1 - Nuclear Sciences and Applications - IAEA

572 WEYNANTS et al.
Important anisotropic tails are produced, leading to an apparent lower confinement of the thermal
plasma component but improved overall confinement. The tail also appears to play a role in sawtooth
stabilization. Excellent agreement is further obtained between theory and experiment with respect to
antenna coupling, wave absorption and tail production.
1. INTRODUCTION
With the installation of the ALT II pump limiter, the ICRH system of TEXTOR
was converted from a high field side (HFS) to a low field side (LFS) launch. At the
same time, an antenna spectrum shaping capability was provided by pairwise
grouping of the total of four antennas, with each antenna in a pair fed in phase or
out of phase with respect to the other. The out-of-phase or dipole mode of operation
is found to be superior to the monopole in all aspects of plasma-wall interaction
(density rise, impurity production) and hence in ease of ICRH operation. Quite
significantly, the antenna coupling in the dipole mode is as good as, or even slightly
better than, the monopole configuration.
Wall and limiter conditioning plays an important role. Appropriate conditioning
techniques prevent high recycling conditions leading to deteriorated confinement
('detaching' conditions [1]) in Ohmic as well as in auxiliary heated discharges, and
strongly affect the RF induced increments in density and particle fluxes. By means
of wall carbonization with the liner brought to a temperature of 400°C during
tokamak discharges or, even better, wall boronization [2], the fraction of the radiated
power during the application of ICRH drops substantially with respect to Ohmic
discharges, and long pulse, low impurity operation is possible.
The confinement scaling of the thermal part of the plasma energy remains in
line with the TEXTOR scaling [3]. In addition, however, and in good agreement with
theory, an energetic hydrogen tail is observed during the usual scenario of hydrogen
minority heating in deuterium plasmas. At low density this tail contribution can
significantly increase the incremental confinement time, as the tail confinement time
is found to be about twice that of the thermal component.
Several observations relate directly to the change in wave launching geometry.
Firstly, the need for adequate wall preparation and antenna spectrum shaping is
greater in the LFS mode. Secondly, the wave physics changes over from a mode
conversion dominated regime with HFS launch to a predominantly minority heating
mode in the LFS case. A detailed study is presented based on sawtooth analysis,
power modulation experiments and hydrogen tail analysis. Finally, although the
power density deposited in the electrons inside the q = 1 surface is very similar in
the two cases, only the LFS mode allows for a significant increase in sawtooth period
amplitude. The period appears to increase in proportion to the tail energy.