Tandem Mirror Research Articles

Observations of temporal and spatial behaviour of plasmas in relation to the interchange stability boundary scaling in GAMMA 10

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.

Effects of Alfvén Ion Cyclotron Waves on End-Loss Electrons from the GAMMA 10 Tandem Mirror

Different time variations are observed for the increases in the flux Ie and the temperature Te of end-loss electrons following excitation of an Alfvén ion cyclotron (AIC) wave in the GAMMA 10 tandem mirror. The increase in Ie is coincident with the end-loss ion flux caused by the AIC wave such that the current balance holds on a floating end plate. On the other hand, the increase in Te is due to direct action of the AIC wave caused by electron Landau damping.

Millimeter-wave two-dimensional imaging optical system for interferometer of the GAMMA 10 tandem mirror

A millimeter-wave optical system for plasma diagnostics for measuring the two-dimensional distribution of electron density in the Tsukuba GAMMA 10 tandem mirror has been developed. Two-dimensional images of plasma density of a cross-sectional area of 83 mm×67 mm in the machine axis direction were successfully measured with the optical system. The position at which the present imaging system is installed corresponds to the plug region, and the plasma radius is relatively narrow (less than 100 mm). Information about the density in the area is valuable in the study of confining-potential formation. This optical system was developed for a millimeter-wave phase-imaging interferometer and was designed with the ray tracing method to reduce aberration. It was evaluated experimentally by measuring images of point sources and phase images of dielectric targets. The receiving optical system from the plasma to detector array was designed with a spatial resolution of 28.7 mm and a magnification of 0.30. Ray tracing indicated that the transverse spherical aberration was 78% of the sampling interval of 5.0 mm on an image plane and that even for the worst image point the distortion was 10% of the sampling interval. In the design of the optical system for plasma diagnostics, the off-focus effect due to plasma size (plasma diameter: 200 mm) with electron density profile is proposed and considered.

Experimental Observation of the Density Limit in Hot Ion Mode Plasmas of the GAMMA 10 Tandem Mirror

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.

SPACE-RESOLVING VUV AND SOFT X-RAY SPECTROSCOPY IN THE TANDEM MIRROR GAMMA 10 PLASMA

Vacuum ultraviolet (VUV) and soft X-ray (SX) spectroscopic measurements are important means to diagnose radiation power loss, impurity ion densities and effective charge of confined plasma, Z eff , in magnetically confined plasmas such as fusion plasmas. We have constructed space- and time-resolving flat-field VUV (150–1050Å) and SX (20–350Å) spectrographs by using aberration-corrected concave gratings with varied line spacing. Absolute calibration experiments have been conducted at the Photon Factory in the High Energy Accelerator Research Organization. Absolute sensitivities of the VUV and SX spectrographs have been obtained for the two (S and P) polarization geometries. Thus, absolute intensities of emission spectra from impurity ions can be measured together with their radial distributions in plasmas. The total radiation power was determined to be less than 6 kW within ±20% of error in our normal plasma operation. Density profiles of impurity ions were reduced by using absolute emissivities of impurity lines and a collisional-radiative model. Moreover, the value of Z eff is estimated to be 1.00 in the tandem mirror GAMMA 10 plasma.

Influence of radial electric field on trajectories of plug potential bounce ion in the tandem mirror

Bouncing ions between the plug potentials play an important role in improvement of the axial confinement in the tandem mirror. We examined the influence of the radial electric field on the trajectories of the ions passed through the anchor cell with nonaxisymmetric magnetic configuration on the assumption that the shape of the magnetic flux tube was shifted from the shape of the equipotential surface of the plasma at the mirror throats of the anchor cells. The discrepancy between the shapes enhanced the radial drift of the bounce ion. Radial potential profile of the core plasma was controlled by adjustment of the radially separated endplate potentials, and it was found that the flattened radial potential profile was effective for the decrease of the radial drift.

Ion Transport in Real and Velocity Space and Confinement in the GAMMA 10 Tandem Mirror.

