M. Laux

ELM energy and particle losses and their extrapolation to burning plasma experiments

Abstract Analysis of Type I ELMs from present experiments shows that ELM energy losses decrease with increasing pedestal plasma collisionality ( ν ∗ ped ) and/or increasing τ Front ∥ , where ( τ ∥ Front =2π Rq 95 / c s ,ped ) is the typical ion transport time from the pedestal to the divertor target. ν ∗ ped and τ Front ∥ are not the only parameters that affect the ELMs, also the edge magnetic shear influences the plasma volume affected by the ELMs. ELM particle losses are influenced by this ELM affected volume and are weakly dependent on other pedestal plasma parameters. ‘Minimum’ Type I ELMs, with energy losses acceptable for ITER, where there is no change in the plasma temperature profile during the ELM, are observed for some conditions in JET and DIII-D. The duration of the divertor ELM power pulse is well correlated with τ Front ∥ and not with the duration of the ELM-associated MHD activity. Similarly, the time scale of ELM particle fluxes is also determined by τ Front ∥ . The extrapolation of present experimental results to ITER is summarised.

Stationary and transient divertor heat flux profiles and extrapolation to ITER

Abstract Experimental results on divertor heat load measurements from ASDEX Upgrade and JET are discussed. Thereby three topics are considered: (i) parameter dependence of steady state heat flux profiles, (ii) spatial distribution of the heat flux profile during type I edge localised modes (ELMs), and (iii) temporal evolution of the energy deposition during type I ELMs. No clear scaling of steady state heat flux profiles with plasma parameters is found. For different data sets a broadening of the heat flux profiles, a constant profile width, as well as a steepening with heating power is found. Extrapolation to ITER requires a review of the data. The heat flux profile is not significantly broadened during type I ELMs. Advantageous is the change of the in/out symmetry. The temporal behaviour of the energy deposition shows a strong increase of the heat flux on time scales of the ion sound speed and an exponential decay with about twice the rise time.

Erosion and redeposition in the ASDEX Upgrade divertor

Abstract The erosion and redeposition of silicon and tungsten evaporated onto the divertor plates of ASDEX Upgrade have been investigated. For comparison with the experimental results the computer program, ERO, has been used. The erosion of Si markers is smaller than expected from plasma ion sputtering, but still exceeds the W erosion by more than an order of magnitude. The observed erosion can only be explained by assuming a protective effect of carbon ions coming from the plasma which are deposited on the marker surfaces, thus reducing the marker erosion. A redeposition of the sputtered tungsten atoms has been found close to the original marker spots mainly because of prompt redeposition during the first gyration.

Experimental investigation of marfes and the density limit in the ASDEX Upgrade

In the ASDEX Upgrade, X-point marfe formation and its behaviour up to the density limit is investigated in gas-fuelled ohmically-heated single-null discharges over a wide range of parameters: Ip=0.6-1.2 MA, BTF=1.35-2.4 T, a plasma elongation of 1.6 and Zeff<2. The standard-ion Del B drift is directed towards the X-point. At medium electron densities inevitably a marfe develops in the vicinity of the active X-point. The marfe formation is consistent with a model of thermal instabilities in the radiating edge plasma. Moreover, stable steady-state operation is demonstrated with marfes which can extend significantly into the bulk plasma. The density limit is always connected with quick marfe expansion and movement followed by mode-locking leading to a major disruption. The limit scales linearly with Ip and is in good agreement with the Greenwald density limit scaling. The resulting experimental Hugill limit is neRq95/BTF=2.8*1020) m-2 T-1. Reversal of the ion Del B drift direction away from the target has a detrimental effect on the density limit.

