1. magnetic confinement - ENEA - Fusione

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1. MAGNETIC CONFINEMENT
1.1 Tokamak Physics
Energy transport and electron temperature profile stiffness with localised ECRH
Off-axis ECRH clearly reveals electron temperature profile stiffness in FTU [1.3],
particularly when absorption is located in the confinement region, i.e. outside the
sawtooth inversion radius (r/a > 0.2) but inside the radiation-dominated periphery
(r/a< 0.6). The typical marker of electron temperature profile stiffness, observed in
all similar experiments on ASDEX-U, D III-D, Tore Supra and TCV, is a step in the
radial dependence of the electron thermal diffusivity. The step is usually positioned
at the EC-wave absorption radius, particularly when the ECRH power density
greatly exceeds the Ohmic input. The step amplitude is just enough to keep the
temperature profile smooth. The gradient length L T =T e /∇T e of the profile hardly
changes from Ohmic heating to ECRH and is not influenced by ECRH intensity and
localisation.
Modulated ECH was applied to study electron temperature profile stiffness in FTU
plasmas during current ramp-up. Modulated ECH experiments at current flat-top on
ASDEX-UG [1.4] have shown that the heat wave propagates much faster outwards
than inwards, confirming the step-wise behaviour of thermal diffusivity at the EC
absorption radius. The experiments during current ramp-up were performed with
ECRH at a much lower power level than Ohmic heating in order to limit as much as
possible the impact of ECRH on profile shapes. In addition, target plasmas with very
different shapes were obtained through control of the breakdown and density buildup
phases. Figure 1.3 shows two typical targets, one with peaked temperature (and
current density) profiles, the other with flat-hollow profiles characterised by the
occurrence of typical double tearing modes. Heat wave propagation is much more
sensitive than power balance analysis to discontinuities in thermal conductivity. In
addition, by looking at the amplitude and phase radial distribution of electron
temperature oscillations, it can be excluded that the apparent drop in diffusivity is
due mostly to a heat pinch.
[1.3] S. Cirant et al., Proc.
14 th AIP Conf. on Radio
Frequency Power in
Plasmas (Oxnard 2001),
Vol. 595, p 338
[1.4] F. Ryter et al., proc.
28 th EPS Conf. on
Controlled Fusion and
Plasma Physics (Madeira
2001), Vol. 25A, p. 685
The experiments showed that in these conditions the low-high diffusivity transition
layer is not strictly positioned at the absorption radius and that it depends to some
extent on the profile shape. For a given position of the absorption layer (r/a≈0.25), in
Te (keV)
Te (keV)
2.5
2
1.5
1
0.5
0
3
2.5
2
1.5
1
0.05
#20144
#20146
a)
a)
b)
ρ ≈ 0.07
r ≈ r dep ≈ 0.28
P ECH
0
0.15 0.25 0.35 0.45
Time (s)
a)
200
100
0
200
100
PECH (kW)
PECH (kW)
Fig. 1.3 - Evolution in time of a) electron temperature on axis and at the deposition radius; b) temperature
profile for two discharges characterised by very different profile shapes. The heat wave is launched at the
EC wave absorption radius, which is well inside the flat region for shot #20144 and in the steep region in for
shot #20146.
Te (keV)
3.5
3
2.5
2
1.5
1
0.5
t = 0.10 ÷ 0.17 s
δt = 0.1 s
#20144
#20146
0
0.7 0.8 0.9 1 1.1 1.2 1.3
R(m)
b)