Sumio Kogoshi

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.

Amplitude squeezing from singly resonant frequency-doubling laser

We analyze amplitude squeezing from a singly resonant frequency-doubling laser oscillating at a single frequency. In this laser system, the cavity loss depends on the intensity of the oscillating fundamental field, so that conventional analyses based on a mean-field approximation become invalid in a highly pumped regime. To avoid this inconvenience, we consider spatial evolution of fields both in a laser medium and in a nonlinear crystal. It is predicted for the first time that a combination of excess nonlinearity and modest laser saturation can increase the output noise. We propose novel indices to evaluate the possible noise enhancement and suggest a design rule for squeezed light generation.

Helical heliac coil reconstruction by higher harmonic modulation on L=1 helical coil

The idea of the higher harmonic modulation on helical coils may provide another possibility for improvement in a helical systems magnetic properties such as the formation of a local magnetic well. The higher harmonic modulation on L=1 Heliotron/Torsatron coil can approximately reproduce one of the optimized helicalheliac coils and its vacuum magnetic properties such as a deep magnetic well

Quantum noise analysis of frequency-doubling ring lasers: nonlinearity-induced noise enhancement and its dependence on gain profile

Using a traveling-wave model that can treat the spatial evolution of light fields, quantum noise characteristics of frequency-doubling ring lasers are analyzed under single-frequency operation. Systems under consideration are the discrete system, in which a laser crystal and a nonlinear crystal are separately installed, and the self-doubling system, which is equipped with a nonlinear crystal with laser gain. It is predicted that the unsaturated gain profile does not affect noise characteristics of the discrete system: nonlinear loss-induced noise enhancement (NINE), i.e., an increase of the spontaneous emission noise associated with enlarged gain that compensates the nonlinear loss, manifests itself at a specific operating point regardless of the gain profile. In the self-doubling system, on the other hand, the manifestation of NINE will arise for higher harmonic output compared with that in the discrete system. For the self-doubling system, moreover, the profile of the unsaturated gain causes a significant difference in NINE; because NINE is associated with a large field variation, it is effectively suppressed by configuring the gain profile to be flat or increasing along the direction of laser field propagation.

Chaotic Approach to Evaluation of Disturbed Magnetic Surfaces

A circle mapping can approximately reproduce the cross section of magnetic surfaces and the value of the periodic driving force ( K ) at a magnetic island varies with the width of the magnetic island, which suggests the value of K is one of measures for the degradation of magnetic surfaces. The profile of a rotational transform has flat regions at the magnetic islands. The width of the flat region is proportional to the width of the magnetic island. Therefore it may be another measure of the degradation of magnetic surfaces. This method requires less data for the estimation than the usual method of calculating the width of magnetic islands. For collapsed magnetic surfaces that are produced by overlapping of two magnetic islands, the fractal dimension can effectively estimate the degradation of them. The fractal dimensions of cross sections of regular magnetic surfaces and clear magnetic islands are nearly 1, while that of a collapsed magnetic surface is about 1.2 in the present study.

Vacuum Property Improvement of a L=1 Heliotron/Torsatron Device by Higher Harmonic Modulation on a Helical Coil

A winding law of a helical coil with higher harmonic modulation can produce a deep magnetic well because of a suitable vertical field formation. The sin ( n φ) modulation ( n =1, 2, 3, 4) on a L =1 helical coil makes magnetic surfaces resembling those of L = n helical coil at θ=0. Also it increases a magnetic ripple and rotational transform on a magnetic axis.

Simplified UV‐preionized long‐pulse TEA CO2‐laser amplifier

A simplified UV‐preionized long‐pulse TEA CO2‐laser amplifier is proposed, which is powered by a common Marx generator for both the preionization arc discharges and the main laser discharge. The measurements of small‐signal gain reveal that there exists an optimum capacitance for the capacitors which are provided in the preionization circuit for the control of preionization level. The physical interpretation for this is also given. A small‐signal gain higher than 3%/cm and an extraction energy of nearly 14 J/l (at the input energy level of nearly 1 J) have been achieved. It has also been demonstrated that the amplifier can amplify a long laser pulse of the order of microseconds without serious distortion of pulse waveform.

Generation of coherent lyman-alpha radiation with an alexandrite laser for diagnostis of neutral hydrogen density

Abstract A frequency-doubled alexandrite laser at 3648 A was frequency tripled in krypton to generate coherent Lyman-alpha radiation (1216 A). Measurement of the output as a function of Kr-gas pressure gives the optimal pressure of 55 Torr.

High-Temperature High-Quality Deuterium Plasma Production by Laser Beam(s) by Means of On-Line Computer Control Technique

The development of a special new technique is described by which high-temperature and high-quality (fully-ionized and impurity-free) deuterium plasma may be produced within any type of magnetic bottles.

Characteristics of pellet injected discharges in TEXTOR

Pellets injected into the TEXTOR tokamak lead to a density profile peaking which is strongest at low plasma current and weakest at high current independent of BT. After the injection two types of density oscillations are excited, the first type follows immediately the injection and the second one is excited with a delay of more than ten milliseconds. The oscillations are also observed in runaway discharges; the synchrotron light from the relativistic electrons drops after the pellet injection and is subsequently modulated due to a trapping of the runaways in magnetic islands. First Faraday measurements have been performed indicating that the distribution of the plasma current is not measurably modified by the pellet.