Thresholds and the role of the radial electric field in confinement bifurcations in the H-1 heliac

The threshold conditions for the confinement bifurcations in the H-1 heliac [S. M. Hamberger et al., Fusion Technol. 17, 123 (1990)] are studied experimentally. The thresholds include the magnetic field, the rf power, and the neutral gas filling pressure. It is shown that in any combination of these parameters it is the radial electric field E_r that is driven to a critical value before the bifurcation. A mechanism of the electric field formation is suggested, which is based on the balance of the electron and ion losses. The ion loss, at low magnetic field and high ion temperature observed in H-1, is dominated by the direct orbit loss mechanism. This is shown by modeling of the exact ion orbits and by comparing a qualitative picture of the E_r formation with the experimental data. Relative efficiency of the electron and ion heating in the inner regions of plasma define the conditions for either low-to-high (L-H) or high-to low (H-L) confinement bifurcations in H-1.