Tandem Mirror Plasma Research Articles

Measurement of effect of electron cyclotron heating in a tandem mirror plasma using a semiconductor detector array and an electrostatic energy analyzer.

Temporally and spatially resolved soft x-ray and end-loss-electron analyses of the electron cyclotron heated plasmas are carried out by using a semiconductor detector array and an electrostatic energy analyzer in the GAMMA 10 tandem mirror. The flux and the energy spectrum of the end loss electrons are measured by a multi-grid energy analyzer. Recently, the electron cyclotron heating power modulation experiments have been started in order to generate and control the high heat flux and to make the edge localized mode-like intermittent heat load pattern for the divertor simulation studies by the use of these detectors for electron properties.

Wave excitation by nonlinear coupling among shear Alfvén waves in a mirror-confined plasma

A shear Alfvén wave at slightly below the ion-cyclotron frequency overcomes the ion-cyclotron damping and grows because of the strong anisotropy of the ion temperature in the magnetic mirror configuration, and is called the Alfvén ion-cyclotron (AIC) wave. Density fluctuations caused by the AIC waves and the ion-cyclotron range of frequencies (ICRF) waves used for ion heating have been detected using a reflectometer in a wide radial region of the GAMMA 10 tandem mirror plasma. Various wave-wave couplings are clearly observed in the density fluctuations in the interior of the plasma, but these couplings are not so clear in the magnetic fluctuations at the plasma edge when measured using a pick-up coil. A radial dependence of the nonlinearity is found, particularly in waves with the difference frequencies of the AIC waves; bispectral analysis shows that such wave-wave coupling is significant near the core, but is not so evident at the periphery. In contrast, nonlinear coupling with the low-frequency background turbulence is quite distinct at the periphery. Nonlinear coupling associated with the AIC waves may play a significant role in the beta- and anisotropy-limits of a mirror-confined plasma through decay of the ICRF heating power and degradation of the plasma confinement by nonlinearly generated waves.

The Improvement of the ECH Antenna System in the GAMMA 10

This paper shows that the design improvement of the ECH antenna system for high efficient heating of GAMMA 10 tandem mirror plasma. The previous ECH antenna was not large enough to cover the microwave because it was limited by the port size. New antenna system clears this limit and is enlarged as large as possible. And its size becomes twice, which enables to inject the microwave with the efficiency up to 95%. It is about 20% increase as compared with the previous system.

Flute-Mode Stability of Quadrupole-Anchored Tandem Mirror Plasmas

We study the flute-mode stability of quadrupole-anchored tandem mirror plasmas. The present analysis is based on Newcomb's Lagrangian density with an assumption of small Larmor radius of ions for paraxial approximation. A radial eigenmode equation for flute perturbations is derived without an eikonal approximation in flux coordinates, where effects of E × B plasma rotation due to an ambipolar electric field are considered. The obtained eigenmode equation is applicable to a mode with an arbitrary azimuthal mode number and can describe interchange, E × B rotational, and Kelvin-Helmholtz modes driven by the shear effect of E × B plasma rotation. A quadratic dispersion equation in perturbation frequency ω is derived based on a simplified cylindrical plasma model and is used to discuss the flute stability of quadrupole-anchored GAMMA 10 tandem mirror plasmas, where the anchor beta required for plasma stability is calculated for the m = 1 and 2 modes for different values of central-cell ambipolar potential.

Upgrade of the analyzer design for multipoint measurements by a gold neutral beam probe on the GAMMA 10 tandem mirror

The study of the mechanisms of both transport and fluctuations is one of the main issues of fusion plasma researches. Spatial fluctuation structure measurements are important for studying of cross correlation between the two fluctuations. Gold neutral beam probe systems are used in GAMMA 10 tandem mirror plasma device. Currently, the plasma potential and its fluctuations cannot be measured at different radial positions simultaneously. This paper presents the upgrade design of the 45 degrees electrostatic energy analyzer for the simultaneous multipoint measurements of plasma parameters.

