Shun-ichi Oikawa

Magnetic field structure and confinement of energetic particles in the LHD

The Large Helical Device (LHD) achieves high-performance plasma confinement by the coordination of the magnetic surface region and the chaotic field line layer. It is theoretically and experimentally shown that drift surfaces exist for highly energetic particles being extended over the last closed flux surface (LCFS) in the LHD. These particles are considered lost particles due to the loss-cone in the previous theories, where the analyses are limited inside the LCFS. The present theory predicts that the loss-cone is strongly reduced in the LHD and that highly energetic particles confined over the LCFS exist. These are consistent with the LHD experimental results in both the ICRF heating experiments and the low magnetic field neutral beam injection heating experiments. From particle orbit analyses and studies on the connection length of diverter field lines, it is also shown that plasma can exist in the chaotic field line layer located outside the LCFS in the LHD. The plasma in the chaotic field line layer is clearly detected by CCD-cameras in the LHD experiment. This ambient plasma might be expected to play the role of a kind of impregnable barrier for the core plasma, which suppresses both the MHD instabilities and the cooling of the core plasma due to charge exchange processes. The line-tying effects of diverter field lines that are slipped out from the chaotic field line layer can also stabilize the ballooning mode and the vertical displacement events of the plasma column.

Construction of neoclassical transport database for large helical device plasma applying neural network method

A neoclassical transport database for the large helical device (LHD) plasma, DCOM/NNW, is constructed using the neural network method. Monoenergetic neoclassical transport coefficients evaluated by the Monte Carlo code, DCOM, are used as training data of the neural network. The databases for two typical magnetic field configurations in LHD, namely, standard and inward-shifted configurations, are constructed and transport coefficients for thermal plasma are evaluated. The plasma parameter dependencies and the ambipolar radial electric field are investigated.

Boundary-only integral equation approach based on polynomial expansion of plasma current profile to solve the Grad–Shafranov equation

On lines 3 and 5 in the right-hand column of page 433, the following corrections should be made: (1 - βpR20) should be (1 - β p)R20 βp (=0.60) and R0 (=3.32 m) should be βp (=0.70) and R0 (=3.50 m)

Characteristics of magnetic island formation due to resistive interchange instability in helical plasma

Focusing attention on the magnetic island formation, we investigate the characteristics of the resistive interchange magnetohydrodynamics instabilities, which would limit a high beta operational regime in helical type fusion reactors. An introduction of a new index, i.e., the ratio of the magnetic fluctuation level to the radial displacement, enables us to make a systematic analysis on the magnetic island formation in the large helical device-like plasmas during the linear growth phase; (i) the interchange instability with the second largest growth rate makes the magnetic island larger than that with the largest growth rate when the amplitude of the radial displacement in both cases is almost the same as each other; (ii) applied to a typical tearing instability, the index is smaller than that for the interchange instability with the second largest growth rate.

Quantum mechanical grad-B drift velocity operator in a weakly non-uniform magnetic field

This paper presents the analytical solution for quantum mechanical grad-B drift velocity operator by solving the Heisenberg equation of motion. Using the time dependent operators, it is shown the analytical solution of the position operators in x(t) and y(t) of the particle in the presence of a weakly non-uniform magnetic field. It is also shown numerically that the grad-B drift velocity operator agrees with the classical counterpart.

Particle Orbit Analysis under the Ion Cyclotron Range of Frequency Heating in the Large Helical Device

Particles under the influence of the ion cyclotron range of frequency (ICRF) electromagnetic field were analyzed in the large helical device (LHD) by numerically solving the equation of motion instead of the guiding-center equation. Behaviors of the ICRF-heated particles in three cases of slowing-down by thermal electrons were compared. We also compared the characteristics of the ICRF-heated particles in the standard magnetic configuration (Rax = 3.75 m) with those in the inwardly shifted magnetic configuration (Rax = 3.6 m). It was found that the maximum energies of particles starting from the core plasma region exceed 300 keV and that such particles are confined within the vacuum vessel wall for 10-4 s. It was confirmed that the ICRF-heated particles with energies of around 400 keV are lost through the divertor field lines. The maximum energy of the ICRF-heated particles starting from the core region rises with increasing electron temperature. As a result, the energy level relevant to the proton (p)–boron (11B) fusion reaction ( 650 keV) was obtained when Te > 30 keV. Through acceleration in both the parallel and the perpendicular direction to the magnetic field, the high-energy chaotic orbit particles were produced by ICRF heating. It was also found that the energy at which the transition to the high-energy chaotic orbit particle occurs determines the upper energy limit of the ICRF-heated particle in LHD. A high confinement performance was found for the high-energy chaotic orbit particles produced by the ICRF field. The particle orbits in the inwardly shifted magnetic configuration were more widespread within the core region. Thus, it was concluded that the ICRF heating efficiency of the core plasma region in the inwardly shifted magnetic configuration exceeds that in the standard magnetic configuration.

