1. magnetic confinement - ENEA - Fusione

1. MAGNETIC CONFINEMENT
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the neutron production of the best FTU performances, deuterate diborane (B 2 D 6 ) is
going to be used as the working gas in the near future.
1.1.7 Radiative improved mode in Ohmic plasmas
1.1 Tokamak Physics
In many tokamaks, controlled injection of gas impurities (mainly noble gases such as
Ne, Ar and Kr) into the plasma has been found to lead, in certain conditions, to an
improved confinement regime, the so-called radiative improved (RI) mode. The
interest in this regime for FTU lies in the fact that it can be obtained with different
magnetic configurations (circular or elongated plasmas, limiter or divertor) and
different heating systems (neutral beam injection, ICRH and Ohmic). Moreover, it
couples good energy confinement with a large fraction of radiation losses (up to 90%
of total input power), thus alleviating the problems of plasma-wall interactions. Of
course the price to be paid is a larger Z eff and greater plasma dilution.
[1.16] M.Z. Tokar et al.,
Plasma Phys. Control.
Fusion 41, B317 (1999)
[1.17] M. Bessenrodt-
Weberpals et al., Plasma
Phys. Control. Fusion 34,
443 (1992)
The strategy to look for the RI mode in FTU is based on the interpretation given by
TEXTOR [1.16]: impurity injection attenuates the growth rate of the ion temperature
gradient (ITG) instability. This leads to a smaller particle outflow and hence to
peaking of the density profile. As a consequence, ITG turbulence is further
attenuated, or even quenched. In cases where ITG turbulence is the dominant heatloss
mechanism, an increase in energy confinement is achieved. In addition, it has
been found that in TEXTOR the energy confinement time increases with density.
An experimental campaign was started at FTU at the end of 2001 to explore the
possibility of a RI mode in Ohmically heated plasmas. The aim was to reproduce the
improved Ohmic confinement (IOC) regime of ASDEX. [1.17]. The plasma target was
chosen so as to clearly see this regime, if it really did exist in FTU. At magnetic field
B T =6 T, the plasma current was programmed to be at 0.8-0.9 MA to avoid the
insurgence of MARFEs. The operational density was set at 10 20 m -3 in order to be
well into the saturated Ohmic confinement (SOC) regime, where energy confinement
is independent of density. In FTU the critical density to access SOC is ~0.8 10 20 m -3 .
Deuterium gas puffing was interrupted at 0.45 s, just at the beginning of the current
flat–top, according to the experience on ASDEX. A neon puff (10-30 ms duration)
was injected at 0.6 s, just at the beginning of the current flat–top.
The standard FTU diagnostics was used to obtain the experimental results.
Fig. 1.12 - a) Lineaveraged
density; b) Z eff
from bremmstrahlung; c)
radiated power; d)
neutron yield for a
discharge without Ne
(red) and with Ne (violet)
puffing of 20 ms at 0.6 s.
1012 (s-1) 105 (W) Zeff 1019 (m3)
10
5
3.5
3
2.5
2
1.5
10
5
1
0.5
a)
b)
c)
d)
x
x
0
0 0.5
1
Time (s)
x
x
x
x
x
x
x
x
01
02
03
04
x
Figure 1.12a shows the
central line-averaged
density for a discharge
with a Ne puff of 20 ms
compared to a reference
discharge without Ne.
Two different phases
can be seen: First, there
is a slow increase in
density after Ne
injection, which cannot
be fully accounted for
by the electrons
contributed by Ne
ionisation. The Ne
concentration can be
roughly derived from
the variation in Z eff
(fig. 1.12b). Radiation