Table of contents

There are plans at ASDEX Upgrade to cover the heat shield of the inner main chamber wall with tungsten coated graphite tiles. Four tiles were installed at different poloidal positions of the heat shield and removed after a full experimental campaign. The tiles were analysed by means of Rutherford backscattering (RBS) analysis before insertion and by both RBS and particle induced X ray emission after removal. This is an investigation of the poloidal and toroidal erosion profiles on the tiles due to sputtering by charge exchange neutral and ion impact.

The trajectories of ELMy H mode discharges in the pedestal temperature-pedestal density plane form cycles characterized by high pressure and low pressure boundaries. While at least for type-I ELMs the high pressure boundary seems to be associated with the ballooning limit in the pedestal, the low pressure boundary is poorly understood. The scaling of the low pressure boundary is assessed for JET type-I H modes. The discussion is guided by a semi-empirical model which emphasizes the role of SOL physics. Our results suggest that the high pressure and low pressure boundaries are governed by distinct physics mechanisms.

The magnetic topology and particle drift orbits are analysed in a monotonic q profile and in a reversed shear TEXTOR equilibrium that is subject to a magnetic perturbation driven by the Dynamic Ergodic Divertor (DED). The main results prove that there exists a transport barrier for magnetic field lines and for circulating particles in the reversed shear case when the DED is applied. This transport barrier occurs near the surface of minimum q value where the KAM theory may be invalid. Moreover, it should be noted that trapped particles are lost due to the presence of ripple and that the DED does not affect their trajectories. To analyse magnetic topology, a symplectic perturbed map is used and the guiding centre orbits are evaluated in Boozer co-ordinates.

During current rampup discharges with early auxiliary RF heating, reversed magnetic shear profiles have been obtained on Alcator C-Mod. MHD oscillations are often observed on ECE diagnostics, as well as magnetics. Although no direct q profile measurement was available, a hollow current density profile with q_min = 2.5 was reconstructed by the EFIT program. The location of the largest electron temperature fluctuations was close to the outer q = 3 rational surface. On the basis of a resistive linear stability code (MARS), the MHD oscillations were identified as n = 1 resistive `multiple' tearing modes. Since the pressure profile was hollow and q > 1 in the core, a resistive interchange mode was also predicted to be unstable at the inner q = 4 rational surface, despite the low β_N.

Normally pellet injection in strongly heated discharges leads at most to a relatively short improvement of the energy and particle confinement times. In contrast to this finding, the radiative improved (RI) mode plasma of TEXTOR-94 is a very well suited target for pellet injection: the interaction of a pellet with the high density plasma together with the radiatively cooled edge leads in the best cases to an improved energy confinement time which lasts to the end of the heating phase. The pellet injection causes at first a density increase; this phase is followed by a quick partial particle loss and later an increase to a quasi-stationary value. Because of the linear density dependence of RI mode confinement this leads to an additional increase in energy confinement time.

Parasitic absorption of the short wavelength modes of the LH spectrum is a probable reason for the hot spots seen in the grill region of several tokamaks. Experiments suggest that the heat loads on the wall structures depend on the coupled power. In this work, the parasitic absorption of LH power was studied with self-consistent particle-in-cell simulations. The launched spectra were obtained from the SWAN coupling code. The power and temperature dependences of the absorption in the near field of the LH grill were investigated with a series of simulations. The parasitic absorption was found to grow from 0.6 to 1.1% when the coupled power increased from 26 to 67 MW/m². When the edge temperature rose from 12.5 to 100 eV, the absorption increased from 0.4 to 1.7%. The maximum kinetic energies were between 0.6 and 1.8 keV. Estimates for the heat loads and surface temperature of the grill limiter are also obtained. The absorption leads to heat loads between 1.5 and 13 MW/m² and surface temperatures of 510-2390^° C.

Resistive wall mode (RWM) instabilities are found to be a limiting factor in advanced tokamak regimes with low internal inductance. Even small amplitude modes can affect the rotation profile and the performance of these ELMing H mode discharges. Although complete stabilization of the RWM by plasma rotation has not yet been observed, several discharges with increased beam momentum and power injection sustained good steady state performance for record durations. The first investigation of active feedback control of the RWM has shown promising results: the leakage of radial magnetic flux through the resistive wall can be successfully controlled and the duration of the high beta phase can be prolonged. The results provide a comparative test of several approaches to active feedback control, and are being used to benchmark the analysis and computational models of active control.

