Pioneering work before becoming mainstream research

Various pioneering works in plasma physics in the last 30 years are reviewed. The following findings are discussed: 1) radial electric field shear in H-mode pedestal, 2) curvature of radial electric field and temperature profile, 3) intrinsic torque for plasma rotation, 4) hysteresis of flux-gradient relation in dynamic transport, 5) bifurcation of magnetic island states and 6) a trigger mechanism also known as a trigger problem for abrupt events. In the several years since the findings, some of these topics have already become mainstream research and others are state-of-the-art frontier research

Summary of the 27th IAEA Fusion Energy Conference in the categories of EX/W, EX/D, and ICC

This is a summary paper of the 27th IAEA Fusion Energy Conference which was held from 22–27 October 2018 in Gandhinagar, India. The results in the categories of EX/W (wave–plasma interactions, current drive, heating, energetic particles), EX/D (plasma–material interactions, divertors, limiters, scrape-off layer (SOL)), and ICC (innovative confinement concepts) in magnetic confinement experiments are summarized. In total, 121 papers have been contributed to these categories. Interesting results on the coupling between energetic particles, magnetohydrodynamics (MHD), wave–particle interaction, turbulence, SOL physics, and divertors are presented at this conference. For example, control of energetic particle driven MHD by electron cyclotron heating and resonance magnetic perturbation, mitigation of disruption by energetic particle driven MHD, control of decay length by SOL turbulence, and wave scattering by SOL density fluctuations were discussed in this conference. Deeper understanding of these couplings will be essential for the sustainment of high performance steady-state plasma in ITER

On the interplay between MHD instabilities and turbulent transport in magnetically confined plasmas

The interplay between MHD and turbulence is an interesting topic in magnetically confined plasma and solar plasma. The experimental observations made recently, shown below, suggest coupling and interplay between MHD and turbulence in magnetically confined toroidal plasmas. (1) Turbulence spreading into the magnetic island, (2) there is a self-organized change in topology and turbulence in the magnetic island, (3) the flow is damped by a stochastic magnetic field, (4) the trigger mechanism for the MHD bursts, (5) MHD bursts have an impact on the ion velocity distribution and potential, and (6) turbulence exhausts are created at the MHD burst event. In this paper, experimental evidence for the interplay between MHD and turbulence in toroidal plasmas is reviewed. The physics mechanism of the interplay and a possible link to astrophysical plasma physics are also discussed

Non-local transport nature revealed by the research in transient phenomena of toroidal plasma

The non-local transport nature revealed by the research in transient phenomena of toroidal plasma is reviewed. The following non-local phenomena are described: core temperature rise in the cold pulse, hysteresis gradient–flux relation in the modulation ECH experiment, and see-saw phenomena at the internal transport barrier (ITB) formation. There are two mechanisms for the non-local transport which cause non-local phenomena. One is the radial propagation of gradient and turbulence. The other is a mediator of radial coupling of turbulence such as macro/mesoscale turbulence, MHD instability, and zonal flow. Non-local transport has a substantial impact on structure formations in a steady state. The turbulence spreading into the ITB region, magnetic island, and SOL are discussed

Structure of the radial electric field and toroidal/poloidal flow in high temperature toroidal plasma

The structure of the radial electric field and toroidal/poloidal flow is discussed for the high temperature plasma in torodidal systems, tokamak and Heliotron type magnetic configurations. The spontaneous toroidal and poloidal flows are observed in the plasma with improved confinement. The radial electric field is mainly determined by the poloidal flow, because the contribution of toroidal flow to the radial electric field is small. The jump of radial electric field and poloidal flow are commonly observed near the plasma edge in the so-called high confinement mode (H-mode) plasmas in tokamaks and electron root plasma in stellarators including Heliotrons. In general the toroidal flow is driven by the momentum input from neutral beam injected toroidally. There is toroidal flow not driven by neutral beam in the plasma and it will be more significant in the plasma with large electric field. The direction of these spontaneous toroidal flows depends on the symmetry of magnetic field. The spontaneous toroidal flow driven by the ion temperature gradient is in the direction to increase the negative radial electric field in tokamak. The direction of spontaneous toroidal flow in Heliotron plasmas is opposite to that in tokamak plasmas because of the helicity of symmetry of the magnetic field configuration

