Speaker
Ms
Lorella Carraro
(Italy)
Description
High current (I_p up to 2 MA) operations in RFX-mod access the Single Helical Axis (SHAx) regime, during which the magnetic dynamics is dominated by the innermost resonant mode (m=1, n=-7): the magnetic chaos level is reduced and internal magnetic field configuration is close to a pure helix.
The best plasma performances at high Ip with pure SHAx states featuring electron transport barriers have been reached with shallow values of the reversal parameter F=B_t(a)/<B_t>(-0.05<F<0).
The SHAx states show back transitions to Multiple Helicity (MH) regime; at I_p>1.5 MA their total persistence is greater than 90% of the plasma current flat-top. During the SHAx state strong electron temperature (Te) gradients can show up, identifying an electron transport barrier. The dynamics of the thermal structure has been characterized with a new high time resolution T_e profile diagnostic, SXR double-filter multichord system. The high T_e gradients last up to 10-15 ms, more than the energy confinement time.
The strongest barriers in the central helical region are achieved at the lowest total amplitudes of the m=1 secondary modes. Improvements of error field control capability has been recently identified in the correction of systematic errors in the edge field measurements and of the fields induced by the presence of a conductive wall with 3D structures.
The T_e profile measured inside the barrier is flat; to describe the transport in this region, electrostatic turbulence and subsequent vortical drift have been taken into account in a simplified model, in alternative, the residual m=2 mode activity has been considered as the source of the magnetic chaos.
In the region of the T_e gradients, the main gas diffusion coefficient is one order of magnitude lower than the MH case and the convection term is negligible, in agreement with the removal of the stochastic transport.
The large plasma volume external to the barrier is crucial to improve the global confinement. The Te gradients in the region 0.7<r/a<0.95 increase at lower amplitudes of the m=0 modes, likely connected to a lower edge turbulence. Lithium wall conditioning experiments are ongoing aiming at producing higher edge temperature and temperature gradients through a reduced radiation and recycling: promising experiments with more peaked density profiles and good density control up to n/n_G = 0.5 have been produced.
Country or International Organization of Primary Author
Italy
Primary author
Ms
Lorella Carraro
(Italy)
Co-author
Dr
Paolo Innocente
(Consorzio RFX, Associazione EURATOM-ENEA sulla fusione)
