The TCV tokamak offers outstanding variability of the plasma shape. Using x-ray tomography, the shape of the inner flux surfaces of a poloidal cross section of the plasma can be reconstructed, including fast variations due to MHD activity. Both the hardware and the software of the 200 channel system developed for TCV are described.A new 'dynamical' calibration takes actual plasma parameters into account to determine the spectrum-dependent detector efficiency, resulting in an enhanced quality of reconstructions.Tomographic inversions are obtained using a variety of methods such as maximum entropy, linear regularization and a newly developed method based on the Fisher information. The merits of the different algorithms, which have been implemented as MATLAB functions, are compared.Inversion results are analysed with the help of singular-value decomposition, allowing, for example, identification of MHD modes without using any a priori information on the poloidal mode structure.Recent results on the dependence of sawtooth activity on the plasma triangularity are presented to demonstrate the performance of the soft x-ray tomography system.

Magnetohydrodynamic (MHD) instabilities in the Wendelstein 7-AS stellarator (W7-AS) [G. Grieger et al., Phys. Fluids B 4, 2081 (1992)] are characterized experimentally in various plasma parameter regimes and heating scenarios. The observations are compared with theoretical predictions for particular cases. In the high-β range (〈β〉⩽2%) no clear evidence of a stability β-limit could be found yet. In the lower β regime fast particle driven global Alfvén modes are the most important instabilities during neutral beam injection (NBI). Besides of coherent modes with almost no effect on the plasma performance additional Alfvén modes appear at higher frequencies up to 400 kHz, which show nonlinear phenomena-like bursting, frequency chirping, and MHD induced energy and fast particle losses. The activity of edge localized modes (ELMs) is investigated in NBI heated discharges. The issue of current driven instabilities and their potential stabilization by a stellarator field has been investigated with regard to the design of compact hybrid stellarator systems.

We have investigated the collisional excitation of hydrogen atoms by protons and multiply charged ions at intermediate velocities applying the optical method. The population or the np levels (n = 2,. . . ,6) has been determined for a variety of projectiles with charge states q ranging from I + to I I + and scaled velocities U/& between 0.7 and 5.6au. The cross &ions are shown to fulfil the scaling relation o / q = f ( d / q ) with respect to the projectile charge q and velocity U for q>3. The relevan= of scaled intermediate velocities for a classification o f excitation mechanism is pointed out.

At W7-AS, confinement properties are analysed and compared mainly with neoclassical predictions for quite different conditions. Low-density electron cyclotron resonance heating (ECRH) discharges allow access to the very-long-mean-free-path regime for electrons (T e up to 6 keV) whereas pure neutral beam injections (NBI) and combined NBI/ECRH discharges at high density (T i ≈ T e 1 keV at n e ≈ 10 20 m −3 ) lead to high performance (τ E up to 50 ms). Depending on the achieved temperatures, the experimental transport analysis in the plasma core is consistent with the neoclassical predictions. The experimentally observed 'electron root' feature with strong E r > 0 is driven by the convective flux of ripple-trapped suprathermal electrons generated by the ECRH absorption. 'Optimum' confinement is obtained in discharges with narrow density, but broad temperature profiles with steep gradients in the region of low densities and strong E r < 0 close to the plasma edge. The large radial electric fields, both positive and negative, strongly reduce neoclassical transport. The achieved temperatures, however, are limited by the strong temperature dependence of the neoclassical transport.

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