Tadashi Sekiguchi

Cusp confinement of high‐beta plasmas produced by a laser pulse from a freely‐falling deuterium ice pellet

Preliminary results of the experimental determination of the particle loss aperture sizes of the spindle cusp magnetic bottle for laser‐produced high‐beta deuterium plasmas are presented. The fully ionized dense plasma (with maximum density ∼1016 cm−3 and ion energy ∼200 V) is produced at the null‐field center of the magnetic bottle, from a freely falling isolated deuterium ice pellet by a focused giant laser pulse. In preparation, the free expansion of laser‐produced plasma has been studied experimentally, and the results indicate that the plasmas produced can maintain the fully ionized condition with prescribed plasma parameters until it expands to a certain prescribed volume if the combination of the physical parameters is properly chosen by considering the electron‐ion recombination as one of the important factors. The experimental results of cusp confinement for the plasma thus produced suggest that the loss aperture size of the ring cusp is much smaller than the local ion gyroradius, in sharp contra...

Re-thermalisation and flow of laser-produced plasmas in a uniform magnetic field

Detailed time- and spatially-resolved measurements of the electron temperature and plasma density have been made, by means of laser Thomson scattering and other plasma diagnostics, for plasmas produced by laser from a thin wire target in an almost uniform magnetic field ( approximately 1 T). The physical processes involved in plasma/magnetic-field interactions are discussed. It has been found that a part of the translational kinetic energy of the ions in an earlier rapid plasma expansion phase is reconverted into the thermal energy of electrons as the results of the interactions, through the ohmic heating, by a circulating diamagnetic current flowing near the plasma outer boundary, as well as through subsequently shock heating (and additional compression). Also, the plasma expansion along magnetic field lines may be regarded as almost one-dimensional, and the time variation of electron temperature is adiabatic.

Observation of a Slow-Surface Wave in Millimeterwave Solid-State Plasma Waveguide

Detailed numerical analyses on the propagation characteristics of a slow-surface wave mode in the transversely-magnetized partially-filled solid-plasma waveguide have been extended to the frequency region of 70 GHz on the bases of the theory developed in the previous paper. The results indicate the existence of at least two modes of propagation which exhibit the properties of the slow-surface wave. The experimental substantiation of the theoretical results has been carried out at 4 mm wavelength by employing two separate interferometric methods, employing the n-type InSb at the liquid-nitrogen temperature. The experimental results are in good quantitative agreement with the theoretical prediction for one of the two modes. The reduction of the wavelength in the plasma waveguide has been observed directly, from which the phase velocity as slow as 1/3~1/4 of the velocity of light has been obtained.

Possibility of Lower-Hybrid-Drift Instability in Laser Produced Plasma in a Uniform Magnetic Field

It is shown theoretically that Lower-Hybrid-Drift instability could occur on the surface of a laser-produced plasma column in a uniform magnetic field. Dispersion relation based on a linearized Vlasov equation with local approximation is numerically solved to demonstrate the dependence of this instability on the characteristic bouncing motion of a laser-produced plasma in a uniform magnetic field.

Computational studies on ionization processes of laser-produced high-Z plasmas.

A computer simulation code has been developed for plasmas produced by laser pulse from a solid target pellet which consists of material with a high atomic number. The code is based upon a spherically-symmetric, one-dimensional Lagrangian hydrodynamic code, combined with detailed atomic transition processes. Emphasis is placed on the ionization processes involved. The computed results indicate that the spatial region within the target volume where the production of multiply-charged ions prevails depends strongly upon the combination of target material and laser frequency. Also, it is shown that two distinctly different categories may exist as for ionization processes; one is the single-step process between the ground levels of each multiply-charged ion, and the other is through stepwise excitations among a series of excited levels of each ion.

Computational and Experimental Studies on Spontaneous Magnetic Field Generation Associated with Laser-Produced Plasmas in Vacuum

Experimental as well as Computer Simulation studies have been systematically performed on the spontaneous generation of magnetic fields associated with laser-produced plasmas from massive solid targets in vacuum. The experimental studies have been performed by employing a pulsed ruby-laser beam, aluminum plane target and magnetic probe(s) located in the vicinity of target surface to measure generated magnetic fields. Numbers of new interesting facts have been found experimentally on the time evolution of spatial distribution of magnetic field generated, and possible physical interpretations for them are given. The comparisons of our experimental results are made with those from our computer simulations, as well as with the other experimental results which have been performed in the existence of background gases, rather than in vacuum.

Development and Performance Characteristics of a UV-Preionized, High-Power TEA Pulsed CO2-Laser

This paper presents a series of systematic studies on a UV-preionized, high-power TEA pulsed CO2-laser with a total laser output energy exceeding 300 joules, a peak output power of GW level and a duration of the first spike of about 50 ns. The studies include (1) the preionization characteristics under UV-irradiation generated by an intense arc-discharge-gap assembly, (2) the main laser discharge characteristics in combination with the UV-preionizing scheme, (3) the optimum relative concentration of the laser gas mixture for maximum peak value of the small-signal gain, and (4) the laser oscillation characteristics using appropriate optical resonator systems which maximize the laser output energy contained in the first spike.

Behaviour of laser-produced high-β plasmas in a spindle-cusp magnetic container

Detailed experimental measurements have been made of the time-dependent behaviour of high- beta plasmas ( beta <or approximately=0.6) produced by a focused laser beam irradiating thin solid target wires of polyethylene or aluminium at the null-field of a spindle-cusp magnetic container. The ring cusp width is shown to increase from 0.6 to 2.9 times the local ion gyro-radius as the value of plasma beta decreases from 0.6 to below 0.2 during the confinement phase. The re-thermalisation of particle energy in the cusp container, the relatively rapid penetration of magnetic fields into the cusp plasma volume, and the containment properties of spindle-cusps for high- beta plasmas are discussed.

Theoretical Analysis on Formation of High-Beta Plasma Sheath

Mainly in connection with the recent results of spindle-cusp confinement experiments, the structure of “high-beta plasma sheath” in the thermal equilibrium as well as in the steady-state is theoretically examined. For the analysis, the velocity distribution function of plasma particles and the electric field induced by the charge separation within the sheath are taken into account. Various cases of the sheath thickness ranging from ton gyro-radius to electron gyro-radius are considered. On the basis of the results, the stability of sheath is discussed in relation to possibly excited micro-instabilities.

Plasma stability of a nearly spherical field‐reversed configuration

A nearly spherical field‐reversed configuration (FRC) is created by employing a combination of a double‐cusp bias magnetic field and laser‐produced plasma, and its plasma stability is studied. The FRC equilibrium configuration is found to last for 20–25 μsec, which is not determined by any plasma instability in the present experiment and is estimated to be longer than the Alfven time by a factor of more than 40. That is, the occurrence of neither the n=2 rotational instability nor the active magnetohydrodynamic (MHD) (tilting) mode is observed during this time period. The stabilization of the rotational mode may be attributed to the effect of magnetic field line bending. Although the exact reason for stabilizing the tilting mode is not known, it has become clear that the geometry of an equilibrium FRC has no significant effect on its stability, based upon comparison with the other experimental results obtained thus far from theta‐pinch‐based FRCs with a very elongated cross section. It is suggested that t...