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TECHNOLOGY OF FAST-WAVE CURRENT DRIVE ANTENNAS*
purpose*
u. J. Hoffman, F. W. Baity, R. H. Goulding, G. R. Haste, P. M. Ryan, D. J. Taylor,
D. W. Swain, M. J. Mayberry,t and J. J. Yugo
Oak Ridge National Laboratory, P. O. Box 2009, Oak Ridge, TN 37831-8071
ABSTRACT
The design of fast-wave current drive (FWCD)
antennas combines the usual antenna considerations (e.g.,
the plasm a/antenna interface, disruptions, high currents and
voltages, and thermal loads) with new requirements for
spectral shaping and phase control. The internal
configuration of the antenna array has a profound effect on
the spectrum and the ability to control phasing. This paper
elaborates on these considerations, as epitomized by a
proof-of-principle (POP) experiment designed for the DIII-
D tokamak. The extension of FWCD for machines such as
the International Thermonuclear Engineering Reactor
(ITER) will require combining ideas implemented in the
POP experiment with reactor-relevant antenna concepts,
such as the folded waveguide.
INTRODUCTION
The long-term engineering requirements for fusion
reactors include the development of a noninductive means
to drive current in the plasma because of the need for
steady-state operation. The ITER design calls for 115 MW
of power to generate 18 MA of plasma current for pulse
lengths of >120 s.1 Negative ion beams and lower hybrid
waves are being considered for this function. Both
techniques have serious limitations. Experiments on
Alcator-C, PLT, and JT-60 have shown that lower hybrid
current drive faces an effective plasma density limit that is
below the desired operating density of ITER. The negative
ion beams face a different type of problem: they require
large amounts of space near the machine, and they are
extremely expensive to manufacture and develop
(historically, beams have cost roughly two to four times as
much as ion cyclotron heating). Nonetheless, the negative
ions are perceived as the choice for current drive on ITER
because there is some proof that beams can drive currents.
The cost of beams is so high and the risk of high-energy
operation is so great that there is some impetus to develop
an alternative method of current drive. Recent analyses by
Ehst,2 Batchelor,3 and Mau4 have shown that ion
cyclotron waves could be used to drive the currents. The
predicted efficiencies for FWCD in ITER are virtually the
same as those for negative ion beams. The FWCD looks
especially attractive because the equipment is compact and
the power is inexpensive and easily transported. However,
FWCD suffers from the lack of any experimental
verification. Thus, a POP experiment was designed for the
DHl-D experiment to confirm that FWCD works and is
efficient
The physics analyses are based on asymmetrically
modifying the electron velocity distribution with a
nonresonant, traveling ion cyclotron wave. The electrons
must be be warm enough to absorb the power by either
Landau damping or transit-time magnetic pumping
(TTMP). These physical mechanisms translate into
technology requirements in the following manner. The
antenna should be a phased array with arbitrary phasing to
generate the traveling wave. The majority of the wave
power needs to be in the low spectral range (n|| - 2-5) with
directivity so that the interaction with electrons results
primarily in net electron current and not heating. During
the current drive, the collisionality must be low so that
energy is not dumped into the bulk of the velocity
distribution. This generally means that densities will be
low, and so will the rf loading. The FWCD frequency is in
the range from 20 io 120 MHz for ITER. The net result of
all these considerations is that FWCD should be possible
with an array of rf antennas that face the standard tokamak
conditions, under the additional constraints of phase
control and spectral content centered at low wave numbers
for current drive with little content at higher wave numbers
(which result oily in heating).
THE Dm-D PROOF-OF-PRINCIPLE DESIGN
A POP experiment5,6 has been devised for the DIII-D
tokamak to demonstrate FWCD on a large tokamak. The
target plasma parameters areB-lT.noil.3x 10*3 cm-3.
Te = 4 keV, and Ip = 0.5 MA. The expected plasma
ctsrent resulting from -2 MW of coupled rf power for 1 s
is predicted to be =0.25-0.5 MA, which is a significant
fraction of the total ohmic current The antenna is a 50- by
100-cm, four-strap array that fits into a vacuum wall
recess, as shown in Fig. 1. The antenna is designed for
•Research sponsored by the Office of Fusion Energy, li.S. Department of Energy, under contract DE-AC05-840R21400 with
Martin Marietta Energy Systems, Inc.
^General Atomics, San Diego, California 97.138.
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Hoffman, D. J.; Baity, F. W.; Goulding, R. H.; Haste, G. R.; Ryan, P. M.; Taylor, D. J. et al. Technology of fast-wave current drive antennas, article, January 1, 1989; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1088456/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.