Radio frequency current drive for small aspect ratio tori Page: 1 of 6
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Radio Frequency Current Drive for Small
Aspect Ratio Tori
M.D. Carter, E.F. Jaeger, D.B. Batchelor, D.J. Strickler, R. Majeski*
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-8071
*Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543
Abstract. Non-inductive current drive (CD) is required during plasma initiation and for
current sustainment in NSTX[l]. The physics of high harmonic fast waves (HHFW)
and the design of an antenna system for NSTX are studied. It is found that the theoretical
current drive efficiency for HHFW can be high, and a general survey of parameters gives
a good target for the antenna design. The primary issue for HHFW during plasma initi-
ation is loading since the CD efficiency is very high for low density plasmas. For high=
beta operation at full current, launching in the usual manner from the equatorial plane
may lead to marginal CD performance. However, advanced antenna designs exploiting
the theoretical results show some promise for high beta operation. Two methods to
optimize the CD efficiency have been explored. The first, non-zero poloidal mode ex-
citation, provides enhanced efficiency because of improved penetration and a reduction
of detrimental trapped particle effects. A second, placement of the antenna away from
the equatorial plane, can also be used to reduce trapped particle effects. These methods
can be used separately or together, yielding potential improvements of more than a factor
of 2 in CD efficiency for NSTX.
Radio frequency (RF) power in the ion cyclotron range of frequencies is a useful
tool for plasma heating and current drive that has not yet been extensively tested in
a small aspect ratio torus (ST). Non-inductive current drive (CD) is required during
plasma initiation and for current sustainment in NSTX. In this paper, we use the
RANT3D, GLOSI and PICES computer models to suggest two methods
for optimizing current drive efficiency in a ST using RF power in the high har-
monic fast wave (HHFW) regime. One method uses poloidal antenna phasing with
launch from the equatorial plane, while the other launches waves from a location
significantly above or below the equatorial plane. These methods enhance the cur-
rent drive efficiency for scenarios with strong damping by enhancing wave
penetration and reducing the power absorbed by trapped electrons.
The PICES and GLOSI plasma models both rely upon a warm plasma approxi-
mation where klp is assumed to be small, and only second order terms are retained.
This assumption becomes suspect for the proposed initial NSTX parameter regime
because k p=(o,;)/(QivA) ao(n;T;) /B2l 112/B can be large. Thus, we have
checked the validity of the warm ion approximation for "NSTX-like" parameters
by comparing the dispersion relation obtained from a full hot plasma dielectric,
retaining 40 Bessel functions, with that obtained from the warm ion approximation.
The results of this study for various fi conditions at 41 MHz with "NSTX-1
APR 2 2 1997
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Carter, M.D.; Jaeger, E.F.; Batchelor, D.B.; Strickler, D.J. & Majeski, R. Radio frequency current drive for small aspect ratio tori, article, December 31, 1994; United States. (digital.library.unt.edu/ark:/67531/metadc675554/m1/1/: accessed November 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.