Effects of net toroidal current profile on the Mercier criterion in heliotron plasmas

The effects of variations in net toroidal current on the Mercier criterion are studied in a heliotron configuration under the free boundary condition. The plasma column and the magnetic axis are shifted horizontally by the net toroidal current because the free boundary condition changes the effective vertical field. The direction of the shift depends on the direction of the current. In the case of a subtractive current density peaked at the axis, which decreases the rotational transform, the equilibrium is more stable than the no net current equilibrium, although the plasma axis is shifted inwards by the current itself. This is because the Shafranov shift is greater than the inward axis shift caused by the current itself, and the magnetic well is enhanced. In the case of additive currents which increase the rotational transform, there is a tendency for the stability to be improved as the peak position of the current density varies from the axis to the peripheral region. This improvement is due to the enhancement of the magnetic shear at low beta and the magnetic well at high beta. As in the no net current case, the equilibrium with an additive hollow current under the free boundary condition is more stable than that under the fixed boundary condition because magnetic well stabilization is enhanced. When the subtractive current peaked at the axis and the additive hollow current flow simultaneously so that the total current is zero or slightly additive, the interchange mode can be stabilized even if the corresponding equilibrium with no net current is strongly Mercier unstable.

Field line and Particle orbit Analysis in the Periphery of the Large Helical Device

Magnetic field lines and particle orbits were analyzed in the periphery of the Large Helical Device (LHD), which is called the chaotic field line region in this paper. The widths of the chaotic field line region were numerically identified for the standard LHD configuration with the magnetic axis position R ax = 3.75 m and for an improved confinement configuration with R ax = 3.6 m. It was found that the reflected particles include of what we have named chaotic particles and non-chaotic particles. Most of the reflected particles are mirror-confined with strong adiabaticity in the chaotic field line region. The remaining reflected particles, named type-A and type-B particles, are harmful to confinement. We found by detailed analysis of the vacuum magnetic field in the LHD that there exist loss canals that are the open intersections of | B | = const. and B ·∇ B = 0 with ( B ·∇) 2 B > 0. The type-A particles, which are deeply trapped in the intersections and exhibit non-chaotic behavior, are lost along the l...

Quantum mechanical expansion of variance of a particle in a weakly non-uniform electric and magnetic field

We have solved the Heisenberg equation of motion for the time evolution of the position and momentum operators for a non-relativistic spinless charged particle in the presence of a weakly non-uniform electric and magnetic field. It is shown that the drift velocity operator obtained in this study agrees with the classical counterpart, and that, using the time dependent operators, the variances in position and momentum grow with time. The expansion rate of variance in position and momentum are dependent on the magnetic gradient scale length, however, independent of the electric gradient scale length. In the presence of a weakly non-uniform electric and magnetic field, the theoretical expansion rates of variance expansion are in good agreement with the numerical analysis. It is analytically shown that the variance in position reaches the square of the interparticle separation, which is the characteristic time much shorter than the proton collision time of plasma fusion. After this time, the wavefunctions of ...

Introduction of the vector potential to a linear MHD simulation code based on a real coordinate system

Here, the vector potential was introduced to a linear magnetohydrodynamics (MHD) simulation code, and the modified and original simulation results were compared. The effects of the broken solenoidal condition on the perturbed magnetic field in linear MHD simulations based on real coordinates were investigated. The results showed that the modified code can successfully remove the error in this condition, and the modified code was confirmed to work appropriately. Incorrect results can be yielded by an error in the condition, especially in analyses of the mode structure.