Direct plasma potential measurements by a novel probe

0103225 - UFP-V 20040094 RIV GB eng C - Konferenční příspěvek (zahraniční konf.)
Adámek, Jiří - Stöckel, Jan - Hron, Martin - Ryszawy, Jiří - Tichý, M. - Schrittwieser, R. - Ionita, C. - Balan, P. - Martines, E. - Van Oost, G.
Direct plasma potential measurements by a novel probe.
[Přímé měření potenciálu plazmatu pomocí nové sondy.]
Proceedings of the 31th EPS Conference on Controlled Fusion and Plasma Physics. London, 2004, s. 514-515.
[EPS Conference on Controlled Fusion and Plasma Physics /31./. London (GB), 28.06.2004-02.07.2004]
Grant CEP: GA ČR GA202/03/0786
Výzkumný záměr: CEZ:AV0Z2043910
Klíčová slova: tokamak, probe
Kód oboru RIV: BL - Fyzika plazmatu a výboje v plynech
http://eps2004.clf.rl.ac.uk/pdf/P5_120.pdf

For the determination of the plasma potential in tokamaks emissive probes and heavy ion beam probes have been used, but these diagnostics are still not used as a routine technique. In practice the floating potential Vfl is usually measured by Langmuir probes. The plasma potential Fpl is then derived from the simple formula of the Langmuir probe theory (without magnetic field): Vfl = Fpl - Te ln(Ri), where Te is the electron temperature in eV. The quantity Ri represents the ratio of the electron and ion saturation currents: Ri = . The theoretical value of ln(Ri) in hydrogen plasma is about 2.5. The basic idea of the direct plasma potential measurements is to adjust Ri equal to one. If this can be achieved, the floating potential of the probe is equal to the plasma potential. This contribution presents the design as well as the first experimental results of a novel probe, by which the value of Ri can be modified. The probe head, shown in the figure, consists of a conically shaped collector, which is shielded by an isolating tube of boron nitride. The collector is moveable inside the tube and is either completely shielded or partially exposed to the plasma. In the ideal case, when the collector is hidden inside the tube, as shown in the figure, only ions with sufficiently large Larmor radii reach the collector surface. In this case the collecting area for electrons is negligible. When the collector is shifted towards to the plasma the electron current increases. Experiments at the CASTOR tokamak (R = 40 cm, a = 8.5 cm, BT = 1.3 T, IP @ 10 kA) have verified that Ri can indeed be adjusted close to one, when the position of the collector is chosen properly. The floating potential of the probe also varies with the position of the collector. Therefore, such a configuration can be used for a direct determination of the plasma potential in a tokamak environment as an alternative for the emissive probe.

V článku je navržena metoda pro přímé měření potenciálu plazmatu pomocí nové sondy. Princip měření je založen na zredukování elektronového nasyceného proudu na sondu, tak aby byl přibližně roven iontovému nasycenému proudu. V takovém případě je podle Langmuirovy teorie plovoucí potenciál sondy roven potenciálu plazmatu. První systematická měření byla provedena na tokamku CASTOR.
Trvalý link: http://hdl.handle.net/11104/0010541