1. magnetic confinement - ENEA - Fusione

14
1. MAGNETIC CONFINEMENT
1.1 Tokamak Physics
partial CD. The central field was varied in the range 4.8-5.2 T, with I p =400–600 kA
and =0.5–0.8×10 20 m -3 . In line with theoretical predictions, the single-pass
absorption was well localised, occurring only on the low-field side where the wave
beam encounters (at r/a ≈0.5) the resonant fast electrons before it is fully absorbed
by the bulk resonant layer. The resulting EC current produced a local modification of
J(r), as observed from the reduced MHD activity and the widening of the fast
electron bremsstrahlung emission profile. The increase in the driven current
(∆I≥100kA) as calculated from the drop in loop voltage was larger than that
calculated from theory. The resulting increase in CD efficiency was above the error
bars and indicates a synergy process between the two waves.
1.1.4 MHD behaviour in improved confinement regimes and new
phenomena by fast MHD analysis
Discharges that exhibit improved confinement after pellet injection are characterised
by a change in the central MHD behaviour [1.7, 1.8]. The optimum condition is an
increase in the sawtooth period to values (20-100 ms, the typical pre-pellet value
being 5 ms) that are a significant fraction of the energy confinement time. The main
parameter controlling the post-pellet period is the pre-pellet central temperature
(fig. 1.5), for a wide range of plasma densities and plasma currents. This dependence
can be easily understood because pellet penetration is a strong function of electron
temperature. If pellet ablation is completed well outside the q=1 surface, the
sawtooth period barely changes. In the other extreme case, if part of the pellet is
ablated inside the q=1 surface, the sawtooth is completely suppressed. In the
intermediate case, the optimum condition is attained. Complete sawtooth
suppression can give transient confinement improvement, but in this case impurity
accumulation takes place and this can lead to central radiative collapse.
The temperature dependence was exploited to attain controlled access to pellet
enhanced performance. The pre-pellet temperature decreases with increasing
density, and in a first stage gas puffing was used for control. Another method that
proved more efficient at plasma currents I p >1 MA was based on pellet sequence
timing: a first pellet was used to cool the plasma, and the timing of the second pellet
was optimised to meet the optimum temperature in the subsequent re-heating phase.
In addition to sawtooth period modification, pellet injection produced MHD
phenomena of fundamental interest [1.9, 1.10]. In particular, macroscopic structures
with dominant m=1 poloidal mode number were observed to saturate at large
amplitudes and to survive across sawtooth collapses for times exceeding the resistive
diffusion period (fig. 1.6).
These structures were
recognised as m=1
120
magnetic islands with a
x
very strong soft-x-ray
100
emission from the o-point
region (fig. 1.7). The nonlinear
stability of these
The sawteeth are stabilised
80
islands seems to be due to
60
x
radiative cooling around
the o-point. In the absence
40
of sawtooth reconnection,
x
locking of the m=1 was
20
observed in some cases.
x
x
x x
x x
x
This phenomenon was due
x xx
x x x
x x x
0
to toroidal mode coupling. 1 1.5 2 2.5 3 3.5
T e (keV)
τst (ms)
x
x x x
I p < 1 MA
I p < 1 MA
[1.7] E. Giovannozzi et al.,
Proc. 28 th EPS Conf. on
Contr. Fusion and Plasma
Phys. (Madeira 2001), Vol.
25A, p. 69
[1.8] P. Buratti et al., Bull.
Am. Phys. Soc. 46, 156
(2001)
[1.9] E. Giovannozzi et al.,
Am. Phys. Soc. 46, 156
(2001)
[1.10] P. Buratti,
Turbolenza e strutture
non lineari coerenti in
FTU, invited oral
presentation, SIF,
LXXXVII Congresso
Nazionale (Milano 2001)
Fig. 1.5 - Sawtooth period
just after pellet injection
as a function of electron
temperature.