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A method is proposed for obtaining radial profiles of plasma temperatures of both plasma ion (Ti) and electron (Te) simultaneously by the use of a semiconductor detector array. The method is based on our developed particle-response model for a semiconductor detector; in particular, the response theory is constructed for giving the applicability in particle energies ranging down to a kiloelectronvolt. Calculated results from our model are in fairly good agreement with experimental data on the detector response of incident particle beams with energies in the range 100 eV to a few kiloelectronvolts. On the basis of the verification of the proposed model, an idea of the use of a developed semiconductor detector array covered with `reliably unbreakable' ultrathin SiO2 `dead-layer filters' having various nanometre-order thicknesses is applied for simultaneous Ti and Te analyses by using charge-exchange neutral particles and x-rays from plasmas. Radial profiles of Ti and Te are obtained in a single plasma discharge alone, and the data reliability is independently cross-checked by a radial scan of a conventional charge-exchange neutral-particle analyser system as well as a 50-channel microchannel plate x-ray diagnostics system in the GAMMA 10 tandem mirror.

The minimum energy state under the constraint of the total toroidal angular momentum is examined. The result is applied to actual tokamak plasmas with an internal transport barrier (ITB). The study shows that tokamak plasmas with ITB are in the minimum energy state under the constraint of the total toroidal angular momentum and that they are likely to be self-organized.

We revealed experimental observation indicating density limit with gas-puff fueling in hot ion mode plasmas of the GAMMA 10 tandem mirror. The responses of the plasmas for the various gas fueling were studied. Despite the increase in the quantity of the particle source, the line average density was saturated with the degradation of the radial particle confinement at the periphery of the central plasma. It was clarified that the line density obtained with the pellet fueling is about 20–50% higher than that with gas-puffing in the same average ion temperature. The clear difference between the pellet fueling and the gas fueling was observed in the radial profile of particle deposition at the central midplane. From the consideration based on the experimental observation, a radial-loss mechanism of ions by charge exchange reaction was discussed as an explanation for the degradation of particle confinement at the plasma periphery.

A new method for a simultaneous observation of both plasma ion and electron temperatures is proposed using one semiconductor-detector array alone. This method will provide a new application of semiconductor-detector arrays for monitoring the key parameter set of nuclear-fusion triple product (i.e., ion temperatures, densities, and confinement time) as well as for clarifying physics mechanisms of energy transport between plasma ions and electrons under various plasma confining conditions. This method is developed on the basis of an alternative “positive” use of a semiconductor “dead layer”; that is, an SiO2 layer is employed as a reliable ultra-thin energy analysis filter for low-energy charge-exchanged neutral particles from plasmas ranging in ion temperatures from 0.1 to several tens of kilo-electron-volts. Using recent fabrication techniques for the thin and uniform SiO2 layers of the order of tens to hundreds of angstrom, our computer simulation and its experimental verification show the availability of such semiconductors for distinguishing neutral particles (for ion temperatures) from X-rays (for electron temperatures). These are simultaneously emitted from the plasmas into semiconductor detectors; however, we employ their quite different penetration lengths and the resultant different deposition depths and profiles in semiconductor materials. As a result, their output signals are distinguishable for these two different and fundamental species of plasmas.

The effects of nuclear fusion produced neutrons on the X-ray energy responses of semiconductor detectors are characterized. The degradation of the response of position-sensitive X-ray tomography detectors in the Joint European Torus (JET) tokamak is found after neutron exposure produced by deuterium–deuterium and deuterium–tritium plasma fusion experiments. For the purpose of further detailed characterization of the neutron degradation effects, an azimuthally varying-field (AVF) cyclotron accelerator is employed using well-calibrated neutron fluence. These neutron effects on the detector responses are characterized using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory (KEK). The effects of neutrons on X-ray sensitive semiconductor depletion thicknesses are also investigated using an impedance analyser. Novel findings of (i) the dependence of the response degradation on X-ray energies as well as (ii) the recovery of the degraded detector response due to the detector bias application are found and interpreted by systematically combined methods using a synchrotron facility, a cyclotron facility, a fusion plasma device, material analysis methods along with our recent theory on semiconductor X-ray responses.

