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The electronic amplification gain of a microchannel plate (MCP), as employed for detector and image amplifier of an x-ray pinhole camera, tends to decrease as the output current increases, posing problems both in quantitative analyses and in construction of a three-dimensional emissivity distribution. We report that the output-current dependent MCP gain is described in a simple empirical formula that is determined by an in situ calibration experiment using a steady-state low-temperature discharge plasma. We examine the validity of the formula affirmatively in correcting raw data of x-ray images of magnetically trapped hot plasma. It is also demonstrated that the correction leads to a three-dimensional distribution of soft x-ray emissivity in a quadrupole-mirror-trapped hot plasma that is consistent with other indirect measurements.

In open systems, velocity distribution functions of end-loss ions provide information not only about ion axial confinement but also about the noticeable structure which appears on the ion distribution in loss regions. An end-loss energy component analyzer (ELECA) has been developed to measure the velocity distribution functions of the end-loss ions in the tandem mirror GAMMA 10. The distribution functions and the physical quantities related to the end-loss ions are derived as functions of the output currents of the ELECA. The ion temperature in the loss region is estimated on the assumption that the distribution function is Maxwellian in consideration of the loss region characterized by the magnetic field strength and the electrostatic potentials in the tandem mirror.

Mesh bias experiments, in which secondary electron emission from an endplate is suppressed, suggest that simple balance between an end loss electron current and an end loss ion current does not hold at the endplate. An ion current flowing into the endplate from its back side is observed, and a theoretical model is constructed which takes account of this current and secondary electrons emitted from the mesh wires. Mirror reflection of secondary electrons off an outer mirror throat is also considered in this model. The model includes the current balance and the endplate potential self-consistently. The current balance at the endplate is well accounted for by this model.

A response function [Y. Kiwamoto et al., Phys. Plasmas 1, 834 (1994)] for higher harmonic cyclotron resonance heating is constructed through the analysis of electron trajectory under harmonic heating in an inhomogeneous magnetic field. General expressions of orbital displacement and energy gain for higher harmonic resonance are obtained. Phase-averaged energy gain of electrons by n-th harmonic electron cyclotron resonance heating (ECRH) is larger by a factor of n, if the effect of the orbital displacement from the unperturbed orbit is correctly included, than that evaluated based on an unperturbed orbit assuming small k⊥ρ. The energy gain for finite k⊥ρ is also obtained. These results are confirmed with a numerical calculation for second harmonics.

We investigate the ion axial confinement time and the ion endloss spectrum in a wide range of collision frequency in a tandem mirror by using Monte-Carlo simulation. It is found that the ion endloss spectrum and the effective temperature determined from the spectrum have a transition depending on the collision frequency. Particularly, in cases where the collision frequency is low, the effective temperature deviate from the field temperature.

Hot ions with a strongly anisotropic distribution are created by radio-frequency waves and are sustained in the GAMMA 10 tandem mirror. The main power drain from the hot ions is accounted for by classical transport processes. The dominant loss is electron drag, which is confirmed in auxiliary electron heating experiments. The anisotropic distribution of hot ions is also examined in terms of neutron yield from D - D fusion reactions. The ion pressure and the anisotropy drive unstable waves near the ion cyclotron frequency. A dispersion relation and spatial distributions of the unstable waves are examined by use of magnetic probes and reflectometry cross-polarization scattering measurements. The hot-ion plasma contains a multi-shell structure of impurities creating concentric layers of pressure-driven azimuthal velocity with strong shears.

Imaging characteristics of channel plate x-ray collimators have been investigated in the low energy range for applications of two-dimensional x-ray spectrometer. Improvement of the degradation of spatial resolution due to x-ray reflection inside a hollow channel has been considered by utilizing a focusing action of the channel plate collimator. Spatial extent of images through the channel plate collimator have been measured systematically by using a low energy point x-ray source and have been compared with results of ray-tracing calculations. It is shown that the spatial extent of the image consisting of the focusing component is limited by the geometrical divergence of the channel plate collimator. The possibility of two-dimensional imaging spectroscopy as well as the focusing action is discussed.

