村上 朝之

We describe high-density energy conversion using a compact closed-cycle magnetohydrodynamic electrical power generator. The slightly divergent disk-shaped generator exhibits improved energy-conversion performance, in particular, a high power density of 0.76 GW/ m(3). The application of high- and uniform-density magnetic flux to the entire generator, the high electrical conductivity, the symmetric plasma structure and stable plasma behavior, and the sufficient pressure gradient used to drive the fluid contribute to the outstanding performance of the generator. (C) 2007 American Institute of Physics.

A time dependent two-dimensional numerical simulation has been carried out in order to clarify the magnetohydrodynamic (MHD) flow behavior and performance of a disk MHD generator installed in a new closed-loop experimental facility at the Tokyo Institute of Technology. The numerical investigation is not limited to the generator channel only, but also includes an inlet duct and a downstream 90'-bend diffuser. A set of possible generator inlet-outlet pressure ratio (PR) was selected, and the influence on flow physics and generator performance was examined. Maximum enthalpy extraction (EE) ratio was obtained at high PR. In this case, an oblique shock wave appeared in the 90'-bend diffuser for both non-MHD and MHD flow regimes. The EE, however, did not vary monotonically with the PR. Rather a local minimum point for the EE was observed at moderate PR. In this case, either an oblique shock wave or a normal shock wave would appear in the generator channel depending upon whether the flow was in the non-MHD or MHD regime. The results predicted in the present simulation are valuable and important for setting the working gas conditions and evaluating generator performance in the closed-loop power generation experiment.

The present experiment has aimed at the improvement in the performance of a shock-tube-driven disk CCMHD generator. For that purpose, an experimental setup was arranged as follows: (1) An inlet swirl was introduced. (2) The area ratio of the disk generator was small. (3) A cesium-seeded helium gas was used as a working medium. Not only these factors but also the production of a homogeneous plasma contributed to outstanding performance characterized by an isentropic efficiency of 63% and an enthalpy extraction ratio of 30.8% at a stagnation temperature of 2250 K and stagnation pressure of 0.14 MPa. Furthermore, a maximum electrical power output of 1.23 MW and a maximum power density of 297 MW/m3 were obtained. © 2006 Wiley Periodicals, Inc.

R-z two dimensional numerical simulation with a large eddy simulation (LES) model has been carried out in order to clarify, for the first time, the typical MHD flow behavior and the performance of the disk MHD generator installed in the new closed loop experimental facility at Tokyo Institute of Technology. The results show thick separated flow regions in the generator channel both for non-MHD flow and MHD flow. The separated region influences the MHD interaction because of its low electrical conductivity. The MHD flow streamlines, however, tend to widen in the generator channel, with reduction of thickness of non-MHD flow separation. The typical performance of the generator have been predicted for several load resistances and seed fractions. The study is important to prepare as better as possible and to assure the success of the future MHD power generation experiment.

We describe the suppression of ionization instability and the control of a magnetohydrodynamic electrical power-generating plasma by coupling with a radio-frequency (rf) electromagnetic field. The rf heating stabilizes the unstable plasma behavior and homogenizes the nonuniform plasma structure, whereby the power- generating performance is significantly improved. (c) 2005 American Institute of Physics.

We describe seed-free pure-argon-plasma magnetohydrodynamic (MHD) power generation assisted by an external radio-frequency electromagnetic field to enhance the nonequilibrium plasma excitation process. The rf heating induces nonequilibrium ionization under a low total argon-gas temperature at which thermal ionization is insufficient. The rf-assisted plasma, the behavior of which is rather stable, contributes to continuous MHD energy conversion. © 2005 American Institute of Physics.

Three-dimensional numerical simulation has been carried out to investigate the performance of a large scale disk magnetohydrodynamics (MHD) generator coupled with radio frequency (RF) electromagnetic field. The RF technique is verified to be useful for the improvement of performance and widening the actual operating conditions even for a large scale MHD generator. The skin effect of plasma influences the distribution of RF electric field, however, under the present plasma condition in the MHD generator, this effect will not induce the failure of the RF application. When the plasma does not achieve the fully ionized seed condition only by self-induced Joule heating, the plasma is unstable and nonuniform three-dimensionally. Coupling with the RF power can stabilize the plasma and improve the performance of MHD generator. An optimal value of the RF power under which the plasma structure becomes completely uniform in azimuthal direction, and the performance of MHD generator can be most effectively improved is evaluated. The energy cost of RF Joule heating is smaller than the power output increased by coupling with the RF power. The improvement of MHD generator performance by the RF power is efficient.

