Angular dependence measurements of Magnum-PSI plasmas using MAST-U angled-tip Langmuir probes

Highlights: • Probes measure higher T _e and lower n _e compared to Thomson scattering. • Measurements show suppression of sheath expansion down to small incidence angles ($\theta$). • Some angle-dependence in n _e measurements for small $\theta$. • Potential upper limit on reliability of measurements at $\theta$ = 8°. Measurements made using flush-mounted Langmuir probes in tokamaks are difficult to interpret when operating at grazing angles of magnetic field incidence due to the effects of sheath expansion on the probe collection area. The Super-X divertor on the upgraded Mega Amp Spherical Tokamak (MAST-U) can have very shallow angles of magnetic field incidence to plasma facing components (1–10$°$), making the use of conventional flush-mounted probes problematic. A novel probe tip geometry, based on the angled-tip design used successfully on JET and DIII-D, has therefore been used in MAST-U to mitigate sheath expansion effects by increasing the projected probe extent. To verify whether the new design of probe tip allows temperature ($T_e$) and density ($n_e$) measurements to be performed accurately at low angles of incidence, a 4-probe array based on this design was used on Magnum-PSI. Parameter scans were made on a range of hydrogen plasmas in conditions comparable to those expected in MAST-U. Plasma parameters were extracted from the measured IV characteristics – using a novel fitting approach which minimised a goodness-of-fit parameter ($T_e\delta T_e$) – and compared to Magnum-PSI Thomson scattering measurements. The measured plasma parameters show that the MAST-U angled-tip design seems to successfully mitigate the effects of sheath expansion at low angles of magnetic field incidence. The standard MAST-U Langmuir probe also shows an apparent upper operational limit of $\theta =8°$. This covers the vast majority of expected plasma configurations in MAST-U when running both conventional and Super-X configurations.