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Ion Orbits and Ion Confinement Studies on ECRH Plasmas in TJ-II Stellarator

F. Castejón, J. M. Reynolds, J. M. Fontdecaba, R. Balbín, J. Guasp, D. López-Bruna, I. Campos, L. A. Fernández, D. Fernández-Fraile, V. Martín-Mayor, A. Tarancón

Fusion Science and Technology / Volume 50 / Number 3 / October 2006 / Pages 412-418

Technical Paper / Stellarators

It was observed previously that the ion temperature profile of low-density electron cyclotron resonance-heated TJ-II plasmas is almost flat and that energetic ions are present well outside the last closed magnetic surface. The heat diffusivity obtained for such ion temperature profiles is very high, and therefore, transport cannot be described by Fick's law. In this work, ion trajectories with different pitches and starting points have been calculated for the relevant magnetic configuration. It is found that a feasible explanation for such a flat mean energy profile is that ion orbits are wide enough to communicate distant parts of the plasma radius, thus giving an effective flat ion temperature profile, for these low-density (<1019-m-3) plasmas. The distribution function is also obtained without considering collisions; thus, non-Maxwellian features are found. The final particle density shows inhomogeneities on a magnetic surface.

 
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