1. magnetic confinement - ENEA - Fusione

1. MAGNETIC CONFINEMENT
17
1.1 Tokamak Physics
Fig. 1.10 - #12744.
Particle fluxes integrated
on the 0.16 m flux
surface: a) neoclassical
diffusion; b) neoclassical
diffusion minus Ware
pinch; c) experimental.
1.5
time is longer than a neoclassical
Particle flux at r = 0.16 m
prediction that includes the Ware
pinch. Figure 1.10 shows that across
the r=16 cm surface, where no particle
a)
1
source can be present after ablation,
the experimental particle losses are
b)
lower than the net computed
neoclassical flux. If the neoclassical
0.5
value is regarded as a lower limit for
c)
radial diffusivity, an anomalous
inward pinch is required to explain
the experimental observation.
0
0.60
0.65
0.70
The plasma impurity content plays an
Time (s)
important role in the evolution of the
discharge after strong pellet perturbation [1.12]. In some cases, hollow temperature
profiles are produced, usually leading to a major disruption when a further pellet is
injected. This happens when the density is raised close to a critical value determined
by the power balance between Ohmic heating and radiation losses from
molybdenum, at the plasma centre This criterion made it possible to prepare a
suitable target for the injection of a given number of pellets. Further investigation of
impurity control and the effect of additional heating is in progress. In 2002 another
injector is going to be installed in collaboration with Padua RFX in order to have also
a high-field-side pellet injection track. This should allow first-time studies on the
effect of the particle radial drift at high density and high field in view of an
extrapolation to ITER.
1021 s-1
1.1.6 Boronisation: plasma results
[1.12] D. Frigione et al.,
Proc. 28 th EPS Conf. on
Control. Fusion and Plasma
Phys, (Madeira 2001), Vol.
25A, p. 73
[1.13] M.L. Apicella et al.,
Proc. 27 th EPS Conf. on
Control. Fusion and Plasma
Physics (Budapest 2000),
Vol. 24B, p. 1573
The boronisation system for overcoming the problem of high Z eff values at low
electron density was tested for the first time in FTU in October 2001.
Regarding vacuum performance, the getter action of boron on low-Z impurities
causes a strong reduction (up to a factor of 2.5) in the overall degassing rate and in
the pressure limit, which is reduced by a factor of 1.7 after 1-2 days of operation
following a fresh boronisation. This condition lasts for a long time (≥ 300 discharges).
After boronisation, restart of operations as well as recovery from plasma disruptions
are immediate and are a strong indication of the reduction in light impurities.
Regarding plasma characteristics, there are two main results: for an Ohmic plasma
(n _ e ≤1.0×1020 m -3 ), the total radiated power typically drops from 70-90% to 35-40%,
and for I p =0.5 MA and n _ e =0.3−0.4×1020 m -3 , Z eff decreases from 6.0 to 2.2 (see
fig. 1.11). These results are due to the strong decrease in heavy-metal concentrations
(up to a factor of 5 for molybdenum) and to the getter action of boron on light
impurities (oxygen concentration in the plasma is reduced from 2.5% to 0.5% and the
carbon flux from the walls drops from 1.0x10 18 to 1.1x10 17 part/s/m 2 ).
However, an unfortunate consequence of boronisation with cold walls is that it is
difficult to control the plasma density during the first two days of operation (about
60 discharges). The wall can either pump or release a large amount of H, depending
on the saturation degree of the surfaces facing the plasma. This is similar to what has
been observed after strong titanisation [1.13]. These phenomena do not occur in
other tokamaks, which operate at high wall temperature (>150°C), because hydrogen
is pumped by the B film during a pulse and then released immediately after it.
Preliminary observations suggest that high recycling or high edge neutral density
could hinder ITB formation in FTU.