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AbstractIn GAMMA 10/PDX, the divertor simulation experiment has been performed recently. Ion cyclotron range of frequency (ICRF) waves are used for plasma production and ion heating. It is necessary for obtaining better controllability and extending the operational regime to understand the excitation, propagation, and absorption of the waves. The density fluctuations accompanied by the ICRF waves propagating in the inner region of plasma has been measured by using a microwave reflectometer on the antenna-phasing experiments, where the propagation of the ICRF waves is actively controlled. The phase difference between two axially separated ICRF antennas remarkably affects the global plasma parameters. The density fluctuation caused by the interfered ICRF waves is shown to depend clearly on the phase difference between the waves excited from two antennas. The availability of a reflectometer for the evaluation of the internal wave propagation is shown.

AbstractPropagation behavior of 3–4 kHz electron density fluctuation in the core and edge regions was investigated in the GAMMA 10 tandem mirror device. Frequency analysis was employed for high spatiotemporal-resolution fluctuations measured by gold neutral beam probe (GNBP) in the central cell and Langmuir probes in the end region. We found same-frequency periodic fluctuations in both signals and applied the correlation analysis technique. By considering the geometry of the measurement system, propagation speeds of the periodic fluctuation along and across the magnetic field lines were estimated.

AbstractThis paper describes the recent results of divertor simulation research toward the realization of the detached plasma using the end-mirror of a large tandem mirror device. The additional ion cyclotron range of frequency heating in the anchor-cells for higher particle flux generation significantly increases the density, which attained the highest particle flux up to 1.76 × 1023particles/·m2at the end-mirror exit. Massive gas injection (H2and noble gases) into the divertor simulation experimental module (D-module) was performed, and a remarkable reduction of the electron temperature on the target plate was successfully achieved associated with the strong reduction of particle and heat fluxes in D-module. Two-dimensional images of Hα emission in D-module observed with a high-speed camera showed strong emission in the upstream region and significant reduction near the target plate. These results clarified the effect of radiation cooling and formation of detached plasma due to gas injection. It is also found that Xe gas is much more effective in achieving detached plasma than Ar gas. Numerical simulation studies also have been performed toward the understanding of the cooling mechanism of divertor plasma. The above results will contribute to establishment of detached plasma control and clarification of the radiation cooling mechanism toward the development of future divertor systems.

AbstractIn GAMMA 10/PDX, divertor simulation studies have been started as a new research plan by using end-loss plasma flux at the end-mirror region. A divertor simulation experimental module (D-module) was installed in the west end-cell. Here we introduce a novel microwave interferometer system with a phase imaging method, and all the crucial components are designed and tested. Finally, we tested the interferometer system using a Teflon plate in substitution for plasma. In order to improve the system accuracy, the phase detection circuit was modified and the phase difference of each channel was verified.

AbstractIn tandem mirror GAMMA 10/PDX, H2gas has been injected to a divertor simulation experimental module (D-module) for radiation cooling and detachment study. Electron temperature (Te) and density (ne) distributions have been measured by Langmuir probes in the D-module. With increase of H2gas amount, Tehas decreased especially at the V-shaped target plate from 30-40 eV to 3-5 eV. Moreover, neand ion saturation current Iishave increased at the inlet but decreased at the target and had a steeper distribution toward the corner. These results suggest detachment. The movement of the ionization front to the upstream side with increase of the gas is suggested from the change in the plasma parameters profiles along the magnetic field line.

In GAMMA 10 two types of low frequency plasma fluctuations have been observed in the central cell by several diagnostics. One of which is the electron drift mode rotating azimuthally in the direction of electron diamagnetic drift rotation observed during ion cyclotron range of frequency heating (ICH). Recently a new interferometer has been installed in the mid-plane of the west anchor cell in order to investigate plasma density and density fluctuations in the anchor cell. By using the new interferometer, line-integrated density fluctuations are measured in the anchor cell and are compared to fluctuations in the central cell obtained by conventional interferometer systems. Density fluctuations around 10 kHz are observed in both cells during the ICH period and they are suppressed during the formation of axial confining potentials by electron cyclotron heating (ECH) in the plug regions.

