2D ion velocity distribution measurement in merging spherical tomakas using glass-tube-pair type Doppler probe with deconvolution method

We extend our glass-tube-pair type Doppler probe to 2D velocity distribution measurement using deconvolution method. Our probe uses just two parallel glass tubes, collecting ion emissions from four different directions, and can reconstruct the ion outflow velocity from the spectral Doppler drift with low perturbation. We now incorporate the instrument temperature into the tomographic technique, extending our Doppler probe to 2D velocity distribution measurement. Phantom numerical simulation showed great increase of reconstruction accuracy compared to previous ones. By using this improved method, we successfully measured the velocity distribution of the ion outflow of two merging spherical tokamaks on TS-6. We found that in the downstream of reconnection, the ion velocity distribution is mostly Maxwellian, and its radial component is consistent with the E_p×B_t drift, suggesting that ions are mostly accelerated by the in-plane electric field E_p in the downstream. However, inside the current sheet, we observed great deviation from Maxwell distribution, especially when the current sheet thickness δ is comparable to the ion gyroradius ρ_i, suggesting strong kinetic behavior of the ions inside the current sheet.