We present the structures of nonequilibrium-seeded plasmas in a closed-cycle magnetohydrodynamic generator. The features of cesium-seeded helium plasmas and cesium-seeded argon plasmas created in a swirl-flow mode operation are examined. The transition of the plasma-fluid structure controlled by seeding is closely coupled with the plasma stability and the flow configuration.

The present paper describes the performance of a radio-frequency (rf) electromagnetic-field-assisted magnetohydrodynamics (MHD) electrical power generator and compares it to a multiple-load MHD generator. rf heating which is superimposed on self-excited Joule heating preionizes cesium-seeded helium gas independent of the in situ loading conditions, whereby Hall potential profile is improved and electron temperature is increased. Furthermore, the dynamic stabilization effect cancels ionization instability and homogenizes plasma structure, which is the most important superiority of the rf-power assistance over a conventional plasma actuation by loading-rate control. The power-generating performance is significantly improved with the aid of the rf power under wide seeding and loading conditions, where the electrical load-matching characteristic is slightly changed. © 2005 American Institute of Physics.

We describe a radio-frequency (rf) electromagnetic-field-assisted magnetohydrodynamic power generation experiment, where an inductively coupled rf field (13.56 MHz, 5.2 kW) is continuously supplied to the disk generator. The rf power assists the precise plasma ignition, by which the otherwise irregular plasma behavior was stabilized. The rf heating suppresses the ionization instability in the plasma behavior and homogenizes the nonuniformity of the plasma structures. The power-generating performance is significantly improved with the aid of the rf power under wide seeding conditions: insufficient, optimum, and excessive seed fractions. The increment of the enthalpy extraction ratio of around 2% is significantly greater than the fraction of the net rf power, that is, 0.16%, to the thermal input. © 2005 American Institute of Physics.

We describe the structure and behavior of a nonequilibrium cesium-seeded helium plasma in a disk-shaped magnetohydrodynamic generator. Excellent time-resolved optical measurements clarify the spatial distribution of the electron temperature. A correlation analysis and the visualization of the plasma structure reveal its propagation phenomena from upstream to downstream and the transformation from a homogeneous quasisteady state to a self-consistent periodical state. A linear perturbation analysis suggests that the inhomogeneous plasma structure associated with the unique electron temperature behavior is closely related to ionization instability due to weak seed ionization. (C) 2004 American Institute of Physics.

In this paper we describe the highest-performance closed-cycle magnetohydrodynamic (CCMHD) power generator driven by a shock-tube. Despite the use of a small-scale device having a power-generating volume of approximately 4 liters, outstanding performance, namely, the isentropic efficiency of 63%, the enthalpy extraction ratio of 30.8%, the electrical power output of 1.23 MW; and the power density of 297 MW/m(3), is achieved at the stagnation temperature of 2250 K and the applied magnetic flux density of 3.0 T. The following facts contribute to the world's highest performance of the CCMHD power generator: a less-divergent generator shape; the employment of cesium-seeded helium gas; the introduction of an inlet swirl; and the production of homogeneous plasma.

We describe the plasma-fluid behavior and performance of a less divergent disk-shaped radial-flow magnetohydrodynamic (MHD) generator using cesium-seeded helium (He-Cs) and compare it to a cesium-seed argon (Ar-Cs) generator. The less divergent generator using He-Cs provides a high isentropic efficiency of 45% and an enthalpy extraction ratio of 15.9%. The generator performance using He-Cs peaks at a low seed fraction of 1.7 × 10-4, which was reduced by a factor of four compared to the Ar-Cs generator. The proper He-Cs operation doubles the Hall parameter (3.7) but reduces the electrical conductivity to one-quarter in comparison with the Ar-Cs generator. The electron temperature of the He-Cs plasma is no more than 4000 K in a wide seed fraction range this suggests that the plasma is in a fully ionized seed condition. Thus, the He-Cs plasma forms a stable and homogeneous structure without any specific nonuniformity. Although radiation from the He-Cs plasma is perturbed, its characteristic frequency is high (from 60 to 90 kHz). In contrast, the Ar-Cs plasma forms a localized structure at low seed fractions and a dense concentric circular structure at high seed fractions. © 2004 IEEE.

