Current drive and profile control in low aspect ratio tokamaks Page: 6 of 8
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Chan et at. CURRENT DRIVE AND PROFILE CONTROL IN LOW ASPECT RATIO TOKAMAKS
Table 1
[A = 1.2, elongation = 2.2, triangularity = 0.7]
Case A Case B
ON 8 7.57
#t 0.329 0.224
AP 1.36 1.79
I, (MA) 8.226 5.913
Id 1.054 0.692
IP-, -0.552 71.1% -0.367 88.6%
It. 5.347 4.915
use rf waves for heating and current drive. The very large At causes the total B-
field JBJ to be non-monotonic in R (Fig. 2). Also the ohmic loss constraint on the
centerpost limits on-axis Bt to relatively low values (- 1.2 T for Cases A and B). The
tight inboard space eliminates launching rf on that side. For electron cyclotron ordinary
waves (0-mode), the low Bt limits the maximum density to ne < 9.74x1012(BT)2 cm-3.
Because of the non-monotonic (BI, one can use the extraordinary wave (X-mode)
launched from the outside but the density is still limited to ne < 1.95 x 1013 (BT)2 cm-3
where BT is normalized to 1 T. For lower hybrid waves, the accessibility condition is
n2 = (ck, /w)2 > 1 + w2 /f2. To avoid strong Landau damping at the edge and for
efficient current drive, w/kip has to be much larger than the thermal velocity. Typically
np ~ 1.5 which limits the density to n, < 1.2 x 1013(BT)2 cm-3.
At the ion cyclotron frequency range, the challenge is to find a frequency window
which avoids cyclotron resonances at the plasma edge because of the non-monotonic
B. For the equilibria in consideration, we have found only one frequency range near
f ~ 8.5 MHz which has a single fundamental tritium resonance at the inboard side
[Fig. 3(a)] for a 50-50 D-T plasma. We have not found a scenario with a cyclotron res-
onance or two-ion hybrid resonance on-axis with no edge cyclotron resonance. However,
with the high At and a relatively flat BI well, electron Landau damping and cyclotron
damping are strong enough for the rf energy to be damped in the core before reaching the
resonance [Fig. 3(b)]. Because BI increases toward the outer edge, a hybrid resonance
exists in the outer half of the plasma similar to high field launch in standard tokamaks.
Near the resonance layer, the magnetosonic wave is described by the coupled equations
E11 - n2 E12 Ex
2 2 = 0 (1)
-E12 E22 _ ,2 + ( X E,GENERAL ATOMICS REPORT GA-22083 2
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Chan, V. S.; Chiu, S. C.; Lin-Liu, Y. R.; Miller, R. L. & Turnbull, A. D. Current drive and profile control in low aspect ratio tokamaks, article, July 1, 1995; San Diego, California. (https://digital.library.unt.edu/ark:/67531/metadc619071/m1/6/: accessed April 22, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.