Toshikazu Kawamoto

The squeezing potential of rocks around tunnels; Theory and prediction

SummaryThe deformational behaviour of tunnels, which underwent large deformations, socalled squeezing, have been recently receiving great attention in the field of rock mechanics and tunnelling. Contrary to rockbursting phenomenon in which the deformation of the medium takes place instantaneously, the deformation of the surrounding rock in squeezing phenomenon takes place slowly and gradually when the resulting stress state following the excavation exceeds the strength of the surrounding medium. Although there are some proposals for the definition of squeezing rocks and prediction of their squeezing potential and deformations of tunnels in literature, it is difficult to say that they are concise and appropriate.In the first half of this paper, the squeezing phenomenon of rock about tunnels and its mechanism and associated factors are clarified by studying carefully observed failures in-situ and laboratory model tests. Then, an extensive survey of tunnels in squeezing rocks in Japan is presented and the results of this survey are summarised. In the second half of the paper, a new method is proposed to predict the squeezing potential and deformations of tunnels in squeezing rock. Then, the method is applied to actual tunnelling projects, where squeezing problems have been encountered, to check its validity and applicability. As a concrete example, an application of the method to predict the squeezing potential and deformations of the rock along a 300 m long section of an actual tunnel was made.

The stability of slopes and underground openings against flexural toppling and their stabilisation

SummaryThe stability of slopes and underground openings during and after excavation is always of great concern in the field of rock engineering. Depending upon the geologic conditions and material properties of rock and discontinuities, and the geometry of excavations and topography, various kinds of instabilities are likely to be encountered. One of these is the flexural toppling failure which has become to be known recently. A stability analysis method for slopes and underground openings under various loading conditions against the flexural toppling failure is proposed. In addition, for the stabilisation of structures, a method is suggested to take into account the reinforcement effect of fully grouted rockbolts. The applicability and validity of the proposed method is checked through model tests carried out in laboratory under well controlled conditions.

PLASMA PRODUCTION WITH ROTATING ION CYCLOTRON WAVES EXCITED BY NAGOYA TYPE-III ANTENNAS IN RFC-XX

A method of plasma production by ion cyclotron wave heating has been developed in RFC-XX. Nagoya Type-III antennas were used for wave excitation, and gas was supplied through a gas box. The effect of the rotating field excitation on plasma production and heating was investigated. In the m = − 1 rotational mode (rotation in the direction of ion cyclotron motion), the plasma density profile is flat within the gas box bore with a line integrated density nl = 3 × 1013cm−2, and the ion temperature is Ti ≈ 150 eV. For the m = +1 mode, a high density plasma was obtained with a different profile having a peak at the centre. In this mode, the line integrated density is nl = 3 × 1014cm−2, with the peak density n(0) = 7 × 1013cm−3.

A slow-wave heating experiment on RFC-XX using an array of phased antennas

Experimental data for ion cyclotron resonance heating in the RFC-XX machine in IPP-Nagoya are presented. The achieved ion temperature is as high as 100 eV at n = 1013 cm−3 and 1 keV at n = 1012 cm−3. The ion energy confinement becomes worse by the application of a longer pulse, which is found to be due to the enhanced charge-exchange loss and/or electron drag. Axially and azimuthally arrayed antennas are used in the heating, and the importance of the phasing is demonstrated. A simple model of the multiple-antenna problem is also given and used to interpret the experimental data.

Measurements of Atomic Hydrogen-Density Profiles in the RFC-XX-M Machine Using Laser Fluorescence Spectroscopy at the H? Transition

To assist in the study of the power balance in a linear confinement machine, atomic hydrogen-density profiles in plasmas of the RFC-XX-M device were measured using laser fluorescence spectroscopy tuned to the Balmer alpha line. Measured profiles showed marked decreases of neutral densities in the centre of the plasma, which are explained by charge-exchange and electron ionization pumping by the plasma. It is important to note that the neutrals are well burned out in the central cell, Nl/Ne \lesssim0.01. Further, charge-exchange losses are quantitatively evaluated and discussed from the neutral-density profiles and parameters of the plasma.

Tunnelling through squeezing rock in two large fault zones of the Enasan Tunnel II

SummaryIn spite of the high content of clay in the rock, blasting technique had to be adopted for excavation because the hydrothermally altered clay was interbedded in the hard rock. Problems with groundwater did not arise owing to the dewatering effect of the pilot tunnel.

Azimuthal non-uniformities induced by ICH and ECH in the RFC-XX mirror plasma

The loss flux to an azimuthally segmented limiter in the central cell of the cusp end cell stabilized, axisymmetric mirror machine, RFC-XX, shows that non-axisymmetric illumination by either ICH or ECH can induce azimuthal non-uniformities in the plasma. Measurements of the floating potential in the vicinity of the limiter and by the limiter itself suggest that a large stationary azimuthal electric field is induced which can convectively transport plasma across magnetic field lines. The local radial component of the electric drift velocity is as large as 7 × 104 cms−1. Examinatio n of the plasma end loss versus azimuth and probe measurements deep in the plasma show that the non-uniformities extend into the plasma and are not localized near the outer boundary. The data suggest that the RF fields are driving an anomalous radial loss.

Developments and applications of laser-fluorescence for studies of particle-behaviours in plasma-surface interactions

Abstract The paper is concerned with systematic developments of laser-fluorescence systems for studies of particle-behaviours in plasma-surf ace interactions, namely from (i) appropriate laser developments needed for the fluorescence studies, and (ii) benchmark experiments to check the plausibilities of the techniques, to (iii) applications in measurements on high-temperature plasmas.

Stability studies of a hollow plasma in the double cusp experiment

Axisymmetric cusp end cells have application for the stabilization and plugging of a tandem‐mirror‐type reactor. Experiments have been performed on the RFC‐XX double cusp in Nagoya, Japan to measure the stability of the hollow plasma in the adiabatically confined region of the device. Experiments were performed with both a uniform field central section and with a mirror central section connecting the two cusps. The plasma is produced by an rf discharge of gas puffed into the vessel. Plasma fluctuations were measured by Langmuir probes and magnetic probes. Stable regions of operation with a hollow plasma in the cusp are found for both magnetic configurations. Various types of instabilities are observed: electrostatic drift type waves with m≥1, lower‐frequency waves with a magnetic perturbation associated with the density perturbation, an m=0 ion‐acoustic mode, and a large amplitude relaxation oscillation.

Scaling Studies on RF Plugging Potential in the Cusp-Anchored Mirror Device, RFC–XX–M

A scaling relation for the RF plugging potential is investigated in the line cusp plasma of a cusp-anchored mirror device, RFC-XX-M. The RF plugging potential is measured by two independent methods: One is measured by multi-grid energy analyzers and the other is deduced from the measured ion density under RF electrodes and the ion energy distribution in terms of the Boltzmann relation. These values agree well each other. The scaling relations of the RF plugging potential for an applied RF field and an ion density are investigated. These are compared with theoretical calculations. The RF plugging potential is formed under RF electrodes by the enhanced RF electric field associated with an eigenmode of the ion Bernstein wave with a perpendicular mode number, n =1 and is in accordance with the ponderomotive potential expression.