Improved Particle Confinement Mode in the H-1 Heliac Plasma

Abstract

The first experimental observation of the sudden transition to the improved particle confinement mode in the H-1 heliac is reported and shows a clear dependence on magnetic configuration. In a lowtemperature plasma a transition to improved confinement followed by a twofold increase in the electron density is observed when the magnetic field exceeds a critical value B cr. At B ഠ B cr the transition occurs spontaneously (within 1 ms). This B cr strongly decreases with increasing rotational transform. The improvement in confinement correlates with the suppression of the fluctuations and fluctuationinduced particle flux and with the increase in the radial electric field. [S0031-9007(96)01636-5]

High-speed 2-D image measurement for plasma-wall interaction studies

“…Edge plasma oscillations have already been reported in many machines [10][11][12], however this oscillation is not always associated with H-mode in Heliotron J plasma.…”

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

“…Fluctuations of the plasma potential are suppressed as well. This leads to about a 100 times reduction in the outward-directed fluctuation-driven radial particle flux and confinement improves [19]. An example of a transition to the fluctuating high mode is illustrated in figure 9 for a power step discharge very similar to that just described.…”

“…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.…”

Effects of plasma parameters on viscosity

“…These experiments indicated that the viscosity in tokamaks is nonlinear. Spontaneous L-H transitions have also been reported in the Wendelstein VII-AS [8], compact helical system (CHS) [9], and H-1 [10] stellarators. The experimental results from the interchangeable module stellarator (IMS), presented previously [11], indicate that viscosity in a stellarator is nonlinear too.…”