Internal Transport Barrier in Tokamak and Helical Plasmas

The differences and similarities between the internal transport barriers (ITBs) of tokamak and helical plasmas are reviewed. By comparing the characteristics of the ITBs in tokamak and helical plasmas, the mechanisms of the physics for the formation and dynamics of the ITB are clarified. The ITB is defined as the appearance of discontinuity of temperature, flow velocity, or density gradient in the radius. From the radial profiles of temperature, flow velocity, and density the ITB is characterized by the three parameters of normalized temperature gradient, $R/{L}_{T}$, the location, ${\rho }_{\mathrm{ITB}}$, and the width, W/a, and can be expressed by \u27weak\u27 ITB (small $R/{L}_{T}$) or \u27strong\u27 (large $R/{L}_{T}$), \u27small\u27 ITB (small ${\rho }_{\mathrm{ITB}}$) or \u27large\u27 ITB (large ${\rho }_{\mathrm{ITB}}$), and \u27narrow\u27 (small W/a) or \u27wide\u27 (large W/a). Three key physics elements for the ITB formation, radial electric field shear, magnetic shear, and rational surface (and/or magnetic island) are described. The characteristics of electron and ion heat transport and electron and impurity transport are reviewed. There are significant differences in ion heat transport and electron heat transport. The dynamics of ITB formation and termination is also discussed. The emergence of the location of the ITB is sometimes far inside the ITB foot in the steady-state phase and the ITB region shows radial propagation during the formation of the ITB. The non-diffusive terms in momentum transport and impurity transport become more dominant in the plasma with the ITB. The reversal of the sign of non-diffusive terms in momentum transport and impurity transport associated with the formation of the ITB reported in helical plasma is described. Non-local transport plays an important role in determining the radial profile of temperature and density. The spontaneous change in temperature curvature (second radial derivative of temperature) in the ITB region is described. In addition, the key parameters of the control of the ITB and future prospects are discussed

Two-dimensional beam emission spectroscopy for hydrogen isotope negative neutral beam in Large Helical Device

A new beam emission spectroscopy system that has improved lines of sight is installed in the Large Helical Device (LHD), and routine measurement has been started in the 21st LHD experiment campaign in 2019–2020. The new system is optimized for hydrogen isotope experiments by equipping a rotatable large-diameter interference filter to be compatible with either the hydrogen or the deuterium beam emission component. An avalanche photo diode detector array having 8 × 8 pixels is used for obtaining a radial–vertical image of electron density fluctuation covering the mid-radius to the plasma periphery. Spatial resolution and wavenumber cutoff are derived from equilibrium reconstruction and plasma kinetic profiles. Obtained fluctuation data is presented for a low field high beta discharge. The spatiotemporal structure of the fluctuations is clearly shown by Fourier correlation analyses

Observation of Toroidal Flow on LHD

In order to investigate the formation of toroidal flow in helical systems, both NBI driven flow and spontaneous toroidal flow were observed in Large Helical Device (LHD). The toroidal flow driven by NBI is dominant in plasma core while its contribution is small near plasma edge. The spontaneous toroidal flow changes its direction from co to counter when the radial electric field is changed from negative to positive at plasma edge. The direction of the spontaneous toroidal flow due to the radial electric field near plasma edge is observed to be opposite to that in plasma core where the helical ripple is small

Bifurcation physics of magnetic islands and stochasticity explored by heat pulse propagation studies in toroidal plasmas

Bifurcation physics of a magnetic island was investigated using the heat pulse propagation technique produced by the modulation of electron cyclotron heating. There are two types of bifurcation phenomena observed in a large helical device (LHD) and DIII-D. One is a bifurcation of the magnetic topology between nested and stochastic fields. The nested state is characterized by the bi-directional (inward and outward) propagation of the heat pulse with slow propagation speed. The stochastic state is characterized by the fast propagation of the heat pulse with electron temperature flattening. The other bifurcation is between the magnetic island with larger thermal diffusivity and that with smaller thermal diffusivity. The damping of toroidal flow is observed at the O-point of the magnetic island both in helical plasmas and in tokamak plasmas during a mode locking phase with strong flow shears at the boundary of the magnetic island. Associated with the stochastization of the magnetic field, the abrupt damping of toroidal flow is observed in LHD. The toroidal flow shear shows a linear decay, while the ion temperature gradient shows an exponential decay. This observation suggests that this flow damping is due to the change in the non-diffusive term of momentum transport

Method for estimating the frequency-wavenumber resolved power spectrum density using the maximum entropy method for limited spatial points

A combination of the Fourier transform and the maximum entropy method for estimating the frequency-wavenumber resolved power spectrum density is proposed. After illustrating the physical insight of the maximum entropy method by using synthetic test data, capability of the proposed method is tested using numerical simulation data. The method is also applied to experimental data obtained by the beam emission spectroscopy in the Large Helical Device. All of those examinations show that the proposed method provides more plausible results than conventional methods when the available spatial points are limited