TJ-II:Biasing probe

At TJ-II, a 2-D carbon composite mushroom shaped electrode (12 mm high with a diameter of 25 mm) has been developed and installed on a fast reciprocating probe drive. Typically, the electrode is inserted to a position 2 cm inside the last closed flux surface (LCFS) and biased positively (200-300 V) with respect to one of the two TJ-II limiters located in the Scrape-Off Layer region (about 0.5 cm beyond the LCFS). Measured electrode currents are in the range of 30-50 A.

Conceptual diagram of the biasing probe, mounted on the reciprocating probe drive

The impact of biasing on confinement properties has been studied to some depth. ^[1] ^[2] ^[3]

In 2009, the power source (originally DC) has been upgraded to allow modulation with a frequency of up to a few kHz.

Location: sector A8 (φ = 174.4 °). Signal names in the TJ-II database: 'PolI', 'PolV'.

Photo of the reciprocating drive of the biasing probe (top right)

References

↑ C. Silva et al, Transport and fluctuations during electrode biasing on TJ-II, Czech. J. Phys. 55 (2005) 1589
↑ M.A. Pedrosa et al, Transport and fluctuations during electrode biasing experiments on the TJ-II stellarator, Proc. 12^st Fusion Energy Conf. (2006) EX-P4/40
↑ Melnikov, A. V., Hidalgo, C., et al. (2004). Plasma Potential Measurements by the Heavy Ion Beam Probe Diagnostic in Fusion Plasmas: Biasing Experiments in the TJ-II Stellarator and T-10 Tokamak. Fusion Science and Technology, 46(2), 299–311

[1] C. Silva et al, Transport and fluctuations during electrode biasing on TJ-II, Czech. J. Phys. 55 (2005) 1589

[2] M.A. Pedrosa et al, Transport and fluctuations during electrode biasing experiments on the TJ-II stellarator, Proc. 12^st Fusion Energy Conf. (2006) EX-P4/40

[3] Melnikov, A. V., Hidalgo, C., et al. (2004). Plasma Potential Measurements by the Heavy Ion Beam Probe Diagnostic in Fusion Plasmas: Biasing Experiments in the TJ-II Stellarator and T-10 Tokamak. Fusion Science and Technology, 46(2), 299–311

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