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A technique used for the external modification of the radial electric field in thermonuclear plasma devices consists on biasing of plasma facing components like limiters, or inserting biased electrodes into the edge plasma. Electrode biasing experiments performed on the tokamak ISTTOK (R = 0.46 m, a = 0.085 m, B = 0.5 T, I = 6-9 kA) shows a strong modification of the electric field during positive bias when a large current is drawn by electrode (~20 A), and almost no effect with negative bias as this current is very low (~1 A). In order to obtain the larger current necessary to modify the radial electric field of the ISTTOK plasma during negative bias a small movable emissive electrode has been developed.
This paper presents the description and operational characteristics of an emissive electrode (EE). The EE consists of a LaB 6 disk with a diameter of 18 mm attached to the tantalum cylinder and heated by a tungsten filament. A boron nitride housing protects the EE from the plasma. The EE is aligned along the magnetic field and oriented in the opposite direction of the plasma current to avoid possible damage caused by runaway electrons. A bellow mechanism allows the radial movement of the electrode in a range of 40 mm. The bias voltage is applied between the electrode and the vacuum vessel. The emissive characteristics of EE have been investigated both in a simple vacuum diode configuration and in the plasma. The difference between the current-voltage characteristics in these two cases is discussed. Emissive currents up to 30 A have been observed during negative bias in plasma accompanied by a strong modification of the electric field. The EE proved to be a valuable tool on the control of the plasma electric field, allowing a detailed investigation of its importance in plasma confinement. |
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