A semiconductor detector is applied for measurement of high energy end loss ions (E⩾15 keV) from a tandem mirror. Its energy sensitivity to protons calibrated by using a monochromatic ion source is found to be the same as that to electrons. The semiconductor detector is installed on an end wall of the GAMMA 10 tandem mirror and high energy end loss ions are observed with the detector when ions in the confinement region are strongly heated by ion cyclotron waves. Counts of high energy end loss ions are closely related with a diamagnetic signal representing ion heating. The energy of the end loss ions spreads with time and attains to 50–60 keV. Time behavior of the ion count suggests that unknown loss processes other than classical Coulomb scattering are relevant to ion end loss.

In order to measure velocity distribution functions of end-loss ions in the tandem mirror, a diagnostic device named ELECA (End-Loss Energy Component Analyzer) has been developed. The ELECA enables us to measure simultaneously both the pitch angle and the energy of the end-loss ions without mixing of the high energy electrons even during strong plugging. By sweeping the electrode voltages of the analyzer with high frequency, time evolution of the ion distribution in the loss region is obtained during one shot. The ELECA was located on one end of the tandem mirror GAMMA 10, and it was found that a complicated structure appeared in the distribution functions of the end-loss ions. The structure is caused by the effects of the electrostatic potential, the strong magnetic field and ion transport in the velocity space. It was also verified from a microscopic viewpoint that the confining potential was created by turning on the electron cyclotron resonance heating (ECRH) without the production of sloshing ions by neutral beam injection.

Two-dimensional correlation reflectometry of plasma fluctuations is investigated. It is shown that a multi-dimensional approximation removes the constraint of low resolution of correlation analysis. A correlation technique of fluctuation drift velocity measurements is analytically studied.

A millimetre wave two-dimensional detector array has been newly developed for application to a phase-imaging interferometer installed in the GAMMA 10 tandem mirror. The interferometer consists of two sources and a quasi-optical transmission system. The transmitted wave is mixed with the local oscillator (LO) wave to provide a 150 MHz intermediate frequency (IF) signal. The detector has 4 × 4 channels consisting of beam-lead GaAs Schottky barrier diodes bonded to bow-tie antennas fabricated using photolithographic techniques on a fused quartz substrate of size 38.1 mm × 38.1 mm. The interferometer successfully measures the time evolution of line density profiles with a phase resolution of less than 1 200 fringe and a spatial resolution of 16 mm in the vertical direction and 5 mm in the horizontal direction. The system is also applied to the measurement of electron cyclotron emission (ECE) as a heterodyne radiometer. The axial and radial dependence of the second-harmonic ECE is observed simultaneously. The fluctuation spectra of the ECE signals are compared with those obtained from the interferometer.

Our recently proposed theory on X-ray energy responses of semiconductor detectors is based on the physics principle of the three-dimensional diffusion of X-ray-produced charges in a semiconductor field-free substrate region. In this paper, this diffusion process is experimentally verified using a spatially distributed charge profile produced by an X-ray beam in a multi-channel semiconductor detector. Actual X-ray profile data are distorted because of the effect of the diffusion. Using this theoretical principle, a new X-ray analysis method for obtaining plasma electron temperature profiles (that have no dependence on plasma densities) is proposed.

A broad band frequency modulation reflectometer system has been installed and operated on the GAMMA 10 tandem mirror. A fast sweep hyperabrupt varactor-tuned oscillator (HTO) is used as a source, which can be swept over the full frequency band (12–18 GHz) in less than 5 μs. The high frequency fringes due to the change in the cut-off layer can be distinguished from those due to the existence of density fluctuations, since the fluctuation level with frequency components larger than the ion cyclotron range of frequency (10 MHz) is much smaller than the average density (10−4 or less). The reliability of density profile measurement is investigated for various sweep times of HTO from 5 μs to 5 ms. It is observed that the reliability is greatly improved and the reconstructed density profiles agree well with that obtained from an interferometer when the sweep time is faster than 10 μs. The present reflectometer is routinely applied to determine the density profiles of the central cell plasma in GAMMA 10.

