinear system. The properties are compared to plasmas in a boronized machine. Silicon and neon are compared with respect to radiation characteristics and transport to the centre. Remarkable differences in the penetration depths and central densities are reported.

Evidence of hot spot formation on carbon limiters due to thermal electron emission

Carbon test limiters have been exposed in TEXTOR to high heat loads up to about 30 MW/m2. The evolutions of the surface temperature distribution and of the carbon release have been observed by means of local diagnostics. A sudden acceleration of the rise of the surface temperature has been found at a critical temperature of approximately= 2400 degrees C. The increase of the rate of the temperature rise is consistent with an enhancement of the power loading by a factor of 2.5-3. Following the temperature jump (hot spot), a quasi-equilibrium temperature establishes at approximately= 2700 degrees C. The development of the hot spot is explained by an increase of the local power loading due to the breakdown of the sheath potential by thermal emission of electrons from the carbon surface. Simultaneously with the appearance of the hot spot, the carbon release from the surface increases sharply. This increase can be explained by normal thermal sublimation. Sublimation cooling contributes to the establishment of the quasi-equilibrium temperature at about 2700 degrees C

Measurement and monte carlo computations of Hα profiles in front of a TEXTOR limiter

In order to assess the existing data for hydrogen reflection and penetration into boundary plasmas, the spectral profile of Hα emitted in front of a TEXTOR limiter has been measured with a high resolution spectrometer and calculated with the EIRENE code. The emission has been investigated for standard recycling conditions of hydrogen, but also for the case that this recycling is superimposed by a strong puff of hydrogenic molecules injected through a hole at the limiter tip. This latter case was used to calibrate the molecular transport model. The Hα profiles during recycling have been observed and calculated for both a graphite and a stainless steel limiter. As expected, the number of reflected particles strongly increases in going from the C to the Fe limiter and seems to be slightly overestimated by present surface reflection databases.

Investigation of the impact of transient heat loads applied by laser irradiation on ITER-grade tungsten

Cracking thresholds and crack patterns in tungsten targets after repetitive ITER-like edge localized mode (ELM) pulses have been studied in recent simulation experiments by laser irradiation. The tungsten specimens were tested under selected conditions to quantify the thermal shock response. A Nd:YAG laser capable of delivering up to 32 J of energy per pulse with a duration of 1 ms at the fundamental wavelength λ = 1064 nm has been used to irradiate ITER-grade tungsten samples with repetitive heat loads. The laser exposures were performed for targets at room temperature (RT) as well as for targets preheated to 400 °C to measure the effects of the ELM-like loading conditions on the formation and development of cracks. The magnitude of the heat loads was 0.19, 0.38, 0.76 and 0.90 MJ m−2 (below the melting threshold) with a pulse duration of 1 ms. The tungsten surface was analysed after 100 and 1000 laser pulses to investigate the influence of material modification by plasma exposures on the cracking threshold. The observed damage threshold for ITER-grade W lies between 0.38 and 0.76 GW m−2. Continued cycling up to 1000 pulses at RT results in enhanced erosion of crack edges and crack edge melting. At the base temperature of 400 °C, the formation of cracks is suppressed.

Experience with bulk tungsten test-limiters under high heat loads: melting and melt layer propagation

The paper provides an overview of processes and underlying physics governing tungsten melt erosion in the fusion plasma environment. Experiments with three different bulk tungsten test-limiters were performed in TEXTOR: (i) thermally insulated solid plate fixed on a graphite roof-like limiter heated up by the plasma to the melting point, (ii) macro-brush of the ITER-relevant castellated structure and (iii) lamellae structure developed for the JET divertor. The main objectives were to determine the metal surface damage, the formation of the melt layer and its motion in the magnetic field. PHEMOBRID-3D and MEMOS-1.5D numerical codes were used to simulate the experiment with the roof-like test-limiter. Both experiments and simulation showed that the melting of tungsten can lead to a large material redistribution due to thermo-electron emission currents without ejection of molten material to the plasma.

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.

Localized recycling as a trigger of MARFE

An analytical model taking into account the plasma cooling due to localized hydrogen recycling is proposed to interpret the conditions of the MARFE onset above a critical plasma density in TEXTOR-94. Results of numerical modeling confirm that under conditions of a good plasma contact with the inner wall this mechanism of the MARFE triggering is more important than the usually considered cooling instability on impurity radiation.