TJ-II:Instabilities

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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] [2]

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. [3] [4]

Influence of the magnetic well

MHD theory predicts that instabilities inside the plasma are stabilized by increasing the "magnetic well". This idea is supported by magnetic well scan experiments in TJ-II. The level of fluctuations, the degree of intermittency and the radial correlation of relevant quantities increase as the magnetic well is reduced. [5] [6]

References

  1. R. Jiménez-Gómez et al., Analysis of Magnetohydrodynamic Instabilities in TJ-II Plasmas, Fusion Science and Technology 51, 20 (2007)
  2. R. Jiménez-Gómez, A. Könies, E. Ascasíbar, et al, Alfvén eigenmodes measured in the TJ-II stellarator, Nucl. Fusion 51 (2011) 033001
  3. I. García-Cortés et al, Edge-localized-mode-like events in the TJ-II stellarator, Nucl. Fusion 40 (2000) 1867-1874
  4. J.A. Jiménez et al, Localized electromagnetic modes in MHD stable regime of the TJ-II Heliac, Plasma Phys. Control. Fusion 48 (2006) 515-526
  5. C. Hidalgo et al, On the radial scale of fluctuations in the TJ-II stellarator, Plasma Phys. Control. Fusion 43 (2001) A313-A321
  6. J. Castellano et al, Magnetic well and instability thresholds in the TJ-II stellarator, Phys. Plasmas 9 (2002) 713