Ion temperature and plasma flows in improved confinement mode in the H-1 heliac

Abstract

Low and improved confinement modes in the H-1 heliac ͓M. G. Shats et al., Phys. Rev. Lett. 77, 4190 ͑1996͔͒ are studied experimentally in rf-sustained ͑Ͻ100 kW, 7 MHz͒ argon discharges at low magnetic fields ͑Ͻ0.15 T͒. Surprisingly high ion temperature, measured using a retarding field energy analyzer, is found which increases across the transition to improved confinement mode from 40 to 80 eV, while the electron density increases by about 50%. Both toroidal and poloidal plasma flow velocities do not change across the transition. The increase in a radial electric field in high mode is balanced on average by a corresponding increase in the ion pressure gradient.

Effect of the radial electric field on the fluctuation-produced transport in the H-1 heliac

“…Thus, the fluctuation propagation in the H fl -mode is also correlated with V E×B , similar to the L-mode. Note that the plasma fluid velocity in the H-1 is significantly lower than the corresponding V E×B velocity [22]. We may thus conclude that it is modifications to the fluctuation propagation velocity due to the E × B electron drift that affect transport properties of the fluctuations, rather than the sheared mass flow.…”

“…Confinement transitions are observed in the H-1 heliac at low magnetic fields (<0.15 T), moderate heating power (<100 kW of RF at 7 MHz), and relatively high ion temperatures (up to 150 eV) [19,22]. In the discharges in heavy gases (argon, neon, helium), line radiation dominates the energy confinement and limits the electron temperature to less than 20-30 eV in argon and less than 40-60 eV in helium plasma.…”

“…In these conditions probes can be used in the inner plasma. Previous studies in the H-1 [19,22] have shown that: (i) close to the critical plasma conditions the density and ion temperature spontaneously increase across the transition by 50-100% in about 1 ms; (ii) the radial electric field doubles in the outer plasma region (r/a = 0.7-1.0); (iii) transitions are correlated with the reduction in the fluctuation level and the turbulent transport; (iv) transitions to improved confinement are not correlated with any significant changes in the ion poloidal or toroidal rotation velocities, which were found to be about ten times lower than the corresponding E × B drift velocities.…”

“…Visible spectroscopy has also been used in these experiments [22] for the neutral particle characterization. In this paper we describe experimental results on the characterization of the fluctuations in the low-confinement mode (L-mode), and study the correlation of the turbulence modifications with the evolution in the radial electric field.…”

Electrode and Langmuir probe tools used for flow damping studies in the Helically Symmetric Experiment

“…Recent work by Hutchinson has criticized 26 the Hudis and Lidsky model for its one dimensional nature, and provided a two-dimensional model 27 for the interpretation of data. Other expressions for the Mach probe calibration can be found in works by Shats et al 28 or Hsu et al 29 for the unmagnetized case.…”