The divertor simulation experimental module (Dmodule) has been installed in the west end region in GAMMA 10/PDX. By use of Langmuir probes and spectroscopic measurement of intensity ratios of He I lines, temporal evolution of electron temperature and that of electron density of the plasma in the D-module with the V-shaped tungsten target are obtained. When the additional ICRF heating is applied to the anchor cell, the electron temperature evaluated with He I intensity ratios decreases from ~60 eV to ~25 eV and that from the probe measurement decreases from ~27 eV to ~14 eV. The difference between both measurements seems to be attributed to the difference of their measurement positions. The electron density measured by the Langmuir probe increases 2.3 times due to the RF3 power but it is rather low (< 1017m-3). The electron density at the end region is expected to be increased by enhancement of ICRF heating and additional gas puffing at the plug/barrier cell which is the upstream cell of the end region.

Thomson scattering (TS) is the most reliable diagnostic to measure the electron temperature and electron density radial profiles. In GAMMA 10, the yttrium-aluminium-garnet (YAG)-TS system was constructed with the large solid angle of TS collection optics. We carried out the Rayleigh and Raman scattering experiments for system settings and applied the YAG-TS system to the GAMMA 10 plasma. We can successfully measure the electron temperature and density radial profiles in the central cell of GAMMA 10 by using YAG-TS system. The supersonic molecular beam injection (SMBI) experiments have been started for plasma fueling study in GAMMA 10. In SMBI experiments, we observed the electron temperature decrease and electron density increase in the plasma center.

In GAMMA 10, a divertor simulation study has been started with open magnetic field configuration in the end region. High heat and particle fluxes are required along the magnetic field line to the end region. Plasmas with high ion-temperature of several keV and strong temperature anisotropy of more than 10 have been produced by using ion-cyclotron range of frequency (ICRF) heating in the central cell. Direct anchor heating experiments with new anchor antennas have been performed and the enhancement of the MHD stabilization has been observed. High energy ions whose energy is more than 50 keV have been observed in the end-loss ions. The axial transport of high-energy ions due to loss processes other than the classical Coulomb scattering has been discussed. Alfvén-ion-cyclotron (AIC) waves are spontaneously excited owing to such the strong temperature anisotropy and considerable energy transport along the magnetic field line due to the AIC waves is expected. In this review, recent ICRF heating experiments for the divertor simulation study are described.

In the new research plan of Plasma Research Center of the University of Tsukuba, a high heat-flux divertor simulator (E-Divertor) was proposed by using an end-mirror exit of a large tandem mirror GAMMA 10/PDX device. Experiments for characterization of end-loss plasma flux have been extensively performed at the end-mirror region of GAMMA 10/PDX and detailed behavior of end-loss particles has been investigated. In standard hot-ion mode plasmas (ne0~ 2×1018m-3, Ti0~ 5 keV), the energy analysis of ion flux was performed by using end-loss ion energy analyzer (ELIEA). It was found that the high ion temperature (100 - 400 eV) is generated and has a liner relationship between diamagnetism in the central-cell. The ion temperature determined from the probe and calorimetric measurements gives a good agreement with the ELIEA measurement. Additional ICRF heating in the anchor-cell showed a significant increase of particle flux, which indicated an effectiveness of additional plasma heating in adjacent cells. Superimposing the ECH pulse of 380 kW, 5 ms induces a remarkable enhancement of heat flux and a peak value in the net heat-flux density more than 10 MW/m2was attained during the ECH injection, which comes up to the heat-load level of the divertor plate of ITER. Recently a large-scale divertor simulation experimental module (D-module) was installed in the west end-cell and the first plasma irradiation experiments onto a new tungsten V-shaped target were successfully performed. A number of interesting results such as neutral compression, enhancement of recycling and impurity radiation during noble gas injection, have been observed.

