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Yoshihiko Nagashima Last modified date:2018.06.15

Graduate School
Other Organization

Home page of Fujisawa-Nagashima Lab in Advanced Fusion Research Center .
Academic Degree
Doctor of Science
Field of Specialization
plasma physics
Outline Activities
Our research group explores experimental study on fluctuations and waves in plasmas. Plasma is the universal state in nature and laboratory, and has complex self-regulated medium. There are various kinds of fluctuations and waves in plasmas, and they play significant roles in structural formation of plasma media. Turbulence plasmas, excited from fluctuations, contribute loss of particle, energy, and momentum of plasmas. In addition, energy partition among waves and between wave and particle via linear and non-linear interactions. Therefore, study on fluctuations and waves in plasmas can develop understanding of structural formation of plasmas in nature, and innovation of fusion plasmas.
Research themes in our laboratory are as following,
1. learning skills of experiment,
2. measuring fluctuations and waves,
3. applying data analysis techniques to observed data,
4. extracting deep physics of fluctuations, waves and turbulence.
In addition, we also explore
5. controlling actively plasmas by radio-frequency heating and current drive, and observing response of RF to fluctuation data.
Our experimental devices are QUEST spherical tokamak for fusion plasma study, and PANTA linear device for basic plasma turbulence study, in Research Institute for Applied Mechanics, Kyushu University.
Our group has research collaborations with the University of Tokyo.
Research Interests
  • Experimental study on plasma wave, fluctuation, and turbulence
    keyword : plasma turbulence, zonal flow, bispectrum, rediofrequency wave heating
Academic Activities
1. Yoshihiko Nagashima, Shigeru Inagaki, Akihide Fujisawa, Hiroyuki Arakawa, Takuma Yamada, Yudai Miwa, Tatsuya Kobayashi, Sanae-I Itoh, Kimitaka Itoh, Observation of Parallel Force Balance for Drift Wave Fluctuation by a Fast Voltage Sweeping Method in a Linear Plasma, Journal of the Physical Society of Japan, 10.7566/JPSJ.84.063501, 84.0, 6.0, 3.0, Article No. 063501, 2015.05, Electron parallel force balance for drift-wave fluctuation is evaluated using fluctuation data obtained by a single probe fast voltage sweeping method in a linear plasma. Waveforms of the floating potential obtained by the method were validated by conventional high-impedance floating potential measurement. It is found that consistency in amplitude and phase relationships in the force balance is high when the electron temperature fluctuation is taken into account..
2. Nagashima, Yoshihiko; Itoh, Sanae-I.; Inagaki, Shigeru; Arakawa, Hiroyuki; Kasuya, Naohiro; Fujisawa, Akihide; Kamataki, Kunihiro; Yamada, Takuma; Shinohara, Shunjiro; Oldenbuerger, Stella; Yagi, Masatoshi; Takase, Yuichi; Diamond, Patrick H.; Itoh, Kimitaka, Non-Gaussian properties of global momentum and particle fluxes in a cylindrical laboratory plasma, PHYSICS OF PLASMAS, 10.1063/1.3601767, 18.0, 7.0, 1.0, 70701.0, 2011.07, Non-Gaussian statistical properties of the azimuthally averaged momentum and particle fluxes driven by turbulence have been simultaneously observed in inhomogeneous magnetized plasmas for the first time. We identified the stretched Gaussian distribution of the both fluxes and the transition from the point-wise distribution to averaged ones was confirmed. The change of the particle flux precedes that of the momentum flux, demonstrating that the momentum flux is induced by the relaxation of density gradient. (C) 2011 American Institute of Physics. [doi:10.1063/1.3601767].