Loss regions in the plasma particles' velocity space are decreased by the electrostatic potentials created on both sides of the plasma, which results in the improvement of the axial confinement in the tandem mirror. We found experimentally that the ion loss region was in contact mainly with both a plug potential bounce region and the outer mirror throat bounce region in which ions were bounced near the outer mirror throat of the plug/barrier cell. These bounce ions play an important role in tandem mirror confinement. The outer mirror throat bounce region is caused by the relatively high potential in the neighborhood of the outer mirror throat. The core plasma's radial potential profile was controlled by changing the potential of coaxially separated end plates, and it was found that control of the radial potential profile was useful for retardation of the radial loss of the bounce ions. We observed a hump structure on the energy distribution function of the end-loss ions, and found that the enhancement of the end-loss ions was caused by ion flow from the trapped region to the loss region due to Alfvn Ion Cyclotron (AIC) fluctuations. Although the ion diffusion due to the fluctuations enhances the axial ion loss, the axial loss can be reduced by creation of a higher confining potential.

タンデムミラーＧＡＭＭＡ １０によるプラズマ閉じ込め

The plasma confinement in the GAMMA 10 tandem mirror is studied. The mirror confinement is much improved owing to the potential confinement. The high density plasma confinement was recently attained. Relations between confinement time and confining potential on the plasma density have been obtained in the high density region.

Potential Formation and High Density Plasma Production on the Tandem Mirror GAMMA 10

GAMMA 10 experiments have advanced in high density experiments after the last EPS Workshop where we reported high density plasma production by using an ion cyclotron range of frequency heating at a high harmonic frequency and neutral beam injection in the central cell. Recently a high density plasma was obtained with much improved reproducibility than before and without degradation of diamagnetic signal. The high density plasma was attained by adjusting the spacing of the conducting plates installed in the anchor transition regions. Dependencies of particle confinement time, ion energy confinement time and plasma confining potential on plasma density were obtained for the first time in the high density region.

Plasma Polarization Spectroscopy. Plasma Polarization Spectroscopy on Magnetically Confined Plasmas.

Plasma polarization spectroscopy experiments are reviewed. The OV and CV impurity emission lines from the WT-3 tokamak are found to be polarized. In the GAMMA 10 tandem mirror machine, the OV and low-ionization stage Fe and Cr lines are also found to be polarized. This polarization is ascribed to the anisotropic distribution of electron velocities. The PACR (Population-Alignment Collisional- Radiative) model is introduced, which interprets the experimental polarization data in terms of the anisotropic velocity distribution. The present status of development of the model is presented with the CV ion taken as an example.

Tomographic reconstruction of plasma electron temperature profiles using semiconductor detector arrays in the elliptic transition region and the circular central cell of the GAMMA 10 tandem mirror

Elliptically shaped transition-region plasmas located between circularly shaped central cell and fish-tail shaped anchor-cell plasmas have been observed using X-ray tomography techniques for the first time in the world largest tandem mirror device, GAMMA 10. These three regions are connected through the lines of magnetic forces. Electrons are, therefore, anticipated to move easily through these regions along the magnetic field lines when neither appreciable electron losses into the transverse direction across the magnetic field lines nor the formation of electron transport barrier potentials in the parallel direction are expected. From this viewpoint, comparisons of electron temperature ( T e) profiles in these regions, thus, give information on one of the most critical issues in tandem mirror plasma confinement. The first data on good agreement in T e profiles of the central cell and the transition region have supported this basic and essential concept of tandem mirror confinement. Development of position sensitive semiconductor-detector arrays and an upgraded X-ray tomography algorithm for analysing elliptically shaped transition-region plasmas are also reported.

Generalization and consolidation of scaling laws of potential formation and associated effects in the GAMMA 10 tandem mirror

Generalized scaling laws for the formation of plasma confiningpotentials and the associated effectiveness of the potentials produced aresystematically investigated to find the physics essentials common to therepresentative tandem mirror operational modes of GAMMA 10, and toexplore novel extended operational modes from the scaling bases constructed.(a) The potential formation scalings are generalized using a novel finding ofwider validity of Cohen's strong ECH theorycovering the representative modes. (b) The potentials produced, in turn,provide a favourable novel scaling of the increase in the central cell electrontemperatures Te with increasing thermal barrier potentials ϕb, limited by theavailable ECH power. The scaling of Te with ϕb is well interpreted in terms ofthe generalized Pastukhov theory of plasma potential confinement. A detailedcomparison 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 iscarried out by the use of an electron energy balance equation for the first time.In addition, (d) an empirical scaling of ϕc with ECH power in the plug regionand the central cell densities are studied to discover whether there is the possibility of extending these theoretically well interpreted scaling data to parametersin the future scalable regime. There is also a discussion about numerical scalings in thethree dimensional parameter spaces.