In order to investigate the effects of plasma conditions on fluid-dynamical prediction of the performance of an MHD generator, local steady-state calculations are employed. The effective Hall parameter and effective electrical conductivity are estimated by taking the linear theory of ionization instability into account. The results of analytical calculations are compared with experimental ones. Although a fully ionized seed condition, which suppresses instability, provides the highest power generation performance, the condition could be realized only at a high seed fraction in the experiments. It is suggested by the analysis that the fully ionized seed plasma produced at a low seed fraction is desirable in order to achieve high performance. The analysis implies that instability due to insufficient or excessive electron temperatures is a performance-limiting factor. The effects of plasma conditions on performance are clearly explained by the present simple analysis. © 2003 Wiley Periodicals, Inc.

Closed cycle magnetohydrodynamics (CCMHD) power generation experiments using a shock-tube driven disk generator have been conducted to achieve high performance. Despite of a small-scale generator having a generating volume of 4.1 liter, outstanding performance: an isentropic efficiency of 63 % and an enthalpy extraction ratio of 30.8 % is achieved at a stagnation temperature of 2250 K and a stagnation pressure of 0.14 MPa, where an applied magnetic flux density is 3.0 T. Furthermore, a maximum electrical power output of 1.23 MW and a maximum power density of 297 MW/m(3) are obtained. The following facts contribute to a world's highest performance of the MHD power generation: a small area ratio of the disk generator, an employment of a cesium seeded helium gas, an introduction of an inlet swirl, and a production of a homogeneous plasma.

The possibility of the improvement in performance of a nonequilibrium disk magnetohydrodynamics (MHD) generator by externally applying a radio frequency (RF) electromagnetic field (RF preionization) was examined experimentally. The MHD power generation experiments were carried out using a shock-tube driven disk MHD generator with the RF induction coils for the two inlet stagnation temperatures of 2275(±75) K and 2650(±50) K. As a result, under the conditions of both high and low stagnation temperatures, the output power was increased by the RF preionization. The increase was markedly observed for the low inlet stagnation temperature, where the plasma could not be produced only by the Joule heating attributed to a self-excited electromotive force. For the high inlet stagnation temperature, the plasma state in the MHD channel was improved by the RF preionization, leading to the increase in the output power. For the low inlet stagnation temperature, the marked increase in the output power by the preionization was not only because applying the RF electromagnetic field triggered the plasma production by the Joule heating attributed to a self-excited electromotive force but also because the plasma with the high electrical conductivity was produced in the more upstream region of MHD channel by the Joule heating attributed to the RF electromagnetic field.

Power generation experiments of a nonequilibrium disk MHD generator have been conducted under segmented load conditions. Effects of the load segmentation on the plasma fluid behavior and the generator performance are made clear. The MHD channel is divided into upstream and downstream parts. Although an MHD interaction along the fluid flow is divided by a middle electrode, each individual power generating part influences each other electrically and fluid dynamically. A higher loading condition in the upstream part is preferable in comparison with that in the downstream one to obtain higher electrical conductivity of the plasma. An excessive enthalpy extraction from the upstream part deteriorates the isentropic efficiency of the generator. It is possible for the segmented loading to maintain a comparable performance to that of the single-loading case by controlling each electrical efficiency in the segmented generation channels.

Cooling performance of superfluid helium (He II) in a channel installing a porous material has been investigated experimentally. Instead of fiberglass reinforced plastic (FRP), the porous material is installed as a part of spacers in the channel expecting advantages of fountain effect.. When heat generation occurs. in the superconducting coil, the porous material can induce He 11 flow toward heated side with fountain effect which results in forced convection heat transfer. The test channel made of FRP is 170 nun long. Both ends of the channel are kept open to an atmospherically pressurized He 11 bath. Heat generation larger than the A transition heat flux causes a steep temperature rise, because it generates He I of very low heat conductivity compared to that of He II. It is confirmed that the He II flow induced by fountain effect can suppress the steep temperature rise in the channel. Even if heat generation exceeds the lambda transition heat flux, fountain effect arises. This effect combined with natural convection of He I to improve the heat transfer in the channel.

