Optimization of fast-ion diagnostic sets in tokamaks and stellarators using diagnostic weight functions.

The fast-ion phase-space distribution function in the magnetic fusion devices is always underdiagnosed, and every new fast-ion diagnostic should be carefully assessed before installation to minimize redundancies in measurements and maximize the information from the yet undiagnosed part of the fast-ion phase space distribution function. Here, we present a novel method of assessing the added value of a considered fast-ion diagnostic, taking actual geometry and an existing set of fast-ion diagnostics into account. The new method is based on a reformulation of the diagnostic weight functions in constants of motion (COM).

Simulation of a scintillator-based fast ion loss detector for steady-state operation in Wendelstein 7-X (invited).

A quantitative theoretical framework has been created to model neutral beam injection and fast ion losses in the Wendelstein 7-X (W7-X) stellarator, including a novel method to develop synthetic diagnostics for fast ion loss detectors (FILDs) of many types, such as scintillating and Faraday Cup FILDs. This is the first time that this has been done in stellarator geometry with this level of fidelity, providing a way for fast ion losses to be predicted more precisely in future stellarator experiments and in W7-X. Simulations of the signal seen by a Faraday Cup FILD have been completed for multiple W7-X plasmas and show close agreement with the measured signals.

Layer thickness characterization of Faraday cup fast ion loss detectors.

Faraday cup fast ion loss detectors have attractive properties for fusion applications, as they can measure wide ranges of energy, are intrinsically neutron-hardened, and can be packaged in very small form factors. The latter allows them to be installed as arrays, offering opportunities to decouple fast ion loss location and magnitude in fully three-dimensional magnetic fields. In this work, we characterize the layer thicknesses of detector prototypes using spectral reflectance measurements, confocal laser scanning microscopy, as well as raster electron microscopy with a focused ion beam.

Characterization of correlations of fast-ion H-alpha measurement volumes in Wendelstein 7-X by particle tracking.

This paper characterizes the correlation of simultaneous measurements with different fast-ion H-alpha (FIDA) spectroscopy sightlines on Wendelstein 7-X. Using a collisionless guiding-center code, it is shown that, for two investigated volumes in the bulk of the plasma, some regions of phase space are correlated and the magnetic configuration has little influence on this correlation. For the sightlines of the FIDA system, the correlation between these is explained well by the magnetic configuration.

Impact of Magnetic Field Configuration on Heat Transport in Stellarators and Heliotrons.

We assess the magnetic field configuration in modern fusion devices by comparing experiments with the same heating power, between a stellarator and a heliotron. The key role of turbulence is evident in the optimized stellarator, while neoclassical processes largely determine the transport in the heliotron device. Gyrokinetic simulations elucidate the underlying mechanisms promoting stronger ion scale turbulence in the stellarator.