1 - Nuclear Sciences and Applications - IAEA

634 ITOH et al.
30
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10
n
H=0.2\
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A I 20
a 1 30
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i
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I I
(a)
i Bt=5T
40=90* .
PRF=27kW
1 2 3 4 5 6 7
ne(10 12 cm" 3 )
- 4
FIG. 6. Parametric dependence of plasma current and line average density: (a) plasma current versus
line average density; (b) plasma current and line average density versus injected RF power.
3. HIGHER DENSITY OPERATION AND CURRENT DRIVE EFFICIENCY
The above mentioned steady state operations were mainly carried out in the low
electron density region of H,. = (1.5-2.5) X 10 12 cm" 3 ; the current drive
efficiencies, -q = 1^(10 l9 m" 3 ) R(m) Ip(kA)/PRF(kW), had the relatively low values
of 0.15-0.25. Hence, we attempted to sustain the plasma current at a higher density
level and to improve the current drive efficiency. The results are shown in Fig. 6.
Figure 6(a) shows the dependence of the plasma current on the line average density
for higher density operations. In the low density region of He < 3.5 x 10 12 cm" 3 ,
there is little dependence of the plasma current on the density, and the current drive
efficiency rises up to 0.3 with increasing density. The plasma current, in the region
of 3.5 X 10 12 cm" 3 < ne ^ 4.5 X 10 12 cm" 3 , falls rapidly with increasing density;
the resulting efficiency drops to 0.2. This seems to be related to the result that the
current density changes rapidly towards a broader profile with increasing electron
density as is to be expected from the Shafranov A (see the dashed line in Fig. 1). In
the high density regime of n,, > 4.5 X 10 12 cm" 3 , the plasma current shows an n,.
dependence, and the efficiency rises up to ij = 0.3 and is then kept nearly constant.
Eventually, although the efficiency was not improved remarkably, the current was
stably driven at higher densities of up to a. = 6 X 10 12 cm" 3 . It is, however, very
difficult to maintain the plasma current constant for several tens of seconds at such
a high density level. Figure 6(b) shows the strong dependence of the electron density
on the injected RF power. The density rises almost linearly up to 3 x 10 12 cm" 3 ,
together with the plasma current, with increasing RF power. Therefore, we may
expect to achieve a steady state current drive at higher density levels with a more
powerful RF system.
To find the optimum current drive conditions, we have also studied the dependence
of the current drive efficiency on the phase shift, A, and the toroidal field
Bt. The results are shown in Fig. 7. Figure 7(a) shows the dependence of the