Kunihiro Sato

Conceptual design of the D-3He reactor artemis

AbstractA comprehensive design study of the D-3He-fueled field-reversed configuration (FRC) reactor Artemis is carried out for the purpose of proving its attractive characteristics and clarifying the critical issues for a commercial fusion reactor. The FRC burning plasma is stabilized and sustained in a steady equilibrium by means of preferential trapping of D-3He fusion-produced energetic protons. A novel direct energy converter for 15-MeV protons is also presented. On the bases of consistent fusion plasma production and simple engineering, a compact and simple reactor concept is presented. The D-3He FRC power plant offers a most attractive prospect for energy development. It is environmentally acceptable in terms of radioactivity and fuel resources, and the estimated cost of electricity is low compared with a light water reactor. Critical physics and engineering issues in the development of the D-3He FRC reactor are clarified.

Correlation between potential well structure and neutron production in inertial electrostatic confinement fusion

The electrostatic potential well in inertial electrostatic confinement (IEC) is studied using two approaches. First, the equilibrium potential profile is obtained by solving the charge neutrality condition, i.e. ni=ne, assuming the appropriate distribution functions for the ions and the electrons. The formation of a double well structure is demonstrated, with a depth depending upon the ratio between the focus radii of the electrons and the ions. The correlations between the well depth and the volume integrated neutron production due to deuterium-deuterium (DD) reactions are obtained. Second, in order to study the stability of the well, the dynamic behaviours of the potential well are calculated by performing time advancing numerical simulations on the basis of the particle in cell method. Single, double and triple wells, depending on the amount of injected ion current, are observed to be formed for ions with a monoenergetic distribution. The well in the centre of the multiwell structure is unstable and oscillates with a period much longer than the inverse ion plasma frequency. A double well structure can be formed even for ions with a spread out energy distribution when the ion current is larger than the threshold value. The time averaged neutron production by DD fusion events is proportional to a power of the ion current involved in forming the double well structure. The results strongly suggest that the high neutron production rate should be attributed to not only the well depth but also the unstable behaviour of the potential, i.e. the intermittent peaking of the density in the centre region. A numerical simulation reveals that IEC possesses a favourable dependence of fusion reactions on the injected ion current for the application to a neutron source or a fusion reactor

Formation of presheath and current‐free double layer in a two‐electron‐temperature plasma

Development of the steady‐state potential in a two‐electron‐temperature plasma in contact with the wall is investigated analytically. It is shown that if the hot‐ to cold‐electron temperature ratio is of the order of 10, the potential drop in the presheath is allowed to have either a small value characterized by the cold electrons or a large value by the hot electrons, and discontinuously changes at a critical value for the hot‐ to total‐electron density ratio. It is also found that the monotonically decreasing potential structure, which consists of the first presheath, a current‐free double layer, the second presheath, and the sheath, can be steadily formed in a lower range of the hot‐ to total‐electron density ratio around the critical value. The current‐free double layer is set up due to existence of the two electron species and cold ions generated by ionization so as to connect two presheath potentials at different levels.

Sheath and presheath in a collisionless open-field plasma

The plasma–sheath equation for a collisionless plasma with a finite‐temperature particle source in a nonuniform open magnetic field is formulated. The plasma equation is solved analytically and the plasma–sheath equation is also solved numerically for various profiles of the magnetic field. The potential formed in the plasma depends considerably on the profile of the magnetic field strength; the potential drop in the presheath increases and the ion distribution function widens as the magnetic field strength decreases in the direction of the plasma flow. Application of the generalized Bohm criterion shows that the solution of the plasma equation always satisfies this criterion when the magnetic field monotonically decreases in the outside direction. The potential drop in the sheath is almost independent of the magnetic field profile, and the ion energy at a wall for a decreasing magnetic field is slightly large when compared with that for a uniform magnetic field.

Multi-potential well formation and neutron production in inertial-electrostatic confinement fusion by numerical simulations

The electrostatic potential well in an inertial-electrostatic confinement (IEC) is calculated by performing the numerical simulations based on the particle-in-cell method. The single, double and triple wells, depending on the amount of the injected ion current, are observed to be formed for the ions with a mono-energetic distribution. The well in the center of the multi-well structure is unstable and oscillates at the period much longer than the ion plasma frequency. A double well structure can be formed even for the ions with a spread energy distribution when the ion current is larger than the threshold value. The time-averaged neutron production in D-D fusion events is found to be proportional to the third power of the ion current where the double well structure is formed. The numerical simulation reveals that an IEC possesses the favorable dependence of fusion reactions on the injected ion current for the application to a neutron source or a fusion reactor.

Electrostatic Potential in a Collisionless Plasma Flow near the Magnetic Throat

Electrostatic potential formed in a collisionless plasma flow near the magnetic throat is investigated by kinetic analysis under the condition that a particle source in a plasma can be neglected. It is found that the plasma flow is required to satisfy the generalized Bohm criterion at the throat for the formation of a stationary continuous potential, and then the speed of plasma flow must exceed the acoustic speed. A monotonical potential profile falling from the inside of the throat to the wall can build up only if the Bohm criterion is marginally satisfied at the throat.

Effects of a nonuniform open magnetic field on the plasma presheath

Effects of a nonuniform magnetic field on the plasma presheath are numerically investigated using the plasma equation for a collisionless plasma with a finite‐temperature particle source. The present calculation confirms that previously published analytical solutions [Phys. Fluids B 1, 725 (1989)] are available over a wide range of mirror ratios. Potential drop in the presheath, which depends considerably on both the magnetic strength profile and the spatial distribution of the particle source, is remarkably increased by applying an expanding magnetic field when plasma particles are generated in the inner part of the plasma. An effect of a nonuniform magnetic field on sheath formation is also discussed by using a calculated ion distribution function. If the plasma equation has no singularity at the sheath edge, its solution satisfies the generalized Bohm criterion with the inequality sign in the expanding magnetic field.

Sheath and Heat Flow of a Two-Electron-Temperature Plasma in the Presence of Electron Emission

The electrostatic sheath and the heat flow of a two-electron-temperature plasma in the presence of electron emission are investigated analytically. It is shown that the energy flux is markedly enhanced to a value near the electron free-flow energy flux as a result of considerable reduction of the sheath potential due to electron emission if the fraction of hot electrons at the sheath edge is much smaller than one. If the hot- to cold-electron temperature ratio is of the order of ten and the hot electron density is comparable to the cold electron density, the action of the sheath as a thermal insulator is improved as a result of suppression of electron emission due to the space-charge effect of hot electrons.

Excitation of Flute-Type Negative-Energy Ion-Cyclotron Waves due to Ion Gradient-B Drifts in High-β Mirror-Confined Plasmas

An excitation of flute-type negative-energy ion-cyclotron waves by means of an ion gradient-β drift is analyzed on the basis of the linearized local dispersion relation. The excitation mechanism of these negative-energy waves is found to be a kind of an ion-cyclotron damping. This instability occurs even in axially homogeneous plasmas, and is still not stabilized even at a radial scale length large enough to stabilize the DCLC (drift-cyclotron loss-cone) instability. This instability is stabilized in the same way as the DCLC instability by an addition of a warm plasma.