Haruo Obayashi

On the generation of runaway electrons and their impact to plasma facing components

Runaway electrons generated by inductive or non-inductive plasma currents in a tokamak have severe interactions with plasma facing materials of a first wall, influencing the first wall structure due to activation and damage. In this paper, modelling of runaway electron generation near the wall in a tokamak is carried out. This includes the evaluation of acceleration along magnetic surfaces for relativistic electrons with energies larger than the runaway threshold. The penetration of runaway electrons of energy ranges from a few MeV to several tens of MeV leads to gamma ray photon production by bremsstrahlung. One of the specific features of their impact on first wall technology is that these runaway electrons give rise to activation due to the giant resonance of the (γ, n) nulcear reaction and, as a consequence, cause a requirement of remote maintenance. Additionally, these runaways induce energy deposition at brazing areas between low Z material and metal, or at a metal itself, leading to melting, cracking and grain growth. The methods to estimate these effects of runaway electrons on plasma facing components are introduced and examples of estimation are given.

POLARIZATION OF RECOIL PROTONS FROM NEUTRAL PION PHOTOPRODUCTION.

The polarization of the recoil proton in the process γ+ P → p +π° was measured for a pion angle of 120° in the center of mass system at photon energies 450, 500, 550 and 600 Mev. Magnetically-analyzed protons were detected by a counter telescope consisting of three scintillation counters, which triggered the spark chamber system designed as a polarization analyzer. About 150 thousand spark chamber photographs were taken, and the tracks of protons quasi-elastically scattered by the carbon plates of the spark chamber and by a carbon block in its front were selected by manual scanning. Data from about 5000 photographs thus selected were processed by an electronic computer. Results obtained are: -0.07±0.18 for 450 MeV, -0.25±0.14 for 500 MeV, -0.38±0.12 for 550 Mev and -0.43±0.18 for 600 meV.

Radiation protection concepts of Large Helical Device (LHD) facility

Abstract Preliminary radiation protection design of Large Helical Device (LHD) was performed. At first, neutron source strength was estimated. Then, shield wall thickness was designed to match the dose limitation of non-controlled area. Shield door was checked from the simple slit streaming calculation. Duct streaming contributions are calculated to estimate the doses at the cellar and the adjacent room. Finally neutron and γ-ray skyshine was calculated. Air and water radioactivity were estimated to decide controlled area. Activation of the device was estimated for accessibility consideration after operation.

Radiation shielding analysis of a large helical device

Abstract Radiation shielding analysis of a large helical device (LHD) has been performed to provide information on a radiation environment needed for equipment design and operation planning. First, the applicability of the JENDL-3 library which was applied to the shielding calculations of the LHD was tested by means of a benchmark experiment on the shielding efficiency of ordinary concrete carried out at the fusion neutronics source facility at JAERI. Benchmark analysis was carried out with the MCNP continuous energy Monte Carlo code to test the JENDL-3 library itself. The same benchmark calculation was carried out with the DOT3.5 code to examine the groupwise cross-section library of fusion-40 processed from JENDL-3. Secondly, the evaluation of the following items with the DOT3.5 code was revised, using fusion-40 instead of the formerly used GICX-40 library: (1) dose distribution inside and outside the LHD building; (2) dose distribution inside the cellar of the LHD building resulting from steamed radiations; (3) Environmental dose distribution resulting from sky-shine effect; (4) activity and dose rate levels of vacuum vessel and superconducting magnet.

Loss Mechanism of Electrons from a Magnetic Bottle

The motion of electrons in a static magnetic field of mirror geometry was investigated by injecting a beam of KeV electrons with a small electron gun in the central region of the magnetic bottle. The first order orbit theory was well verified by high vacuum experiment. The The mirror loss of KeV electrons was studied by filling the vaccum chamber with helium and argon gases. The mean lifetime of trapped electrons was measured by making use of the azimuthal drift of electrons as a function of gas pressure. The elastic scattering of single or plural type is concluded to be the most responsible for the mirror loss at pressure of 10 -4 mmHg or higher.

On the Possibility of the Two-Fermion, Interaction

As the general source for non-leptonic weak interactions, an elementary two-fermion interaction is introduced and investigated. The calculated asymmetry factors and decay rates for LAMBDA and SIGMA decays show that this interaction, with some corrections due to the strong pion-baryon interactions, does not seem sufficient to describe these decay processes. Hence the main features of weak interactions cannot be ascribed to the two-fermion interaction only. (auth)

STUDIES ON SITE AND FACILITIES IN R-PROJECT

Task problems on the site and facilities for a proposed DT-reacting plasma experiment (R-project) are given and discussed, with emphasis of minimizing environmental impacts, especially from the radiological safety point of view.

Recoil Proton Polarization and Differential Cross Section in Eta-Meson Photoproduction at 890 MeV

The differential cross section and recoil proton polarization for the process γ+ p →η°+ p were measured by detecting photons from η-decay and recoil protons. The differential cross sections are 0.75±0.03 and 1.04±0.07 µb/sr at the CM angles θ η * =82.3° and 108.3°, respectively, at the incident photon energy k =890 MeV, † and 0.20±0.05 µb/sr at θ η * =83.1° and k =1050 MeV. Thus the differential cross sections at k =890 MeV seem to show a backward peaking. The value of recoil proton polarization is -0.15±0.30 at θ η * =82.3°, whereas it is 0.27±0.25 at θ η * =108.3°, both at k =890 MeV, thus suggesting a sign change around θ η * =90°. In addition to the S 11 (1535), a comparably large contribution of the D 13 (1520) is needed to explain the experimental results.

Monte Carlo calculation of energy spectra on a γ-counter in detection of η0 and π0 mesons

Abstract Energy spectra of gamma-rays from η 0 and π 0 mesons produced in the γ+p reactions are computed by the Monte Carlo method. The detector system consists of a Cerenkov counter with large aperture and some electronic circuits. In the course of the calculation the measured characteristics of the system, such as the energy resolution of the counter, the sensitivity of the acceptance area and the signal modification in the circuit system, are taken into account. The kinematical efficiency for detecting the gamma-rays from η 0 and π 0 mesons is given in terms of a weight function proportional to the Lorentz invariant phase volume. This saves much of the computing time of the Monte Carlo treatment. The comparison with the experimental energy spectra shows the usefulness of the present computational scheme.

Motion of Electrons in a Magnetic Bottle, II

In order to investigate the motion of electrons as single particles, we have constructed a magnetic bottle, into which electrons are injected by an electron gun, The magnetic bottle field was produced by a pair of air core coils excited by a steady current, A cylindrical vacuum chamber was inserted through the coils, so that its axis coincides with the magnetic axis, Electrons were injected at the center of the bottle and were observed at various positions.The present paper is the first report of our experiment, by which we aim at understanding qualitative features of the motion of electrons in our device, We are here interested mainly in the effect of scattering by gases on the mirror loss, In order to avoid the shadow effect of the gun as far as possible, electrons were injected as short pulses, so that a fraction of electrons getting rid of colliding with the gun system could be distinguished from those which have collided, The shadow effect was further eliminated by varying the pressure of a gas (He or A) filled in the chamber, The trapping time of injected electrons was deduced essentially from the intensity of slow electrons produced by their collisions with gas atoms and it was found to be accounted for in terms of the single Coulomb scattering. The lifetime of the slow electrons can be qualitatively explained by their atomic scattering.