Investigations of the global confinement of stellarator devices are done in the frame of the international stellarator scaling (ISS) activity. The objective is to deduce a scaling expression for the global energy confinement time that predicts confinement of future stellarator devices. To this end, data from the world-wide existing stellarators are collected and regression analyses are performed.
At TJ-K, experimental confinement studies are carried out to understand density and temperature profiles on the basis of diffusion coefficients. The obtained values are compared to estimates derived from fluctuation measurements. Experimental data are measured with movable probes and the SPIDER array. The data are analysed with a fluid code solving the combined particle and energy-balance equations.
Power modulation experiments are used to identify the power deposition region. Simulations have shown (see figure) that the dynamic response of the plasma density can be used to estimate the power deposition profile. At lower modulation frequencies, the local diffusion coefficients can be deduced.
In most heating scenarios used in TJ-K, the microwave power is deposited at the plasma edge. This leads to slightly hollow electron temperature profiles. Electron heat diffuses from the edge to the centre, where it is lost due to ionisation processes. Thus the particle source is determined leading to centrally peaked density profiles. Particle diffusion is outward directed. The pressure profile is centrally peaked.