1. magnetic confinement - ENEA - Fusione

24
1. MAGNETIC CONFINEMENT
1.1 Tokamak Physics
studies were focussed
mainly on the role played
by magnetic shear, under
the hypothesis that
turbulence is stabilised
when the ExB shearing rate
exceeds the linear growth
rate of ITG modes.
Task Force M
0
T io (keV)
12
Due to problems with the
8
plasma position feedback
(pickup from the generated 4
T eo (keV)
mode), the experiments on 0
Z eff (0)
LHCD stabilisation of the 6
NTM had just a couple of 4
n
useful shots. Only a slight
eo (1019m-3)
2
H
destabilising effect was
89 β N
observed in these 0.9
discharges, suggesting 0.6
li
Vs
interaction between LH 0.3
waves and the mode, with 0
inappropriate localisation
2 4 6 8 10 12 14
of the power. This is
Time (s)
encouraging in view of
continuing the experiments in the 2002 campaigns.
The aim of other Task Force M experiments was to study the behaviour and
threshold scaling for error-field-induced locked modes at high beta poloidal. In past
experiments on DIII-D, it was observed that the penetration threshold was lower at
high beta. Although the observations made at JET are still inconclusive, a
phenomenology has been observed that differs both from the classical penetration
observations and from the onset of neoclassical tearing modes. Lack of power made
it impossible to perform experiments far from the natural threshold for the onset of
2/1 neoclassical tearing modes.
Internal transport barrier analysis in JET
3
2
1
0
12
8
4
P LHCD (MW)
P NBI (MW)
I p (MA)
R nt (1015n/s)
P ICRH (MW)
Analysis of the ITB in JET discharges was continued during the C4 experimental
campaign (January-February 2001) and was focussed in particular on the effect of
magnetic shear on ITB formation. The IDL code previously developed [1.23] to
calculate the radial electric field (E r ) in the plasma, the E×B flow shearing rate (ω s )
and the linear growth rate of ITG modes (γ ηi ) in ITB discharges was upgraded. In
fact, γ ηi can be now calculated using an explicit dependence on the magnetic shear s
(as given either by gyrokinetic and gyrofluid codes or by theoretical predictions). The
results were applied to the analyses of ITB discharges from the C2 and C4
campaigns. It was found that by taking into account the dependence of γηi on s, it is
qualitatively possible to explain the radial location, the time of formation and the
time evolution of different kinds of transport barriers in terms of the E×B shear flow
suppression of ITG-driven electrostatic turbulence [1.24]. In addition, the carbon
poloidal velocity (which is another output of the above IDL code), calculated
according to the neoclassical theory, was compared with the results of the new JET
spectroscopic diagnostic system (currently being commissioned), which provides a
measurement of the impurity poloidal velocity. Reasonable agreement between the
data was found [1.25].
Fig. 1.17 - Shot #53521.
a) Plasma current - LHCD
power; b) NBI and ICRH
power - neutron yield; c)
central ion and electron
temperature; d) central
electron density and
effective Z-H 89 β N ; e)
V loop and l i .
[1.23] F. Crisanti et al.,
Nucl. Fusion 41, 883
(2001)
[1.24] B. Esposito et al.,
Proc. 28 th EPS Conf. on
Contr. Fusion and Plasma
Phys. (Madeira 2001), Vol.
25A, p. 553
[1.25] F. Sattin et al.,
Proc. 28 th EPS Conf. on
Control. Fusion and Plasma
Phys. (Madeira 2001), Vol.
25A, p. 373