Makoto Sasaki | Last modified date：2019.12.24 |

Graduate School

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##### https://sites.google.com/site/kosugagroup/

Theoretical and experimental research for plasma turbulence is done. The basic physics research in thermonuclear fusion plasmas, the confinement physics (e.g. the self-organization mechanisms in nonlinear and non-equilibrium systems) is the current topic. Research on nonlinear dynamics of waves and fluctuations is also made, which includes the topics on solitons and chaos as well as on turbulence. .

Phone

092-583-7724

Fax

092-583-7723

Academic Degree

PhD (Science)

Field of Specialization

Plasma Physics, Fluid Physics

Outline Activities

Theory and simulation on turbulence in magnetized plasmas, and education

Theoretical study: Importance of zonal flows, which suppress turbulent transport, has been widely recognized. In the presence of magnetic curvature such as toroidal plasmas, oscillatory components (GAMs) exist in combination with the stationary zonal flows (ZFs). We study nonlinear theory of GAMs and ZFs. The theoretical understandings of the nonlinear saturation mechanism of GAMs and of the energy partition between ZFs and GAMs have developed. Recently, We focus on the properties of GAMs driven by energetic particles.

Simulation study: We study nonlinear fundamental processes of drift waves, using a three-dimensional turbulence simulation in linear magnetized plasma devices (such as PANTA), and collaborate with experiments. Recently, we obtain nonlinear competition among zonal flows, streamer and flute structure, which are linearly stable and driven by nonlinear coupling of the drift waves. The formation mechanism of a solitary drift wave has been theoretically understood, whose results are consistent with the simulation and the experiments.

Theoretical study: Importance of zonal flows, which suppress turbulent transport, has been widely recognized. In the presence of magnetic curvature such as toroidal plasmas, oscillatory components (GAMs) exist in combination with the stationary zonal flows (ZFs). We study nonlinear theory of GAMs and ZFs. The theoretical understandings of the nonlinear saturation mechanism of GAMs and of the energy partition between ZFs and GAMs have developed. Recently, We focus on the properties of GAMs driven by energetic particles.

Simulation study: We study nonlinear fundamental processes of drift waves, using a three-dimensional turbulence simulation in linear magnetized plasma devices (such as PANTA), and collaborate with experiments. Recently, we obtain nonlinear competition among zonal flows, streamer and flute structure, which are linearly stable and driven by nonlinear coupling of the drift waves. The formation mechanism of a solitary drift wave has been theoretically understood, whose results are consistent with the simulation and the experiments.

Research

**Research Interests**

- Data analysis of plasma turbulence

keyword : Data driven science, Machine learning

2019.04～2019.06. - Numerical Simulation on turbulence in linear magnetized plasma

keyword : plasma turbulence, nonlinear process, zonal flow, streamer, meso-scale structure

2011.11. - Theoretical study of nonlinear processes in plasma turbulence

keyword : plasma turbulence, zonal flow, geodesic acoustic mode, structural formation, meso-scale structure

2007.04.

**Current and Past Project**

- The purpose of this study is to elucidate the kinetic effects of energetic particles on the turbulence transport and particle heating. Recently, it has been pointed out that oscillatory zonal flows (geodesic acoustic modes, GAMs) driven by energetic particles heat bulk ions via ion Landau damping. In addition to this, nonlinear coupling between Alfven eigenmodes (AEs) driven by energetic particles and stationary zonal flows (ZFs) has been observed. Previous studies on zonal flow-turbulence theory do not include such effects. This study will extend the zonal flow-turbulence theory to include the particle heating and turbulence suppression effects of energetic driven modes self-consistently, and will discuss the controllability of heating and transport.
- The purpose of this study is to evaluate the effect of zonal flow (including low-frequency zonal flows (ZFs), geodesic acoustic modes (GAMs)) on the energy confinement time in turbulent toroidal plasmas. In previous studies, the assumption has been used that the wavelength of zonal flows is much shorter than the scale length of density or temperature profile. Therefore, although the energy density of zonal flows and turbulence has been reported, the energies of them, which are the volume integrals of the energy densities, have not been understood. In this study, first, we elucidate the nonlocal spatial structures of ZFs and GAMs and energy partition between them, considering the nonlinear coupling among turbulence, ZFs and GAMs. Second, the effect of ZFs and GAMs on the energy confinement time is investigated. Finally, we will present the minimum model that can describe the turbulent transport in toroidal plasmas.

