Polarimetric spectra analysis for tokamak pitch angle measurements

Abstract

Measurements of the internal magnetic field structures using conventional polarimetric approaches are considered extremely challenging in fusion-reactor environments whereas the information on current density profiles is essential to establish steady-state and advance operation scenarios in such reactor-relevant devices. Therefore, on ITER a hybrid system is proposed for the current density measurements that uses both polarimetry and spectral measurements. The spectrum-based approaches have been tested in the Korea Superconducting Tokamak Advanced Research (KSTAR) during the past two plasma campaigns. As such, KSTAR is a test-bed for the proposed ITER hybrid system. Measurements in the plasma core are based on the motional Stark effect (MSE) spectrum of the neutral beam emission. For the edge profiles, the Zeeman effect (ZE) acting on the lithium emission spectrum of the newly installed (2013) Lithium-beam-diagnostic is exploited. The neutral beam emission spectra, complicated by the multi-ion-source beam injection, are successfully fitted making use of the data provided by the Atomic Data and Analysis Structure (ADAS) database package. This way pitch angle profiles could be retrieved from the beam emission spectra. With the same spectrometer/CCD hardware as on MSE, but with a different wavelength range and different lines of sight, the first ZE spectrum measurements have been made. The Zeeman splitting comparable to and greater than the instrumental broadening has been routinely detected at high toroidal field operations ( ∼ 3 Tesla).

GS-DeepNet: mastering tokamak plasma equilibria with deep neural networks and the Grad–Shafranov equation

“…We examine this quantity with measured local magnetic pitch angles via the motional Stark effect (MSE) diagnostic system 6 . The MSE system in KSTAR 54 measures the pitch angle at 25 different radial positions as , where the coefficients , and are fixed by the geometrical configuration of the system. Note that the MSE system requires a neutral beam injection, and because the pitch angle is a field quantity rather than a species quantity, we do not have to restrict ourselves only to ohmic discharges, which was the case for the pressure comparisons.…”

Effect of coherent edge-localized mode on transition to high-performance hybrid scenarios in KSTAR

“…Using kinetic-EFIT [47], equilibria were reconstructed to fit profiles in the normalized poloidal flux coordinates, ψ N , and to obtain safety factor profiles, q. Kinetic-EFIT employs measured kinetic and magnetic pitch angle profiles as supplementary constraints in the equilibrium reconstruction based on external magnetic measurements. Unfortunately, Motional Stark Effect diagnostics [48] were not available in these experiments for measuring magnetic pitch angle profiles. Instead, the synthetic magnetic pitch angles calculated by the equilibrium solver, CHEASE [49], and the external current drive modules were used as a constraint for the kinetic-EFIT reconstruction.…”

Overview and recent progress of KSTAR diagnostics

“…Several research activities are underway to improve the MSE instrumentation and thereby the measurement reliability both in-house and via international collaborations. These include the real-time MSE measurements associated with the feedback control of the safety factor and current density profiles [24], the spectral MSE analysis technique directly related with the ITER MSE diagnostic system development [25], the in situ polarized background measurement and correction using the polychrometer technique [26], the systematic evaluation of the effect of multiple ion-source neutral beam injections on the MSE measurements [27], and finally the R & D activities to seek for a second MSE view for the measurement of the intrinsic radial electric field in tokamak plasmas.…”