Plasma and Fusion Research: Rapid Communications

Volume 10, 1201080 (2015)

Tomography for Local Plasma Turbulence Measurements
Akihide FUJISAWA, Yoshihiko NAGASHIMA and Shigeru INAGAKI
Research Institute for Applied Mechanics and Research Center for Plasma Turbulence, Kyushu University,Kasuga Kohen, Kasuga, Fukuoka 816-8580, Japan(Received 8 August 2015 / Accepted 25 August 2015)
A prototype system for plasma turbulence tomography was tested in linear plasma with aiming at futureapplication to toroidal plasma. The line-integrated plasma emission lines of ArI and ArII were measured withsuﬃciently high signal-to-noise ratios, and the signals were reconstructed using the maximum likelihood expectation maximization (MLEM) algorithm and parallel computing. Here, the first results for local plasma emissions
and fluctuation spectra obtained with the prototype system are reported.
c 2015 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: plasma turbulence, turbulence tomography, MLEM, local emission fluctuation
DOI: 10.1585/pfr.10.1201080
Plasma turbulence is ubiquitous in the laboratory, nature and the universe. At present, plasma turbulence has
been recognized as a system of microscale drift waves
with meso and macrostructures, such as zonal flows andstreamers [1–3]. These elemental components are interacting with each other to form the turbulent plasma structureand dynamics. Progress in understanding of plasma turbulence requires new innovative diagnostic methods thatcan measure the plasma turbulence structures in the entire
plasma region from micro- to macroscales.
Computed tomography (CT) satisfies such requirements, and can measure the entire plasma region, in a sufficiently wide range of spatiotemporal scales. A prototype
system has been built and installed on the linear cylindricalplasma, plasma assembly for nonlinear turbulence analysis(PANTA) [4]. Trial experiments have been performed using argon plasma discharges. In this study, we describe thedevelopment of the diagnostic system, and present the first
results successfully obtained with this system, i.e. local
plasma emission distribution and its local emission fluctuations.
The PANTA linear cylindrical device produces helicon plasmas of 5 cm radius with discharge duration of approximately 0.6 s in a 0.09 T straight magnetic field. Figure 1 shows a schematic view of the tomography system, with which we observe the line-integrated plasma
emissions from four azimuthal directions at 45 degree toeach other. Each light guide array contains 33 fiber channels, and consists of collimators, optical filters, and optical feedthrough that transfers the optically filtered lightto the atmosphere side. The collimators (or fiber channels) are aligned by every 5 mm in the range from L = −8to L = 8 cm. The plasma light is passed through optical fibers to the photodiode detectors with gain and frequency bandwidth of 108 V/A and up to 50 kHz, respecauthor’s e-mail: fujisawa@triam.kyushu-u.ac.jp

Fig. 1 (a) Geometry of the tomography system and corresponding definition of coordinates. (b) Schematic view of thelight guide. (c) Sinogram of ArI.

tively. The noise of the detection system depends on theamplifier noise level, which is less than ±50 mV. Plasma
emission was observed with two wavelength bands aroundthe strong lines of ArI (810 nm) and ArII (476.5 nm) lines
for the discharges of low-filling pressure of ∼2 mT. The
ArI and ArII emissions are measured at the plasma crosssection at a distance of 62.5 cm and 37.5 cm, respectively,from the plasma-producing antenna. Line-integrated data
are plotted on θ and L axes and shown as sinograms. Figure 1 (c) shows the sinogram of the temporally averaged
ArII emission in a steady state from 0.2 s to 0.4 s.
To deduce local emissions, the series expansion methods [5, 6] may regulate the reconstructed images using assumed function forms but can not account for spatially discrete changes that are expected in plasma with strong nonlinearity. Therefore, the maximum likelihood expectation
maximization (MLEM) algorithm [7] without any a priori
assumptions, which has been developed for medical applications, was selected for the tomography reconstruction.
The reconstructed tomography images using MLEM are

1201080-1

c 2015 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Rapid Communications

Volume 10, 1201080 (2015)

