Chikara Ichihara

Newly Designed 3 T MRI Magnet Wound With Bi-2223 Tape Conductors

We have designed and fabricated a 3T magnetic resonance imaging magnet system for the human brain, which was wound with Bi-2223 tape conductors. Cooled by a Gifford-McMahon cryocooler, it was operated at 20 K with a stored energy of 2.3 MJ. A magnetic-field homogeneity of 5 ppm was attained at 1.5 T, which was our target value. Using this Bi-2223 high-temperature superconducting magnet, we obtained magnetic resonance images in 1.5 T at 8.5 K. The system was successfully magnetized to 3 T, which is the final target field in our project. This work demonstrates the potential of the high-temperature superconducting magnet for use in human magnetic resonance imaging experiments.

Effect of Mg or Ag addition on the evaporation field of Al.

It is known that the distribution of the charge-states as well as the evaporation field shift to higher values as the specimen temperature is decreased at a constant rate of evaporation. This study has explored the effect of Mg or Ag addition on the evaporation field of Al in terms of the charge state distribution of the field evaporated Al ions. The fractional abundance of Al(2+) ions with respect to the total Al ions in Al-Mg alloy is lower than that in pure Al, whereas it shows higher level in the Al-Ag alloy at lower temperatures. The temperature dependence of the fractional abundance of Al(2+) ions has been also confirmed, suggesting that Al atoms in the Al-Mg alloy need lower evaporation field, while higher field is necessary to evaporate Al atoms in the Al-Ag alloy, compared with pure Al. This tendency is in agreement with that of the evaporation fields estimated theoretically by means of measurements of the work function and calculations of the binding energy of the pure Al, Al-Mg and Al-Ag alloys.

Depth profile analysis of helium in silicon with high-resolution elastic recoil detection analysis

Helium depth profiling in silicon was investigated by high-resolution elastic recoil detection analysis (HERDA) using a high-resolution Rutherford backscattering spectrometry system. A 0.7 μm Mylar film was installed in front of the detector of this system to eliminate the background signals originating from incident nitrogen ions scattered at the sample surface. This film successfully eliminated the background signals without the loss of helium signals or serious degradation of the depth resolution. The HERDA has several attractive features for helium depth profiling in silicon, including high sensitivity (detection limit of 1 at. % or less), excellent depth resolution (less than 1 nm), and good quantitative accuracy, giving it significant advantages over other surface analysis methods such as secondary ion mass spectrometry.

Discharge characteristics of helium under inhomogeneous pressure distribution

Discharge characteristics of helium gas under inhomogeneous pressure distribution were examined both analytically and experimentally. The purpose of the study is to generate plasma within a limited space introducing gas through a very small aperture. First we derived an analytical expression of the Paschens law for the discharge under inhomogeneous pressure distribution. It was obtained that larger pd product, where p is the pressure and d is the distance between the anode and cathode, is necessary, but the lowest spark voltage may become lower as compared to that of normal homogeneous pressure distribution. Experiments were also done, and current-voltage characteristics and relationship between the pressure and the discharge voltage were obtained. It was also confirmed that localized plasma could be formed. The voltage rapidly increased with a decrease in the gas pressure. Lowering of the discharge voltage was, however, not observed in the present study

DESIGN OF A NEW HIGH VOLTAGE GENERATOR FOR ION BEAM ANALYSIS

Abstract A new type of high voltage generator has been designed, with the aim of developing a compact, inexpensive and easily-maintained generator for energetic ion beam analyses. It has a similar structure to a rotating disk-type generator, such as the Disktron, and has the potential to have a compact size and satisfactory generating voltage stability. Its generating mechanism is, however, rather similar to rectifier type generators, such as the Cockcroft-Walton and the Schenkel types, so that there are no friction components which might produce a lot of dust, so opening the high pressure tank for cleaning is an infrequent operation. The proposed generator can be produced relatively inexpensively. A single module model has been built and it was confirmed that the proposed generating mechanism functions successfully.

New MV-class generator

A new type of high voltage generator has been designed with the aim of developing a compact, inexpensive and easily maintained generator for energetic ion beam analyses such as Rutherford backscattering and particle induced X-ray emission. It consists of several rotating disks and fixed plates made of insulator with metal plates. Since the structure is similar to a rotating disk-type generator, such as the Disktron, it should basically have the potential to display indicators such as compact size and reliable generated voltage stability. Its generating mechanism is, however, rather similar to rectifier-type generators, such as the Cockcroft- Walton and the Schenkel types, so that there are no friction components which might produce a lot of dust, thus opening the high pressure tank for cleaning is an infrequent operation. The proposed generator can be produced relatively inexpensively. A single module model has been built and its generating ability was studied. As a result, it was confirmed that the proposed generating mechanism functions successfully.