A two-channel reflectometer has been developed and successfully applied to the GAMMA10 central cell to investigate the spatial structure of spontaneously excited Alfven ion-cyclotron (AIC) waves. At the frequencies of externally applied ICRF waves and AIC waves, good correlations of well above the statistical noise level are obtained between two density fluctuations at axially separated positions in the central cell. Density fluctuations at various radial positions and two axially separated positions show that the power distribution among the AIC waves changes much both in radial and axial directions especially in the earlier period just after the excitation and in the core region of r/a< 0.3. Phase differences of the AIC waves at axially separated positions show features of the transformation of the AIC waves from propagating to standing waves. The pass of the node of the standing wave is also observed. The time of the pass is different by AIC waves, which clearly indicates difference of the axial wavelengths of the simultaneously excited AIC waves.

GAMMA 10/PDX project is the development of the mirror devices to aim the fusion-reactor relevant research using the potential control and the end loss as the particle and heat fluxes for transport, divertor plasma physics and Plasma Wall Interaction (PWI) studies. It was obtained that more than 10 MW/m2heat flux density for PWI studies. The closed divertor-simulator module has been newly installed. The first studies of this module have been done and obtained the expected plasma flow. We will strengthen the module functions and diagnostic capability and heating systems to enable to be more reactor-relevant simulator. In the core transport control by the ECH, correlation studies of the radial electric field profile and the fluctuations are made and the high coherency is confirmed by using the new two-point potential detector of the Gold Neutral Beam Probe(GNBP). The maximum electron temperature obtained so far is - 150 eV during the ECH by Thomson Scattering, but we need more studies for the high power ECH optimization. As one of the key tools for the new GAMMA 10/PDX project, 2 MW gyrotron development at 28 GHz has been started, based on the success of 77 GHz-1.9 MW output. The maximum outputs of 1 MW at 28 GHz in short pulse and 540kW for 2 s are obtained.

(1) Four-time progress in ion-confining potentials Φcto 3.0 kV in comparison to Φcattained 1992-2002 is achieved in the hot-ion mode (Ti=several keV). A scaling of Φc, which favorably increases with plug electron-cyclotron heating (ECH) powers (PPECH), is obtained. (2) The advance in Φcleads to a finding of remarkable effects of radially sheared electric fields (dEr/dr) on turbulence suppression and transverse-loss reduction. (3) A weak decrease in Φcwith increasing ncto ˜1019m-3with the recovery of Φcwith increasing PPECHis obtained. (4) The first achievement of active control and formation of an internal transport barrier (ITB) has been carried out with the improvement of transverse energy confinement. Off-axis ECH in an axisymmetric barrier mirror produces a cylindrical layer with energetic electrons, which flow through the central cell and into the end region. The layer, which produces a localized bumped ambipolar potential Φc, generates a strong Ershear and peaked vorticity with the direction reversal of Err× Bsheared flow near the Φcpeak. Intermittent vortex-like turbulent structures near the layer are suppressed in the central cell. This results in Teand Tirises surrounded by the layer. The phenomena are analogous to those in tokamaks with ITB. (5) Preliminary central ECH (170 kW, 20 ms) in a standard tandem-mirror operation raises Te0from 70 to 300 eV together with Ti[perpendicular]0from 4.5 to 6.1 keV, and Ti//0from 0.5 to 1.2 keV with τp0=95 ms for Φc(=1.4 kV) trapped ions. The on-axis particle to energy confining ratio of τp0/τE0is observed to be 1.7 for Φctrapped ions (consistent with Pastukhov’s theory) and 2.4 for central mirror-trapped ions with 240-kW plug ECH and 90-kW ICH (ηICH˜0.3; nlc=4.5×1017m-2). (6) Recently, a 200 kW central ECH with 430 kW plug ECH produces stable central-cell plasmas (Te=600 eV and Ti=6.6 keV) with azimuthal Er×Bsheared flow. However, in the absence of the shear flow, hot plasmas migrate unstably towards vacuum wall with plasma degradation.