**Academic Activities**

**Papers**

1. | M. Sasaki, Y. Kawachi, R. O. Dendy, H. Arakawa, N. Kasuya , F. Kin, K. Yamasaki, S. Inagaki, Using dynamical mode decomposition to extract the limit cycle dynamics of modulated turbulence in a plasma simulation, Plasma Physics and Controlled Fusion, 61, 112001-1-112001-6, 61, 112001 (2019)., 2019.10. |

2. | M. Sasaki, N. Kasuya, Y. Kosuga, T. Kobayashi, T. Yamada, H. Arakawa, S. Inagaki, K. Itoh, Turbulence simulation on zonal flow formations in the presence of parallel flows, Plasma Fusion Research, 14, 1401161-1-1401161-7, 2019.10. |

3. | M. Sasaki, Y. Camenen, A. Escarguel, S. Inagaki, N. Kasuya, K. Itoh, T. Kobayashi, Formation of spiral structures of turbulence driven by a strong rotation in magnetically cylindrical plasmas, Phys. Plasmas, 10.1063/1.5085372, 26, 042305, 2019.03. |

4. | M. Sasaki, K. Itoh, Y. Kosuga, J. Dong, S. Inagaki, T. Kobayashi, J. Cheng, K. Zhao, S.-I. Itoh, Parallel flow driven instability due to toroidal return flow in H-mode plasmas, Nuclear Fusion, https://doi.org/10.1088/1741-4326/ab1292, 59, 066039, 2019.03. |

5. | M. Sasaki, K. Itoh, T. Kobayashi, N. Kasuya, A. Fujisawa, S.-I. Itoh, Propagation direction of geodesic acoustic modes driven by drift wave turbulence, Nuclear Fusion, 10.1088/1741-4326/aad251, 58, 112005, 2018.10, Selection rule of the radial propagation direction of geodesic acoustic modes (GAMs) is investigated. Here, we study the influence of nonlinear coupling with drift wave turbulence on the propagation direction of GAMs. Based on wave-kinetic equation for the turbulence, the phase-space dynamics is numerically solved and the nonlinear saturated states are obtained, where the phase-space consists of the real space and the wavenumber space. A wave pattern of the GAM in a nonlinear saturated state varies to form a standing wave, outward and inward propagating waves, depending on the peak radial wavenumber of the turbulence. The impact of nonlinear coupling with turbulence is discussed by deriving the GAM dispersion relation that includes the effect of the turbulence.. |

6. | M. Sasaki, K. Itoh, T. Ido, A. Shimizu, T. Kobayashi, H. Arakawa, N. Kasuya, A. Fujisawa, S.-I. Itoh, Evaluation of measurement signal of Heavy Ion Beam Probe of energetic-particle driven geodesic acoustic modes, Plasma Fusion Research, https://doi.org/10.1585/pfr.13.3403040, 13, 3403040, 2018.05. |

7. | Makoto Sasaki, Naohiro Kasuya, Tatsuya Kobayashi, Hiroyuki Arakawa, Kimitaka Itoh, Kouhei Fukunaga, Takuma Yamada, Masatoshi Yagi, Sanae-I. Itoh, Formation mechanism of steep wave front in magnetized plasmas, Physics of Plasmas, 10.1063/1.4916490, 22, 032315-1-032315-10, 22, 032315-1, 032315-10 (2015), 2015.03. |

8. | Makoto Sasaki, Naohiro Kasuya, Kimitaka Itoh, Masatoshi Yagi, Sanae-I. Itoh, Nonlinear competition of turbulent structures and improved confinement in magnetized cylindrical plasmas, Nuclear Fusion, 54, 114009, 2014.11. |

9. | Makoto Sasaki, Naohiro Kasuya, Masatoshi Yagi, Kimitaka Itoh, Yoshihiko Nagashima, Inagaki Shigeru, SANAE INOUE（論文のみ） ITOH, Statistical analyses of turbulent particle and momentum fluxes in a cylindrical magnetized plasmas, Plasma and Fusion Research, 8, 2401113 (2013) , 2013.09, [URL], A nonlinear simulation of resistive drift wave turbulence in a cylindrical plasma is carried out. Long time evolution of turbulence with formation of a zonal flow is obtained for more than 1000 times of the typical drift wave period, which is sufficient for statistical analyses. Dynamical particle and momentum balance for the formation of the mean reveals that the radial turbulent fluxes are dominant contributors for the evolution of fluctuations. The particle flux is found to precede the momentum flux for 0.4 times of the typical drift wave period with large temporal variance. The analyses of the time series data of 3-D fields give the understanding of the dynamical structural formation mechanism.. |

