Kei-ichi Hirano

Experimental studies on energy transport in a reversed-field theta pinch

The transport of energy and particles from a field-reversed configuration (FRC) produced by a theta pinch has been studied experimentally under the following typical plasma parameters in the quasi-static phase: Ti ~ 300 eV, Te~100eV, e ~ 5 × 1015 cm−3, β ~ 1, Zeff ~ 3 and separatrix radius rs ~ 1.8 cm. Energy transport through various processes has been evaluated, consistent with the observed volume-averaged quantities which are changing in time in a quasi-static manner. As a result, it has been found that the dominant energy loss mechanism is particle loss, which accounts for more than 50% of the power flow out of the FRC. The second important process is the electron thermal conduction loss, which is estimated to be roughly 30% of the total loss if impurity radiation losses other than by carbon and oxygen are not important. In contrast, the ion thermal conduction loss is observed to be so small as to be hidden by the experimental errors. Radiation loss has been estimated to play a minor role if the observed Zeff can be ascribed to contaminations of only carbon and oxygen. The inferred energy confinement time τE is 7 is μs, which compares with the observed particle confinement time τp of 17 μs. The data indicate that τp is consistent with classical diffusion theory.

Fast power ''crowbar'' system using a current transformer with extremely low leakage inductance

A power crowbar system for fast θ‐pinch experiments is operated successfully. A current transformer with extremely low leakage inductance is used to reduce the effective impedance of series capacitor system. The leakage inductance of the transformer is reduced to 2 nH by employing a special arrangement of windings. It is demonstrated that a current of 40 kA, the rise time of which is 4 μsec, is maintained for 500 μsec.

A steady-state axisymmetric toroidal system

Conditions for achieving a steady state in an axisymmetric toroidal system are studied with emphasis on a very-high-beta field-reversed configuration. The analysis is carried out for the electromotive force produced by the Ohkawa current that is induced by neutral-beam injection. It turns out that, since the perpendicular component of the current ⊥ to the magnetic field can be generated automatically by the diamagnetic effect, only the parallel component || must be driven by the electromotive force. The drive of || generates shear in the field line so that the pure toroidal field on the magnetic axis is rotated towards the plasma boundary and matched to the external field lines. This matching condition determines the necessary amount of injection beam current and power. It is demonstrated that a very-high-beta field-reversed configuration requires only a small amount of current-driving beam power because almost all the toroidal current except that close to the magnetic axis is carried by the diamagnetic current due to high beta. A low-beta tokamak, on the other hand, needs very high current-driving power since most of the toroidal current is composed of || which must be driven by the beam.

Slow formation of field reversed configurations by colliding high beta counter flows

To produce a field reversed configuration (FRC) without using the very high voltage techniques required in a conventional theta pinch, the author has studied the steady head-on collision of counter plasma streams, flowing along the magnetic field, as ejected from two identical co-axial plasma sources mounted at each end of the apparatus. The study was motivated by the fact that such a flow can be regarded as a steady state theta pinch. Ideal Poisson and shock adiabatic flow models were used to analyse the steady colliding process. It was demonstrated that an FRC involving a large number of particles can only be produced by the weak shock mode, which is achieved when energetic plasma flow is decelerated and heated up exclusively through the Poisson adiabatic process before the streams collide.

Ignition and reactor application of deuterium based fuel cycles in field reversed configurations

Self-consistent plasma modelling, on the assumption that all particle species have the same confinement time, is applied to a field reversed configuration (FRC) to show that plausible designs of an ignited advanced fuel reactor are possible within the framework of present day technology. FRCs in the β ~ 1 state are predicted to be microscopically and macroscopically stable; here, any ignited state must be of this kind since the plasma is heated up by a vast amount of nuclear power which even exceeds the electric output. In the β ~ 1 state, the steep pressure gradient appears to concentrate dissipation into a thin sheath formed on the plasma surface; hence, the problem can be solved independently of the detailed local dissipative properties, in analogy to the case of a supersonic flow decelerated through an Hugoniot adiabatic shock. It was found out that, in the ignited state, self-pinching of the electrons takes place in the scrape-off layer towards the X-point so that the size of the loss hole from the scrape-off layer is reduced, which raises the edge plasma pressure to a high level; thereby, serious impurity concentration is eliminated. In the paper, possible D-based steady state fuel cycles are studied systematically by using a formulation which predicts each important parameter of a simple toroidal FRC reactor if only the electric power output and the plasma radius are specified. In optimizing the burning temperature, care was taken to satisfy the criteria for thermal stability. A study has also been pursued on a complex closed reactor system which is fed by D alone and exhausts the final fusion products only. It is found that flexible design is possible for such a complex system to minimize construction cost or environmental effects. Advanced fuel reactors are demonstrated to be very compact in size so that a field strength and a plasma volume approximately equal to those of JT-60 may be sufficient for a fusion reactor constituting the basis of a 1000 MW(e) power plant

Fast air gap crowbar switch decoupled by a low pressure gap

A high pressure air gap is used as a fast ’’crowbar’’ switch with the aid of a low pressure gap decoupler. In order to trigger the switch, a special trigger circuit is designed, in which a single pulse breaks down the high and low pressure gaps successively. The jitter of breakdown time of this switch is less than 20 nsec over a wide range of operating conditions, pressures in both gaps, and working voltage. The switch is designed for 40 kV operation. Its inductance is 22 nH, and the resistance at a current of 40 kA is 4 mΩ. Seventy switches are installed in a 40 kV, 210 kJ fast bank. Up to now they have been running for about 20u2009000 shots with little maintenance.

Simulation of a Reversed-Field Theta-Pinch Plasma by a Zero-Dimensional Model Including Effect of Impurities

A zero-dimensional rigid rotor model is useful in investigating post-implosion behavior in a reversed-field theta-pinch plasma including impurities. The thermal sheath length δ and the inverse of the gradient length en can be determined for the numerical calculation to give better agreement with the experimental results on the initial time behaviors of τp, τeth, Te and Ti, where τp is the particle confinement time, τeth the thermal conduction time, and Te and Ti are the electron and ion temperatures, respectively. The numerical results of the total electron number Ne, electron density ne, energy confinement time τE, τeth, Te, and Ti, and energy flows with impurity for Zeff=2 are closer to the experimental results than those without impurity for Zeff=1, where Zeff=2 is the effective ionic charge taken from the experimental measurement. The calculation shows that τE and τeth are much more sensitive to Zeff than τp is.