Measurement of spectra in the wavelength range from vacuum ultraviolet (VUV) to soft x ray is an important means to diagnose impurity behavior in magnetically confined plasmas used in fusion plasmas. Recently, a space- and time-resolving flat-field grazing-incidence VUV spectrograph was constructed for the simultaneous observation of spatial, temporal, and spectral distributions of plasma radiation in the wavelength range of 150–1050 A. Absolute calibration experiments were performed at beamline 11C and beamline 12A in the Photon Factory at the High Energy Acceleration Research Organization. The absolute sensitivity of the VUV spectrograph was measured under both P and S polarized light conditions.

Generalized scaling laws for the formation of plasma confining potentials and the associated effectiveness of the potentials produced are systematically investigated to find the physics essentials common to the representative tandem mirror operational modes of GAMMA?10, and to explore novel extended operational modes from the scaling bases constructed. (a)?The potential formation scalings are generalized using a novel finding of wider validity of Cohen's strong ECH theory covering the representative modes. (b)?The potentials produced, in turn, provide a favourable novel scaling of the increase in the central cell electron temperatures Te with increasing thermal barrier potentials b, limited by the available ECH power. The scaling of Te with b is well interpreted in terms of the generalized Pastukhov theory of plasma potential confinement. A detailed comparison of the results from several related modified theories is also made. (c)?Consolidation of the two major scalings of (a) and (b) in a tandem mirror is carried out by the use of an electron energy balance equation for the first time. In addition, (d)?an empirical scaling of c with ECH power in the plug region and the central cell densities are studied to discover whether there is the possibility of extending these theoretically well interpreted scaling data to parameters in the future scalable regime. There is also a discussion about numerical scalings in the three dimensional parameter spaces.

A grazing incidence flat-field soft X-ray (20–350 A) spectrograph was constructed and applied for impurity diagnostics in the GAMMA 10 fusion plasma. The spectrograph consisted of a limited height entrance slit, an aberration-corrected concave grating, a microchannel-plate intensified detector and an instant camera/a high speed solid state camera. An absolute calibration experiment for the SX spectrograph was performed at the Photon Factory in the High Energy Accelerator Research Organization with monitoring the incident synchrotron beam intensity by using an absolutely calibrated XUV silicon photodiode. From the results of absolute calibration of the spectrograph, the radiation loss from the plasma was obtained.

An operational regime for achieving high ion temperatures in the GAMMA10 central cell has been experimentally investigated by analysing the correlation between ion temperatures and electron density profiles measured under various experimental conditions performed over the last 10 years. The analysis shows that broadened density profiles or excessive high ion temperatures (or both) induce significant degradation of the stored energy. It has been found that the peaked density profile is a necessary condition to achieve high ion temperatures and high stored energy. The analysis reveals the presence of the operational regime to achieve high ion temperature plasmas. The change of the magnetic field line shape in the central cell has successfully contributed to achieve a high ion temperature of about 10 keV at the plasma centre due to the formation of the peaked density profile in the GAMMA10 central cell.

The main operations from 1979 to 2000 in the GAMMA 10 tandem-mirror, characterized in terms of the high-potential mode having kV-order plasma-confining potentials and the hot-ion mode yielding fusion neutrons with 10-20 keV bulk-ion temperatures, are summarized and generalized as a result of scalings of the formation and the effects of the potentials. The wide validity of potential-formation physics from Cohen's theory and the validity of the generalized Pastukhov's theory for the effects of thermal-barrier potentials on electron confinement are verified and consolidated through electron-energy balance.