The ion energy balance in the central cell of the GAMMA10 tandem mirror is investigated for ICRF-heated plasmas on the basis of ion temperatures determined from charge exchange neutral particle measurements. The radial profiles of perpendicularly heated ions are measured using a neutral particle energy analyser (NPA). An experiment on ICRF power modulation is performed in order to evaluate the energy confinement. The resultant temporal change in the ion temperature is analysed from the charge exchange neutrals and central cell diamagnetism. A time evolution code of ion energy balance based on the radial profiles of measured plasma parameters is developed to predict the temporal behaviour of the ion stored energy and the calculated results are compared with the measurements.

Reflectometric techniques have been applied to the GAMMA 10 tandem mirror to measure density and magnetic fluctuations associated with electromagnetic plasma waves in the ion cyclotron range of frequency (ICRF). The excitation and propagation of ICRF-driven waves are studied by reflectometers in order to achieve efficient heating of plasmas. The radial profile of the fluctuation level is expected to give the deposition profile of the heating power. A cross-polarization scattering diagnostic system is also installed in the central cell of GAMMA 10. An incident ordinary (extraordinary) wave is converted to an extraordinary (ordinary) wave by magnetic fluctuations. The converted wave can propagate through a cut-off layer and reach the detector. The level of magnetic fluctuation (about 10 −4 ) is evaluated for the ICRF-driven waves.

Behavior of neutral hydrogen in the central-cell of the GAMMA 10 tandem mirror is investigated by measuring spatial-profiles of Hα line-emission and neutral transport simulation. Hα-emission detectors are newly installed horizontally in the vacuum chamber and detailed profiles of the Hα emission are measured. It is found that hydrogen atoms introduced from the gas puffer at the mirror throat are localized around the gas puffer in the steady state phase of ICRF-heated plasmas. The DEGAS neutral transport code is applied to calculate axial density profiles of atomic and molecular hydrogen. In the DEGAS code, the mesh model is modified to take into account variations along the magnetic-field line. The simulation result fairly agrees with the above experimental result. In a standard ICRF-heated plasma, an experiment with modulated gas-puffing is performed. Characteristics of particle confinement in the main plasma are discussed by using the experimental and the simulation results.

The cross-polarization scattering (CPS) technique was applied to the GAMMA 10 tandem mirror for measurements of internal magnetic fluctuations associated with waves in the ion cyclotron range of frequency. The CPS is related to the mode-conversion effect consisting of a polarization difference of a scattered wave with regard to an incident wave. In GAMMA 10, the incident ordinary (extraordinary) wave is transferred to the extraordinary (ordinary) wave by magnetic field fluctuations near the cutoff layer of a plasma. The transferred wave penetrates through the cutoff layer and is detected. The frequency spectra and density and magnetic field fluctuations of the waves are evaluated from the CPS diagnostic and reflectometry. The ratio of the magnetic field to density fluctuation level depends on the phase velocity of the slow Alfven wave. The CPS data are also compared with those obtained from magnetic probes.

This paper describes a modified method of analysis of the charge exchange neutrals for determination of the radial profile of ion temperature including the effect of the ion finite Larmor radius. It also includes the effect of the charge exchange reaction with neutral hydrogen molecules. The ICRF-heated plasmas in the central cell of the GAMMA10 tandem mirror were analyzed by taking into account these effects. The obtained energy spectra and the count-rate spatial profiles of charge exchange neutrals were consistent with the experimental results. The present method, thus, proved to be effective in determining the radial profile of ion temperature.

Examination is made of the relation between the distribution function of the ions confined in a tandem mirror and the current–voltage (I–V) characteristics of the end loss ions collected on a gridded energy analyzer. To do this, a model ion distribution function characterized by two parameters, an anisotropy parameter χ and a decay parameter Y is used. The decay parameter represents depletion of ion distribution in the loss cone. A Maxwellian distribution as the limit of Y=∞ is shown not to reproduce the measured I–V curve as an energy spectrum of the ions lost from the central cell of the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 939 (1985)]. The ions in the central cell are subject to strong ICRH and have a highly anisotropic velocity distribution. These ions are deeply trapped in the central cell and the loss cone is not fully occupied. The model distribution with a small value of Y represents such features of the ion distribution. From comparison between a measured loss current and calculated one, ...

