Electron cyclotron resonance heating during plasma current ramp
The control of the current density radial profile can be a tool for reducing the level of turbulent fluctuations causing anomalous transport. The formation of transport barriers with flat or hollow current density profiles has indeed been shown by several tokamaks. On FTU, the transport behaviour in these conditions has been investigated by heating the plasma with Electron Cyclotron Resonant Heating (ECRH) during fast current ramps. If the ramp rate of the plasma current is sufficiently fast, the skin effect drives the plasma current preferentially in the outer part of the discharge producing a non-monotonic current density radial profile. In addition, during plasma current ramp-up sawteeth are absent, and the underlying heat transport mechanisms can be disclosed.
The FTU ECRH system allows to perform such an investigation at high magnetic field values (B=5T) and high plasma density (n<2.4 10 20 m-3). Power deposition by ECRH is extremely intense and well localised; this allows to check the radial temperature profile response to variations of the location of the resonant absorption radius, and to determine the influence of the electron free energy sources on turbulent heat transport. Furthermore, by changing the current ramp rate a variety of current density profiles can be obtained, and the stability of the discharge is accordingly affected.
In the following figures the electron temperature radial profile is shown at different times during the ECRH phase for two discharges with different location of the resonant absorption radius. No MHD activity was present during the profile evolution shown, so that the profile shape is governed by heat transport only.
a - Off-axis ECRH (R=1.05). Electron temperature profiles before ECRH (black dashed); 5 ms during ECRH (blue); 10ms during ECRH (cyan-dashed); 15 ms during ECRH (red).
b - Central ECRH (R=0.98). Electron temperature profiles before ECRH (black dashed); 5 ms during ECRH (blue); 10ms during ECRH (cyan-dashed); 23 ms during ECRH (red).
The main results can be summarised as follows:
-Very high central electron temperature values (up to 16 keV at densities around 0.7 1020 m-3) have been achieved with central resonance. These are the largest electron temperature values achieved so far in a tokamak. Although the electron distribution function is non-maxwellian, the temperature values are representative of the average electron energy.
- Electron heat transport is dominated by thermal diffusion as exemplified when off-axis resonance is used.
- Tearing modes and fast mixing events have been observed which can affect the transport properties in the internal region. Core confinement improvements have been observed when the minimum q was close to integer values and a stability window for MHD modes is expected.
- Deviations of the energy distribution function from the maxwellian shape are in agreement with theory (in collaboration with CIEMAT).
- The thermal diffusivity remains low even at extremely high values of the electron temperature gradient, i.e. in the presence of strong free energy sources which might drive turbulence. This may be due to the beneficial effect of a flat/hollow current density profile on plasma turbulence theoretically predicted and experimentally observed also on other tokamaks. Note that improved confinement in other tokamaks was mainly achieved either at high Ti /Te ratio or at low densities, whereas in FTU low thermal diffusivities are obtained at high density and high electron temperature.