3. Yoshihiko NAGASHIMA, Akira EJIRI, Yuichi TAKASE, Masateru SONEHARA, Hidetoshi KAKUDA, Takuya OOSAKO, Junichi HIRATSUKA, Osamu WATANABE, Takashi YAMAGUCHI, Hiroaki KOBAYASHI, Takuma WAKATSUKI, Takuya SAKAMOTO, Kentaro HANASHIMA, Takanori AMBO, Ryota SHINO and Shigeru INAGAKI, Evaluation of Edge Electron Temperature Fluctuation by the Use of Fast Voltage Scanning Method on TST-2, Plasma and Fusion Research, 6.0, RA, Article No. 2402036, 2011.07, Edge electron temperature fluctuation is evaluated by the use of fast voltage sweeping technique on TST-2. The validity of obtained current-voltage characteristic curve was checked by comparing the time evolutions of floating potential between that obtained from the fast voltage sweeping technique and that measured with floating probe method. Good agreement between them was confirmed. We also found that fitting errors in the evaluation of the electron temperature itself are less than 10% of fluctuation levels of the electron temperature. Therefore the accuracy of the technique is applicable to study of plasma fluctuations..
4. Nagashima, Yoshihiko; Inagaki, Shigeru; Kamakaki, Kunihiro; Arakawa, Hiroyuki; Yamada, Takuma; Shinohara, Shunjiro; Kawai, Yoshinobu; Yagi, Masatoshi; Fujisawa, Akihide; Itoh, Sanae-I; Itoh, Kimitaka; Takase, Yuichi, Development of radially movable multichannel Reynolds stress probe system for a cylindrical laboratory plasma, REVIEW OF SCIENTIFIC INSTRUMENTS, 10.1063/1.3557384, 82.0, 3.0, 1.0, Article No. 033503, 2011.03, A new radially movable multichannel azimuthal probe system has been developed for measuring azimuthal and radial profiles of electrostatic Reynolds stress (RS) per mass density of microscale fluctuations for a cylindrical laboratory plasma. The system is composed of 16 probe units arranged azimuthally. Each probe unit has six electrodes to simultaneously measure azimuthal and radial electric fields for obtaining RS. The advantage of the system is that each probe unit is radially movable to measure azimuthal RS profiles at arbitrary radial locations as well as two-dimensional structures of fluctuations. The first result from temporal observation of fluctuation azimuthal profile presents that a low-frequency fluctuation (1-2 kHz) synchronizes oscillating Reynolds stress. In addition, radial scanning of the probe system simultaneously demonstrates two-dimensional patterns of mode structure and nonlinear forces with frequency f = 1.5 kHz and azimuthal mode number m = 1. (C) 2011 American Institute of Physics. [doi:10.1063/1.3557384].
5. Yoshihiko NAGASHIMA, Jun'ichi OZAKI, Masateru SONEHARA, Yuichi TAKASE, Akira EJIRI, Kotaro YAMADA, Hidetoshi KAKUDA, Shigeru INAGAKI, Takuya OOSAKO, Byung Il AN, Hiroyuki HAYASHI, Kentaro HANASHIMA, Junichi HIRATSUKA, Hiroaki KOBAYASHI, Hiroki KURASHINA, Hazuki MATSUZAWA, Takuya SAKAMOTO, Takashi YAMAGUCHI, Osamu WATANABE and Takuma WAKATSUKI, Fluctuation measurement in the edge plasma on TST-2, Plasma and Fusion Research, 5.0, RA, Article No. S2049, 2010.12, A new technique is used to estimate the amplitude of temperature fluctuation in the edge plasma on TST-2. Langmuir probe current-voltage characteristic curves are conditionally reconstituted in terms of the intensity of the floating potential. High/low electron temperatures are obtained in low/high floating potential phases. As a result, normalized temperature fluctuation levels are found to be about 19 % or larger..
