New method for studying the vacuum magnetic configuration in a stellarator

A steady-state source of electrons in a stellarator magnetic field reacts to a change in the electron confinement as the source moves away from the boundary of the region with closed magnetic surfaces toward the magnetic axis. A method for stydying the vacuum magnetic configuration in stellarators can thus be based on simple measurements of the current drawn from a source of electrons moving in the stellarator by the vacuum chamber or by an additional transparent conducting grid spanning the magnetic-confinement region. The characteristics of a ''stellarator diode'' and of a ''stellarator diode with an auxiliary grid anode'' in the case of a moving point cathode have been studied. A procedure for studying the magnetic surfaces in some fusion devices, the Uragan-2 stellarator and the Uragan-3 torsatron, has also been studied. The method proposed in this paper for studying the magnetic surfaces in a stellarator does not require searching for the wake of an electron bunch, as is required in, for example, techniques involving a pulsed electron beam. The measurement time is accordingly shorter by more than an order of magnitude. This method has been used at the Uragan-3 torsatron to optimize the magnetic configuration by varying the transverse magnetic field of the torsatron