10. | Makoto Sasaki, Kimitaka Itoh, Klaus Hallatschek, SANAE INOUE（論文のみ） ITOH, On a nonlinear dispersion effect of geodesic acoustic modes, Plasma Fusion Research, 8, 1403010 (2013), 2013.02. |

11. | M. Sasaki, K. Itoh, S.-I. Itoh and N. Kasuya, Zonal flows induced by symmetry breaking with existence of geodesic acoustic modes, Nuclear Fusion, 10.1088/0029-5515/52/2/023009, 52, 023009, 2012.01, [URL], The nonlinear dynamics of zonal flows (ZFs) is investigated when geodesic acoustic modes (GAMs) have substantial influence on plasma states. Simultaneous existence of multiple GAMs with different radial phase velocities gives additional nonlinear mode couplings, and asymmetry of the turbulence spectrum induces energy exchanges between GAMs and a ZF. A set of model equations is derived to describe the nonlinear dynamics of a ZF, GAMs and ambient turbulence. The model includes the mechanism of ZF generation by a pair of GAMs, and the back interaction from the turbulence is solved self-consistently. Two stationary solutions are obtained; one is the known solution that the GAM propagates as a travelling wave with no ZF excitation, and the other is a new solution that a pair of GAMs forms a standing wave, which induces a ZF. The accessibility to the steady states, and the threshold for the transition between them are obtained. The GAM and ZF formation in the new state affects the background plasmas. The effects on the turbulent heat diffusivity and the ion heating rate are discussed.. |

12. | Makoto Sasaki, Kimitaka Itoh, SANAE INOUE ITOH, Energy channeling from energetic particles to bulk ions via beam driven geodesic acoustic mode - GAM channeling, Plasma Physics and Controlled Fusion, 10.1088/0741-3335/53/8/085017, 53, 085017-1-085017-11, 2011.06, [URL]. |

13. | M. Sasaki, K. Itoh, A. Ejiri, Y. Takase, Transient excitation of Zonal Flows by Geodesic Acoustic Modes, Plasma Phys. Control. Fusion, 51, 085002, 51 085002 (2009)., 2009.07. |

14. | M. Sasaki, K. Itoh, A. Ejiri, Y. Takase , Poloidal Eigenmode of Geodesic Acoustic Mode in the limit of high safety factor, Journal of Plasma Phys., 75, No.6, 721, 75 No.6 721 (2009)., 2009.01. |

15. | M. Sasaki, K. Itoh, Y.Nagashima, A. Ejiri, Y. Takase , Nonlinear self-interaction of Geodesic Acoustic Modes in toroidal plasmas, Phys. Plasmas, 16, 022306, 16, 022306-1, 022306-8 (2009), 2009.01. |

**Presentations**

1. | M. Sasaki, K. Itoh, N. Kasuya, S. Inagaki, Y. Kasuga, T. Kobayashi, A. Fujisawa, S.-I. Itoh, Parallel flow driven instability due to toroidal return flow in H-mode plasmas, EFTSOMP, 2019.07. |

2. | M. Sasaki, S. Inagaki, Y. Kawachi, F. Kin, H. Arakawa , Extraction of spatial structures of intermittent events using dynamical mode decomposition, 2nd International Conference on data driven plasma science, 2019.05. |

3. | M. Sasaki, On the spatial profile of turbulence in magnetically confined plasmas , ISEE, 2019.02. |

4. | M. Sasaki, T. Kobayashi, K. Itoh, N. Kasuya, Y. Kosuga, A. Fujisawa, S. Inagaki, S.-I. Itoh, Spatiotemporal dynamics of turbulence with zonal flows, European Physical Society, 45th Conference on Plasma Physics, 2018.07. |

5. | M. Sasaki , Summary talk on Model reduction and experimental validation (for session E) , Asia-Pacific Transport Working Group (APTWG) international conference, 2018.06, 国際会議のセッションリーダとして「model validation and experimental verification」というグループをオーガナイズし、会議最終日にサマリートークを行った。また、会議のサマリー論文を共著で執筆し、Nuclear Fusion誌から出版された。. |