A substantial amount of Yushmanov‐trapped electrons are generated as a result of electron cyclotron resonance heating employed for generating a thermal dike in an open magnetic field configuration. The production rate and the energy spectrum of the trapped electrons are calculated by using a heating response function. The calculated energy spectrum of the end‐loss electrons driven from the Yushmanov‐trapped region indicates a reduction in power efficiency of the thermal dike. The potential profile modified by the Yushmanov‐trapped particles tends to reduce their production rate, leading to a stationary state. Their significance to a thermal dike basic experiment in the end region of the GAMMA 10 tandem mirror [Plasma Physics and Controlled Nuclear Fusion Research 1992 (International Atomic Energy Agency, Vienna, 1993), Vol. 2, p. 651] is discussed.

A gold neutral beam is perpendicularly injected into a plasma column along a main chord near an inner mirror throat of a plug/barrier cell in the tandem mirror GAMMA 10, in order to directly measure time evolution of the electrostatic radial potential profile during one shot. The new method pays attention to the fact that trajectories of the ions, which are ionized at each point on the path of the neutral beam in the plasma column, are shifted due to the magnetic field gradient and the electrostatic potential. After traveling about a half period of Larmor motion, ions are detected by a plate type of ion detector. The radial profiles of the plasma potential are determined analyzing positions of the ion beam spots on the detector plane. The main advantage of this method is that radial potential profiles in a strong magnetic field region can be measured as a function of time during one shot without sweeping incident beam angles.

Plasma diagnostics using vacuum ultra violet (VUV) and X-ray radiation in the world largest tandemmirror plasma-confinement-device GAMMA 10 are reported. In particular, the detection characteristics of microchannel plates (MCP) with 50 channels (anodes) are investigated using synchrotron radiation ranging from VUV to an X-ray regime (12 eV to 82.5 keV) for the purpose of detector fundamental researches as well as their development studies. These investigations also provide applications of MCP as imaging detectors including applications for spatially and temporally resolved plasma VUV and soft X-ray diagnostics. It is found that the detection efficiency of MCP remains within one order of magnitude through dozens eV to several tens keV along with the physical mechanism interpretations of the MCP energy response, and the response uniformity of each channel for an imaging use is experimentally investigated. Using these detailed and precise MCP calibration data, we apply the MCP not only for a plasma-radiation imaging detector so as to reconstruct spatial profiles of emissivity but for a plasma diagnostic tool to measure plasma-electron velocity distribution functions.

The three dimensional diffusion effect in a semiconductor photon detector is directly demonstrated using a spatially distributed charge profile produced by a monochromatic energy beam in a multichannel semiconductor detector. When we substitute the value of the diffusion length obtained by the data on the charge profile and the depletion layer thickness from the data on its capacitance vs applied voltages into our theoretical formula, then the photon energy response data are well fitted by the theory. The effect of this diffusion on the plasma VUV and X-ray data is also demonstrated for obtaining actual plasma data.

Neutron diagnostics are applied to a deuterium plasma mixed with protons in the central cell of the GAMMA 10 tandem mirror [Inutake et al., Phys. Rev. Lett. 55, 939 (1985)]. The deuterium ions are heated with a slow ion cyclotron wave tuned to the fundamental deuterium resonance near the mid‐plane of the central cell while the plasma is sustained with the fundamental resonance heating of protons in the minimum‐B anchor cells. The measurement is based on in situ calibration to determine the transmission efficiency through machine walls and the counting efficiency of the detection system for neutrons emitted from 252Cf with energy close to that in the deuterium–deuterium (D–D) fusion reaction. The observation shows that the count rate of neutrons increases with diamagnetism, and this relation is accounted for in terms of fusion reaction between deuterium ions with a transverse temperature exceeding 10 keV. Discrepancies among ion temperatures determined with different diagnostics are mostly attributed to insufficient knowledge of the profiles of plasma parameters. The results indicate that the neutron measurement can be added as a powerful diagnostic tool for hot ions if combined with more detailed profile measurements.