The improvement of potential confinement reported at the last IAEA Fusion Energy Conference in 1998 was attained by axisymmetrization of the heating pattern of ECRH. It was experimentally shown that the axisymmetrization of ECRH actually produced an axisymmetric potential profile. GAMMA 10 experiments on potential confinement have advanced in the direction of longer sustainment and higher density. Experiments to achieve long sustainment of potential confinement were carried out in order to study problems of steady state operation of a tandem mirror reactor. A confining potential was sustained for 150 ms by sequentially injecting two ECRH pulses into the plug region. It had previously been difficult to increase the central cell density to higher than about 2.5 × 1012cm-3 with or without potential confinement, owing to some density limiting mechanism. In order to overcome this problem, a new, higher frequency ICRF system (RF3: 36-76 MHz) has been installed. A higher density plasma has been produced with RF3. In addition to RF3, NBI in the anchor cells was recently made more effective through the reduction of neutral gas from the beam injectors. Potential confinement experiments have advanced to higher central cell densities of up to 4 × 1012 cm-3 with RF3 and NBI. A 35% density increase due to the potential confinement was obtained in the high density experiments.

application/pdf, The improvement of potential confinement was attained in the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 939 (1985); Proceedings of the 13th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Washington, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 2, p. 539] by axisymmetrization of heating systems for the plasma production, heating, and potential formation. A significant increase of the density and diamagnetism by the potential confinement was observed. In the previous experiment, it was difficult to increase the central cell density higher than 2.7×1018 m–3. One of the possible mechanisms is the density clamping due to the eigenmode formation of the ion–cyclotron-range of frequency (ICRF) waves in the axial direction. With high harmonic ICRF waves (RF3), the experiments to overcome this problem have been performed. In preliminary experiments with RF3 and NBI the maximum density of 4×1018 m–3 was attained.

Measurements of spectra in the wavelength range from vacuum ultraviolet (VUV) to soft X-ray are important means to diagnose impurities in magnetically confined plasmas used in fusion plasmas such a...

A numerical simulation of cross-polarization scattering (CPS) was performed by using frequency dependent finite difference time domain [(FD)2TD] method. In the CPS process, an incident ordinary (extraordinary) wave is converted to an extraordinary (ordinary) wave by magnetic fluctuations in a plasma. It is confirmed in the simulation that an incident wave propagates in an anisotropic dispersive media properly. The relation between scattered electric field and magnetic fluctuations is also obtained.

After the last Novosibirsk Conference (1998), GAMMA 10 experiments have advanced in long sustainment of confining potential and higher density experiments. An experiment which attained doubling of ...

By using a calcite plate and a bundle of optical fibres, the entrance surfaces of which are aligned in two columns, we resolved the two linearly polarized components of the emission lines. We found significant polarization for the Fe II and Cr I lines, while the O V lines showed little polarization. This finding suggests that the electron velocity distribution is significantly anisotropic at the periphery of the plasma.

In the GAMMA10 tandem mirror, the first pellet injection experiments in open systems have started.1 The tandem mirror machine is suitable for measuring propagation of pellet-fueled particles along ...

The eigen-function and stability boundary of the ideal ballooning modes with various axial plasma pressure profiles and boundary conditions at the end-plates are investigated in the magnetic configuration of the GAMMA10 tandem mirror.

Ion distributions in a loss cone are measured in the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 939 (1985)]. When Alfven ion cyclotron (AIC) modes are excited in the central cell, enhanced end-loss of ions is observed for a characteristic energy region which agrees well with the resonant ion cyclotron condition with the AIC wave. From measurements of pitch angle distributions of end-loss ions, it is found that the loss cone is filled for this characteristic energy region, indicating an ion diffusion in a velocity space induced by the AIC wave. Magnitude of the observed additional end-loss increases with increase in the amplitude of the AIC modes. In the present experimental parameters in GAMMA 10, the AIC wave effect on the axial ion confinement is not serious.

Radiofrequency waves in the ion cyclotron range of frequency (ICRF) are mainly used for plasma production and heating in the central cell of the GAMMA 10 tandem mirror. GAMMA 10 has minimum-B anchor cells with a non-axisymmetric magnetic field configuration. The ICRF heating system in the central cell has been improved to create a more axisymmetric plasma. A high ion temperature is attained with the system, and high energy ions with energies of more than 50 keV are detected both parallel and perpendicular to the magnetic field lines. Strong temperature anisotropy is observed and strong Alfven ion cyclotron (AIC) modes are excited due to the anisotropy. With the AIC modes, the number of high energy ions detected at the end of the mirror increase and the number of high energy ions with a pitch angle in the central cell of nearly 90° decrease.

