1. magnetic confinement - ENEA - Fusione

1. MAGNETIC CONFINEMENT
13
1.1 Tokamak Physics
Fig. 1.4 - Key elements
showing consistency
between critical gradient
modelling and experimental
data. The
effective gradient length
(open dots) saturates (at
≈10) when a critical value
derived from ETG
turbulence (x) is
exceeded (at r=5-7 cm).
Both steady-state and
transient thermal
diffusivity switch from
low to high values at
almost the same radial
position.
[1.5] A. Jacchia et al.,
Proc. 14 th AIP Conf. on
Radio Frequency Power in
Plasmas (Oxnard 2001),
Vol. 595, p.342
[1.6] G.T. Hoang et al.,
Phys. Rev. Letts 12,
125001 (2001)
R/LT,e
slow
low χ e,hp
#20145(0.140 − 0.160 s)
heat wave
fast
high χ e,hp
R/L T,e -experiment
R/L T,e,crit = 5 + 10 s/q (T.S.)
χ e (p.b.)
χe (m 2 /s)
the case of peaked discharges the
narrow EC deposition occurs mostly
in the high-diffusivity region, while
for flat-hollow discharges it is
located well inside the lowdiffusivity
central volume. The step
in diffusivity appears, therefore, to
depend on the gradient profile
shape, which is consistent with the
assumption that the maximum
temperature gradient length is
limited below a critical value.
In fact, the critical gradient length
model gives a good description of
most experimental findings on
profile stiffness in steady state [1.5].
The results of modulated ECH
experiments on current ramp-up can be consistently included within this
framework, as shown in figure 1.4. Firstly, the plasma column appears to be divided
in two regions, each with different confinement properties. Secondly, the radial
position of the step in both transient and steady-state electron thermal diffusivity
almost coincide. Thirdly, the low-high diffusivity transition layer is located where
the effective gradient length, which decreases with increasing radius, stabilises
around a critical value. All these features can be explained if it is assumed that
electron thermal transport is enhanced in the plasma region where 1/L T exceeds a
critical value 1/L T,c that depends on local plasma parameters.
Assuming as the critical gradient length L T,c the value of the actual L T at the
transition layer, data from different discharges can be correlated with the
corresponding local magnetic shear, as also observed on Tore Supra [1.6]. FTU data
show a dependence of L T,c on the s/q parameter very similar to Tore Supra, in spite
of the different electron heating methods (ECRH for FTU, fast wave in the ion
cyclotron frequency range for Tore Supra). This dependence is consistent with
theoretical predictions based on electron temperature gradient turbulence.
1.1.3 Downshifted and upshifted experiments with ECRH in LHCD
plasmas
To increase CD efficiency, the EC wave can be injected on a LHCD sustained plasma
by exploiting the suprathermal absorption mechanism. The presence of fast electrons
generated by LH waves allows the cyclotron resonant frequency to be shifted up or
down from the cold resonance, depending on the launched N ⎟⎟EC , the magnetic field
and the fast electron distribution.
In the downshifted configuration (B T in the range of 6.9 -7.2 T and the cold resonance
outside the plasma), up to 80% of EC power absorption is observed, with increments
in electron temperature (∆T~ 1 keV) and driven plasma current (up to ∆I p ~ 35 kA for
a plasma with I p =350 kA, =0.5×10 20 m -3 ). Electron cyclotron power absorption
results in a loop voltage drop and in an increase in fast electron energy. The EC
power absorption is closely related to the fast electron tail density that corresponds
to the absorbed LH power, and is in agreement with a linear model of suprathermal
interaction of the EC wave.
In preliminary experiments on the up-shift scheme, 700 kW of EC waves were
injected with 30° of toroidal angle in plasma with LHCD (1.5–2 MW), resulting in