Influence of magnetic well on electromagnetic turbulence in the TJ-II stellarator

A magnetic well scan has been performed in the TJ-II stellarator to investigate the confinement properties with different values of the well, or even of the hill, and to explore the properties of electromagnetic turbulence. Stable plasmas have been obtained in theoretically Mercier-unstable configurations, and the electrostatic turbulence levels in the edge are increased. Three families of modes appear during the experiments: (1) a family of modes of Alfvénic nature with high frequencies; (2) a second set of modes of middle frequencies (tens of kHz) and (3) an oscillation at f  ≈  10–20 kHz happens in several cases. In spite of the fact that the vacuum rotational transform is very similar in all of the cases, the Alfvénic mode family changes drastically when decreasing the magnetic well, showing a non-monotonic behaviour of the amplitude, and a decrease of the typical frequencies. This behaviour cannot be explained only by current or density variations, so the effect of the modification of the configuration is playing a key role. Regarding the intermediate frequencies, a coherent mode appears with decreasing frequency as the magnetic well decreases. This mode is a candidate for a GAM, which can survive in these TJ-II plasmas, despite of the strong damping these modes should suffer in this device.

[1]  S. E. Lysenko,et al.  Transition from chirping to steady NBI-driven Alfvén modes caused by magnetic configuration variations in the TJ-II stellarator , 2016 .

[2]  F. Castejón,et al.  Magnetic well scan and confinement in the TJ-II stellarator , 2015 .

[3]  M. Ochando,et al.  Alfvén eigenmodes including magnetic island effects in the TJ-II stellarator , 2015 .

[4]  Á. Cappa,et al.  Limit cycle oscillations at the L–I–H transition in TJ-II plasmas: triggering, temporal ordering and radial propagation , 2015 .

[5]  C. Cook,et al.  Analytical theory of the shear Alfvén continuum in the presence of a magnetic island , 2015 .

[6]  S. E. Lysenko,et al.  Effect of magnetic configuration on frequency of NBI-driven Alfvén modes in TJ-II , 2014 .

[7]  F. Castejón,et al.  Simulations of fast ions distribution in stellarators based on coupled Monte Carlo fuelling and orbit codes , 2013 .

[8]  F. Castejón,et al.  Stellarator optimization under several criteria using metaheuristics , 2012 .

[9]  Xavier Sáez,et al.  Collisionless damping of flows in the TJ-II stellarator , 2012 .

[10]  E. Ascasíbar,et al.  Plasma potential and turbulence dynamics in toroidal devices (survey of T-10 and TJ-II experiments) , 2011 .

[11]  L. G. Eliseev,et al.  Internal measurements of Alfvén eigenmodes with heavy ion beam probing in toroidal plasmas , 2010 .

[12]  Á. Cappa,et al.  Doppler reflectometer system in the stellarator TJ-II. , 2009, The Review of scientific instruments.

[13]  W. A. Cooper,et al.  Integrated physics optimization of a quasi-isodynamic stellarator with poloidally closed contours of the magnetic field strength , 2006 .

[14]  F. Castejón,et al.  Influence of low-order rational magnetic surfaces on heat transport in TJ-II heliac ECRH plasmas , 2004 .

[15]  Donald A. Spong,et al.  Shear Alfvén continua in stellarators , 2003 .

[16]  A. Melnikov,et al.  Development of heavy ion beam probe diagnostics , 1994 .

[17]  V. Lynch,et al.  Ideal Mercier stability for the TJ-II flexible Heliac , 1990 .

[18]  V. Krivenski,et al.  TJ-II Project: A Flexible Heliac Stellarator , 1990 .

[19]  J. C. Whitson,et al.  Steepest‐descent moment method for three‐dimensional magnetohydrodynamic equilibria , 1983 .

[20]  V. Shafranov.  Magnetohydrodynamic theory of plasma equilibrium and stability in stellarators: Survey of results , 1983 .