Design of a Four-Pin Triple Langmuir Probe for the Pi3 Spherical Tokamak at General Fusion Inc.

A four-pin triple Langmuir probe has been developed for General Fusion’s spherical tokamak PI3. The probe head is boron nitride with four 0.6 mm diameter, 3mm length, 4mm interspace tungsten wires exposed to the plasma. A DC voltage of 50-100V is applied between two pins while floating potential is measured by the other two. The triple probe method allows the electron temperature and density, and plasma potential to be inferred from the probe. The four-pin configuration additionally allows poloidal E field fluctuations to be inferred, thus the fluctuation-driven radial particle flux.

The plasma facing components were selected to avoid failure during the ~30 ms plasma lifetime. Hexagonal boron nitride was chosen due to its machinability and high thermal conductivity favoring thermal shock resistance. Using preliminary Langmuir probe data, a 1-D thermal simulation confirmed the probe will remain below the material’s operating temperature.

The electronic readout and bias circuitry floats at Pi3’s vessel wall potential which can vary over 1kV during a shot. EMI is minimized by continuous electrostatic shielding and passive signal conditioning.

The probe will assist transport studies for Pi3 [1] with results expected next campaign.

[1] C. Ribeiro et al., in this conference.