High-density plasma production with potential confinement in the GAMMA 10 tandem mirror

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.

Potential confinement experiments with long sustainment and high density on GAMMA 10

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.

Numerical Simulation of Neutral Beam Injection in the GAMMA10 Anchor Cell

The GAMMA10 tandem mirror contains neutral beam injectors at the anchor mirror cells to increase the number of high-energy ions there for magnetohydrodynamic stability. We calculate the population of high-energy ions by using the bounce-averaged Fokker-Planck code for various background neutral densities and electron temperatures. We compare the simulation results with results of a point model and make clear the validity of the point model.

Studies of edge plasmas in an anchor minimum-B region of the GAMMA 10 tandem mirror

Results of edge plasma investigation in a minimum-B region of the GAMMA 10 tandem mirror are described. In this experiment, the spatial profile of the ion saturation current and the floating potential in the edge plasmas are measured by using arrays of Langmuir probes installed on conducting plates in anchor cells together with a movable probe near the plate in the east anchor cell. From these measurements, a significant asymmetry to the axis of the plasma was observed in both probe currents and floating potentials. It is also found that there are some impurity-deposited layers partially observed on all the conducting plates and that their locations show a rotational symmetry to the magnetic axis at 180°. In this region, a strong gradient of the magnetic field strength toward the axis exists. The direction of the expected plasma shift corresponds to the ∇ → B and curvature drift for ions. The results of ion orbit calculation taking account of the above effect show a good agreement with the measured results.

Observation of a Radial Current at a Plug/Barrier Cell in GAMMA10

This paper presents a picture of current flow relevant to the plug/barrier cell and an end plate installed on the end wall of the GAMMA 10 tandem mirror. Detailed analyses show that, during electron cyclotron resonance heating for plug potential formation, a net electron current flows through the end plate. In the plug/barrier cell, on the other hand, ion currents are detected during ECRH on ring electrodes newly installed near the side wall of the vacuum vessel. A part of the radial ion currents is connected to the axial net electron current flowing through the end plate. This indicates that a residual non-ambipolar diffusion still remains. The radial ion current in the plug/barrier cell includes another component that originates form the axial ion flux emanating from the central region. This suggests radial diffusion of ions during potential plugging.

Measurement of End Loss Electrons and Ions from a Hot Ion Plasma in a Tandem Mirror

Measurement of End Loss Electrons and Ions from a Hot Ion Plasma in a Tandem Mirror

Observation of Fine Structure in the Velocity Distribution Function of the End-Loss Ions in the Tandem Mirror

Reduction of the loss regions existing in the velocity space plays an important role in improvement of the confinement in the tandem mirror. Particle flow into the loss region affects the axial confinement. The ions being trapped in the magnetic mirror field are scattered from the trapped region into the loss region through the loss boundaries. In order to investigate the fine structure appeared in the loss region, the end-loss energy component analyzer (ELECA) devices have been constructed and located on both sides of the tandem mirror. Three kinds of structures in the loss region were observed in the RF-driven tandem mirror plasma.

Potential Measurement at the Inner Mirror Throat of the Tandem Mirror GAMMA10

In order to improve the axial confinement, in the tandem mirror machine, the electrostatic potentials are created on both sides of the machine. Usually, thermal barrier potential is created at the midplane of the plug/barrier cell. We note that the electrostatic potential at the Inner Mirror Throats (IMT) is able to play an important role acting as a barrier of the electron flow from the central cell to the plug cell. We examined the influence of the IMT potential on the electron flow by calculation, and it was found that the IMT potential acted effectively as the thermal barrier. In consideration of the undesirable condition at the IMT region, that is the tight accessibility and the strong magnetic field, we adopted a newly designed gold neutral beam probe system including a new type of MCP detector. The characteristics of the MCP detector were obtained successfully using a teststand with strong magnetic field created by the nuclear magnetic resonance (NMR) magnets.This beam probe system was applied to the tandem mirror GAMMA 10, and the electrostatic potential was measured successfully by the beam probe system located in the neighborhood of the IMT region.