A gold neutral beam probe system was improved by adopting a multichannel ion detector and adding sweeping functions of beam energy, deflector voltage, and electrode voltage of the analyzer to the system to measure the fluctuations and time evolution of the two-dimensional space potentials with fast resolving time during one shot. Positions of the beam spot on the multichannel detector corresponding to the ionizing points of the beam in the plasma were simulated precisely as a function of the beam energy and the injection angle. A potential derivation formula was determined taking into consideration both the numerical and experimental results, and the reproducibility of the potential profile was checked. Two-dimensional potential profiles were measured in the optimization experiment of the microwave injection angles for formation of the plug potential.

The central cell density and the diamagnetic signal were doubled as a result of plug potential formation by ECRH in hot ion mode experiments on the GAMMA 10 tandem mirror. In order to obtain these remarkable results, the axisymmetrized heating patterns of ECRH and ICRF heating were optimized. Furthermore, conducting plates were installed adjacent to the surface of the plasma along the flat shaped magnetic flux tube located in the anchor transition regions; the plates may contribute to the reduction of some irregular electric fields produced possibly with ECRH in these thin flux tube regions. The conducting plates contributed to reducing the radial loss rate to less than 3% of the total particle losses, along with improvements in the reproducibility of the experiments and the controllability of the potential confinement. The increases in central cell density and diamagnetism in association with the increase in plug potentials scaled well with increasing ECRH power. A plug potential of 0.6 kV and a density increase of 100% were achieved using an ECRH power of 140 kW injected into both plug regions. The plasma confinement was improved by an order of magnitude over a simple mirror confinement owing to the tandem mirror potential formation.

A gentle hump structure on energy distribution function of end-loss ions was observed in the rf-driven tandem mirror plasma by use of an end-loss energy component analyzer. Alfv\'en ion-cyclotron (AIC) fluctuations are excited spontaneously due to anisotropic ion heating in the central cell of the tandem mirror. We observed that the excitation of the AIC waves caused the hump structure on the energy distribution function. The enhancement factor of the end-loss ions is estimated, and the gentle hump structure is discussed from the viewpoint of the heating characteristics of the AIC waves. It is pointed out that the hump structure decreases the ion confinement.

A gold neutral beam probe system which uses a microchannel plate (MCP) as a beam detector is being constructed in order to measure the electrostatic potential at the inner mirror throat (IMT) of the plug/barrier cell in the tandem mirror. The MCP detector was newly designed so as to be able to be used in the IMT strong magnetic field. The characteristics of the MCP gain were measured on a test stand with the strong magnetic field. The characteristic curves of the relative MCP gain were described as functions of the magnetic field strength, the MCP bias voltage, and the angle between the magnetic field and the capillary axis of the MCP. It was found that the newly designed MCP detector was useful for the potential measurement at the IMT region.

A charge exchange neutral particle analyzer including a newly designed multichannel 63.6° cylindrical electrostatic energy analyzer was constructed. Since the energy analyzer can simultaneously adopt a small radius of cylindrical electrodes and a large width of an entrance slit, high incident current can be analyzed while the size of the analyzer remain compact. Characteristics of the analyzer were investigated using hydrogen beam with energies from about 0.5 to 5 keV. The experimental results are compared well with the numerical analysis.

In order to understand the behavior of neutral hydrogen in ICRH plasmas of the GAMMA 10 tandem mirror, we tried new simulations of neutral transport by using the DEGAS Monte Carlo code in which the effects of hydrogen recycling were taken into account. This simulation has been performed by introducing a multiplication coefficient in DEGAS to control the amount of desorbed test particles, which enables us to simulate the neutral transport for the first time under the conditions in which the recycling coefficient exceeds unity. By using the multiplication coefficient, it was found that the axial profile of the atomic hydrogen density obtained from the simulation changed significantly. On the other hand, there is little influence on the radial profile of the simulated hydrogen density. The simulation result, taking the axial variation of the multiplication coefficient into consideration, well predicted the result of the H α -emission measurements. This indicates that strong hydrogen recycling is localized near the midplane. These calculation results are discussed from the view point of the wall-reflux coefficient deduced from the DEGAS simulation.

