TJ-II:Biasing probe

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Layout of the biasing probe, mounted on the reciprocating probe drive

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.

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

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)


  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. 12st Fusion Energy Conf. (2006) EX-P4/40