Generalized scaling laws for the formation of plasma confining potentials are investigated to find the physics essentials common to representative tandem-mirror operational modes in GAMMA 10. These modes are characterized in terms of (i) a high-potential mode and (ii) a hot-ion mode. The potential-formation scalings in these modes are consolidated and generalized on the basis of the consistency with finding of the wider validity of Cohen’s strong electron-cyclotron heating (ECH) theory covering over both modes. A plateau-shaped electron distribution function is observed when a plug electron-confining potential is formed in the hot ion mode of GAMMA 10, as predicted in terms of the strong ECH theory.

Scaling laws of potential formation and associated effects are theoretically and experimentally investigated in the GAMMA 10 tandem mirror. In GAMMA 10, the main tandem-mirror operations from 1979 to 2003 are characterized in terms of (i) a high-potential mode having kV-order plasma-confining potentials, and (ii) a hot-ion mode yielding fusion neutrons with 10-20 keV bulk-ion temperatures. In this paper, the externally controllable parameter scaling including electron cyclotron heating (ECH) powers for potential formation covering over these two representative operational modes is investigated; that is, the construction of “the central-cell plasma-confining potentials” Φcformation scaling with plug ECH is studied on the basis of the electron energy-balance equation and Cohen’s strong electron cyclotron heating (ECH) theory for investigating the formation physics of plasma confining potentials.It is found that our proposed scaling formulae are in good agreement with the experimental data in the two representative operational modes of the high-potential and hot-ion modes in the GAMMA 10 tandem mirror.This scaling shows a favorable increase in confining potentials with installing more powerful ECH sources by the use of ECH powers over the present 250 kW. On the basis of the scaling prediction, we also report the design of a newly developed 500 kW gyrotron for an application to investigate the validity of the abovedescribed Φcformation scaling with plug ECH aiming at achieving higher plasma parameters.

Following the 2002 Conference on Open Magnetic Systems for Plasma Confinement, a three times progress in the formation of ion-confining potential height is achieved in the hot-ion mode. The advance in the potential formation leads to a finding of remarkable effects of radially produced shear of electric fields dEr/dr on the suppression of not only coherent drift waves but turbulence-like fluctuations for the first time in GAMMA 10. Also, the progress in the potential formation is made in line with the extension of our proposed physics scaling of potential formation covering over representative tandem-mirror operational modes, characterized in terms of (a) a high-potential mode having kV-order plasmaconfining potentials and (b) a hot-ion mode yielding fusion neutrons with 10-20 keV bulk-ion temperatures (Ti).

GAMMA 10 experiments have advanced in high density experiments after the last IAEA fusion energy conference in 2000 where we reported the production of the high density plasma through use of ion cyclotron range of frequency heating at a high harmonic frequency and neutral beam injection in the anchor cells. However, the diamagnetic signal of the plasma decreased when electron cyclotron resonance heating was applied for the potential formation. Recently a high density plasma has been obtained without degradation of the diamagnetic signal and with much improved reproducibility than before. The high density plasma was attained through adjustment of the spacing of the conducting plates installed in the anchor transition regions. The potential confinement of the plasma has been extensively studied. Dependences of the ion confinement time, ion-energy confinement time and plasma confining potential on plasma density were obtained for the first time in the high density region up to a density of 4×10¹⁸ m⁻³.

Scaling laws of potential formation and associated effects along with their physical interpretations are consolidated on the basis of experimental verification using the GAMMA 10 tandem mirror. A proposal of extended consolidation and generalization of the two major theories—(i) Cohen's strong electron cyclotron heating (ECH) theory for the formation physics of plasma confining potentials and (ii) the generalized Pastukhov theory for the effectiveness of the produced potentials on plasma confinement is made through the use of the energy balance equation. This proposal is then followed by verification using experimental data from two representative operational modes of GAMMA 10, characterized in terms of (i) a high-potential mode having plasma confining potentials of the order of kilovolts and (ii) a hot ion mode yielding fusion neutrons with bulk ion temperatures of 10–20 keV. The importance of the validity of the proposed physics-based scaling is highlighted by the possibility of extended capability inherent in Pastukhov's prediction of requiring an ion confining potential of ~30 kV for a fusion Q value of unity on the basis of an application of Cohen's potential formation method. In addition to the above potential physics scaling, an externally controllable parameter scaling of the potential formation increasing with either plug or barrier ECH powers is summarized. The combination of (i) the physics-based scaling of the proposed consolidation of potential formation and effects with (ii) the externally controllable practical ECH power scaling provides a new direction for future tandem mirror studies.