6. Yoshihiko NAGASHIMA, Kenichi NAGAOKA, Kimitaka ITOH, Akihide FUJISAWA, Mitsutaka ISOBE, Tsuyoshi AKIYAMA, Chihiro SUZUKI, Shin NISHIMURA, Yasuo YOSHIMURA, Keisuke MATSUOKA, Shoichi OKAMURA, Yuichi TAKASE, Akira EJIRI, Sanae -I. ITOH, Masatoshi YAGI, and CHS Group, Observation of edge Reynolds stress increase preceding an L-H transition in Compact Helical System, Plasma and Fusion Research, 5.0, RC, Article No. 022, 2010.10.
7. Yoshihiko Nagashima, Takuya Oosako, Yuichi Takase, Akira Ejiri, Osamu Watanabe, Hiroaki Kobayashi, Yuuki Adachi, Hiroshi Tojo, Takashi Yamaguchi, Hiroki Kurashina, Kotaro Yamada, Byung Il An, Hiroshi Kasahara, Fujio Shimpo, Ryuhei Kumazawa, Hiroyuki Hayashi, Haduki Matsuzawa, Junichi Hiratsuka, Kentaro Hanashima, Hidetoshi Kakuda, Takuya Sakamoto, and Takuma Wakatsuki, Observation of beat oscillation generation by coupled waves associated with parametric decay during radio frequency wave heating of a spherical tokamak plasma, Physical Review Letters, 104.0, 24.0, Article No. 245002, 2010.06.
8. Nagashima Yoshihiko, A. Ejiri, Fujisawa Akihide, Y. Takase, Calculation of Beam Trajectories of a Heavy Ion Beam Probe Diagnostic in the TST-2 Spherical Tokamak, IEEJ Transactions on Fundamentals and Materials, 129.0, 9.0, 2009.09.
9. Yoshihiko NAGASHIMA, Sanae-I. ITOH, Kimitaka ITOH, Akihide FUJISAWA, Shigeru INAGAKI, Yoshinobu KAWAI, Shunjiro SHINOHARA, Masayuki FUKAO, Takuma YAMADA, Kenichiro TERASAKA, Takashi MARUTA, Kunihiro KAMATAK, Hiroyuki ARAKAWA, Masatoshi YAGI, Naohiro KASUYA, George R. TYNAN, Patrick H. DIAMOND, and Yuichi Takase, Reynolds Stress Measurements for Investigation of Nonlinear Processes of Turbulence in the Large Mirror Device and in the Large Mirror Device-Upgrade, JOURNAL OF NUCLEAR MATERIALS, 8.0, 2009.08.
10. Nagashima, Y.; Itoh, K.; Fujisawa, A.; Shinohara, K.; Itoh, S-I; Ido, T.; Yagi, M.; Hoshino, K.; Ejiri, A.; Takase, Y.; Uehara, K.; Miura, Y., Boundary of the geodesic acoustic eigenmode in the vicinity of the magnetic separatrix, PLASMA PHYSICS AND CONTROLLED FUSION, 10.1088/0741-3335/51/6/065019, 51.0, 6.0, 10.0, Article No. 065019, 2009.06, We present a detailed investigation of the geodesic acoustic eigenmode(GAEM) profile in the vicinity of the last closed flux surface (LCFS) in the JFT-2M tokamak. Envelope and spectral peak frequency of the GAEM vanish near the LCFS obtained by an equilibrium reconstitution code. Spatial changes in mean plasma gradients (the electron temperature, electron density and floating potential) are correlated with the boundary. This does not contradict that the GAEM can have a node at the LCFS. Discussions of an inverse problem and the application of this GAEM measurement to an experimental test of the LCFS location calculated by the equilibrium reconstitution code are presented..
