Hisatoshi Nakashima

Recent progress on high performance steady state plasmas in the superconducting tokamak TRIAM-1M

An overview of TRIAM-1M experiments is presented. The current status of issues related to steady state operation is presented with reference to the achievement of super-ultra-long tokamak discharges sustained by LHCD for over 2 h. The importance of control of the initial phase of the plasma, the avoidance of high heat load concentration, wall conditioning and the avoidance of abrupt termination of long discharges are discussed as the crucial issues for the achievement of steady state operation of the tokamak. A high ion temperature (HIT) discharge fully sustained by 2.45 GHz LHCD with both high ion temperature and steep temperature gradient was successfully demonstrated for longer than 1 min in the limiter configuration. The HIT discharges can be obtained in a narrow window of density and position. The avoidance of heat load concentration on a limiter is the key point for the achievement and long sustainment of the HIT discharge. As the effective thermal insulation between the wall and the plasma is improved for the single null configuration, HIT discharges with peak ion temperature > 5 keV and a steeper temperature gradient of up to 85 keV/m can be achieved through the fine control of density and position. Plasmas with high κ ≈ 1.5 can also be demonstrated for longer than 1 min. The current profile is also well controlled for a time about 2 orders of magnitude longer than the current diffusion time using combined LHCD. The serious damage to the material of the first wall caused by energetic neutral particles produced by charge exchange is also described. As the neutral particles cannot be affected by a magnetic field, this damage by neutral particles must be prevented by a new technique.

Non-inductive current start-up assisted by energetic electrons in Q-shu University experiment with steady-state spherical tokamak

After intensive discharge cleaning of the chamber wall, non-inductive current start-up experiments have been successfully performed in QUEST in moderate vertical fields of about 1.0–1.5 mT with positive n-index. Simultaneously, with increasing plasma current, an asymmetric toroidal flow of energetic electrons was observed and direct measurements of current driven by this asymmetric flow were taken with a newly developed Langmuir probe technique. A numerical study of the energetic electron orbits indicates that the total current is enough to play a dominant role in the formation of a closed flux surface in QUEST.

Cascading Bifurcations to Chaos in a Current-Carrying Ion Sheath

A theory is developed to explain cascading bifurcations to chaos observed in a current-carrying ion sheath. The basic equation is derived to describe ion dynamics in an ion sheath which obeys the Child-Langmuir law for a large potential drop and is numerically solved to show the same bifurcation sequences as those in the experiments.

Direct measurement of energetic electron flow in Q-shu University experiment with steady-state spherical tokamak

In magnetized plasmas, the presence of a significant number of energetic electrons has been observed but quantitative characteristics of these electrons are proving difficult to investigate. A Langmuir probe offers a means to provide quantitative measurement of these energetic electrons that takes into account electron emissions (secondary electron emission and electron reflection) from the probe tips and sheath expansion around the probe tips caused by a considerable negative potential. In this paper, these effects are experimentally confirmed and an analytical means to measure energetic electron characteristics are proposed. An analysis of plasmas produced by a high frequency wave is then applied leading to the successful detection of an asymmetric flow of energetic electrons. The estimated electron temperature and current density were approximately 4-5 keV and 2-3 kA/m(2).

Investigations of the radial propagation of blob-like structure in a non-confined electron cyclotron resonance heated plasma on Q-shu University Experiment with a Steady-State Spherical Tokamak

A study of radial propagation and electric fields induced by charge separation in blob-like structures has been performed in a non-confined cylindrical electron cyclotron resonance heating plasma on Q-shu University Experiment with a Steady-State Spherical Tokamak using a fast-speed camera and a Langmuir probe. The radial propagation of the blob-like structures is found to be driven by E × B drift. Moreover, these blob-like structures were found to have been accelerated, and the property of the measured radial velocities agrees with the previously proposed model [C. Theiler et al., Phys. Rev. Lett. 103, 065001 (2009)]. Although the dependence of the radial velocity on the connection length of the magnetic field appeared to be different, a plausible explanation based on enhanced short-circuiting of the current path can be proposed.

Investigation of Non-inductive Plasma Current Start-up by RF on QUEST

Formations of a closed flux surface (CFS) on QUEST are achieved by fully non-inductive current start-up driven by RF, which is 8.2GHz in frequency and more than 40kW in power. It found that appropriate magnetic configuration with positive n-index and reduction of particle recycling was crucial to achieve the non-inductive plasma current start-up (PCS) successfully. Especially the controllability of particle recycling should be improved by wall conditioning based on successive plasma production and wall cleaning with electron cyclotron resonance heating (ECR) plasmas induced by RF in frequency of 2.45GHz.