The stability of three types of CuNb reinforced Nb 3Sn wires has been experimentally studied in order to clarify effects of critical current density, J c, and Nb fraction in CuNb reinforcements. The stabilities, i.e., minimum quench energy, MQE, and normal zone propagation velocity, v p, were evaluated for sample wires having low J c and low Nb fraction (A) and high J c and low Nb fraction (B) with in-situ processed CuNb reinforcement and high J c and high Nb fraction with jelly-roll processed CuNb reinforcement (C). The MQE decreased with increase in transport current density normalized by J c, critical generation density, and Nb fraction, on the other hand, v p increased with increasing the transport current density, regardless of Nb fraction. We obtained basic data for the design of magnets having both a high stability and mechanical strength.

For a high field tokamak device, we have developed force-balanced coils (FBCs) which have nonuniform-pitch multi-pole helical-configuration to reduce centering electromagnetic forces without using cancellation coils. As a superconductivity magnet, the FBC configuration is favorable to the achievement of high critical current densities because of a Jc∥B effect. In the tokamak operation with the FBCs, the rise of the toroidal field synchronizes with the plasma current ramp up because the FBCs function both as toroidal field coils (TFCs) and as primary coils for ohmic heating. The torsional force exerted on the FBCs is comparable and the net centering force is reduced to 20% compared with those on conventional TFCs of the same dimensions.

In order to clarify effect of utilizing a Nb rich CuNb reinforment on superconducting stability, r-z two-dimensional time-dependent numerical simulations on composite CuNb/Nb3Sn wires are conducted. The time variations of temperature and current density distributions, minimum quench energy (MQE), and normal zone propagation velocity (νp) of a Cu-17vol%Nb/Nb3Sn wire, a Cu-63vol%Nb/Nb3Sn wire, and a conventional Cu/Nb3Sn wire are investigated. The increase of the volume fraction of an outermost Cu stabilizer provides high MQE but decreases the total current density. Although the νp is not significantly influenced by the Nb fraction, the Nb rich CuNb reinforcement sacrifices the MQE for its high tensile strength. It is important for magnet design to control the volume fraction of the Cu stabilizer and Nb fraction in the CuNb reinforcement to balance the desired current density, tensile strength, and superconducting stability.

By applying a radio-frequency (RF) electromagnetic field, the feasibility of improvement in the performance of a nonequilibrium disk-shaped magnetohydrodynamic (MND) generator suffering from mater vapor contamination in the,working gas is investigated with tau-theta two-dimensional numerical simulations Attention is paid to the relation between the behavior of MND plasma in the presence of the RF electric field and the generator performance. The water contamination causes the strongly nonuniform and unsteady plasma, and deteriorates its performance. The fluctuations of the electron temperature and of the ionization degree of seed atoms are found to be suppressed by applying the RF electric field. As a result, the enthalpy extraction ratio and the isentropic efficiency of the generator improve. The ratio of the required additional joule heating by the RF electric field to the thermal input to the generator for the stabilization of the plasma and the improvement in the performance is estimated to be about 0.9%.

The influence of azimuthal non-uniformity of the seed fraction on plasma structure and performance in a non-equilibrium disk MHD generator is investigated with a two dimensional r-theta numerical simulation. It is found that a locally high seed fraction causes mainly non-uniformity of gas-dynamical properties, whereas a locally low seed fraction develops a non-uniform plasma. Both locally high and low seed fractions reduce generator performance considerably. These results suggest a spatially uniform seed fraction should be required for high power generation. (C) 1999 Scripta Technica.

When a high magnetic field tokamak reactor is designed with reasonable sizes, the force-reduction technique is necessary. We have proposed Force-Balanced Coils(FBCs)for the purpose of vanishing of Lorentz forces exerted on the conductors and simplifying the construction of coil systems. The forcebalanced winding provides not only the toroidal magnetic fields but also the poloidal magnetic flux to induce the plasma current. The net radial force is reduced by a factor of 10 from that exerted on the toroidal magnetic field coils. We have manufactured a small tokamak device with FBCs(R_<major>/a_<minor>～0.3/0.1m)to demonstrate plasma production and confinement. The toroidal magnetic field of ～0.9T at the vessel center and the plasma current up to ～10kA were achieved by two-step FBC excitation. The plasma column with T_e of ～50eV and n_e of ～2×10^<19>m^<-3> was well centered in the vacuum vessel.

