Recent progress in fast-ion diagnostics for magnetically confined plasmas

Abstract

On the road to a fusion reactor, a thorough control of the fast-ion distribution plays a crucial role. Fusion-born -particles are, indeed, a necessary ingredient of selfsustained burning plasmas. Recent developments in the diagnostic of fast-ion distributions have significantly improved our predictive capabilities towards future devices. Here, we review key diagnostic techniques for confined and lost fast ions in tokamak and stellarator plasmas. We discuss neutron and gamma-ray spectroscopy, fast-ion D-spectroscopy, collective Thomson scattering, neutral particle analyzers, and fast-ion loss detectors. The review covers physical principles of each diagnostic, sensitivities, basic setups, and operational parameters. The review is largely (but not exclusively) based on the contributions from ASDEX Upgrade and JET. Finally, we discuss integrated data analysis of fast-ion diagnostics by velocity-space tomography which allows measurements of 2D velocity distribution functions of confined fast ions.

Bi-Maxwellian, slowing-down, and ring velocity distributions of fast ions in magnetized plasmas

“…In diagnostic applications we are often interested in velocity distribution functions projected onto one particular direction, often given by the line-of-sight of the diagnostic. A few examples are charge-exchange recombination spectroscopy including fast-ion D α -spectroscopy, Thomson scattering and collective Thomson scattering, neutron emission spectroscopy and γ-ray spectroscopy [17]. The projection of isotropic functions is independent of the direction and can hence be found by integration of f 3D (v) over two coordinates as often shown in textbooks.…”

Collective Thomson scattering diagnostic at Wendelstein 7-X

“…12,13 A CTS diagnostic will also be installed on ITER. [14][15][16] CTS measures directly the bulk plasma (i.e., usually deuterium, helium, or hydrogen) temperature rather than the temperature of an impurity. These two temperatures can sometimes be quite different.…”

“…These two temperatures can sometimes be quite different. 16 Recently, the feasibility of T i measurements by CTS, having the second harmonic electron-cyclotron resonance in a direct view of the receiver, was demonstrated on ASDEX Upgrade. 18,19 CTS is scattering of electromagnetic waves off the collective fluctuations in the plasma, and its principles are sketched in Fig.…”

Forward modeling of collective Thomson scattering for Wendelstein 7-X plasmas: Electrostatic approximation

“…This paper concerns the development of a diagnostic system for W7-X based on the collective Thomson scattering (CTS) of a probing beam of mm-waves. This technique in principle gives access to a variety of fundamental core plasma properties, including the ion temperature, [4][5][6] the radial electric field, the fast ion distribution function, [7][8][9][10] and the composition. [11][12][13][14] The CTS diagnostic for fusion plasmas has originally been proposed in Ref.…”