Tomographic reconstructions of the fast-ion phase space using imaging neutral particle analyser measurements

Abstract

In this paper we demonstrate how the inversion, in energy and major radius (E, R) coordinates, of imaging
neutral particle analyser (INPA) measurements can be used to obtain the fast-ion distribution. The INPA is
most sensitive to passing ions with energies in the range (20-150) keV and pitches near 0.5 in the core and 0.7
near the plasma edge. Inversion of synthetic signals, via 0th -order Tikhonov and Elastic Net regularization,
were performed to demonstrate the capability of recovering the ground truth fast-ion 2D phase-space distri-
bution resolved in major radius and energy, even in the presence of moderate noise levels (10%). Finally,
we apply our method to measure the 2D phase-space distribution in an MHD quiescent plasma at ASDEX
Upgrade and find good agreement with the neoclassical fast-ion distribution predicted by TRANSP.

Measurement of toroidal Alfvén eigenmode-driven fast-ion flows using an imaging neutral particle analyzer at ASDEX Upgrade

“…Up to t = 4.2 s, during each beam blip, the plasma density peaks up to the same values (near 4 • 10 19 m −3 at the core), while the temperature is gradually changing from blip to blip but less than 10% in the time range. This small increase should not have any impact on the INPA sensitivity [37]. Only a small increment in the INPA signal at the plasma core is expected, as predicted from the increase in the slowing-down time due to the electron temperature raise.…”

“…This pitch profile, calculated for discharge #41090, can be seen in figure 1, together with the fast-ion distribution calculated with the TRANSP code [35]. Full details of the diagnostic design, installation, validation and instrument functions can be found at [29,30,36,37].…”

“…To visualize this particle transport caused by the TAE, two frames measured by the INPA diagnostic are compared: one without significant TAE activity (highlighted in magenta in figure 2(c)) and a second one with a large amplitude of the TAE, in a subsequent blip. The differences in the signal will correspond to the differences in the fast-ion distribution function, as the density profile is similar among them and hence, the INPA sensitivity will be equivalent [37].…”

“…The predicted changes in the INPA signal are depicted in figures 4(h) and (i). This is calculated with the workflow detailed in [29]: TRANSP [35] simulations are used to predict the neoclassical fast-ion distribution function, FIDASIM [45,46] simulations to model the CX influx at the detector entrance, and an upgraded version of FILDSIM [37] to track the markers until the scintillator plate. The comparison between the energy-integrated INPA measurement and the neoclassical simulation is depicted in figure 6.…”

Validation of a synthetic fast ion loss detector model for Wendelstein 7-X

“…These saved markers are divided into bins in 5D space, where the dimensions include the radial and vertical distances from the magnetic axis, R and Z, along with three velocity space coordinates: the pinhole to the detector, and backward in time from the pinhole to the plane. These orbits were simulated and the image was created using FILDSIM [45,46].…”