11. Nagashima, Yoshihiko; Itoh, Sanae-I.; Shinohara, Shunjiro; Fukao, Masayuki; Fujisawa, Akihide; Terasaka, Kenichiro; Kawai, Yoshinobu; Tynan, George R.; Diamond, Patrick H.; Yagi, Masatoshi; Inagaki, Shigeru; Yamada, Takuma; Itoh, Kimitaka, Observation of the parametric-modulational instability between the drift-wave fluctuation and azimuthally symmetric sheared radial electric field oscillation in a cylindrical laboratory plasma, PHYSICS OF PLASMAS, 10.1063/1.3078075, 16.0, 2.0, 1.0, 20706.0, 2009.02, Observation of the parametric-modulational interaction between the drift-wave fluctuation (7-8 kHz) and azimuthally symmetric sheared radial electric field structure (similar to 0.4 kHz) in a cylindrical laboratory plasma is presented. Oscillation of the sheared radial electric field is synchronized at modulations of the radial wave number and Reynolds stress per mass density of the drift-wave spectrum. Bispectral analysis at the location where the sheared radial electric field has finite radial wave numbers shows that nonlinear energy transfers from the drift wave to the sheared radial electric field occur. Nonlocal energy transfers of fluctuations via "channel of the azimuthally symmetric sheared radial electric field" in spectral space as well as real space are discovered..
12. Nagashima, Yoshihiko; Itoh, Sanae-I; Shinohara, Shunjiro; Fukao, Masayuki; Fujisawa, Akihide; Terasaka, Kenichiro; Kawai, Yoshinobu; Kasuya, Naohiro; Tynan, George R.; Diamond, Patrick H.; Yagi, Masatoshi; Inagaki, Shigeru; Yamada, Takuma; Itoh, Kimitaka, Coexistence of Zonal Flows and Drift-Waves in a Cylindrical Magnetized Plasma, JOURNAL OF NUCLEAR MATERIALS, 10.1143/JPSJ.77.114501, 77.0, 11.0, Article No. 114501, 2008.11, Spatiotemporal structures of fluctuations with frequencies lower than the ion cyclotron frequency in a cylindrical magnetized plasma are investigated. Drift-wave and low-frequency zonal flow coexist. Electrostatic potentials of the zonal flow and the drift-wave are distributed widely in radius. The radial wave number profile of the zonal flow has a shear structure at the radial location where the drift-wave has a maximal normalized fluctuation amplitude. On the other hand, the radial wave number profile of the drift-wave shows vortex tilting, resulting in the generation of stationary turbulence Reynolds stress gradient per mass density. The envelope and bispectral analyses indicate significant nonlinear interactions between the zonal flow and the drift-wave..
13. Yoshihiko NAGASHIMA, Sanae -I. ITOH, Shunjiro SHINOHARA, Masayuki FUKAO, Akihide FUJISAWA, Kenichiro TERASAKA, Yoshinobu KAWAI, Naohiro KASUYA, Masatoshi YAGI, Shigeru INAGAKI, Takuma YAMADA and Kimitaka ITOH, Nonlinear energy transfer of drift-wave fluctuation in a cylindrical laboratory plasma, Plasma and Fusion Research, 3.0, RC, 2.0, Article No. 056, 2008.10, Nonlinear energy transfer of the drift-wave spectrum is investigated experimentally in a cylindrical laboratory plasma. Using the Hasegawa-Mima equation, the nonlinear transfer function of electrostatic potential energy in frequency space is estimated by bispectral analysis. The transfer function derived from the drift-wave spectrum indicates a forward cascading of energy transfer from fundamental to higher harmonic modes..
14. Nagashima, Y.; Itoh, K.; Itoh, S-I; Fujisawa, A.; Yagi, M.; Hoshino, K.; Shinohara, K.; Ejiri, A.; Takase, Y.; Ido, T.; Uehara, K.; Miura, Y., In search of zonal flows by using direct density fluctuation measurements, PLASMA PHYSICS AND CONTROLLED FUSION, 10.1088/0741-3335/49/10/002, 49.0, 10.0, 15.0, 2007.10, A method for zonal flow study by using direct density fluctuation measurements is proposed. When ambient drift-wave turbulence is modulated by zonal flows (i.e. in the drift-wave-zonal flow systems), an envelope of the ambient density fluctuations has spectral peaks around zonal flow frequencies. A spectral peak at the geodesic acoustic mode (GAM) frequency is observed in the envelope of the ambient density fluctuations measured in edge plasma of the JFT-2M tokamak. The significant cross-bicoherence is also found between the ambient density fluctuations and its envelope in the GAM frequency. This result demonstrates that we can measure the GAM only by using density fluctuation data. This method provides a possibility of zonal flow research in burning core plasma by density fluctuation diagnostics such as microwave reflectometry..