In the GAMMA 10 tandem mirror [M. Inutake et al., Phys. Rev. Lett. 55, 939 (1985)], plasmas with a high beta value (ratio of plasma pressure to magnetic pressure) are produced by using an ICRF (ion cyclotron range of frequency) heating. Magnetic field fluctuations identified as an Alfven ion cyclotron (AIC) mode are excited with strong dependence on a perpendicular beta value and a temperature anisotropy T⊥/T∥ (a ratio between the temperatures perpendicular and parallel to the magnetic field line). Spatial structures and excitation conditions of the AIC mode are experimentally studied. AIC modes are spontaneously excited as axial eigenmodes, of which boundary is determined by the reflection point in anisotropic plasmas. The AIC mode in the GAMMA 10 is observed well below the theoretically predicted threshold.

A novel end‐loss ion‐energy spectrometer is designed for plasma‐ion diagnostics in open‐ended plasma‐confinement devices. This analyzer significantly upgrades a previous slanted grid end‐loss analyzer (SELA) to essentially eliminate secondary‐electron current, and to provide a differential‐spectrum mode of operation, in addition to the usual integral‐spectrum operation of gridded ion‐energy analyzers. The upgraded SELA does not perturb the ambient magnetic field due to its electrostatic operation. Either the differential or integral spectra are obtained by a time sweep of grid voltages, collecting the ion current on a single‐channel plate. Because the angular alignment of the SELA is not critical, it can be used as a spatially scannable diagnostic of ion‐energy distributions and plasma potentials. It is characterized using computer simulations of ion trajectories, monoenergetic ion beams, and end‐loss plasma from the world’s largest tandem mirror—GAMMA 10.

The monotonic electrostatic potential profile is found to be realized around the plug region of a tandem mirror, even if the ion drift velocity is zero at the point of potential maximum.

Surface temperature-driven erosion instability of plasma-facing material is analytically investigated. It is shown that for shallow incidence the influence of temperature disturbance consists primarily of growth rate enhancement for the fluctuations with small wave numbers. For larger incidence angles surface temperature-related effects dominate over ion flux-driven deposition instability. Mean scale of surface roughening is estimated to be 1–2 mm.

Confinement of ion cyclotron resonance heating (ICRH) plasmas in the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 159 (1985)] has been investigated. Magnetohydrodynamic (MHD) stability is provided by the axisymmetrized minimum‐B system. Ion confining and thermal barrier potentials are produced only by electron cyclotron resonance heating (ECRH). Ions are heated to several keV and hot ion modes, where Ti/Te≳(mi/me)1/3, are produced. Ion pressure in the central cell increases approximately linearly both with the power radiated from the antennas and with the magnetic field. A record ion temperature of about 10 keV is obtained at the maximum magnetic field (∼0.57 T). Thermonuclear fusion neutrons are detected in a deuterium/hydrogen mixture plasma for the first time in a tandem mirror. In typical GAMMA 10 operations, strong radial electric fields are naturally produced, and density fluctuations are suppressed. Axial confinement of the core plasma is consistent with the classical potential confinement, indicat...

Loss-cone boundaries have been directly measured using a newly developed diagnostic device in the tandem mirror GAMMA 10. Double loss boundaries clearly appeared in the velocity space of the end-lo...

Magnetohydrodynamic (MHD) stability of the GAMMA 10 tandem mirror is extensively studied in ICRF-heated, hot ion plasmas. Stability boundary for a flute interchange mode is predicted to depend on a pressure-weighted curvature integrated along the magnetic field line. It is found that the upper limit of the central-cell beta {beta}{sub C} increases linearly with the anchor-cell beta {beta}{sub A}. The critical beta ratio {beta}{sub C}/{beta}{sub A} above which the plasma cannot be sustained strongly depends on the pressure anisotropy P{sub PRP}/P{sub PLL} of hot ions. Stronger anisotropy greatly expands the stable region up to a higher critical beta ratio, owing to the reduction of the pressure weighting in the bad curvature region of the central cell. On both sides of the quadrupole anchor cells, there are flux-tube-recircularizing transition regions where the normal curvature is highly bad. Then the density and ion temperature of the cold plasma in the transition region are measured. Theoretical prediction on the flute stability boundary calculated by using the measured axial pressure profile of the hot-ion and the cold-plasma pressure can explain well the experimental results. 16 refs., 7 figs.

