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 low-temperature 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 $B_{\mathrm{cr}}$. At $B\approx B_{\mathrm{cr}}$ the transition occurs spontaneously (within 1 ms). This $B_{\mathrm{cr}}$ strongly decreases with increasing rotational transform. The improvement in confinement correlates with the suppression of the fluctuations and fluctuation-induced particle flux and with the increase in the radial electric field.

• Received 9 September 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.4190