Correlation Between MHD-Activity, Energetic Particle Behaviour and Anomalous Transport Phenomena in WENDELSTEIN 7-AS Page: 1 of 6
6 p.View a full description of this article.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
24th European Physical Society Conference on Controlled Fusion and Plasma Physics
r"- (Berchtesgaden, Germany, 9-13 June 1997)
conF- 9'7 0 6 I3)- /
Correlation between MHD-activity, energetic particle behaviour
and anomalous transport phenomena in WENDELSTEIN 7-AS
A. Weller, M. Anton, J. Geiger, C. Grner, R. Jaenicke, REC ED
C. Konrad, F.P. Penningsfeld, N. Rust, D.A. Spong*) , C.Y. Teo
W7-AS Team, NBI Group, ECRH Group **) SEP 2 3 197
Max-Planck-Institut fir Plasmaphysik, EURATOM-Association,
D-85748 Garching, Germany 0. T I
*) Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA
**) Institut fur Plasmaforschung, Universitdt Stuttgart, D-70569, Germany
Introduction - Energy and particle transport in W7-AS exhibits a resonance-like dependence on the edge
rotational transform (iota) as long as the magnetic shear is relatively weak (low 3, no significant net toroidal
currents). MHD modes at resonant surfaces may cause enhanced radial transport depending on the magnitude
and radial extent of the magnetic perturbations. In many cases discharges in W7-AS are very quiescent, or in
case of mode activity, often no influence on energy and particle confinement is found. In the high beta regime
(($) < 1.8 %) shear is increased due to the effect of the Shafranov shift leading to the formation of rational
surfaces inside the plasma. Pressure driven mode activity appears at corresponding resonant surfaces. These
modes could be resistive interchange instabilities since the respective stability criterion can be violated at least
in the outer part of the plasma. Only around the highest beta values and in cases, where the magnetic well of the
configuration was reduced, relaxations of the plasma energy are observed, indicating the vicinity of a soft beta-
limit. In most cases, however, the maximum achievable beta is determined by the available heating power.
Effect of NBI driven global Alfven Eigenmodes on plasma confinement - Most of the MHD activity
observed occurs during neutral beam injection (NBI) in the lower beta regime, where low order rational
surfaces can be avoided due to the very low shear of the configuration. Therefore, away from but close to
resonant surfaces gaps in the shear Alfven spectrum are present, where global Alfven eigenmodes (GAE) can
be excited by resonant fast ions of the beam distribution. In NBI heated plasmas GAE activity often coincides
with the iota-range of degraded confinement. However, this is also the range where formation of low (m,n)
Alfven gaps becomes possible. Since the resonant confinement degradation also occurs during ECRH, where
no fast ion population is present and only very weak MHD-activity is observed, it is conjectured, that magnetic
turbulence phenomena around the dense high (m,n) resonant surfaces are causing this effect. From the analysis
of iota-profiles it is concluded, that the main low (m,n) resonances are less unfavourable because their
neighbourhood is free of resonances. In the period after switch on of NBI very pronounced GAE activity
causing enhanced transport can be excited due to transiently unstable fast ion velocity distributions. In
particular, when the target plasma has very low density or the discharge is initiated by the neutral beams, even
more unfavourable fast ion distributions can be formed due to charge exchange effects. In this case in addition
to the low frequency (20 - 40 kHz), low (m,n) GAE-activity a broad frequency range of activity extending up to
500 kHz is found by magnetics and various fluctuation diagnostics. The origin of this activity can be explained
by Alfv6n modes of higher mode numbers. GAE's, but also global Alfven modes in gaps induced by ellipticity
or higher non-symmetry (EAE, NAE) could play a role. Fig. 1 shows the change of the frequency spectrum
(Mirnov coil) with the change of the Alfven speed. Simulations of a (virtual) antenna loading spectra with the
DS R1BtUTON O ThS DOCUMENT IS UNUMrTED A T E
DTIC QUALITY IINCJ TED 3
Upcoming Pages
Here’s what’s next.
Search Inside
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Weller, A.; Anton, M.; Geiger, J.; Goerner, C.; Jaenicke, R.; Konrad, C. et al. Correlation Between MHD-Activity, Energetic Particle Behaviour and Anomalous Transport Phenomena in WENDELSTEIN 7-AS, article, December 31, 1997; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc696304/m1/1/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.