Triple Langmuir Probes in Lithium-Driven Low Recycling Regimes in the Spherical Tokamak Pi3 at General Fusion Inc

A 3-pins triple Langmuir probe has been used in the spherical tokamak (ST) Pi3 whose characteristics are: R_o=0.67m, a=0.37m, A=1.8, k=1.7, δ~0.4, B_T(R_o)=0.23T, I_p =300kA, n_e(bar)=4.3x10¹⁹m^-3, T_e(0)=170eV and T_i(bar)=230eV, and pulse of < 25ms. The Pi3 plasmas are formed exclusively via coaxial-helicity injection (CHI) and the walls are regularly lithium coated. This device will guide the ST Pi4 (under construction) and the Fusion Demonstration Plant (FDP) to be constructed at the Culham Laboratory site, UK.

This probe body is made of alumina with 3 tungsten wires of 0.9mm diameter (3mm length) with an interspace of 4mm and a bias of 75-100V between pins. The sample rate was 1ms. The probe was moved about 4cm. (cp. with a=37cm) into the plasma scrap-off-layer (SOL). The SOL thickness is the result of a programable minor detachment of the plasma outboard last-close-surface (LCS) to respect the quasi-spherical inner radius R=1.00m thick (50mm Al) vacuum wall, acting as flux-conserver.

With CHI, a transitory high load power at the probe head was observed in the early times of the discharge. This prevents the probe from being inserted beyond 2cm inside of the LCS accordingly to the equilibrium reconstruction.

At 2.5cm inside of SOL, the probe measured a maximum value of T_e=9eV at the formation which reduces to 2eV just after 10ms. The density remains almost constant over such a time interval, that is n_e=0.4x10¹⁹m^-3. This value of density is overestimated because of the effect of the sheath collection area.

In such a position, this probe also identifies the onset of the internal reconnection events (IREs) prior to the appearance of the same signature from the magnetic poloidal pick-up (Mirnov) coils.

A new probe with an h-BN head has been created with the same 4mm pin-interspace but with 4-pins and 0.6mm diameter. Now, the anomalous particle radial particle losses are expected also to be inferred. The results of both probes will be compared.