A method is proposed for obtaining radial profiles of plasma temperatures of both plasma ion (T_i) and electron (T_e) 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 SiO<sub>2</sub> `dead-layer filters' having various nanometre-order thicknesses is applied for simultaneous T_i and T_e analyses by using charge-exchange neutral particles and x-rays from plasmas. Radial profiles of T_i and T_e 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.

Generalization and consolidation of scaling laws of potential formation and associated effects are investigated in the GAMMA 10 tandem mirror. The scaling covers over representative tandem-mirror operational modes, characterized in terms of (i) a high-potential mode having kV-order plasma-confining potentials, and (ii) a hot-ion mode yielding fusion neutrons with 10-20 keV bulk-ion temperatures. A novel proposal of extended consolidation and generalization of the two major theories of (i) Cohen's strong electron cyclotron heating (ECH) theory for the formation physics of plasma confining potentials, and (ii) the generalized Pastukhov theory for the effectiveness of the produced potentials on plasma confinement has been made through the use of the energy-balance equation. This proposal is then followed by the verification on the basis of experimental data from the above two representative modes in GAMMA 10. The importance of the validity of this proposed consolidation is highlighted by a possibility of extended capability inherent in Pastukhov's prediction of requiring ion-confining potential (ɸc) of 30 kV for a fusion Q value of unity through an application of Cohen's potential formation method. In addition to the above potential physics scaling, an externally controllable parameter scaling including ECH powers for potential formation is investigated: The construction of the ɸcformation scaling with both plug and barrier ECH is carried out. Data on ɸc. [kV] (or equivalently ɸhwith np/ncfrom the strong ECH theory) as a function of externally controllable plug and barrier ECH powers, (PPECH[kW] and PBECH[kW], respectively), and nc[1018m−3] are summarized as follows:ɸc= 1.0×10−4(1+5.0×10−3×PBECH1.04±0.02) PPECH1.73±0.02× [c (np/nc)2/3-1]exp [-(0.33±0.05) nc]. Here, c=9-11 and 7-9 for the hot-ion and high-potential modes, respectively. The present paper covers the following updated results: (i) A verification of our novel proposal for potential formation and effects is carried out so as to consolidate two major (Pastukhov's and Cohen's) theories by the use of the “third mode” with the central ECH. The validity of the theory provides a future scenario for combining present representative modes so as to upgrade to hot-ion plasmas with high potentials. (ii) A novel efficient scaling of ɸcformation with both plug and barrier ECH is summarized. The combination of the physics scaling of (i) with the externally controllable practical power scaling of (ii) provides a scalable way for the future tandem-mirror researches.

An electron density profile is observed by using an ultrashort-pulse reflectometry in the central cell of the GAMMA 10 device. The pulse having 65 ps FWHM is launched into the plasma in the O-mode and reflected at the cut off layer. The frequency range of the receiving system is 6 to 11 GHz. Time of flight of the received signal is measured via a time to amplitude converter and processed by a computer. Here, electron density profile lower than 1.5 × 1018m−3is reconstructed within one-shot data. The time variation of the electron density profile is acquired. Reflected wave has information of fluctuation, simultaneously. Frequency spectrum of the fluctuation is also observed.