15. Yoshihiko NAGASHIMA, Katsumichi HOSHINO, Kenichi NAGAOKA, Kouji SHINOHARA, Akihide FUJISAWA, Kazuya UEHARA, Yoshinori KUSAMA, Katsumi IDA, Yasuo YOSHIMURA, Shoichi OKAMURA, Keisuke MATSUOKA, Akira EJIRI, Yuichi TAKASE, Kimitaka ITOH, Masatoshi YAGI, Sanae-I. ITOH, JFT-2M group and CHS group, Experimental study of nonlinear processes in edge turbulence of toroidal plasmas, Plasma and Fusion Research, 1.0, RA, 15.0, Article No. 041, 2006.09, This paper presents experimental studies of nonlinear processes between meso-scale structures and turbulent fluctuations, by use of bispectral analysis, in the edge region of toroidal plasmas. In ohmically heated plasmas of the JFT-2M tokamak, the drift wave-zonal flow system is investigated by observation of the potential fluctuations. The total bicoherence is composed of two components, i.e., the peak at the geodesic acoustic mode (GAM) frequency and broadband distribution. The biphase is also obtained, which indicates the phase angle of coherent interaction between GAM oscillations and broadband fluctuations. Bispectral functions (e.g., magnitude and phase) give an order-of-estimate agreement for the drift wave-zonal flow systems. Bicoherence analysis is also applied to the edge plasmas of the high-confinement mode (H-mode) of the Compact Helical System (CHS). In the case of CHS plasmas, nonlinear couplings in a system composed of coherent global magneto-hydrodynamic (MHD) oscillations and broadband fluctuations (up to hundreds kHz range) have been observed. Bispectral analysis enhances our understanding of the self-regulation systems of various toroidal plasmas in which strong turbulence and meso-scale structures coexist..
16. Nagashima, Y; Itoh, K; Itoh, SI; Hoshino, K; Fujisawa, A; Ejiri, A; Takase, Y; Yagi, M; Shinohara, K; Uehara, K; Kusama, Y, Observation of coherent bicoherence and biphase in potential fluctuations around geodesic acoustic mode frequency on JFT-2M, PLASMA PHYSICS AND CONTROLLED FUSION, 10.1088/0741-3335/48/5A/S38, 48.0, 0.0, #, 0.0, 2006.05, Two types of small-poloidal wavelength low frequency floating potential fluctuations (similar to 1 kHz, and 10-15 kHz) are observed in the edge region of ohmically heated plasmas in the JFT-2M tokamak. The higher frequency fluctuations are inferred as geodesic acoustic modes (GAMs). Significant bicoherences between both modes and the broad-band fluctuations are observed simultaneously; however, frequency ranges are different. A significant bicoherence range is from 70 to 125 kHz at the lower frequency modes (LFM) and is from 20 to 125 kHz at the GAM. Phase angles at the significant bicoherences are coherent around pi. These results indicate that both modes are nonlinearly coupled to the broad-band fluctuations. Two spectral peaks are observed around the GAM frequency and the difference in peak frequencies is around the LFM frequency, suggesting nonlinear couplings between the LFM and the GAM. However, the modulation of the GAM by the LFM is not conclusive. In this paper, the results of the letter (Nagashima et al 2005 Phys. Rev. Lett. 95 095002) are presented in detail..
