[en] After showing the necessity of an intensely promoted HTR-development for process heat application a programme is presented which is characterized by a relatively small development effort. Some of the main problems are discussed concerning the layout of a nuclear process heat plant. It is proposed to investigate them in the first phase of the development programme. (author)

No abstract available

[en] Full text: Large burning plasma fusion devices such as ITER and DEMO require divertor detachment to not exceed the tolerable power load densities (< 5 MW/m²) for long pulse operation. The understanding of the processes leading to divertor detachment is currently incomplete and a reliable prediction for future large scale devices out of reach. In ASDEX Upgrade divertor detachment has been studied for Ohmic and L-mode density ramp discharges with deuterium and hydrogen as a fuelling species and both toroidal field directions. The effect of seeding nitrogen has been tested. Prior to the detachment of the outer divertor a fluctuating detachment state appears in the SOL of the inner divertor, characterized by strong radiative fluctuations close to the X-point. Simultaneously a region with high ne appears in the inner far SOL and X-point regions. Once these radiative fluctuations disappear detachment occurs along the entire inner target plate and the outer divertor reaches a state of complete detachment. The integrated ion flux at the inner target reaches its peak value at an only ∼ 10 - 20% lower line averaged density than for the outer divertor target. However, the maximum of the integrated ion flux to the inner target remains well below what is observed at the outer target. Numerical transport code packages, such as SOLPS 5.0, contain an as complete as possible model of our current understanding of the basic processes present in the Scrape Off Layer. Under common assumptions for the model it is found that the roll over of the ion flux at the inner and outer targets occurs at a similar n^sep. Contrary to experimental findings the simulated peak values remain comparable for both target plates and no strong reduction of the ion flux density is seen for the inner divertor. The experimental observation of the high n_e in the far SOL during the fluctuating phase indicates that plasma is transported outward into the far SOL and/or strong radiation spontaneously sets in over a large volume in the far SOL. Based on these observations various levels of perpendicular transport in the X-point region of the inner divertor are assumed and recycling impurities such as oxygen and nitrogen are included. This leads to a shift of the simulated peak ion flux into the far inner SOL and a reduction of its peak value at the roll over by more than a factor of 3. (author)

No abstract available

[en] Full text: Independent of the plasma facing materials in future fusion devices impurity seeding will become an inevitable element of the operation to protect the divertor from excessive heat loads. Optimisation of the divertor impurity enrichment and the radiation distribution between core, SOL and divertor will be required for devices with high values of P/R (ratio of loss power and major radius). Another important constraint is the fact that the seeding scenario must be integrated into a small ELM regime or combined with ELM mitigation techniques. Impurity seeding has become necessary in the all-tungsten clad ASDEX Upgrade for high power conditions. A very beneficial behaviour in terms of reduced power loads, moderate impurity concentrations and increased confinement has been found in N seeded discharges. Radiative cooling has been applied in a large variety of plasmas ranging from improved H-Modes at intermediate density and heating power to discharges with very high heating power (P_aux ∼ 20 MW) or high density and radiation fraction exploring the type III ELM regime. Generally, the radiated power from the X-point and divertor region increased in N-seeded discharges by more than a factor of two to about 30% of the total input power. There is also a slight increase of radiation from the edge of the main plasma, but the core radiation is almost unchanged. Similarly, also the radiation during type I ELMs is increased. In unseeded discharges with a pure tungsten wall, about 20% of the ELM energy is radiated, which is clearly less than in earlier discharges with mixed carbon and tungsten PFCs. For nitrogen seeded discharges, however, the ELM energy is generally smaller, and about 40% of the ELM energy is radiated. This value is comparable to that found in former campaigns with mixed C/W PFCs and in line with simulations at JET for ELMs with similar energy loss. Consequently, the power load to the divertor targets during and in between type-I ELMs drops significantly with nitrogen seeding. In seeded type III ELMy discharges with good confinement (H_98(y,2) ∼ 1) at β_N ∼ 2.3, power densities below 2 MWm^-2 in the outer divertor and strongly suppressed W influx could be achieved. Investigations using N₂/Ne or N₂/Ar as seeding gas revealed a change in central transport compared to discharges with N-seeding only. (author)

[en] Using a reciprocating Retarding Field Analyser (RFA), Scrape-Off Layer (SOL) modifications were investigated on ASDEX-Upgrade during heating with waves in the Ion Cyclotron Range of Frequencies (ICRF), suspected for enhanced impurity production in this all-metal machine. Two quantities involved in the sputtering were measured: the current I(slit) on a saturated slit plate, proportional to the parallel ion flux and the mean parallel energy < W_I>_t of collected ions, averaged over many RF cycles. Combining multiple RFA reciprocations over a scan of q₉₅ provided 2D poloidal/radial resolution. In the outer SOL a localized RF-perturbed zone was evidenced on the RFA side magnetically connected to an active ICRF antenna. A flat 2D I(slit) pattern surrounded by steep gradients was observed, correlatively with < W_I>_t exceeding 150 eV. The centre of the zone is connected radially slightly behind the leading edge of antenna side limiters, with a radial extension up to ±2 cm. The zone is broadest and < W_I>_t is largest near the bottom of the active antenna. This is interpreted as a zone of local plasma biasing via sheath rectification, creating density convection around it. The I(slit) pattern is qualitatively consistent with simple considerations about ExB particle convection. (authors)

[en] Full text: In ASDEX Upgrade turbulent transport in the scrape off layer (SOL) was investigated by electrostatic and electromagnetic reciprocating probes. In Ohmic discharges density holes dominate the fluctuations inside the separatrix and density blobs in the SOL. The poloidal velocity of blobs and holes was determined by cross correlation of two Langmuir probe pins. The profile shows a strong velocity shear with an abrupt flow reversal just outside the separatrix, which agrees well with the poloidal density fluctuation velocity profile measurement using the Doppler reflectometry technique. This measurement can also give the radial electric field E_r assuming the phase velocity is small compared to the E x B drift velocity. Using the time averaged floating potential and electron temperature (T_e) profiles to determine the plasma potential and E_r, the Doppler E_r profile cannot be reproduced. Significant T_e fluctuations occur in Ohmic discharges which are out of phase with the floating potential fluctuations. Measuring in L-mode floating and plasma potential simultaneously, the fluctuation power spectrum shows a much higher level for the floating potential at frequencies above 30 kHz. Using a ball pen probe (BPP) the plasma potential can be measured directly. The E_r profile derived from the BPP data agrees well with the E_r calculated from the reflectometer data supporting that the poloidal motion of the density fluctuations is governed by v_E x B. T_e of individual ELM filaments was measured using single probes with a fast swept bias voltage. Density and T_e peaks of 10¹⁹ m^-3 and 30 - 40 eV were detected 4.5 cm outside the separatrix. BPP measurements in the SOL showed the ion temperature is higher than T_e. This still has to be validated in ELM filaments. First experiments with a retarding field analyser at ASDEX Upgrade indicate ion energies exceeding 160 eV in the far SOL during ELMs. ELM filaments are thought to be related to current filaments. In previous experiments magnetic fluctuations measured during ELMs could be explained by mode structures in the confined plasma carrying a bidirectional current. New experiments with improved diagnostic were indicating mono polar current filaments in the SOL with current densities of up to 6 MAm^-2. Additionally, the small ELMs in type II ELMy and N₂ seeded discharges will be compared to type I ELMs. (author)