Katsuji Shimoda

Imaging Bragg spectrometer for pinched plasma experiments

A compact Bragg spectrometer for monochromatic imaging in the soft x‐ray region was designed and examined. With this spectrometer, a pinhole image is simultaneously recorded with a monochromatic image. This makes it possible to easily identify the wavelength when the location of the soft x‐ray source fluctuates from shot to shot, such as in the plasma focus and Z‐pinch plasma. An rubidium acid phtalate (RAP) flat crystal was examined and monochromatic images of the argon K lines around 4 A were successfully obtained and displayed. The resolution power and dispersion obtained in situ were compared with the calculated values.

Neutrons, X-Rays and Charged Particle Beams Emission in a 65 kV Plasma Focus

The correlation between the dynamic behaviour of a focused plasma and the bursts of neutrons, X-rays and charged particle beams have been investigated experimentally using a 65 kV plasma focus device. The dynamic bahaviour has been observed with a Mach-Zehnder interferometric system operating in the framing mode. The ion- and the electron beams are detected by a Thomson parabola analyzer and a Cerenkov detector, respectively. Neutrons, X-rays and charged particle beams are produced when a voltage spike appears. The generation of the electron- and the ion beam occur in an interval of 20 ns around the peak of the voltage spike. The m=0 type instabilities are observed even if the voltage spike does not appear. The plasma inductance increases monotonically but rather slowly during the process of the m=0 type instability growth. The neutron yield is ascribed to a generation of the anomalous resistivity in the plasma.

A Method for Measuring Electron Energy Distribution in a Plasma Focus

A magnetic energy analyzer is designed for pulsed energetic electron beams, and successfully used to obtain energy distribution of the electron beam generated by a plasma focus device. In order to provide quick access of the electron energy distribution in a single shot, an electron detection system consisting of an MCP (microchannel plate) and a PCD (plasma-coupled device) linear image sensor is developed. Sensitivity calibration of the system is carried out using the nuclear emulsion sensitive to energetic electrons. Energy resolution and dynamic range of the detector are presented as functions of the electron energy. The major part of the energy-distributed electrons generated by the plasma focus device are in the relativistic region extending to higher than 1 MeV. The acceleration mechanism of those electrons up to such a huge energy is discussed.

A system for a multiframing interferometry and its application to a plasma focus experiment

A four-framing Mach-Zehnder interferometer system which has variable intervals from frame to frame is developed. TEA N(2) lasers that are operated with atmospheric-pressure N(2) gas are employed as light sources. Applicability of the system is demonstrated for a rapidly changing plasma in the plasma focus discharge.

Cross Check of Electron Density Distribution by Moiré-Schlieren Technique and Mach-Zehnder Interferometry

Simultaneous measurements of the electron density in a plasma focus by a Moire-Schlieren technique and Mach-Zehnder interferometry were carried out to investigate the unavoidable error in the interferometry generated by refraction of the probe beam by the density gradient. The refraction angle was measured precisely by the Moire-Schlieren technique, and the spatial distribution of the electron density calculated from the refraction angle agreed well with that obtained by Mach-Zehnder interferometry. The error produced by the refraction is found to be negligible if an N2-laser is employed as the light source for the plasma focus.

Quantitative measurement of x-ray images with a gated microchannel plate system in a z-pinch plasma experiment

A multiframe, gated pinhole system capable of quantitative acquisition for pulsed soft x rays is described and tested. The system based on a gated microchannel plate (MCP) is employed to observe the evolution of z-pinch plasma in a plasma focus facility with a time resolution of similar to 220 ps. The quantitative relationship between x-ray source intensity and the recorded images has been investigated. To make a quantitative measurement, the phosphor screen current was measured to calculate the total electrons output from the MCP, which is proportional to both the incident x-ray intensity and the intensity of the recorded images. Furthermore, by taking into account the pinhole geometry, MCP gain and system spectral response, a quantitative calibration of the x-ray images has been established. We have employed the system to observe the plasma evolution in a plasma focus facility. An order of similar to 10(16) photons/(s mm(2) mrad(2)) soft x-ray emission within 6-14 Angstrom was observed in a neon puffed experiment with time resolved four successive frames

Characterization of Soft X-Rays Generated in Gas-Puffed Z-Pinch

Soft X-rays generated with a gas-puff pinch device is characterized using a pinhole camera which enables a quantitative intensity measurement, a Bragg imaging spectrometer and an X-ray diode. A neutral argon shell whose line density was 8.1×10 17 atoms/cm ( M =5.5×10 -5 g/cm) was formed between the electrodes. The soft X-rays were emitted for about 10 ns in the early stage of the plasma column formation. Highly localized and intense soft X-ray sources, so-called hot spots, were observed in the pinched plasma. The emitted energy was estimated to be 50±15 J into 4π sr, for Ar K -lines. The reproducibility of the X-ray emission are discussed.

Correlation between Plasma Dynamics and Emmision of Deuteron Beam, X-Rays and Neutrons in a Plasma Focus Discharge

The correlation between the plasma dynamics and the emission of a deuteron beam, X-rays and neutrons in a plasma focus discharge was investigated experimentally. The dynamics were observed using interferometry in the streak mode and framing mode, and the energy distribution of the deuteron beam was measured using a Thomson parabola analyzer. The plasma column was partially disrupted by the m=0 instability. The deuteron emission started some tens of nsec before the disruption of the plasma column, while the X-ray and neutron signals started some Lens of nsec after the disruption. The m=0 instability was observed in each discharge even if neutrons were not produced. It is concluded that the acceleration field of the ion beam giving the neutron yield is generated not by a rapid change in the plasma inductance but by an abrupt rise in the resistivity of the plasma. The fine structure of the bubble formed in front of the current sheet propagating downstream was observed.

Time correlation between plasma behaviour and soft X-ray emission in a plasma focus

Soft X-rays emitted from a plasma focus are investigated experimentally. In contrast to single-pulsive burst of neutron, hard X-rays, ion- and electron beams, the soft X-rays are observed from the collapse phase to the decay phase of the plasma column, and have typically three successive peaks in its signal. Each peak corresponds to the maximum compression, the disruption and the decay phase of plasma column. It is revealed that the first and the second peaks are radiated by plasma itself, whereas the third peak is caused by emission from the inner electrode face.

Measurement of Electron- and Ion Beam Energies and Currents in a Plasma Focus Discharge

Measurements of energetic particle beams in a plasma focus with a Mather type device are presented. Rogowski coils are used for time-resolved measurement, and solid-state nuclear track detectors for time-integrated measurement of the beams. In the upstream direction with respect to the discharge current, only the electron beam with the maximum current of several kA was detected, which was approximately one percent of the discharge current. The electron energies of the beam were spread from 0.1 to 1 MeV. In the downstream direction, two successive emissions of ions were observed. The first emission had an extremely high energy of the order of some MeV and a low beam current of less than 10 A. The second emission, the main part of the ion beam, with energies of 100–800 keV, followed the first one with a time lag of several tens of nanoseconds, and the beam current reached several tens of amperes.