This paper studies modifications of convective heat transport in terms of electric and magnetic forces acting on electrons streaming along open field lines. The heat transport is greatly affected by the potential profile along the field lines. One important mechanism of the transport is associated with the exchange of impingig warm electrons with cold electrons produced near or on the surface of plasma-facing walls such as end plates and divertor plates. A recently developed potential model predicts that the end plate potential deepens associated with reduction of cold electron emission. In the GAMMA 10 tandem mirror a negatively biased mesh placed in front of the end plate has successfully suppressed secondary electron emission and leads to deepening of the end plate potential. In the thermal dike ECRH works to mirror reflect colder electrons back to the plate by increasing their pitch angles. The consequent potential depression enhances reflection of warm electrons reaching the plate.

In this paper we describe the experimental results on the evaluation of fueling in the central region of the GAMMA 10 tandem mirror on the basis of the neutral pressure balance, plasma spectroscopy and atomic and molecular processes with numerical simulations. Quasi steady-state plasmas are achieved by optimizing the combination of gas puffing and ICRF pulse, which simplify the quantitative evaluation of fueling. The total fueling in the central region deduced from the pressure measurement near the plasma boundary is compared with the amount of end-loss flux which is measured by an end loss analyzer (ELA). It is found that the amount of fueling increases slightly with the ICRF power due to the enhancement of recycling in the central region and agrees well with the behavior of end-loss flux. Results of impurity-injection experiments are also described from the viewpoint of recycling processes.

It is analytically shown that interaction of magnetized multicomponent plasma with a target surface causes an instability of material erosion. The wave numbers and increments of growing fluctuations are determined in a linear approximation. The mean time of surface roughening is of the same order or even shorter than that of the erosion process itself. Erosion instabilities are of importance for long-pulse or steady-state operation of future reactor-grade fusion devices.

A system of 2D full wave equations is solved for the case of microwave scattering by plasma density and magnetic field fluctuations near the cut-off layer. The 2D field structure in ordinary-to-ordinary scattering (O to O), extraordinary-to-extraordinary scattering (X to X) and O to X, X to O mode conversion processes is studied as well as the influence of refraction, side scattering, diffraction on both X- and O-mode reflection near the cut-off layer. For the case of relatively large antenna size the expressions obtained reveal high sensitivity of the scattered field to the plasma disturbance wavenumber in the transverse direction. For the horn size of 1-2 wavelengths the sensitivity is weak. Numerical examples exhibit multisource spatial distribution of the scattered field. Resonance features of modes scattered by wave-shaped fluctuations were found to be similar to the 1D case.

Recent experiments related to macro- and micro-instabilities in the GAMMA 10 tandem mirror are described. A stability boundary of a flute interchange mode was revealed experimentally. The pressure anisotropy becomes strong with an increase in the β value in the central cell. The critical β of the central cell for the flute interchange mode depends on the pressure anisotropy. The Alfven ion-cyclotron (AIC) mode which is driven by the anisotropy in the velocity space is spontaneously generated well below the theoretically predicted absolute boundary. The performance of the macro- and micro-stabilities in ICRF-heated plasma of the central cell is related to the pressure anisotropy.

Two aspects of ECRH experiments in the GAMMA 10 tandem mirror are described. One is hot electron production in a minimum- B anchor cell by second harmonic ECRH. When a resonance region is located at the centre of the anchor cell, hot electrons are piled up near the centre region and the β-value attains more than 20%. For off-centre resonance, hot electrons have a spatial profile like a baseball seam. This profile is explained by a drift orbit of an electron in a minimum- B field. The other topic is observation of an intense axial electron flow with energy from hundreds of eV to a few keV generated by fundamental ECRH in the end cells. The force due to the fundamental ECRH driving heated electrons into the mirror loss cone is particularly strong. A bounce averaged Fokker-Planck code with a quasi-linear diffusion operator reproduces the electron flow with the same energies. The electron flow is a predominant ingredient for generation of the electrostatic potential in the GAMMA 10.

