Nobutada Aoki

Underwater Direct Metal Processing by High-Power Copper Vapour Laser

Copper Vapour Lasers (CVLs) have been applied to underwater direct metal cutting and surface stress improvement. CVL’s features of high peak power, high repetition rate and visible wavelength enable fast, clean and thermal-effect free processing. The CVL based processing is expected as an alternative repairing or maintaining method in nuclear facilities.

High-power copper vapor laser development

Copper vapor laser (CVL) devices of 60 mm bore have been developed for atomic vapor laser isotope separation, and recently an average power per device of over 200W (211W maximum) has been achieved by expanding the discharge length to 3000 mm. In order to further improve the power output by increasing the discharge volume, it is important to supply discharge energy efficiently to the laser tube and to maintain the optimum copper vapor density in the larger volume. Now a CVL discharge circuit has been designed using a CVL discharge simulation code able to calculate time-dependent plasma resistance. In addition, a thermal insulation structure that effectively maximizes the laser gain volume has been designed using a thermal simulation code that takes thermal conduction and emission loss from the tube ends into account. This code yields results which show good agreement with experimental data. The results demonstrate that codes which simulate electrical and thermal characteristics are effective tools in the design of high-power CVL devices. In this paper, the methods of designing CVL electrical circuits and laser head structures using CVL simulation codes are reported, and some resulting high-power devices are discussed.

Compact soft X-ray source for biological applications

A compact soft X-ray source with carbon nanotubes (CNTs) based field emission electron source is being developed for applications to biological, medical and material technology fields. In the tests using a diode-type source, the maximum current of 13 mA (density of over 50 mA/cm/sup 2/) was extracted from the CNTs emitters (0.26cm/sup 2/) at electric field of about 6 V//spl mu/m and 2.5 /spl times/ 10/sup -5/ Pa pressure. In addition, a triode-type source was designed to obtain a pencil-like soft X-ray beam. Geometrical parameters in the source were optimized using calculation codes for electric fields and electron beam orbits. The X-ray source was stably operated, and the electron beam current of 0.24 mA was obtained at the applied anode voltage of 35 kV. Based on the numerical estimations, it is expected that the focal size is about 0.5 mm, and the X-ray beam density is 1.5/spl times/ 10/sup 7/ photons/s with the divergence angle of 30 mrad.

Method of Generating Monochromatic Soft X-ray with Small Focal Spot

In this paper, we present a method of generating monochromatic soft X-rays with a small focal spot. We used transmission metal targets bombarded by electrons to generate L X-rays. We developed the transmission targets by optimizing the incident electron energies and the target thicknesses. We surveyed optimal conditions for monochromaticity, for Ag and Mo targets, by Monte Carlo simulation (EGS4). The results predicted that monochromatic L X-rays are obtained by setting the target thickness to about 100 times the electron range and the electron energy to slightly less than 2 times the L X-ray energy. We estimated the size of the focal spot of monochromatic X-rays to be 4 mum (FWHM) at an electron energy of 5 keV, for a target thickness of 5 mum. For the Mo target, we experimentally confirmed that these conditions were optimal. Experimental spectra matched simulated spectra well, except for the higher energy region.

Development of high-power copper vapor laser system

A high power copper vapor laser (CVL) system in master oscillator power amplifier configuration has been developed for laser isotope separation program in Japan. Maximum output power of 650 W has been successfully achieved with 9- cm diameter and 350 cm discharge length amplifier. Also MOPA output power of 2.4 kW has been demonstrated in small master oscillator with 4 cm bore and 4 stage power amplifier with 9 cm bore configuration. The authors developed a thermal calculation code to maintain an optimum copper vapor density throughout a large volume and a new thermal insulation structure design method has been proposed to combine two different heat insulators to make longitudinal temperature distribution of the laser tube as flat as possible. A CVL discharge circuit has been improved by applying an excellent magnetic switch which prove a approximately 90 kV-4000 A pulse to a CVL at 4.4 kHz repetition rate. This paper reports such CVL design methods together with the performance of the designed high power CVL system.

Laser processing to improve residual surface stress of metal components

Laser processing of water-immersed material has been developed to improve the residual surface stress of metal components. The process changes the stress field from tensile to compressive through irradiation by a frequency-doubled YAG laser or copper vapor laser (CVL).