1 - Nuclear Sciences and Applications - IAEA

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IAEA-CN-50/A-I-2 33
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LINE-AVERAGED DENSITY OF TARGET PLASMA (10 19 m 3 )
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A Thermal
L-mode E tot
FIG. 4. Effects of variations in the line-averaged density of the target plasma on the total and the thermal
(ion + electron) components of the stored plasma energy for discharges with 22 MW balanced NBI.
The total energy is determined magnetically, the thermal component from measurements of the electron
and ion profiles.
cay time of the density to its equilibrium level following a small diagnostic
deuterium gas puff (Ane < 1 x 10 18 m~ 3 ) into a reference discharge. The recent
experiments have shown that while still necessary, this degassing is not,
by itself, sufficient for good performance at high plasma current. Even in
discharges with degassed walls, small variations in the line-average density
of the target plasma result in major changes in the energy confinement time
in the subsequent NBI phase. These variations in density are correlated
with the influx of carbon [7]. Though a well-defined procedure has been
developed for modifying the deuterium recycling, the variations in carbon
recycling are not well controlled at this time. Recently, through a combination
of the helium degassing discharges and extensive operation with NBI
at higher plasma currents than previously used, the densities of the target
plasmas at 1.4 and 1.6 MA have been reduced by ~ 25% from the levels
achieved in 1986 and 1987.
The effects of variations in the target plasma conditions on the total
and thermal component of the stored energy are illustrated in Fig. 4. Data
from discharges run under otherwise similar conditions in which the intrinsic
changes in the state of conditioning produced changes in the target plasma
density are supplemented by data from discharges with the same NBI power
in which the overall recycling was increased by puffing helium into the target
plasma. In the discharges without added helium, the deuterium content of
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