Practical solutions for reliable triple probe measurements in magnetized plasmas

Abstract

The triple probe method to obtain local, time-resolved measurements of density, electron temperature and plasma potential is investigated in detail. The difficulties in obtaining reliable measurements with this technique are discussed and overcome. These include phase delay errors, ion sheath expansion and limited bandwidth due to stray capacitance to ground. In particular, a relatively simple electronic circuit is described to strongly reduce stray capacitance. Measurements with the triple probe are presented in a plasma characterized by interchange-driven turbulence in the TORPEX device. The measured time-averaged and time-dependent, conditionally averaged parameters are cross-checked with other Langmuir probe based techniques, and show good agreement. Triple probe measurements show that electron temperature fluctuations are sufficiently large, such that the identification of plasma potential fluctuations with fluctuations of the floating potential is not a good approximation. Over a large radial region, the time-averaged fluctuation-induced particle flux can, however, be deduced from floating potential only. This is because the phase shift between density and electron temperature is close to zero there and temperature fluctuations do not give rise to a net radial particle transport.

Turbulent fluctuations during pellet injection into a dipole confined plasma torus

“…First, these measurements are similar to particle flux measurements made at the edge of other toroidal plasma devices. 35 Second, detailed measurements of interchange-driven turbulence 43 show that the density and electron temperature fluctuations are in phase, and this increases the reliability of using probes to estimate the direction of particle flux. Third, the quasilinear turbulent particle diffusion measured using the interferometer array and the edge floating potential fluctuation intensity 19 are consistent with the inward-directed particle flux measured with the probe array.…”

“…On the other hand, because the phase shift between the density and temperature fluctuations can be small, the direction of the radial particle flux may still be correctly estimated. 43 While recognizing uncertainties in the use of the edge probe array to estimate radial particle flux, these edge measurements are significant for several reasons. First, the spectral characteristics of the probes are similar to the fluctuations measured independently by the microwave interferometer.…”

Blob motion and control in simple magnetized plasmas

“…43 and benchmarked with a triple probe in Ref. 44, can be illustrated as follows. A fixed reference probe in the blob region measures ion saturation current, while one or several movable probes are operated in swept mode.…”

Methodology for turbulence code validation: Quantification of simulation-experiment agreement and application to the TORPEX experiment

“…A discussion on the interpretation of Langmuir probe data together with experimental consideration relative to the TORPEX device can be found in Ref. 16.…”