Kotaro Yamasaki

Localized electron heating by strong guide-field magnetic reconnection

Localized electron heating of magnetic reconnection was studied under strong guide-field using two merging spherical tokamak plasmas in the University of Tokyo Spherical Tokamak experiment. Our new slide-type two-dimensional Thomson scattering system is documented for the first time the electron heating localized around the X-point. Shape of the high electron temperature area does not agree with that of energy dissipation term Et·jt. If we include a guide-field effect term Bt/(Bp+αBt) for Et·jt, the energy dissipation area becomes localized around the X-point, suggesting that the electrons are accelerated by the reconnection electric field parallel to the magnetic field and thermalized around the X-point.

Laboratory study of diffusion region with electron energization during high guide field reconnection

Floating potential profile was measured around the X-point during high guide field reconnection in UTST merging experiment where the ratio of guide field ( Bg) to reconnecting magnetic field ( Brec) is Bg/Brec>10. Floating potential measurement revealed that a quadrupole structure of electric potential is formed around the X-point during the fast reconnection phase due to the polarization by inductive electric field. Also, our floating potential measurement revealed the existence of parallel electric field in the vicinity of the X-point. While field-aligned components of inductive electric field ( E∥ind) and electrostatic electric field ( E∥es) cancel out with each other away from the X-point, E∥ind exceeds E∥es around the X-point, indicating the deviation from ideal MHD criterion within the region. The diffusion region extends in the outflow region and the scale length of region is an order of ion skin depth, which is quite different from the VTF experiment result. Based on the measured magnetic field an...

Three-Dimensional Structure of the Streamer in Linear Plasmas

The three-dimensional structures of the streamer and its carrier waves and mediator were observed in a linear plasma device for the first time. Not only the cross-sectional structure but also the a...

A proposal of Fourier-Bessel expansion with optimized ensembles of bases to analyse two dimensional image

It is a critical issue to find the best set of fitting function bases in mode structural analysis of two dimensional images like plasma emission profiles. The paper proposes a method to optimize a set of the bases in the case of Fourier-Bessel function series, using their orthonormal property, for more efficient and precise analysis. The method is applied on a tomography image of plasma emission obtained with the Maximum-likelihood expectation maximization method in a linear cylindrical device. The result demonstrates the excellency of the method that realizes the smaller residual error and minimum Akaike information criterion using smaller number of fitting function bases.

Magnetic island dynamics in magnetic reconnection in UTST experiments

The dynamics of the magnetic island formed inside the reconnection current layer was investigated under the strong guide field in the UTST spherical tokamak merging experiment. A pair of proximately located O- and X-points was generated at ∼5 cm away from the other X-point. The formed O- and X-points immediately started to move toward the downstream region inside the current layer, but the O-point had larger velocity and caught up the preceding X-point within 0.5 μs. The results from Doppler spectroscopy indicated that the ion flow velocity had the intermediate value between the O- and X-point velocities, suggesting that the ions flowed with approximately the same velocity with the magnetic island.

Extraction of nonlinear waveform in turbulent plasma

Streamers and their mediator have been known to exist in linear cylindrical plasmas [Yamada et al., Nat. Phys. 4, 721 (2008)]. Conditional averaging is applied to extract the nonlinear characteristics of a mediator, which has been simply treated as a linear wave. This paper reports that a mediator should have higher harmonic components generated by self-couplings, and the envelope of a streamer should be generated with not only fundamental but also higher harmonic components of the mediator. Moreover, both the mediator and the envelope of the streamer have common features with solitary waves, i.e., the height should increase inversely as the square of their localization width.Streamers and their mediator have been known to exist in linear cylindrical plasmas [Yamada et al., Nat. Phys. 4, 721 (2008)]. Conditional averaging is applied to extract the nonlinear characteristics of a mediator, which has been simply treated as a linear wave. This paper reports that a mediator should have higher harmonic components generated by self-couplings, and the envelope of a streamer should be generated with not only fundamental but also higher harmonic components of the mediator. Moreover, both the mediator and the envelope of the streamer have common features with solitary waves, i.e., the height should increase inversely as the square of their localization width.