1 - Nuclear Sciences and Applications - IAEA

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transition
S
2 0.5
0
KIYAMA et al.
I I I . I ,
D 0.5
initial
ranfip-up succeeded.
ramp-up failed ]
. . . i i
1.0
8»
FIG. 2. $p immediately after the transition through q, = 2 (at 150 us after the initiation) versus initial
@p (at 50 us after the initiation). Dots and crosses are for the cases of successful and failed ramp-up,
respectively.
the plasma state with the specific q profile might be in a minimum energy equilibrium
state of the high & and low q tokamak.
3.2. Effect of 13 on the transition from high q to low q
The transition through qf ~ 2 from the high q plasma with high S number (S
~ 10 6 ) can also be attained by slow ramping of the plasma current. The transition
is usually accompanied by a rapid (one or two orders of magnitude shorter than the
tearing mode growth time) and violent relaxation process. In this configuration, when
qf = 2, q^ near the axis is about 2, instead of about 1, as in the usual tokamaks. The
transition is sometimes not successful and results in plasma current limitation, especially
for excessively elongated and inward shifted plasmas. The necessary condition
of initial j3p to achieve the transition to the low q regime is shown in Fig. 2. This
figure shows that /3p must be higher than the critical value before the transition. At
the transition, the high /3 plasma causes an outward shift of the plasma column itself,
which results in the reduction of q^ and a deepening of the magnetic well near the
column axis. The initial high @ plasma can supply the energy which may be necessary
to drive the transition.
3.3. Relaxation of density profile
The electron density profile also relaxes to a specific profile. A flat density profile
of a high q plasma relaxes to a peaked profile, and a peaked density profile of
a low q plasma relaxes to a flat profile. This occurs in most cases simultaneously with
the q profile relaxation, and the profile stays roughly constant during the rest of the
discharge. The profile changes drastically at qf ~ 2, as is shown in Fig. 3. At the
high q to low q transition, the profile changes from peaked to flat, and vice versa
at the transition from low q to high q. The volume averaged density of the constant