1 - Nuclear Sciences and Applications - IAEA

764 COHEN et al.
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ECH power P (MW)
FIG. 2. Efficiency of quasi-linear electron cyclotron current drive as a function of the injected power
for three values of Nt.
We have used a relativistic version of the computer code CQL [14] on a radial
array of flux surfaces to solve the bounce averaged Fokker-Planck quasi-linear
equation for the electron distribution. The incoming RF power is damped at each
radius consistent with the quasi-linear distortion of the distribution function, resolved
locally in poloidal angle. For an inside launch with NB ~ 0.34, frequency equal to
100 GHz, central temperature Te = 20 keV, density n,. = 1.0 X 10 20 mf 3 ,
R = 1.8 m, a = 0.5 m, and BT = 5 T, we find that the global electron cyclotron
current drive efficiency degrades to zero at RF power equal to 17 MW. For outside
launch into the same plasma at various launch angles and at frequency 160 GHz, we
obtain the results shown in Fig. 2. For these cases, there is some degradation of
efficiency with increasing power at smaller N|, but we obtain an enhanced efficiency
with increasing power at larger NN (near the optimum N| « 0.7 value
discussed at the beginning of this section). The momentum conservation enhancement,
not included in Fig. 2, is of the order of 25% for these results.
4. INTENSE LOWER HYBRID HEATING AND CURRENT DRIVE
It has been generally assumed that lower hybrid waves are unsuitable for use
in the core of a reactor plasma because an accessibility requirement restricts the phase
velocity to be low enough that electron Landau damping is severe. However, the
absorbed power saturates as a function of applied power as the distribution function
forms a plateau. The advent of intense, pulsed microwave sources in the 10 GHz
range, such as the relativistic klystron [15], allows enhanced lower hybrid wave
penetration in a reactor with high peak but moderate average power.
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