Carbon sheet pump (CSP), which has been developed for reduction of hydrogen recycling in the large helical device (LHD) of National Institute for Fusion Science, is applied to an actual plasma device for the first time to evaluate the pumping characteristics. A small scale of pump module is manufactured and installed in the GAMMA-10 tandem mirror device and the module is exposed to charge-exchange neutrals emitted from ICRF-heated hot-ion plasmas. Temporal evolution of the pressure is measured with a fast ionization gauge installed in the test chamber during the plasma discharge and a significant difference between CSP-on and CSP-off is observed under good reproducibility. The dependence on the ICRF power is investigated and the pressure difference between on and off is found to increase with the ICRF power. These phenomena indicating the pumping ability of CSP are observed to continue during more than 300 plasma shots, which shows the effectiveness of CSP in controlling hydrogen recycling. The result of thermal desorption experiments of CSP after being exposed to charge-exchange neutrals is also described.

A cross-polarization scattering (CPS) diagnostic method was applied to the GAMMA10 tandem mirror in order to study electromagnetic plasma waves with frequency less than 10 MHz. In the CPS process, an incident ordinary (extraordinary) wave is converted to an extraordinary (ordinary) wave by magnetic fluctuations in a plasma. The frequency spectra, and the density and magnetic fluctuations of the waves can be evaluated from both the CPS and reflectometry. The converted wave propagates through the cutoff layer and reaches a different diagnostic port with a receiver horn. The observed waves are identified as the electromagnetic drift wave assuming the wave number matching condition. The low frequency waves nonlinearly coupled with the Alfven ion cyclotron waves are also observed with increase in the plasma pressure.

A time–frequency analysis of plasma turbulence is performed for the GAMMA10 tandem mirror plasma. The method is based on the decomposition of the reflectometer signal into a set of wavelets. Excellent resolution in the time–frequency domain, inherent to wavelet analysis, allowed us to observe fast changes in the instantaneous spectra of fluctuations, which would be averaged out if treated using conventional Fourier analysis. In order to estimate the propagation velocity of turbulent fluctuations, a wavelet cross-correlation function was calculated. Comparison of wavelet spectra, corresponding to different reflectometer channels, demonstrates the existence of short-time flashes of fluctuation intensity, as well as elongation of low-frequency disturbances along the magnetic field lines and their rotation in the E×B direction. Plasma fluctuations in the Alfven range of frequencies sometimes propagate from the center towards the magnetic plugs with velocities close to 1000 km/s. In general, the wavelet spectr...

This paper describes characteristics of end loss electrons (ELE's) from a tandem mirror. Without electron cyclotron resonance heating (plug ECRH), ELE's issue from an ICRF heated hot ion plasma Their temperature increases with the hot ion diamagnetism. Drag heating of electrons on hot ions balances with axial loss due to Coulomb scattering. Landau damping of an Alfven ion cyclotron wave is also observed in this phase. On application of the plug ECRH, an intense loss flux of warm electrons is generated. The axial heat flow carried by the warm electrons significantly increases. Control of the axial flux of the warm electrons is tried A highly transparentmesh is put up over an end plate and a bias voltage negative to the end plate is applied to the mesh. Then secondary electrons emitted from the end plate are suppressed and the loss flux of the warm electrons decreases to about one third of that without the mesh bias. This reduction factor is much smaller than the value expected from the rate of ion plugging. A possible cause of this is an ion current through a plasma behind the end plate.

