HIGH Te0 (~14 KeV) PLASMAS IN CURRENT RAMP-UP EXPERIMENTS WITH ECRH HEATING
Up to 700 kW of Electron Cyclotron Resonance Heating (ECRH) power at 140 GHz have been injected during the current ramp-up phase of 700 kA plasma discharges, Fig. 1.
FIG.1 Fast current ramp traces, pulse 14669 690 kW ECRH: (a) plasma current; (b) central line average density; (c) peak electron temperature from ECE Michelson; (d) electron temperature at R=1.008 m from ECE polycromator.
Heating at the fundamental frequency, with perpendicular, low field side launch with ordinary polarisation has been used, so that the resonant magnetic field BT is 5 T. By changing BT, both on-axis and off-axis experiments have been performed. The fast current ramp-up scenario (5 MA/s) has been selected because it is characterised by a variety of the current density profiles, going from very peaked cases when sawtooth or m=1 MHD activity can be detected already at t=0.050 s, before ECRH injection, to very broad or hollow cases, that often terminate with a reconnection consistent with double-tearing modes at resonance surfaces characterised by an integer q value. In this scenario the ECRH experiments have produced very high peak electron temperature, up to 14 keV for 690 kW injected power, Fig. 2.
Fig. 2 Electron temperature profiles at t=0.095 sec; full points: shot 12658, 350 kW ECRH power, hollow initial current profile; crosses: shot 14669, 690 kW ECRH power, peaked initial current profile.
Due to localisation and the full absorption of the additional heating, these experiments allows to perform a quantitative analysis of the energy transport in a plasma condition where the energy transport is dominated by the electron channel (due to the low density, the electron-ion coupling is very week) and the magnetic shear values change significantly. The information on the current density profile are obtained by the solution of the diffusion equation for the poloidal magnetic field, using the resistivity evaluated from the electron temperature measured by Electron Cyclotron Emission (ECE) diagnostics. It has been checked that the results are consistent with the MHD behaviour of the discharges, that has been studied extensively.
In on-axis heating experiment the interpretative local transport analysis shows, Fig. 3, that the value of the effective electron thermal diffusivity in the plasma core is in the range 0.2-0.3 m2/s.
Fig 3 Radial profile of the electron thermal diffusivity in the plasma core at t= 0.095 sec; full points: shot 12658, 350 kW ECRH power; squares:shot 14669, 690 kW ECRH power. Shaded area indicate the uncertainty on the diffusivity value.
The interpretative analysis results are meaningful only in the plasma core, outside the narrow deposition of the ECRH power (Æ 2 cm FWHM). In the more external plasma region the local analysis is made difficult by the role of the radiation and the uncertainty on the ohmic power evaluation in the transient conditions characterising the fast current ramps.
In ECRH off-axis experiments the plasma attains lower temperature compared to the on-axis cases, as the additional heating is distributed an a larger volume. In off-axis experiments plasma discharges having pre-ECRH peaked or hollow current profiles behave qualitatively in different ways. In the case of a hollow initial current profile, Fig. 4, the temperature profile remains hollow during the ECRH, showing a trend to become flat in the region inside the ECRH deposition layer.
Fig. 4 Electron temperature profile at the times marked on the plot for the shot 12953, 325 kW off-axis ECRH power; the full points indicate the pre-ECRH profile.
This behaviour can be considered as the mark of an underlying diffusive transport mechanism that can be described by a local thermal diffusivity. In the case of plasma discharges having peaked pre-ECRH profiles, the electron temperature profile remains peaked, Fig. 5. This behaviour could be explained by the effect of the residual ohmic heating.
Fig. 5 Electron temperature profile at times marked on the plot for the shot 12616, 290 kW off-axis ECRH power; The full points indicate the pre-ECRH profile.