High-density experiments using newly applied ioncyclotron range of frequency (ICRF) wave and neutral beam injection (NBI) in the GAMMA 10 tandem mirror are described. A new ICRF wave system (RF3) with high harmonic frequency has been introduced for achieving high density. In addition, neutral beam injectors were recently installed at the central and anchor cells for fueling to target plasmas produced by ICRF waves. Arrays of Hα line-emission detectors are installed from the midplane of the central-cell to the anchor-cell in order to evaluate the particle source density around these regions. In a typical ICRF-heated hot-ion-mode plasma, both anchor and central NB's are injected together with the RF3 wave and the significant increase of the line-density in the central-cell up to ~8×1012cm−2was attained during the potential formation. It is confirmed that this high density is achieved under the ion temperature of three times higher than the value expected from the usual empirical boundary without using these new heating systems. An analysis of neutral particle transport using the Monte Carlo simulation code is developed to calculate the spatial profile of neutral density in non-axisymmetric region, such as anchor cell. Particle source rate is estimated based on detailed measurements of Hα line-emission from the central-cell to the east anchor-cell together with the neutral transport simulation.

Observations of internal core plasma structural behaviour during the magnetohydrodynamic (MHD) destabilization of the central cell plasmas are carried out by the use of our developed semiconductor x-ray detector arrays installed in both central cell and anchor regions of the GAMMA 10 tandem mirror. In the present paper, it is found from the developed x-ray diagnostics that the bulk plasmas rotate without a change in its shape and structure with an E×B velocity during the destabilization. The onset of the off-axis rotation is identified to be closely related to a scaling of the MHD stability boundary (i.e. the anchor beta requirements for stabilizing central cell hot ion plasmas). These data confirm pressure driven interchange instability in tandem mirror plasmas, and reveal the rigid rotational bulk plasma structure as the first demonstrated interior plasma property during the destabilization.

A new method is proposed for obtaining radial profiles of both plasma ion (Ti) and electron temperatures (Te) simultaneously using one semiconductor detector array alone. Furthermore, availability of the new idea of the simultaneous Tiand Tediagnostics is experimentally demonstrated by the use of a small-sized semiconductor detector array. This novel method for semiconductor Tidiagnostics is proposed on the basis of an alternative “positive” use of a semiconductor “dead layer” as an energy-analysis filter. Filtering dependence of charge-exchange neutral particles from plasmas on the thickness on the order of nm thick SiO2layer is used for analyzing Tiranging from hundreds to thousands eV. In this report, proof-of-principle plasma experiments for the proposed idea are, at first, demonstrated in the GAMMA 10 tandem mirror to verify the availability of this novel idea of distinguishing and identifying each value of Tiand Teby the use of various thin filtering materials. Furthermore, novel experimental data on radial profiles of Tiand Teare simultaneously observed and analyzed using a semiconductor detector array along with the development of a Monte-Carlo computer simulation code for analyzing interactions between semiconductor materials and incident particles. The radial profiles of Tiand Teobtained from semiconductor detectors by the use of the proposed method are found to be in good agreement with those from a charge-exchange neutral-particle Tianalyzer and a microchannel-plate Tedetector. Detailed data and analysis method are represented in the paper.

AbstractRecently, the energy responses of widely utilized seraiconductor X-ray detectors employed in many plasma-confmement devices were found to show contradictory behavior against the predietion from the conventional X-ray-energy-response theory in textbooks employed over the last quarter of the century for the analyses of plasma electron tem-peratures Te. This problem gave serious confusion for plasma researchers, who were utilizing X-ray data from semiconductor detectors including widely utilized X-ray-tomography data. For solving this problem, we proposed a new theory on the X-ray-energy response using X-ray-produced three-dimensionally diffusing charges from a field-free-substrate region in a detector. This novel theory is verified by the detailed data on a semiconductor X-ray response using synchrotron radiation. On the basis of these significant new findings, a novel method for X-ray diagnostics is proposed for the pur-pose of developing novel Temeasurements. The physics principle is essentially different from those in commonly utilized standard methods; namely, the X-ray pulse-height analysis and the X-ray absorption method. The new method is based on a semiconductor depletion layer as a temporally changeable and externally controllable X-ray “absorber” during one plasma shot, in place of its conventional use as an X-ray sensitive region. The new method is conveniently characterized by no effects of plasma densities on the signal analyses (being different from the above conventional methods) as well as no need of complicated systems for X-ray diagnostics.

