Study of ion viscosity by spontaneous L–H transitions under marginal hot cathode biasing in the Tohoku University Heliac

Abstract

Using the spontaneous transition condition under marginal hot cathode biasing, the ion viscosity at the L–H transition was estimated in various magnetic configurations in the Tohoku University Heliac. The critical viscosity, which is the viscosity at the transition point, was experimentally estimated from the J × B driving force. The critical viscosities in different magnetic configurations were in agreement with the neoclassical predictions within a factor of 2 and were compared with the viscosities obtained in the externally forced biasing experiments. Although the transition points were spread over a wide range, poloidal Mach numbers at the transition point were concentrated near the viscosity maxima predicted by the theory.

Electrode biasing experiment in the Large Helical Device

“…We tried hot cathode biasing on CHS in a low-field and lowtemperature argon plasma (n e ∼ 0.5 × 10 18 m −3 , T e ∼ 8 eV and B 0 ∼ 0.09 T) using the same electrode current scanning method as in TU-Heliac [24]. The target plasma for the biasing experiment was produced by electron cyclotron heating (ECH) (f = 2.45 GHz, P out ∼ 40 kW), as shown in figure 1.…”

“…In neoclassical theories the nonlinearity of ion viscosity plays an important role in the bifurcation phenomena of the L-H transition [1][2][3][4][5][6][7][8], which is observed in tokamaks and stellarators [9][10][11][12][13][14][15]. The effects of the ion viscosity maxima on the transition to an improved confinement mode were experimentally investigated by the externally controlled J ×B driving force for poloidal rotation using hot cathode biasing in Tohoku University Heliac (TU-Heliac) [16][17][18][19][20][21][22][23][24]. Here, J and B are the biasing electrode current and magnetic field.…”

Transition of poloidal viscosity by electrode biasing in the Large Helical Device

“…The nonlinearity of the ion viscosity has the important role in the bifurcation phenomena of the L-H transition in neoclassical theories [1][2][3][4][5][6][7][8], which is observed in tokamaks and stellarators [9][10][11][12][13][14][15]. The effects of the ion viscosity maximum on the transition to an improved confinement mode were experimentally investigated by the externally controlled J × B driving force for a poloidal plasma rotation using the hot cathode biasing in Tohoku University Heliac (TU-Heliac) [16][17][18][19][20][21][22][23][24]. Here, J and B are a biasing electrode current and a magnetic field.…”