Yasuhito Kiwamoto

Plasma Turbulence Generated by Ion-Ion Two-Stream instability

An instability is generated by injecting a high density ion-beam with a large cross-section into an unmagnetized homogeneous plasma. The linear dispersion relation of the wave in the ion-ion two-stream system is experimentally confirmed. Three-dimensional investigations of the turbulence thus produced reveal some new features: The turbulent waves propagate at large angles to the beam with extremely short correlation lengths. At these angles a rapid relaxation of the velocity distribution of the ions is observed until the beam temperature reaches around 0.2 T e . the observed rapid heating of ions can be explained by a theory incorporating the effect of orbit modification.

Study of Potential Formation in an Open Magnetic Field Configuration

A theoretical model is developed on the electrostatic potential which is formed during fundamental electron cyclotron resonance heating in an end region of a tandem mirror. The model includes experimental conditions such as expanding magnetic flux tube, multicomponent electron temperature and secondary electrons emitted from an end plate. The axial potential profile of a collisionless plasma is analyzed with non-Maxwellian velocity distribution functions of ions and electrons. The potential profile in the plasma region, where the charge-neutrality holds, depends predominantly on the magnetic field profile. The potential drop in the sheath region in front of the end plate strongly depends on the secondary electrons and the fraction of the higher temperature component of the primary electrons.

Energy analysis of ECRH-induced end-loss warm electrons in a tandem mirror

The amount of an end-loss flux of warm electrons and its energy spectrum are studied by using an electrostatic energy analyser over a wide range of energy up to 10 keV in the GAMMA 10 tandem mirror. The outgoing electron flux is found to be much larger than the associated ion flux during the strong end-loss plugging under the fundamental electron cyclotron resonance heating. The observed energy spectrum reveals a high-energy tail component ( T eH ∼3 keV) in addition to the already reported bulk warm component ( T eL ∼700 eV). The new spectrum is successfully applied to evaluate the secondary electron flux from end plates so as to account for the charge balance of the net flux of electrons and the ion flux on the floating plates.

Velocity-Space Diffusion of Wave-Heated Electrons in a Mirror Field

A theoretical model is presented which accounts for an enhanced flux of loss-cone electrons observed when fundamental electron cyclotron resonance heating (ECRH) is applied on a magnetic slope in a tandem-mirror plasma. The essential point of the model consists in the velocity-space diffusion across densely converging heating-characteristic curves along which electrons have high mobility under strong ECRH. Remaining shortcomings in the present model are also addressed.

Transport processes of a non-neutral plasma coupled to an external rotating wave

Experimental investigations are carried out on radial transport phenomena of a pure electron plasma under the application of an external rotating wave that belongs to Trivelpiece-Gould (T-G) modes. Substantial radial compression of the density distribution is achieved by application of a properly controlled rotating electric field to one side of the plasma. Analyses of the observed plasma wave indicate that the efficient increase of the on-axis density entails the substantial damping of the T-G wave propagating in the plasma. The radial particle flux observed during the density compression is consistent with that of the theoretical model based on the drift-kinetic Vlasov equation [Y. Kiwamoto et al., Phys. Plasmas 12, 094501 (2005)]. This result implies that the radial compression of the plasma density distribution consists of the transverse E×B drift of particles subject to resonant wave-particle interaction in the axial dynamics.

A Pinhole Camera with Microchannel Plates for Hot Electron Spatial Distribution Measurement

A sensitive X-ray pinhole camera with microchannel plates (MCPs) has been constructed and successfully used to obtain X-ray images of hot electron distributions produced by electron cyclotron resonance heating in the thermal barrier region of the axisymmetrized tandem mirror GAMMA 10. Tomographic techniques have been applied to reconstruct a 2-dimensional X-ray intensity distribution. It is found that hot electrons are accumulated on the magnetic axis in a disc-like shape with a full width at half maximum (FWHM) diameter of about 25 cm.

Determination of equilibrium density distribution and temperature of a pure electron plasma confined in a Penning trap

A fast scheme is proposed for determining the three-dimensional density distribution of a pure electron plasma under the thermal relaxation in a Penning trap on the basis of the observation of the axially integrated density distribution. The analysis that includes the contribution of the conducting wall reveals the appearance of a halo distribution surrounding the core distribution around the midplane. The core is confirmed to approach the Penning-type equilibrium distribution. Also proposed is a new scheme of the temperature determination of the electrons by analyzing a radial profile of the particles extracted with energy selection. This method is available on the basis of the self-consistent potential distribution associated with the equilibrium density distribution. The application of the two schemes of data analysis shows that the electron temperature decreases well below 0.1eV with a 1∕e folding time of ≈4× cyclotron-cooling time in the trap kept at room temperature.

Electrostatic Potential in the Endcells of a Tandem Mirror under Electron Cyclotron Resonance Heating

The electrostatic potential profile in the endcells of a tandem mirror under electron cyclotron resonance heating is studied by including an ionization process. A plug potential is found to form with a combination of created ions by ionization, wave-heated electrostatically trapped electrons and mirror trapped electrons, but without a help of high energy sloshing ions. In the present model the electrostatic potential has its maximum at the outer mirror throat of end mirror cell.

Calculation of Diamagnetic Current in a Quadrupole Minimum B Mirror Field

An analytical expression of the diamagnetic current in a quadrupole minimum B mirror field is derived in the flux coordinates using a paraxial approximation. For various plasma pressure profiles, a diamagnetic flux is calculated which goes through racetrack- and baseball-shaped diamagnetic coils installed in the anchor cell of the GAMMA 10 tandem mirror.

Radial transport in magnetized non-neutral plasma driven by rotating wave

Radial transport in non-neutral plasmas driven by a rotating wave field is examined in terms of the drift-kinetic Vlasov equation. It is shown that the radial flux is generated by the E×B drift of resonant particles subject to Landau damping in the axial dynamics. The rate of change in the canonical angular momentum associated with the radial flux is equal to the torque resonantly exerted by the azimuthal component Eθ of the wave. The absorbed wave energy is shared between the axial kinetic energy of the particles and potential energy of the charged particle system. The basic idea of this model may be extended to a scenario that the radial flux is generated by any other dissipative processes that shift the phase relation between the wave and the particle orbits.