Fig. 2 (a) Reconstructed local emission of ArI with MLEM, and(b) the corresponding image obtained with the fitting ofFourier−Bessel series expansion. (c) Reconstructed local
emission of ArII with MLEM, and (d) the corresponding
image obtained with the fitting of Fourier−Bessel series
expansion. The dashed circles represent the shape of theantenna that generates the plasma.

shown in Fig. 2. The obtained local values are calculated
on 121 (= 11 × 11) grids on a 16 cm × 16 cm observationsquare with grid width of ∼1.5 cm. Increasing the number of the channels is essential to obtain continuous images with finer spatial resolution. Figure 2 also shows themozaic image obtained with MLEM and the fitting image
obtained with the Fourier−Bessel function series.
The experimental sinograms for the whole discharge
can be converted into the reconstructed images for eachsampling time, in this case 1 µs. The tomography data
for a single shot are huge about 0.32 GB for a shot. To
manage such a huge amount of data, the tomographic reconstruction is performed using parallel processing on 10
Power macintosh computers with totally 12-core CPUs
(2 × 2.4 GHz 6-Core Intel Xeon).
The local waveforms of the ArI and ArII emissions
are obtained as the 121 grid values on the observationsquare using MLEM and parallel computing. Figure 3
shows the local waveforms of the ArI emission at (x, y) =(−1.5, − 1.5) cm and the ArII emission at the center of theobservation square, (x, y) = (0, 0) cm. The corresponding
wavelet and fast Fourier transform (FFT) spectra are also
shown in Fig. 3. The wavelet spectra demonstrate the intermittent nature of the emission fluctuations [8], whereas theFFT spectra calculated as averages from 0.2 s to 0.4 s show
the presence of coherent activities in the fluctuations. Thenoise levels owing to the amplifier noise are also shown in
the plots of FFT spectra. This shows that the local emissions and their power spectra can be successfully obtainedfor good signal-to-noise ratio.
In summary, the proposed turbulence diagnostic

Fig. 3 (a) Local emission waveforms of ArI and ArII. (b) The
corresponding wavelet and FFT spectra of ArI at a position of ∼2 cm away from the center of the observationsquare, and (c) wavelet and FFT spectra of ArII emission
at the center. The scale of the wavelet spectra is linear.

method appears promising. Moreover, PANTA is nonperturbative, and it is hence possible to access the plasmacore region where drastic changes in plasma properties inhibit the insertion of traditional probes. The proposed tomography method can address the missing link between
central core and periphery in dynamic phenomena [9, 10].
Finally, the ultimate goal of this study is to apply an extended system on magnetically confined high-temperatureplasmas. This requires a magnetically confined torus
plasma specially designed for turbulence tomography.

Acknowledgments

This work was partly supported by the Grant-inAids for Scientific Research (Nos. 21224014, 23246162,26420852, and 15H02335).
[1] P.H. Diamond, S.-I. Itoh, K. Itoh and T.S. Hahm, Plasma
Phys. Control. Fusion 47, R35 (2005).
[2] A. Fujisawa, Nucl. Fusion 49, 013001 (2009).
[3] G.R. Tynan, A. Fujisawa and G. McKee, Plasma Phys.
Control. Fusion 51, 113001 (2009).
[4] S. Oldenbürger, S. Inagaki, T. Kobayashi et al., Plasma
Phys. Control. Fusion 54, 055002 (2012).
[5] A.M. Cormack, J. Appl. Phys. 35, 2908 (1964).
[6] Y. Nagayama, J. Appl. Phys. 62, 2702 (1987).
[7] L.A. Shepp and Y. Vardi, IEEE Trans. Med. Imag. 1, 113(1982).
[8] B. Ph. van Milligen, C. Hidalgo and E. Sanchéz, Phys. Rev.
Lett. 74, 395 (1995).
[9] T. Yamada, S.-I. Itoh, T. Maruta et al., Nature Phys. 4, 721(01 Sep. 2008).
[10] H. Arakawa, T. Kobayashi, S. Inagaki et al., Plasma Phys.
Control. Fusion 53, 115009 (2011).

1201080-2