Recent experimental results of closed cycle MHD electrical power generation with the 'Fuji-1' blow-down facility are presented. In the experiment with Disk-F4 MHD generator, which was conducted with a modified seed injection system in 1997, an enthalpy extraction ratio of 18.4% was successfully demonstrated with a large output power of 506 kW. This enthalpy extraction ratio is the highest among those achieved with the Fuji-1 facility. The experimental results also revealed the electrical characteristics of the generator installed in the blow-down facility. The decline in the output power and its recovery were observed at the early stage of the power generation run. This fact could be attributed to the attachment of seed material to the generator walls and to its detachment, related to the relatively slow rise in temperature on the wall surface. It was verified for the first time in the Fuji-1 experiment that the reduction of impurity contamination resulted in improvement in the generator performance.

Structures of nonequilibrium inductively coupled plasmas with cesium metal vapor ionization in argon gas are revealed experimentally and are compared with ones from two-dimensional numerical simulations, The main object of the present paper is to clarify the behavior of the plasma in which cesium atoms as seed atoms are completely ionized, and the ionization of the argon gas as a mother gas is negligible, that is, the fully ionized seed (FIS) plasma, produced by the inductive radio-frequency electric fields, The plasmas are generated in a cylindrical quartz discharge tube around which a four-turn induction coil is wound, under the conditions of currents of &lt;10 A, excitation frequencies similar to 10 MHz, argon gas pressures 30-50 torr, and cesium mole fractions on the order of 10(-5), By cesium seeding, the quite uniform plasma in the azimuthal direction of the discharge tube is realized even at small coil currents, Under suitable operating conditions, the electron temperatures are in the range of 5000-7000 K near the tube wall, whereas the temperature around the axis is relatively low (similar to 3000 K), Then, the plasma consists of three layers, that is, the weakly ionized argon plasma, the FIS plasma in which the electron density is maintained almost uniform, and the partially ionized seed plasma, The thickness of the FIS plasma is determined by the distribution of the inductive electric field, The experimental results can be explained well by the numerical simulation based on two-dimensional vector potential and two-temperature plasma models.

Nonequilibrium plasmas with cesium metal vapor ionization in helium and argon gases at moderate pressures are excited with microwave power. The structures and behavior of the seeded plasmas are experimentally examined, particularly under the condition of full seed (cesium atoms) ionization. By cesium seeding, the minimum power sustaining the plasma is reduced markedly, and both a broad plasma observed in pure helium and unsteady filament-like plasmas in pure argon change to the steady and broad plasma locating close to the inner surface of a discharge tube. It is revealed from the electron temperature measurements that the plasma can he in the regime of full seed ionization for suitable microwave powers, where the electron density is kept almost constant. The thickness of the fully ionized seed (FIS) plasma decreases with increasing the mole fraction of cesium vapor, and is almost independent of noble gas pressure. The thickness almost coincides with the skin depth determined from the electrical conductivity almost uniform in the FIS plasma. These facts suggest that the FIS plasma will be easily produced and maintained as long as the microwave power is consumed to the electron heating.

Electron temperatures of nonequilibrium cesium seeded argon plasmas in a disk MHD generator installed in a blow-down facility are measured spectroscopically, and the generator performance is discussed in relation to the electron temperature. The temperature is decreased from ∼9000 K to ∼3000 K when the seed fraction is increased from 1 × 10-4 to 3 × 10-4. For the seed fraction of about 2 × 10-4 corresponding to the maximum power output, the temperature is found to be 4000-5000 K and the temperature fluctuation becomes minimal. For the seed fraction around 2 × 10-4, the electrical conductivity evaluated from the temperature is almost independent of the temperature. These facts suggest that the plasma is almost in the full seed ionization regime. Partially ionized argon and cesium plasmas are dominant at seed fractions below 1.3 × 10-4 and over 2.3 × 10-4, respectively, which degrades generator performance. © 1997 Scripta Technica, Inc.