1 - Nuclear Sciences and Applications - IAEA

IAEA-CN-50/E-IV-10 763
3. QUASI-LINEAR ELECTRON CYCLOTRON CURRENT DRIVE
Here, we assess the prospects for current drive with gyrotron or continuous
wave FEL sources by means of quasi-linear diffusion theory. Optimization studies
of current drive efficiency in a hot tokamak have been performed with the TORCH
ray tracing code derived from TORAY [9]. We consider current drive near the
magnetic axis of a tokamak with major and minor radii of R = 4.0 m and
a = 1.4 m, a non-circular equilibrium, and a toroidal j3 * 6%. The optimum launch
scenario exploits the fact that absorption on the low field side of cyclotron resonance
can limit the wave-particle interaction to electrons with a single sign of parallel
velocity and with pitch angles far from the trapped particle region. The wave
frequency is substantially upshifted (1.3 < u/Q0 < 1.5) from the central cyclotron
frequency ti0, and large values of NB :£ 0.7 are used to selectively heat electrons
with the highest possible energy. Use of the fundamental cyclotron interaction leads
to much higher efficiency than the second harmonic because the second harmonic
competes much less with the fundamental in absorbing the wave power than does the
third harmonic with the second [10]. Even for the fundamental, we find that
harmonic competition begins to limit current drive efficiency for a central electron
temperature Te > 25 keV. Another limiting effect is finite opacity: rays are not
absorbed by the highest energy resonant electrons encountered but by electrons with
energies E ranging over Te < E < 3 Te. The results of optimizing rj = nlR/P
(units of 10 20 Aj-m" 2 -W" 1 by variation of w and toroidal launch angle are r) = 0.16,
0.22 and 0.24 at fe = 15, 25, and 35 keV, respectively, and central density
n = 1.6 x 10 20 m" 3 . These values of rj are based on the current drive efficiency
theory of Ref. [11] and do not include the effects discussed in the next three sections.
Momentum conservation in collisions between resonant electrons and bulk electrons
is an effect of order Te/E and was noted for both lower hybrid and electron
cyclotron waves by Karney and Fisch [12]. Since the optimum 01 and N]: lead to
absorption by electrons in a range near Te/E = 2, we estimate, very roughly, an
enhancement of 50% compared to the theory of Ref. [11] if momentum conservation
is included.
High microwave power can distort the electron distribution and significantly
alter the current drive efficiency [11, 13]. On the high field side of cyclotron
resonance (inside launch), efficiency degrades sharply with increasing power because
of the competition between the Fisch-Boozer (anisotropic resistivity) and Ohkawa
(anisotropic trapping, isotropic detrapping) effects and because of the existence of
resonant electrons with both signs of parallel velocity. On the low field side (outside
launch), the power dependence of the efficiency is determined by the relative
importance of two competing processes. The formation of an extended quasi-linear
plateau of high energy electrons tends to improve efficiency, while decreased opacity
allows the RF power to 'burn through' towards the resonance layer where it is
deposited on lower energy (more collisional) electrons.