1 - Nuclear Sciences and Applications - IAEA

500
BOLTON et al.
SMM
1.0 (s)
0.5 t(s) 0.7
FIG. 5. Dynamic variation of radial position from magnetics (top) and centroid of electron density profile
(bottom). Offset of curves arbitrary.
PLASMA CONTROL
Plasma position, density, and current control is carried out using a digital feedback
system incorporating a programmable high speed controller with a cycle time of 0.4 ms .
This system permits dynamic variation of the gains and transfer functions during rampup,
plateau, and rampdown of the plasma current.
The plasma position is dynamically controlled at the present time within 10 mm.
Fig. 5 shows the plasma position as a function of time deduced from magnetic
measurements and from the in-out asymmetry of the electron density distribution as
measured by the six-channel interferometer. The latter signal is obtained from the
difference of the inboard and outboard channels weighted by the path length and
normalized to the total number of electrons. The absolute position agrees within 5 mm. The
dynamic agreement between these two measurements is remarkable.
Low-q operation has been achieved on the Tokamak de Varennes, as shown in Fig. 6.
The current and the density are simultaneously ramped up, with no observable macroscopic
effect when q=3 (1=195 kA) is passed. A current plateau is maintained for 0.4 s at
1=280 kA (q=2.1±.l) at a line average density of 2.5-10 m , followed by a disruptionfree
rampdown of the current to zero. Further increase of the current to 290 kA, however,
leads to a disruption, presumably because q=2 is reached.