Energy transport to the divertor plates of ASDEX-Upgrade during ELMy H-mode phases

Abstract The energy flux to the ASDEX-Upgrade divertor plates is routinely measured by thermography and Langmuir probes. The thermographically observed power decay length at the target plate is about 1 cm near the inboard separatrix. During an edge localized mode (ELM) of type I the density profiles are significantly, changed; an additional contribution occurs characterized by a power decay length in the order of 10 cm outside the separatrix and additional power is deposited into the private flux region. It is supposed that this is due to the changing, contribution of energy conduction versus convection. Results of ELM-modelling using the coupled B2-EIRENE code reproduce the main features of the experimental observations. The sheath transmission factor is calculated by combining thermography and Langmuir probe data.

Results from a double-sided langmuir probe in T-10, and an extended model of a probe in a streaming magnetized plasma

Abstract By means of a JANUS-type Langmuir probe operated in the scrape-off layer (SOL) in T-10 the temporal evolution and radial profiles of plasma density, electron temperature, and toroidal asymmetry of saturation currents were measured in dependence on discharge parameters. Of particular importance turned out to be the B T field direction and the radial probe position relative to the active limiter structure (rail and aperture limiter). The interpretation of the results is based on the local SOL structure in terms of connection lengths to the active limiter and special flow effects in T-10, and makes use of a theoretical probe model by Hutchinson. As to the latter model, a refinement concerning its basic principles is presented independently.

Operational limits of ASDEX Upgrade H mode discharges in the new closed Divertor II configuration

Systematic investigations of gas fuelled, high density H mode discharges with the new closed Divertor II of ASDEX Upgrade are reported. The focus is on operational limits (H-L transition, density limit) over a wide range of externally controllable plasma parameters. It is found that the H mode threshold power dramatically exceeds the generally accepted prediction Pthreshheat ∝ eBt when high densities are approached. Highest densities (density limit) are achieved close to the H-L mode back-transition and are generally associated with complete divertor detachment. The scaling of the density limit is assessed and discussed with a view to detachment based models and the Greenwald limit.

Langmuir probe measurements in the limiter shadow of T-10=non-linear scaling of edge density with central plasma density

In the aperture limiter shadow of the T-10 tokamak, the temporal evolution of plasma density and electron temperature during single discharges is studied by using a Langmuir double probe. It is observed that the plasma of the scrape-off layer (SOL) passes through a stationary phase when the Ohmic heat input Uloop Ip also does, suggesting that the stationarity of the SOL is correlated with the stationarity of the discharge power balance. During this stationary phase the plasma density at the aperture limiter edge was found to be approximately proportional to the square of the line averaged core plasma density. – It is shown in the paper that this quadratic density scaling can be easily understood within the framework of a stationary particle balance model of the recycling process, i.e. the interaction of plasma and neutral particles of the working gas in the edge region. Quadratic density scaling is expected to hold generally if the plasma density is high enough to prevent the neutrals from reaching the centre. For high densities, on the other hand, there is a density limit defined by the requirement that the discharge should supply enough power to sustain recycling.

Analysis of the stainless steel limiter and of shields of the wasaprobe after operation in T-10

Abstract The stainless steel limiter and shields of the surface analysis probe used in T-10 were analyzed by SEM, EDS, SAM, RBS, and SIMS with respect to modifications of the surface structure and the chemical composition. Results on deposition of surface layers and microparticles and on erosion by arcing are presented.

Deposition probe investigations of the sol of T-10

Abstract Several hundred systematic deposition probe measurements were carried out in the SOL of T-10 to investigate the time development of the Fe-impurity flux at various radial positions. During these investigations the discharge conditions as well as the limiter configuration were varied over a wide range. Some qualitative guiding principles are given for deposition probe measurements and their interpretation. On the basis of the radial dependence of the iron flux during both the plateau and the end phase of the tokamak discharge, the influence of erosion processes as well as of local sources and enhanced transport on the iron deposition is discussed. The experiments show an immediate relation between the direction of the toroidal magnetic field and the evolution of the impurity flux during the plateau phase, no influence of various limiter arrangements could be detected.