Current density calculation from particle orbit in RF-driven divertor plasma on QUEST

We investigate and calculate particle orbits and the effect of particle orbits on plasma current density for nonrelativistic resonance condition in the present RF-driven divertor plasma on QUEST. We surveyed particle orbits for different values of parallel refractive index, particle initial positions and pitch angles on fundamental and second harmonic resonance conditions. We observed that for fundamental harmonic resonance condition when particle orbits are plotted on the poloidal cross-section for positive values of parallel refractive index, these orbits are started from the resonance surface and produced their orbits around the LCFS (Large Closed Flux Surface). These orbits carry positive current. When particle orbits are plotted for negative values of parallel refractive index, these orbits are started from resonance surface, but remained at the inside of the LCFS. These orbits carry negative current that reduced the overall plasma current. For second harmonic resonance condition when particle orbits are plotted on the poloidal cross-section most of the orbits remained in inside the LCFS and carry positive current. When we consider the value of parallel refractive index -0.4 and +0.4 some particle orbits arrived at the limiter and become lost particles. On the other hand, when we consider particle initial positions 0.16 m or more vertically far from the mid plane some banana orbits are produced. These banana orbits make the current density profile maximum at low field side region. From this calculation we got a hollow current density profile with current density peak at the low field side region outside of the LCFS. From this calculation we can infer that parabolic current density profile is possible, if we set the resonance surface outside of the magnetic axis by increasing the toroidal magnetic field coil current and make the plasma position inward by increasing vertical field coil current.

Initial Plasma Production by Townsend Avalanche Breakdown on QUEST Tokamak

On tokamak devices, an induction electric field induced by poloidal field (PF) coils plays a role to produce initial plasma. On a DIII-D tokamak, the required induction electric field for plasma breakdown agrees well with theoretical predictions based on the Townsend avalanche theory. According to the Townsend avalanche theory, the minimum induction electric field for plasma breakdown depends on neutral gas pressure and connection length. For stable plasma breakdown, a sufficiently large induction electric field is required. However, in the case of spherical tokamaks without electric insulation in the toroidal direction, the effect of eddy currents flowing in the toroidal direction should be considered in evaluating a feasible induction electric field because this effect suppresses the generation of an induction electric field. On a QUEST spherical tokamak, the possibility of Townsend avalanche breakdown is studied by evaluating the connection length and achievable induction electric field. The connection length is greater than 100 m in the case where a null point is set to be R=0.56 m with a CS coil current of 2.0 kA and a PF26 coil current of 0.4 kA. Moreover, the induction electric field is about 1.5 V at this point including the effect of eddy currents. With these values, the initial plasma production by the induction electric field is sufficiently possible on QUEST.

Guiding Center Orbit Calculation for Evaluating the Current Density Distributions of the Electrons in Electron Cyclotron Heating on QUEST

In order to evaluate the magnetic surface current density distributions, the electron guiding center orbits were calculated with the equilibrium fitting plasma equilibrium analysis. The current density distributions in electron cyclotron-heated plasma were estimated from the orbit analyses in the equilibrium plasma shaping for the fundamental and the second-harmonic resonant electrons separately. The current density distribution profiles on the equatorial plane were obtained for the electrons with initially positive and negative velocities parallel to the magnetic field direction as well as the trapped electrons. The surface averaged current density profiles of the closed flux surfaces were also evaluated. A significant amount of the positive current density distributions appeared outside the last closed flux surface (LCFS), while all the negative current density distributions were evaluated inside the LCFS. The trapped electrons being the second-harmonic electron cyclotron resonant contributed the negative current inside the LCFS. The positive current density distributions outside the LCFS were significant for the driven current density for both the fundamental and second-harmonic resonances.

Quaternion Analysis of Power Supply for Tokamak Plasma Control

Power supply for plasma control has been developed from condenser bank with ignitron for pulse tokamak plasma to PWM inverter with IGBT for steady tokamak plasma. Quaternion, four-dimensional hypercomplex number is good at describing three-dimensional rotation. Utilizing the performance of the quaternion rotation, we analyze three-phase power electronic circuit for the tokamak plasma control of equilibrium and stability. By further introduction of biquaternion concept, we can deal with phasor of each phase of three-phase AC similarly. But, we have to multiply the rotation operator from the left-hand side and from the right-hand side by using the conjugation. We try to verify the merits beyond the multiplication inconvenience from both sides. Not only symmetrical three-phase AC (positive sequence) but also negative sequence and zero sequence can be dealt with. Concerning quaternion power of three-phase AC, we can obtain the similar result as the one in pq theory. Quaternion can divide three-dimensional vector. The capability is utilized to develop matrix converter strategy based not only on Venturini method but also on space vector method in more detail.