Potential confinement of an ICRF-heated high-ion temperature plasma in GAMMA 10 is experimentally studied. The potential confinement was shown from data of one-end plugging and both-end plugging. The waveform of end loss current and an analysis of end loss ion energies have also indicated potential formation and confinement. The central cell line density increases 50% by the potential confinement. Some radial losses were observed in the anchor andlor plug/barrier regions and a rate of the radial loss was measured by using the data from one-end plugging. Under an experimental condition, the radial loss rate was estimated to be about 3%. In order to reduce the radial loss, conducting plates were installed adjacent to the plasma in the anchor transition region. The density increase of 60 % was attained after installation of the conducting plates and a higher density increase can be expected in the near future. The density increase was 50% before installation of the conducting plates. Controllability and reproducibility of the potential confinement are also improved after installation of the conducting plates.

A millimeter-wave two-dimensional imaging array has been developed for diagnostics of magnetically confined plasmas. It is applied to the plug cell of the GAMMA 10 tandem mirror, and measures time evolution of radial and axial line-density and electron cyclotron emission (ECE) profiles in one plasma shot. The monolithic-type GaAs Schottky diode detector having an antenna and built-in amplifier on a GaAs substrate is newly fabricated using monolithic microwave integrated circuit (MMIC) technology. This novel detector is developed to attain high frequency response up to submillimeter-wave region and wideband heterodyne characteristics, and is expected to be applied to ECE imaging in a large helical device. The response of the detector is compared with that of the hybrid-type detector using beam-lead Schottky barrier diodes. The first experimental result of the ECE measurement is reported using a heterodyne radiometer with the MMIC detector.

Behavior of impurity ions has been investigated using visible spectroscopy in the GAMMA10 tandem mirror. A 40 channel visible spectrometer system has been developed for measurements of the ion temperature and ion flow velocity. The spectrometer consists of a 100 cm monochromator, a 40 channel optical fiber array and an image intensifier tube coupled with a charge coupled device TV camera. The spectra from low ionization states of oxygen and carbon are measured in ion cyclotron range of frequency heated plasmas. High ion temperatures (3–10 keV) of O4+ are observed in the anchor region, where the minimum B mirror field is produced by baseball coils. The O4+ ion is heated by the fourth harmonic frequency of O4+, which is most likely due to the cyclotron higher harmonic damping.

In the limited regions of operation of the GAMMA10 tandem mirror, a saturation or a reduction of the stored energy in the central cell is sometimes observed. By using electrostatic and magnetic probes, the low frequency (

The use of ultrashort pulses (full width at half-maximum=1–100 ps) as a source of broadband, time-resolved radiation has high potential for application in microwave diagnostics for fusion plasmas. Here we report on the ultrashort-pulse reflectometry system developed for electron density profile and fluctuation measurements on the GAMMA 10 tandem mirror. The four-channel system (selectable frequencies of 8–17 GHz at 1 GHz intervals) measures the double-pass time-of-flight from the vacuum window to the cutoff layer. The hardware has an uncertainty of ±40 ps and can be operated at a repetition rate of 400 kHz. The system is operational, however, data from plasma experiments is unavailable at the moment. Results from a single-channel O-mode system show time-of-flight measurements consistent with values calculated from profiles obtained by a scannable single-channel interferometer. Fluctuation measurements are also consistent with results from a Fraunhofer diffraction diagnostic. These results suggest that wit...

Gentle hump structure on energy spectra of end-loss ions was observed in the RF driven tandem mirror plasma by use of an end-loss energy component analyzer (ELECA), which has been developed in order to directly measure velocity distribution functions of the end-loss ions. 1 Ions are heated anisotropically in the central cell of the tandem mirror by the ICRF wave injection, and Alfven ion-cyclotron (AIC) fluctuations are excited due to the anisotropic ion temperature. 2 The correlation between the hump structure and the excitation of the AIC fluctuations was observed experimentally. From the viewpoints of the heating characteristics and the resonance condition, the hump energy was estimated and successfully compared with the experimetal results. Influence of the ICRF waves on the hump structure was investigated using a bounce averaged Fokker Planck code. 3 The AIC fluctuations enhance the end-loss ions scattered from the trapped region to the loss region, and have direct effects upon the ion confinement. The energy flux of the enhanced end-loss ions due to the AIC fluctuations was estimated and the enhancement factor was obtained. The strong anisotropic ion heating is dangerous for the ion confinement in the open systems which contain necessarily the loss regions.