17. Nagashima, Y; Itoh, SI; Yagi, M; Itoh, K; Fujisawa, A; Hoshino, K; Shinohara, K; Uehara, K; Kusama, Y; Ejiri, A; Takase, Y, Convergence study of bispectral analysis in experiments of high temperature plasmas, REVIEW OF SCIENTIFIC INSTRUMENTS, 10.1063/1.2194016, 77.0, 4.0, 6.0, Article No. 045110, 2006.04, A convergence study of bispectral analysis on toroidal plasma turbulence is presented with a theoretical estimate of bicoherence for the system of the drift wave and zonal flow. Examples are chosen from the analysis of floating potential fluctuation in the edge region of the JFT-2M tokamak. The convergence properties of the bispectral functions are clarified for an oscillatory branch of zonal flow (or geodesic acoustic mode) and drift waves. The total squared bicoherence or the biphase converges inversely proportional to the realization number or to the square root of the realization number, respectively. The numbers of realizations to qualitatively converge and obtain a quantitatively significant value of bicoherence are discussed for the drift wave fluctuations and for the zonal flow. (c) 2006 American Institute of Physics..
18. Nagashima, Y; Itoh, K; Itoh, SI; Fujisawa, A; Hoshino, K; Takase, Y; Yagi, M; Ejiri, A; Ida, K; Shinohara, K; Uehara, K; Kusama, Y, Bispectral analysis applied to coherent floating potential fluctuations obtained in the edge plasmas on JFT-2M, PLASMA PHYSICS AND CONTROLLED FUSION, 10.1088/0741-3335/48/4/S01, 48.0, 4.0, 15.0, 2006.04, This paper presents results of bispectral analysis applied to floating potential fluctuations in the edge region of ohmically heated plasmas in the JAERI Fusion Torus-2 Modified (JFT-2M) tokamak. Inside the outermost surface of plasmas, coherent mode fluctuations, (CMs) in floating potential were observed around the frequency of the geodesic acoustic mode. The squared bicoherence shows significant nonlinear couplings between the CMs and background fluctuations. The biphase at the frequency of the CMs is localized around pi, while that at frequencies of background fluctuations distributes in a wide range. The total bicoherence at the frequency of the CMs is proportional to the squared amplitude of the CMs. These observations are consistent with the theoretical prediction on the drift wave-zonal flow systems. Interpretation of the absolute value of the total bicoherence is also discussed..
19. Y. Nagashima, K. Hoshino, A. Ejiri, K. Shinohara, Y. Takase, K. Tsuzuki, K. Uehara, H. Kawashima, H. Ogawa, T. Ido, Y. Kusama, and Y. Miura, Observation of Nonlinear Coupling between Small-Poloidal Wave-Number Potential Fluctuations and Turbulent Potential Fluctuations in Ohmically Heated Plasmas in the JFT-2M Tokamak, Physical Review Letters, 95.0, Article No. 095002, 2005.08.
20. Y Nagashima, K Shinohara, K Hoshino, A Ejiri, K Tsuzuki, T Ido, K Uehara, H Kawashima, K Kamiya, H Ogawa, T Yamada, S Shiraiwa, S Ohara, Y Takase, N Asakura, N Oyama, T Fujita, S Ide, H Takenaga, Y Kusama, Y Miura and JFT-2M group, Coherent edge fluctuation measurements in H-mode discharges on JFT-2M, PLASMA PHYSICS AND CONTROLLED FUSION, 46.0, 5A, 2004.05.
Membership in Academic Society
  • The Physical Society of Japan
  • The Japan Society of Plasma Science and Nuclear Fusion Research
  • The Institute of Electrical Engineers of Japan
  • American Physical Society
  • Study on direct measurement of mechanism behind zonal flow generation using nonlinear analysis on plasma turbulence
  • Study of causal relationship between formation process of plasma flow and global momentum transport driven by turbulence
Educational Activities
Lecture : Lectures on fundamentals of data analyses and fundamentals of vacuum technology, thermodynamics and statistical physics.
Education in laboratory : Education on experimental research of students, and education on basic academic skills for research activity in seminar.