7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Phase-space tomography for charge exchange recombination spectroscopy
1Tatsuya Kobayashi1
National Institute for Fusion Science, 2 The Graduate University for Advanced Studies,
SOKENDAIe-mail (speaker): kobayashi.tatsuya@nifs.ac.jp

In order to resolve the anomalous transport problem inmagnetically confined fusion plasmas, nonlinear
wave-particle interaction in turbulent field has beenstudied [1,2]. In high-temperature and low-collisionfusion plasmas, particles are trapped in a phase of a waveby which the wave potential is nonlinearly evolved andstructures form in the phase-space. The phase-spacestructure can couple with the real space plasma profile,by which a free energy is released and the phase-spacestructure grows [1,2]. Stochastic overlapping andcascade of the phase-space structures are also possible,
resulting in emergence of the so-called phase-space
turbulence. Phase-space turbulence is considered to playa role for driving anomalous transport in hightemperature plasmas, involving enigmatic transportfeatures, e.g., nondiffusivity, nonlocality, subcriticality,and others [2]. According to those theoretical predictions,survey for possible phase-space structures andexamination of their contribution on plasma transport areexperimentally started [3].
One of the issues that make phase-space structure
measurement challenging is the trade-off relationshipamong time, real-space, and velocity-space resolutions
and signal intensity. As the total signal intensity that isdetermined by the diagnostic system and plasma
condition is constant, improving resolution results in adecrease of signal intensity at a single detector pixel [3].
By performing a space-integration (by compromising thespatial resolution), ultrafast measurement of the plasma
ion velocity distribution function was successfully
performed recently [4].
Recently, a new signal processing algorithm is proposed
to overcome the trade-off relationship among resolutions
and signal intensity, that is, the phase-space tomography.
In this algorithm, a set of three integrated signals withthe same viewing sight is used to recover thethree-dimensional resolution in the phase-space, i.e., thespace spanned by time, real-space, and velocity-spacecoordinates. Integrations are performed in each of
dimensions, i.e., time, real-space, and velocity-space.
Schematic view of this algorithm is shown in Fig. 1. As
the tomography technique, the maximum likelihood
expectation maximization (MLEM) method is used.
This concept is applied to a test data created according
to the Large Helical Device (LHD) observation [4]. The
target physics is the energetic-particle driven
magnetohydrodynamic (MHD) burst events. After theMHD burst, the Landau damping is observed, which
induces the non-Maxwellian distortion of the velocity
distribution function. In [4], the spatially integrated

Fig. 1. Schematic of phase-space tomography.
measurement with a high time-resolution was performed.
The main diagnostic system used for the velocity
distribution function measurement is the charge
exchange recombination spectroscopy (CXS). By
combining the ultrafast CXS [4] with the conventional
CXS and the ultrafast line-emission measurement, thedataset for the phase-space tomography is composed:
they correspond to the real-space integrated data, thetime integrated data, and the velocity-space integrated
data, respectively. Diagnostic simulations for thosesystems are performed and three integration data aremade from the test data having full three-dimensional
resolution. By use of the MLEM algorithm, the original
test data is successfully recovered. For the actual use,
robustness against the realistic noise is also examined,
and a reasonable robustness is found.
References[1] P. W. Terry, P. H. Diamond, and T. S. Hahm, Phys.
Fluids B 2, 2048–2063 (1990)[2] Y. Kosuga, S.-I. Itoh, P. H. Diamond, K. Itoh, and M.
Lesur, Nucl. Fusion 57, 072006 (2017)[3] T. Kobayashi, Plasma Fusion Res. 18, 2402059(2023)[4] K. Ida, T. Kobayashi, M. Yoshinuma, et al., Commun.
Phys. 5, 228 (2022)[5] T. Kobayashi, M. Yoshinuma, W. Hu and K. Ida,
Phys. Plasmas 30, 052303 (2023)
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