6. | M. Sasaki, Spatiotemporal dynamics of turbulence with zonal flows, 8th East-Asia School and Workshop on Laboratory, Space, and Astrophysical Plasmas (EASW8) , 2018.08. |

7. | M. Sasaki, Enhancement and suppression of turbulence by energetic‐particle‐driven geodesic acoustic modes, Electric Fields Turbulence and Self-Organization in Magnetised Plasmas (EFTSOMP), 2018.07. |

8. | M. Sasaki, K. Itoh, N. Kasuya, S. Inagaki, Y. Kosuga, T. Kobayashi, A. Fujisawa, S.-I. Itoh, Parallel flow driven instability due to toroidal return flow in H-mode plasmas , Asia-Pacific Transport Working Group (APTWG) international conference, 2018.06, We theoretically investigate turbulence in H-mode plasmas with the pressure gradient and the strong mean flow. The toroidal flow, which is induced by the poloidal mean flow so as to satisfy the divergence free condition, is strong in H- mode, thus the effect of the toroidal return flow on instabilities is considered. The proposed model self-consistently includes not only the destabilization of the drift wave and the parallel flow shear instability, called D’Angelo mode, but also the stabilization due to the poloidal flow shear. Depending on the strength of the flow shear or on the magnetic geometrical parameter, we obtain the stabilization of drift wave and the destabilization of the D’Angelo mode. The competition between different instabilities through coupling of the poloidal flow with the toroidal return flow could be a key concept for understanding the turbulence in H-mode. The characteristics of the instabilities are similar to the observations of the precursor of the type-III ELM.. |

9. | M. Sasaki, N. Kasuya, S. Toda, T. Yamada, M. Yagi, K. Itoh, S.-I. Itoh, Bifurcation between flow driven instability and drift wave in cylindrical plasmas, 5th Asia Pacific Transport Working Group International Conference, 2015.06. |

10. | Makoto Sasaki, Naohiro Kasuya, Tatsuya Kobayashi, Hiroyuki Arakawa, Takuma Yamada, Kouhei Fukunaga, Masatoshi Yagi, Kimitaka Itoh, Sanae-I. Itoh, Formation mechanism of steep wave front in drift wave turbulence, 24th International Toki Conference, 2014.11. |

11. | Makoto Sasaki, Naohiro Kasuya, Kimitaka Itoh, Maxime Lesur, Sanae-I. Itoh, Eigenmode analysis of geodesic acoustic modes induced by energetic particles, 4th Asia Pacific Transport Working Group International Conference, 2014.06. |

12. | Makoto Sasaki, Naohiro Kasuya, Kimitaka Itoh, MASATOSHI YAGI, SANAE INOUE ITOH, Dynamical response of a poloidal flow with particle source modulation, H mode workshop, 2013.10. |

13. | Makoto Sasaki, Naohiro Kasuya, Kimitaka Itoh, MASATOSHI YAGI, SANAE INOUE（論文のみ） ITOH, Nonlinear competition of zonal flow, flute structure and streamer in resistive drift wave turbulence in cylindrical plasmas, 12th Asia Pacific Physics Conference , 2013.07. |

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**Membership in Academic Society**

- The Japan Society of Plasma Science and Nuclear Fusion Research
- The Physical Society of Japan

**Awards**

- Theoretical study on spatio-temporal structures and energy transfer processes of oscillatory zonal flows
- Theoretical study on nonlinear dispersion relation and dynamics of oscillatory zonal flows
- Plasma Conference 2011 was held, which was co-sponsored by the Physical Society of Japan and the Japan Society of Plasma Science and Nuclear Fusion Research. I had a oral presentation, whose title was ''Nonlinear process of geodesic acoustic modes near a plasma edge'', and the presentation was highly evaluated.
- Encouraging prize on doctor thesis in Tokyo University

Educational

**Educational Activities**

Advising methods of data analysis and physical explanation on plasma turbulence experiments in department of advanced energy engineering science

**Other Educational Activities**

- 2018.08.
- 2018.06.
- 2018.07.
- 2013.10, We hold the international conference, 14th H mode workshop 2013, as a local organizer..
- 2007.08.

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