The microwave reflectometry using FM and ultrashortpulse radar techniques have been applied to GAMMA 10 in order to diagnose plasma density profile and fluctuations. The reliability of profile measurement using the FM reflectometer is investigated for various sweep times and local path length. It is demonstrated that the reconstructed density profiles seem to be improved when the sweep time is faster than 50-100 μs. The several reconstruction algorithms are introduced to analyze the fast time-varying data, such as, the maximum-entropy method and wavelet analysis as well as zero-cross fringe counting and digital complex demodulation method. The reflectometers are applied to the measurement of density/magnetic fluctuations. The space- and time-resolving spectra of rf waves as well as low-frequency waves are obtained The density and magnetic-field fluctuations are evaluated from both the reflectometer and cross-polarization scattering diagnostic method.

application/pdf, Charge-exchange (C–X) neutral particle measurements have been carried out in hot-ion-mode plasmas of the GAMMA 10 tandem mirror. In the present experiment, a microwave power of 40 kW in 28 GHz is injected toward a second harmonic ECR layer located in the vicinity of the ICR layer at the central region and the radial profiles of ion temperatures determined from the energy spectrum of the C–X neutrals by using a neutral particle energy analyzer (NPA) are investigated from the viewpoint of ion energy balance. At the onset of the ECRH pulse, a remarkable increase of C–X neutral flux with high energy (few keV to few tens keV) is observed with NPA and the resultant ion temperature on the plasma axis is found to increase from 2.5 to 5.0 keV at the electron line density of 3 × 1013 cm – 2. Based on the measured plasma parameters, radial profiles of ion-energy losses due to classical processes are evaluated and ECRH in the central region is confirmed to reduce the energy loss due to electron drag significantly in the core-plasma region.

We have constructed a 2-dimensional Hα line-emission measurement system in order to study neutral hydrogen behavior. We can obtain 2-dimensional radial profiles of hydrogen density by considering a collisional-radiativ model. We have also constructed space- and time-resolving spectrograph system in the range of vacuum ultraviolet (VUV). 15 - 105 nm. studying ion density profiles which directly relates to impurity transport. We measure both Hα line-emissions and VUV spectra from the hot ion mode plasmas in the GAMMA 10 for studying plasma particle confinement and impurity behavior. We found that both particle and energy confinement of the GAMMA 10 plasma was slightly improved during electron cyclotron resonant heating (ECRH) over that without ECRH. From the VUV measurement it is concluded that the impurity ions mainly came into the plasma during its formation phase.

A new type of compact electrostatic cylindrical energy analyzer originally providing a deflection angle of π/22 has been designed with the aim of measuring energy spectra of charge exchange neutral particles in the tandem mirror GAMMA10. The essential advantage of the use of this analyzer is that a large entrance aperture can be adopted with high energy resolution so that high incident current is analyzed, while the radius of the cylindrical electrodes remain compact. Characteristics of the analyzer are discussed using computer simulations of ion trajectories including an effect of fringing field obtained by finite-element method as well as small-angle scattering in stripping collisions of hydrogen atoms.

Recent results of the plasma confinement and the behavior of the edge plasma in the minimum-B regions of the GAMMA 10 tandem mirror are described. In this experiment conducting plates are newly installed in the anchor-cells in order to improve the performance of plasma confinement. The conducting plates are fixed closely to the plasma surface where the cross-section of the plasma is flatly e]elongated in the anchor transition region. After conditioning the conducting plates, basic characteristics on the conducting plates are investigated. A fairly good symmetry between the two plates facing each other is found in the drain current and in the floating potential. The axial profiles of these measured data also show a symmetry with respect to the central-cell mid plane. The dependence of the resistance connecting the plates to the machine ground shows that the floating condition of the plates provides the better property of particle confinement. As the result of optimization of the heating and gas puffing systems a s significant improvement of the plasma parameters is observed during potential formation. According to the increase of plug ECRH power, the central-cell line density and the diamagnetism increase and both parameters are nearly doubled at the ECRH power of 140kW under the floating condition of the conducting plates.