Thermal dike is a new concept for controlling energy transport carried by electrons along open magnetic field lines by enhancing mirror‐reflection with assistance of electron cyclotron resonance heating (ECRH). Quantitative analyses are made to evaluate merits of thermal dike and to clarify requirements for efficient use of it. Heating response function is applied for trajectory analyses of electrons streaming through the ECRH zone. Substantial power gain is obtained under a configuration with a high mirror ratio between the reflection point and the resonance point and with a large scale length of the magnetic gradient in the resonance zone. Thermal dike is shown to be beneficial both in tandem mirrors and in diverters in tori. Discussions are made also on a basic experiment program of thermal dike in the GAMMA 10 tandem mirror.

A spatial imaging vacuum ultraviolet (VUV) spectrograph has been constructed for simultaneous observation of spatial and spectral distributions of plasma radiation in the wavelength range 150–1050 A. The spectrograph consists of an entrance slit of limited height which provides spatial resolution, an aberration‐corrected concave grating with varied spacing grooves which gives a flat‐field spectral output plane, and an image‐intensified two‐dimensional detector system. The basic characteristics of the spectrograph have been investigated by ray‐tracing calculations. The expected performance has been confirmed through experiments using a dc glow discharge source on the reciprocal dispersion, the spatial resolution, or the incident angle dependence of spectral images. VUV spectra with spatial and time resolution have been obtained successfully in the GAMMA10 tandem mirror experiment.

X-ray energy responses of silicon surface barrier (SSB) semiconductor detectors are found to be explained neither by the commonly believed conventional model using the depletion layer thickness of an SSB detector nor by a recently proposed model using its wafer thickness as the X-ray sensitive layer. Our new theory using both the depletion layer sensitivity and the X-ray response due to a three-dimensional charge diffusion effect in the field-free substrate region of a SSB detector can well fit the response data. This theory is newly extended to analyse each channel response of a multichannel semiconductor X-ray detector fabricated on one silicon wafer; these detectors are widely utilized as position sensitive X-ray detectors for nuclear fusion oriented plasma research as well as for high energy elementary particle studies. A numerical simulation of multichannel detector outputs using our three dimensional diffusion theory is carried out; generalised theoretical formulae are also presented.

The reflection and mode conversion of microwaves are investigated analytically for the case of relatively high field distortion by density and magnetic fluctuations. The polarization ratio of two extraordinary wave components is obtained. A solution of the full-wave equation is derived for the 1D cross-scattering problem. Peculiarities of cross-polarization scattered waves are discussed as well as possible applications to reflectometry of fusion devices.

Numerical studies are made on dynamics of electrons under electron cyclotron resonance heating (ECRH) by a strong monochromatic wave in a nonuniform magnetic field. The response of the electrons is described in terms of gains in energy and magnetic moment. These expressions are employed to define a response function which connects the velocity distributions of electrons before and after heating in a wide range of conditions. The response function is applied to calculate the velocity distribution of cold electrons which stream up the magnetic slope and are heated at the resonance. This configuration simulates a possible scheme for controlling an inward flux of cold electrons in diverter regions of tori and in mirror ends by mirror reflection enhanced by ECRH. The velocity distribution shows wing‐like structures which are not expected from diffusion equations.

A new type of end‐loss‐ion energy analyzer (ELA; a multigridded Faraday cup) is applied to measure plasma ion temperatures Tip and plasma potentials ΦP in the GAMMA 10 tandem‐mirror device. As compared with conventional‐type ELA, this new ELA has obliquely placed ion‐ and secondary‐electron‐repeller grids and a collector plate with respect to external tandem‐mirror magnetic fields. One of the most useful capabilities of this new ELA for precise ion measurements is to greatly reduce the current of high‐energy electrons onto the collector plate. Also, this compact‐sized ELA is scannable in the plasma to obtain radial profiles of Tip and ΦP without providing any perturbations of the tandem‐mirror magnetic fields because it neither adds its own magnetic field nor needs to shield against the ambient magnetic field. Ion trajectories in the new ELA are numerically calculated so as to make a further understanding of its physics principles and to optimize its design. The energy‐calibration experiments for the new ...