We investigate the effect of nozzle on the particle transport in the SMBI experiments for optimization of fueling in the GAMMA 10 tandem mirror. The diffusion particle during SMBI was evaluated from the emission intensity captured by the fast camera. Compared with the results without nozzle, the particle diffusion was more convergent in the case using the straight nozzle. Simulation results by using Monte-Carlo simulation code (DEGAS) roughly reproduce the 2-D image captured by the fast camera.

A new high-speed camera (MEMRECAM GX-1, NAC Inc.) has been installed at the GAMMA 10 central-cell. This camera observes two directions through vertical and horizontal ports by using dual optical fiber bundles. This paper describes the results of 2-D image of visible emission observed from the central-cell plasma during Supersonic Molecular Beam injection (SMBI) in the C-ECRH experiments. Observational results showed that the instability of plasma light-emission on the central-limiter was suppressed during SMBI and the plasma sustainability increased along the SMBI plenum pressure. Relationship between plasma sustainability and power of C-ECRH and/or the quantity of SMBI is also investigated.

A multi-channel microwave interferometer can measure plasma density radial profiles in a single plasma shot. It has six observing channels for measuring the radial electron line integrated density profile. The multi-channel microwave interferometer has been reconstructed for improving the signal intensity increase. We set a spherical mirror between the probe beam transmission horn and the injection port window of the multi-channel microwave interferometer. The insertion of spherical mirror makes signal intensity in all measuring channels to be improved. Then, we can successfully obtain the radial electron density profile in a single plasma shot.

A 16-channel microwave interferometer with the phase imaging method has been reconstructed to obtain the radial plasma density distributions at the west plug cell in the GAMMA 10 tandem mirror. Signals measured by the interferometer have DC offsets, which cause a measurable error in the phase imaging measurement system. We propose the method to eliminate DC offsets from the signals in each channel easily on the analysis software.

We applied Fraunhofer diffraction (FD) method to plasma density fluctuation measurement in GAMMA10. In GAMMA10 the drift-type fluctuation in the electron density and potentials and its suppression with application of electron cyclotron heating (ECH) are observed. The FD method can measure the fluctuation spectra in detail. We successfully obtained the density fluctuation spectra and its suppression with application of ECH by using FD method for the first time.

To study the detailed radial structure of the Alfvén ion-cyclotron (AIC) waves excited in the GAMMA10 central cell, we upgraded the previously used reflectometer system so as to measure radially separated two density fluctuations simultaneously. Successful application of the new reflectometer system reveals existence of non-trivial radial structure of the AIC waves; from the core to about a half plasma radius, the density fluctuations arising from the AIC waves have common phase relationship, while in the external region, they have opposite phase relation with that of the core.

Edge plasma behavior was observed by fast camera at high density central-cell plasma in GAMMA 10. The plasma performance was better when the plasma rotated clockwise to the magnetic field. Also, when the interaction with the central-cell limiter was observed, the plasma stored energy was deteriorated. These results were very similar that of high potential central-cell plasma experiment as previously reported. Several rotation mechanisms exist, and on the empirical basis it is suggested the interaction with the central-cell limiter is important.

An application of electron-cyclotron heating with an off-axis resonance location produces a cylindrical layer with energetic electrons and facilitates the formation for a profile of plasma rotation with a radially localized high-vorticity layer. These phenomena are detected by several types of end-loss ions-and electrons-analyser. Formation of radial transport barrier has been observed in the vicinity of the high-vorticity layer in GAMMA 10.