At the W7-AS stellarator the influence of non-vanishing toroidal current densities on stability is studied over a broad range of externally provided values of the rotational transform (0.301 ⩽ ι_ext ⩽ 0.565). By means of inductive current drive and by varying the heating scenarios, using 1 MW NBI and 0.4 MW ECRH, access is given to a variety of current density radial profiles with significant shear. Disruptive-like events accompanied by fast energy loss are observed as soon as low order major rational resonances appear in the outer region of the plasma in discharges with significant contributions of net toroidal ohmic current in the positive direction (increasing the rotational transform). To some extent the applied loop voltage is able to prevent a total decay of the plasma current. Preceding the partial or total energy loss, a perturbing effect in the equilibrium is observed in the radial profile of the electron temperature, as derived from ECE measurements, and the tearing mode structure is identified by tomographic reconstruction of the emission in the soft X ray wavelength region. A Δ code is used to calculate numerically the stability of the current density radial profiles against tearing modes, and the predictions are compared with experimental observations. ECR heated plasmas with inductive current drive in the negative direction are observed to be stable at ι_ext = 0.48 and 0.52 in a reversed magnetic shear configuration (as defined in the tokamak sense: dq/dr < 0). No significant tearing mode activity is found in the presence of large bootstrap and Ohkawa current densities in plasmas without ohmic current drive. However, a deterioration of confinement is observed due to other effects on rational surfaces at the edge.

A two dimensional, time dependent model has been developed for calculating the time evolution of the vapour layer characteristics over vaporizing surfaces subjected to magnetically confined energetic plasma particles. An essential part of this model is the proper determination of the electromagnetic field distributions in the vapour layer. In this analysis, the electric field and current distributions are determined self-consistently on the basis of a properly posed boundary value problem. The computational results presented show the existence of an E × B type drift which may notably influence the shielding characteristics of the evolving vapour layer. In addition, excessive ohmic heating and/or arc formation at the edges of conductor segments may cause additional erosion. Quantitative results are provided on the effect of the inner (baseplate) and outer (vapour-plasma interface) boundary conditions on the resulting erosion rates. The results show that the vapour shield evolution is a complex coupled electromagnetic-hydromagnetic phenomenon. Ignoring any of the fundamental physical processes present, such as shielding by electrostatic fields and sheaths, affects the reliability of the results obtained.

The standard analysis for rotating magnetic field (RMF) current production in simple fixed density plasma columns is extended to include the critical interaction between the RMF drive and an elongated field reversed configuration (FRC) equilibrium inside a flux conserver. The standard analysis involves penetration of the RMF into a highly conducting plasma through production of synchronous rotation of the electron fluid due to Hall terms in Ohm's law. In the present article the RMF analysis is combined with two dimensional equilibrium constraints, which have a defining role in governing the RMF penetration. Very simple analytic models are developed, based on instantaneous RMF penetration into an edge layer, that illuminate the basic parameters necessary for current drive and flux buildup or sustainment to be successful. The model does not account for time dependent RMF penetration or transport consistent density profiles, but it clearly points out the basic conditions that must be satisfied, and how they scale, for RMF sustainment of FRCs to be effective.

Ideal and resistive MHD instabilities degrade the confinement or even lead to disruptions in ASDEX Upgrade reversed shear discharges. Double tearing modes temporarily arise when the minimum q value passes the q = 2 surface. They usually become stabilized with increasing distance between the rational surfaces or by the large pressure gradient at the inner rational surface provided by central electron heating. Large pressure gradients in the region of weak shear between the two q = 2 surfaces drive infernal modes unstable. These modes can couple to external kink modes if the q value at the plasma edge is close to low order rational values. The resulting global mode usually causes disruptions. An optimized current profile has to avoid, therefore, close double rational q surfaces as well as large pressure gradients in the weak shear region.

Transport in the ignited reduced cost ITER devices IAM (Intermediate Aspect Ratio Machine) and LAM (Low Aspect Ratio Machine) is explored by self-consistent simulations using a special version of the 1.5 dimensional BALDUR predictive transport code and applying empirical transport coefficients. The main objective of the study is to determine the thermal energy confinement time τ_E,r required for operation at the design parameters. It is found that the τ_E,r value of the inductive IAM (with a helium fraction of 9%) is 3.3 s with argon seeding compared with τ_E,p = 3.0 s predicted by global scalings. For operation at τ_E,r = τ_E,p the value of Q = P_fus/P_b must be reduced from 10 to 6.1, where P_fus is the fusion power and P_b is the beam power. The necessary radiative loss from closed flux surfaces is reached in the argon scenario at high and low separatrix densities. The assumption of flat density profiles and high edge densities is supported by simulations using a new scaling law for the anomalous inward drift velocity. A study of the advanced tokamak scenario of IAM resulting in an estimate of the required energy confinement time is also carried out. Simulation of the inductive LAM argon scenario yields τ_E,r = 4.0 s compared with τ_E,p = 3.9 s predicted by global scalings. Here operation at τ_E,p demands a Q value of 8.7.