Table of contents

The influence of foams on laser-produced shocks has been studied experimentally using sub-ns laser pulses smoothed with phase zone plates and focused on layered foam - aluminium targets. A strong pressure increase was measured when the foam was present in comparison with that obtained by focusing the beam directly onto the aluminium target, due to impedance mismatch at the aluminium - foam interface. Results are compared with computer simulations. The impact of these measurements on the possible use of `foam-buffered targets' for direct-drive inertial confinement fusion is briefly discussed.

Experimental results are described from an integrated interferometer-reflectometer system implemented on TEXT-Upgrade. In plasma regions where the systems have spatial overlap, excellent agreement is observed between the independently measured electron density profiles. Combined interferometer-reflectometer data are used to obtain the optimal total density profile. By removing chords from the interferometer, one can examine how resolution is compromised as the channel number decreases and spacing varies. Depending on the expected profile shapes (i.e. flat or peaked), one can design an integrated interferometer-reflectometer system employing a minimum number of chords while still providing crucial profile information.

Core transport barriers can be reliably formed in DIII-D by tailoring the evolution of the current density profile. This paper reports studies of the relative role of magnetic and shear in creating core transport barriers in the DIII-D tokamak and considers the detailed dynamics of the barrier formation. The core barriers seen in DIII-D negative shear discharges form in a stepwise fashion during the initial current ramp. The reasons for the stepwise formation are not known. Their extremely good shot to shot reproducibility suggests the steps are connected to a very reproducible plasma parameter such as the current density profile; however, the simple hypothesis that they occur each time q(0) or crosses an integer value is not consistent with all the data. The data from DIII-D are consistent with previous results that negative magnetic shear facilitates the formation of core transport barriers in the ion transport channel but is not necessary. However, strongly negative magnetic shear does allow formation of transport barriers in particle, electron thermal, ion thermal and angular momentum transport channels. Shots with strong negative magnetic shear have produced the steepest ion temperature and toroidal rotation profiles seen yet in DIII-D. In addition, the shearing rates seen in these shots exceed the previous DIII-D record value by a factor of four.

Time-resolved VUV spectroscopy has been used to investigate the effects of impurities in a reversed field pinch operating with a resistive shell. Results of electron temperature, impurity ion densities, particle confinement time and together with a description of the interpretation and the equipment are presented.

Ideal magnetohydrodynamic (MHD) stabilities are studied for the negative shear plasmas with steep pressure gradient observed in JT-60U. In order to improve the ideal -limit, effects of the location of a minimum value of the safety factor, , and of the location of the maximum pressure gradient on ideal MHD stability are investigated. When the maximum pressure gradient is inside the surface, the ideal -limit is determined by the n = 1 kink-ballooning mode. When the maximum pressure gradient is on or outside the surface, the infernal and high-n ballooning modes become more unstable than the n = 1 mode. It is found that the ideal -limit is improved when the maximum pressure gradient is inside the surface and the edge pressure gradient is high. The experimentally observed -limit in negative shear plasmas in JT-60U is consistent with the numerically obtained -limit determined by the n = 1 mode.

A nonlinear Schrödinger equation is derived by perturbation analysis for a transverse magnetic (TM) mode surface wave propagating on a plasma bounded by free space, taking into account all the nonlinearities including the nonlinear boundary conditions and the kinematics of the rippling boundary, by formulating the boundary conditions in terms of the currents and the surface charges. The nonlinear frequency shift is also obtained for the case of a fixed boundary and is compared with the case of moving boundary. We find that bright solitons can be obtained over the almost entire range of values of the dielectric constant, i.e. , for a moving boundary while the fixed boundary gives bright solitons only for the dielectric constant smaller than -3.

There is not yet a straightforward method for determining the D-T ratio in the centre of a tokamak plasma. One of the simpler measurements put forward in the past is the interpretation of the MHD spectrum in the frequency range of the global Alfvén eigenmodes (GAE). However, the frequencies of these modes do not depend on the plasma mass alone, but are also quite strongly dependent on the details of the current and density profiles; this creates a problem of deconvolution of the estimate of the plasma mass from an implicit relationship between several measurable plasma parameters and the detected eigenmode frequencies. In view of the lack of competitive diagnostics, this method has been revisited to assess its likely precision for the JET tokamak. Our results show that the low-n GAE modes are sometimes too close to the continuum edge to be detectable and that the interpretation of the GAE spectrum is therefore rendered less direct than had been hoped. However, information on the effective plasma ion mass is still available in the detectable modes and we present a statistical study on the precision with which this quantity could be estimated from the GAE spectrum on JET, including other directly measured or simply available plasma parameters.

Toroidicity-induced Alfvén eigenmodes (TAE) were studied systematically in low-q discharges in JT-60U. Low-n TAE modes appeared outside the q = 1 surface at a low internal inductance (li), while high-n multiple TAE modes appeared inside the q = 1 surface at high internal inductance. In addition, the low-n multiple TAE modes were observed outside the q = 1 surface in very low-li plasmas , while the bi-directional TAE modes were observed inside the q = 1 surface in high-li and relatively high electron density plasmas . The high-n multiple TAE modes observed inside the q = 1 surface are much more harmful for energetic ion confinement than the low-n TAE modes observed outside the q = 1 surface.

Cold-pulse experiments were carried out in the W7-AS stellarator for the first time. Carbon was injected by a laser blow-off system into the plasma edge. The electron density increase due to the injected carbon is found to be responsible for the edge electron temperature drop. In all cases, the propagation of the edge temperature perturbation to the plasma centre could be modelled with a local parameter-dependent electron heat diffusivity. Unlike in tokamaks, non-local transport effects were not observed in these experiments.

Drift type and kinetic ballooning modes for negative magnetic shear are studied by taking effects of trapped electrons into account. Eigenvalues of the modes are calculated by using a gyrokinetic shooting code. The trapped-electron- mode is found to have almost the same characteristic features as those for positive shear in the electrostatic limit, except it is not stabilized by finite effects. Trapped electrons can cause appreciable reduction in the growth rate of the kinetic ballooning mode. Complete stabilization may occur in the short wavelength regime.

18 - 24 October 17th International Conference on Fusion Energy Yokohama, Japan Contact: Conference Service Section, IAEA, PO Box 100, A-1400 Vienna, Austria. Tel +43-1-2060-21310, Fax +43-1-20607, E-mail official.mail@iaea.org