TJ-II:Instabilities
At TJ-II, a number of studies have been performed to analyse and understand the various instabilities occurring in fusion-grade plasmas.
MHD and Alfvén modes
The appearance of low-frequency Magneto-HydroDynamic (MHD) modes (some tens of kilohertz) in electron cyclotron heated plasmas depends on the rotational transform profile and the plasma density. In neutral beam injection plasmas, high-frequency modes (150- to 300-kHz) have been found in plasmas with line densities in the range 0.6 × 1019 m-3 to 3 × 1019 m-3 and heated with on/off-axis electron cyclotron heating. They are good candidates for global Alfvén eigenmodes related to the low-order resonance n/m = 3/2. [1]
ELM-like modes
ELM-like activity has been observed in plasmas with a stored energy above 1 kJ. The plasma is observed to develop bursts of magnetic activity (seen in Mirnov coil signals) which are followed by a large and distinct spike in the Hα signal. An increase in electrostatic and magnetic fluctuations at the plasma edge and a cold pulse towards the plasma centre are also characteristic of these events. In addition, the electron temperature profile locally flattens at the plasma radius where the temperature is in the range 100-200 eV. This flattening can be explained in terms of enhanced electron heat conductivity. Between ELM-like events the electromagnetic turbulence at the edge decreases and the Te profiles recover their former shapes. This activity is probably triggered by a resonant m = 2, n = 3 mode. [2] [3]