Validity of averaged minimum- B configuration on Magnetohydrodynamics (MHD) stability of the GAMMA10 tandem mirror are studied in the ion cyclotron range of frequency (ICRF)-heated, hot ion mode plasma. It has been observed that the plasma can not be sustained mostly owing to the flute interchange mode when the high power ICRF is applied for the central cell ion heating. As the ratio of the central cell beta to the anchor cell beta increases, the amplitude of the flute interchange mode increases. The anchor cell plasma is also sustained by the another ICRF source in the central cell. The flute type MHD instability in GAMMA10 is well controlled by the phased ICRF antennas in the central cell.

Vacuum ultraviolet (VUV) and soft X-ray measurements are important means of diagnosing impurities in magnetically confined plasmas used in fusion research. Recently, space- and time-resolving flat-field VUV (150–1050 Å) and soft X-ray (20–350 Å) spectrographs have been constructed by using aberration-corrected concave gratings with varied-spacing grooves which give a wide simultaneous spectral coverage on a microchannel-plate intensified detector. Calibration experiments have been performed at beamlines 11A and 11C at the Photon Factory of the High Energy Accelerator Research Organization. The relative efficiency of the VUV spectrograph has been measured for P-polarization geometry in the spectrograph. In the soft X-ray spectrograph, efficiencies have been obtained for several different points of irradiation on the grating along the groove direction and for two (S and P) polarization geometries.

We investigate the ion axial confinement time and the ion endloss spectrum in a wide range of collision frequency in GAMMA10 tandem mirror by using Monte-Carlo simulation. It is found that in the two limits of collision frequency, the confinement time is smaller than the theoretical value on square well model. Also, unlike the square well model, the ion endloss spectrum consists of two elements where collision frequency is very small.

A new method of plasma density profile reconstruction in microwave reflectometry is proposed and implemented on an X-mode broadband reflectometer of the GAMMA 10 mirror device with an ultrafast sweep rate of 10–20 μs. The proposed method makes use of the wavelet transform of the detected signal. Excellent resolution in the time-frequency domain, inherent to wavelet analysis, allows one to obtain a radial electron density profile for every frequency sweep. The electron density reconstruction algorithm, besides the wavelet transform of the reflectometer signal, also includes the calibration, profile initialization, and the solution of an integral equation, ultimately yielding the local values of the electron density. Calibration of the measured signal phase and profile initialization is performed using the independent results of microwave interferometry. Inversion of the integral equation is implemented utilizing the gradient method, numerically stable even for plasma regions with steep density gradients an...

Temperature of end loss electrons is a base factor in data analysis of an axial potential profile and electron transport in an end region of a tandem mirror. In this analysis, a large ratio of a magnetic field strength at a mirror throat to that at an end wall, where the end loss electron temperature is measured, may bring an error in evaluation of the end loss electron temperature. In addition, anisotropy of the electron velocity distribution is also a source of error. By using a bi-Maxwellian velocity distribution of end loss electrons as a test function, relation between the measured electron temperature and the parallel electron temperature at the mirror throat is given as a function of the temperature anisotropy. Then, theoretical predictions from a potential model of the axial potential distribution and the electron transport are expressed by using the measured electron temperature. The present analysis shows that, when the measured temperature is fixed, the potential difference along a magnetic fie...

Potential distribution in an end region of the GAMMA10 tandem mirror is investigated associated with experiments for control of an electron flow to a floating endplate installed on an end wall of the vacuum vessel. The end region has an open field configuration. Mesh bias and thermal dike experiments have been carried out in order to reduce a heat flow to the endplate by suppression of secondary electron emission from the endplate. Variations of the axial potential distribution in the two experiments are studied. The observed variations agree with predictions of a theoretical potential model. This agreement shows a strong effect of the secondary electrons on the potential distribution in the end region and also indicates that the potential distribution is predominantly determined by axial particle transport. In this study, production of Yushmanov electrons is also shown.