1. magnetic confinement - ENEA - Fusione

MW 1012 s-1 keV MA
1. MAGNETIC CONFINEMENT
11
1.1 Tokamak Physics
[1.2] G. Tresset et al., A
dimensionless criterion
for characterizing internal
transport barriers in
JET, accepted for
publication on Nucl. Fusion
Fig. 1.2 - Time evolution of
main plasma quantities in
an ITB discharge with
LHCD in the current
ramp-up phase and offaxis
ECH power (I p =0.5
MA, B T =5.5 T), compared
with an Ohmic shot. Top
to bottom: time traces of
a) plasma current; b) lineaveraged
density; c)
central electron temperature;
d) neutron yield; e)
LH and ECH power.
density. High central electron temperatures (T e0 >8 keV) were achieved at
n _ e =0.3×1020 m -3 , B T =5.3 T, I p =350 kA with P LH =0.6 MW and P ECH =0.35 MW. The
q profile was not measured, but transport simulations [1.1] showed a magnetic shear
reversal region at r/a≤0.35. At the border of this region, a large gradient developed,
L T
-1 =(dTe /dr)/T e =30 m -1 , corresponding to R/L T =28 (with R the major plasma
radius). The local ρ T * =ρ/L T value (where ρ is the ion Larmor radius for T e =T i ) was
≥ 0.03, which is double the ρ T * value considered in JET discharges as a threshold for
an ITB [1.2]. At higher density, n _ e =0.6×1020 m -3 (n _ e0 =0.9×1020 m -3 ), T e0 (central
electron temperature)=5.4 keV was achieved with P LH =1.7 MW, and P ECH =0.7 MW
at B T =5.3 T, I p =460 kA. The neutron flux also increased by a factor of 2 from the
Ohmic to the LHCD phase and reached a factor of 2.5 in the combined LH+ECH
phase. According to the transport analysis, the WS/NCS region extended up to half
radius because of a broad LH power deposition profile. At the border of this region
(r/a~0.5) L T
-1 =20 m -1 , corresponding to a local ρ T * , again larger than the JET
threshold value. The current was not fully driven by LHCD. The residual V loop was
≈0.4 V (fig. 1.2), corresponding to a residual Ohmic power P OH ≈ 0.2 MW. As usually
found with central ECH heating in conditions close to full LHCD, the hard x-ray
profile emission remained unchanged during the whole heating phase, which
indicates a stationary current density profile.
1020 m-3
0.5
0
0.5
0
5
0
0.2
0
2
1
I p
n e
T e0
Neutrons
0
0 0.1
P LH
PECH
0.2 0.3 0.4 0.5
Time (s)
In the second scenario,
both ECH and LHCD were
applied early on in the
discharge, during the
current ramp-up phase
(dI p /dt=2 MA/s), to take
advantage of any preexisting
WS-NCS associated
with initial non-relaxed
j-profiles. The ECH was
applied off axis, before
LHCD (P ECH ≈0.3 MW,
r dep /a=0.2), thereby
broadening the initial
temperature and possibly
triggering an off-axis
LHCD. Figure 1.2 shows
the time evolution of the
main plasma quantities.
The LH power was
injected in 100 ms in steps
of just over 0.3 MW up to
1.7 MW to compensate for the increasing electron density. In this way an electron ITB
(L T
-1 =20 m -1 , ρ T * >0.03) was sustained for more than 0.2 s (6-7 confinement times) well
inside the current flat-top. In this phase the driven current fraction was I LH /I p ~50%,
the central density increased up to n e0 ~0.8×10 20 m -3 , T e0 exceeded 11 keV, the ITB
footprint expanded from r/a~ 0.3 to ≈0.4 and T i0 went from 1 to 1.6 keV. The neutron
yield also increased during the main ITB phase and was three times larger than in a
reference Ohmic discharge. The ITB was terminated at t>0.34 s by an m=1 MHD
tearing mode related to a change in the current density profile after ECH switch-off.
The local transport analysis showed that, in the weak shear region, transport is
indeed reduced during the main heating. However, the low plasma volume still
involved implies that the global energy confinement time is generally in line with the
ITER89-P scaling, although it exceeds the ITERL-thermal scaling.