In order to clarify the effects of fusion-produced neutron irradiation on silicon semiconductor x-ray detectors, the x-ray energy responses of both n- and p-type silicon tomography detectors used in the Joint European Torus (JET) tokamak (n-type) and the GAMMA 10 tandem mirror (p-type) are studied using synchrotron radiation at the Photon Factory of the National Laboratory for High Energy Accelerator Research Organization (KEK). The fusion neutronics source (FNS) of Japan Atomic Energy Research Institute (JAERI) is employed as well-calibrated D-T neutron source with fluences from 1013to 1015neutrons/cm2onto these semiconductor detectors. Different fluence dependence is found between these two types of detectors; that is, (i) for the n-type detector, the recovery of the degraded response is found after the neutron exposure beyond around 1013neutrons/cm2onto the detector. A further finding is followed as a “re-degradation” by a neutron irradiation level over about 1014neutrons/cm2. On the other hand, (ii) the energy response of the p-type detector shows only a gradual decrease with increasing neutron fluences. These properties are interpreted by our proposed theory on semiconductor x-ray responses in terms of the effects of neutrons on the effective doping concentration and the diffusion length of a semiconductor detector.

In the GAMMA 10 tandem mirror, neutral hydrogen densities in the central-cell and anchor-cell have been estimated by measuring spatial profiles of Hα line-emission with absolutely calibrated Hα line-emission detectors. Absolute measurements of Hα line-emission enable us to evaluate the neutral density based on the collisional-radiative model. Recently, single channel Hα line-emission detectors were installed in plug/barrier cells, where the axial confining potentials are produced by electron cyclotron resonance heating. We studied the neutral particle behavior in the plug/barrier cells for the first time using the newly installed Hα line-emission detectors. Moreover we have designed and installed a multi-channel Hα line-emission detector system to study the neutral hydrogen behavior in the plug/barrier cell.

As an improvement of the central-cell ECRH (C-ECRH) system in GAMMA 10, an ellipsoidal mirror was installed instead of a cylindrical mirror. With this replacement, the microwave beam for ECRH is converged to the GAMMA 10 axis on the resonance surface and Poynting flux reaching the plasma core region becomes about five times larger than the previous one. Effectiveness of this system has been shown in the initial stage of experiment. Finite increment of soft X-ray signal during ECRH operation indicates an increase of the electron temperature. As more improvement, designing a new antenna is now under way.

Significant effects of sheared transverse electric fields in plasmas on both turbulent fluctuations and drift waves are experimentally demonstrated with improvement in plasma confinement for the first time in the tandem mirror GAMMA 10. Here, electron-cyclotron heatings (ECH) for ion-confining potential formation are applied in association with a significant rise in the absolute value of the central-cell potential and the resulting formation of a strong shear of electric fields of the order of 10 kV/m2 in the radial direction of the plasma column (dEr/dr). The central-cell line density increases during ECH in association with decreasing fluctuations. Various fluctuation diagnostics, in particular, the frequency analyses of end-loss ion currents and central soft x-ray brightness, show the consistent features. This encourages the usefulness of potentials and radial electric-field shear for confinement improvements.

Detailed plasma-physics investigations by the use of x-ray-tomography data supported by the fundamental theoretical studies of x-ray-detector responses enhance the importance of x-ray diagnostics for fusion-plasma analyses. However, degradation in responses of semiconductor x-ray detectors after fusion-produced neutron exposure still remains one of the most serious problems in recent fusion experiments even at this time. For the purpose of investigating and characterizing neutron effects on semiconductor x-ray detectors, detection characteristics of n-type silicon semiconductor detectors which are similar to those utilized for x-ray-tomography detectors in the Joint European Torus (JET) tokamak, are studied by the use of synchrotron radiation from a 2.5-GeV positron storage ring at the Photon Factory. The fusion neutronics source (FNS) of Japan Atomic Energy Research Institute is employed as well-calibrated deuterium-tritium (D-T) neutron source with fluences from 1013to 1015neutrons/cm2onto these semiconductor detectors. Degradation in x-ray responses with increasing neutron fluences has been reported; however, our recent detailed investigations of detector responses show nonlinear dependence as a function of the neutron fluence.