Advances in neutral-beam-based diagnostics on the Madison Symmetric Torus reversed-field pinch (invited)

Innovative charge-exchange recombination spectroscopy (CHERS), motional Stark effect (MSE), and Rutherford scattering diagnostics are now in operation on the Madison Symmetric Torus (MST) reversed-field pinch (RFP). The CHERS diagnostic measures impurity ion flow and temperature, localized to 2 cm with high time resolution ( ∼ 100 kHz ) . A spectral MSE diagnostic has been in use for five years, measuring ∣ B ∣ down to 0.2 T with high precision ( ∼ 2 % ) and good time resolution ( 10 kHz ) . The Rutherford scattering diagnostic has demonstrated the robustness of this technique for reliable measurement of majority (D) ion temperature, also with high time resolution. MST is a large RFP ( R = 1.5 m , a = 0.52 m ) operated at moderate current ( I p ⩽ 600 kA ) , with n e typically ( 1 – 2 ) × 10 19 m − 3 and T e , T i ⩽ 2 keV . Two compact and reliable diagnostic neutral beams are installed on MST. These beams are short pulse, intense, monoenergetic, and low divergence. The first, a neutral H beam, is used in combination with ultraviolet and visible spectroscopy to make the CHERS and MSE measurements. For CHERS, the C VI line at 343.4 nm is collected by a custom high-throughput double grating spectrometer which simultaneously records both charge-exchange and background emissions. The spectrum is analyzed using a sophisticated model derived from the Atomic Database and Analysis Structure (ADAS) package. The MSE system records the entire H α Stark spectrum; ∣ B ∣ is derived from the measured splitting of the π + and π − manifolds. Measurement of ∣ B ∣ is critical to accurate equilibrium reconstruction in the RFP. The second diagnostic beam is a 20 keV neutral He beam and is used for the Rutherford scattering measurements. A pair of neutral particle analyzers is used to record the energy spectrum of the small-angle Rutherford scattered He atoms.