Measurements with an emissive probe in the CASTOR tokamak

Abstract

An emissive probe has been used in the edge region of the CASTOR tokamak in order to test the possibility of direct measurements of the plasma potential. The difference between the floating potential of a cold probe and that of an emissive probe has been found to be approximately 1.3 times the electron temperature, which is less than predicted by the probe theory. Several possible reasons to explain this discrepancy are offered, such as secondary electron emission, uncertainties in the ion temperature, different collecting areas for electrons and ions, etc. The possible impact of a space charge formed by the emitted electrons is also discussed.

Characterization of the mutual influence of Ion Cyclotron and Lower Hybrid Range of frequencies systems on EAST

“…In order to investigate those interactions based on ScrapeOff Layer (SOL) plasma parameters, a new reciprocating probe was designed allying a three tips Langmuir probe with an emissive wire. The emissive filament provides a precise measure of plasma potential [2], which can be used to calibrate Langmuir probe's results. This paper reports on experimental results obtained on EAST, where there are two ICRF antennas and two LH launchers.…”

A synchronized emissive probe for time-resolved plasma potential measurements of pulsed discharges

“…[1][2][3] First described by Langmuir in 1923, electron emitting probes consist of a thin metal wire filament, typically made from tungsten, that is heated to high temperature with a current in order to achieve thermionic emission. 4 When immersed in plasma, a sheath forms around the surface of the filament.…”

“…The emission currents needs to be added to the ion saturation current because a current of (negative) electrons emitted from the probe is electrically equivalent to a current of (positive) ions collected by the probe. Equation (2) shows that the floating potential V f of the probe approaches the plasma potential as the emitted current from the probe increases. V f attains the plasma potential  for I em =I es -I is .…”

“…The accuracy of the measurement is typically about 1 V for well-controlled operating conditions. 10 The saturated probe method has been used successfully to make measurements in non-Maxwellian, magnetized plasmas 2,7,11 even as this method is susceptible to perturbations by magnetic fields, which must be taken into account. In the following section we will apply the saturated probe method and show the feasibility of pulsed heating to minimize plasma disturbance and enhance accuracy of measurements.…”

Results of direct measurements of the plasma potential using a laser-heated emissive probe

“…Obviously, for this formula T e has to be determined at first, which is not easy with sufficient reliability, especially in the edge region of a toroidal plasma, where there can be strong gradients and fluctuations of the temperature. The factor α = ln(I es /I is ) is in general around 2.04 for hydrogen in a magnetised plasma [3], but there can be deviations depending on the magnetic field strength B and on the direction of the probe with respect to the vector B.…”