Theoretical analyses are made on the structure of the potential distribution along open field lines in a plasma where the velocity distributions of particles are modified from Maxwellian to include Yushmanov-trapped components and temperature anisotropy. The considered particle species are warm electrons and ions flowing out of the system, secondary electrons flowing backward, and Yushmanov electrons trapped between a stronger side of the magnetic field and reflecting potential. The analyses show systematic variations in the potential profile with the change in the temperature anisotropy and the abundance of the Yushmanov-trapped particles. Moreover, the analyses predict measurable feed-back effects to the local heating for the plasma control.

The electrostatic potential profile around a magnetic mirror throat is studied. Bohm's criterion expresses the necessary condition for the formation of a stationary sheath in front of the absorbing wall. This condition, when applied to the mirror throat of a magnetic mirror, requires that the ion drift velocity flowing into the mirror throat from the confining region must exceed the ion sound velocity for the continuity of the potential. It is found, however, that the ionization around the mirror throat renders the potential continuous even with zero ion drift velocity.

The electron distribution function is derived analytically with the linearized Fokker-Planck equation in order to analyze the electron confining potential in the geometry of mirror ratio R ≪1. The magnitude of the potential is determined by the electron axial loss time (transit time) τ // when τ // is longer than the coulomb collision time τ, on the other hand the magnitude of the potential depends mainly on τ when \(\tau _{//}\lesssim\tau\) . Especially, it is found that the electron confining potential does not depend on the emission rate γ of electrons hitting the wall as long as τ // ≪τ. The bounce-averaged nonlinear Fokker-Planck simulation is carried out in order to investigate the electron confining potential in the case of R =1. The electron temperature ratio T e ⊥ / T e // depends strongly on the ratio τ/τ // . The magnitude of the potential obtained in the simulation has a tendency to decreasing with the ratio τ/τ // .

application/pdf, Alfvén-ion-cyclotron modes are generated over a wide range of anisotropy parameter β⊥(T⊥/T∥)2 in ICRF-heated central cell plasmas of the GAMMA 10 tandem mirror. The fluctuations are observed at a small driving force in the convectively unstable region predicted theoretically in an infinite plasma. Before reaching the β limit of β⊥=3.52×(T∥/T⊥)2, above which the absolute instability is predicted, relaxation of pressure anisotropy is observed for the first time by use of an array of diamagnetic loops together with a Faraday cup and secondary-electron emission detectors.

A method for evaluating wall condition by using plasma impact desorption (PID) technique has been developed and successfully applied to the tandem mirror GAMMA 10 as a monitor for wall conditioning. A magnetically shielded quadrupole mass spectrometer was installed in the vacuum chamber of the GAMMA 10 central cell. The behaviour of the partial pressure of various gas molecules desorbed by ICRF-heated plasma discharges were analyzed. The predominant increase of the partial pressure due to PID (ΔPPID) was hydrogen (M = 2) and a small amount of impurity as CO (M = 28), CH3 (M = 15), H2O (M = 18) and CO2 (M = 44) was observed in the wall-conditioning discharges. The reduction of hydrogen pressure as well as ΔPPID of the above impurities was observed with the progress of wall conditioning. This behavior has a good correlation with the increase of partial pressures due to electron-impact desorption measured at the same period. The relation between ΔPPID and the charge-exchange flux was investigated.

An empirical scaling for the potential difference between the plug potential ΦP and the floating potential ΦEP of an endplate is obtained in terms of end‐loss electron temperatures. Results from the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 939 (1985)] indicate that ΔΦ=ΦP−ΦEP scales much better with an effective temperature Teff≡(1−β)TeL+βTeH than with a single temperature TeL or TeH, where TeL and TeH are temperatures of the bulk and the high‐energy tail components of the end‐loss electrons and β denotes the flux fraction of the TeH component. Moreover, the observed scaling is consistent with a recently developed theoretical model [K. Kurihara et al., J. Phys. Soc. Jpn. 61, 3153 (1992)].