Experiment on direct energy conversion from tandem mirror plasmas by using a slanted cusp magnetic field

A direct energy converter (DEC) designed for thermal ions escaping from a linear or near-linear device consists of a cusp magnetic field and decelerating electrodes. The electrons are deflected along the field lines and consequently separated from ions that are not fully magnetized. The ions are led to the decelerating electrodes to produce dc power. This type of DEC, the CUSPDEC, is applied to the GAMMA 10 tandem mirror in order to investigate the capability of separation of charged particles as well as to demonstrate energy conversion from ions. The separation of electrons and ions with energies of the order of kilo-electron Volt is achieved by using a slanted cusp magnetic field for the first time. It is also demonstrated that the separated ions are decelerated by the electric field in front of ion collectors and flow into the collectors at a high potential to produce electricity.

The Neutral Transport Analysis Based on Visible Light Measurement of Recycling and 3-Dimensional Simulation in GAMMA 10

Neutral transport related to recycling phenomena on two limiters installed in the central-cell was studied in the GAMMA 10 tandem mirror plasmas. Dependence of iris limiter is investigated in terms of plasma diamagnetism and Hα-line intensity. It was found that a plasma diamagnetic signal was improved and Hα-line intensity near the iris limiter increased if the diameter of the iris limiter shrank to 340 mm. Fully 3-dimensional MonteCarlo simulation (DEGAS) was executed with the recycling source on the limiters. The neutral sources on the limiters were consistently given so as to satisfy 2-di mensional (2-d) image obtained by a high-speed camera. The determined Hα-line intensity from DEGAS well explained the experimental result. The simulation result, in the 340 mm case, predicted the reduction in the peak value of Hα-line intensity on the central limiter and the enhancement on the iris limiter. It suggests that the reduction of the recycling on the central limiter makes it possible to lead the better plasma performance.

X-ray tomography systems for observations of electron cyclotron heated plasmas using novel position-sensitive X-ray semiconductor-detector arrays

Tomographic reconstructions of X-ray emission from hot electrons having a temperature of several tens of keV and cold electrons having a temperature ranging below 100 eV have been carried out by the use of the novel position-sensitive X-ray diagnostic system. The X-ray detection system in the thermal-barrier region of the GAMMA 10 tandem-mirror plasmas consists of a 48-channel silicon semiconductor detector array and the 50-channel microchannel plates (MCPs). When we make outward shifts of the second-harmonic electron cyclotron layers radially by intensifying the mirror magnetic fields in the thermal-barrier region, tomographically reconstructed signals by using the MCP show hollow X-ray profiles and the X-ray reconstructed signals obtained with the semicondcutor detector indicate a good axisymmetric radial profile peaked on the magnetic axis. Several applications including investigation of these hot electrons due to the second-harmonic electron cyclotron heating are made with the newly developed X-ray diagnostic systems.

X-ray diagnostics for investigating electron distribution functions in the central cell of the GAMMA 10 tandem mirror

The quantum efficiency of an ultralow-energy-sensitive pure-Ge (ULE Ge) detector is investigated using synchrotron radiation from the storage ring at AIST especially for x-ray pulse-height analyses (PHAs), down to a few hundred eV. Several types of x-ray diagnostics such as x-ray PHA, x-ray absorption methods, and x-ray tomography using the ULE Ge detector, a NaI(Tl) detector, as well as a microchannel-plate tomography system are employed for investigating electron distribution functions and electron temperature profiles with preliminary central electron-cyclotron heating in the central cell of the GAMMA 10 tandem mirror. These measurements play an important role in studying an essential physics scaling of the electron temperature as a function of electron confining potential in tandem mirror plasmas.

Numerical study of microwave imaging reflectometry for measurements of density fluctuations in a tandem mirror plasma

In this paper, numerical experiment is used to study the imaging properties of microwave imaging reflectometry for the case of tandem mirror device geometry. First of all, the alignment of the experimental setup is performed and then the imaging system is applied for density fluctuations measurements. Fluctuations employed in this study have a nonshifted Gaussian wavenumber spectrum with equal poloidal and radial widths. The size of the optics is shown to be a main parameter limiting the performance of the imaging system. Space-imaging and time-imaging modes are considered. In the latter case, for model conditions imaging and conventional (without optics) reflectometers demonstrate comparable performance in general. When root-mean-squared amplitude of the density fluctuations is large (σn = 0.06 and 0.09) the imaging system shows a wider range of measurable parameters.

Progress in potential formation and findings in the associated radially sheared electric-field effects on suppressing intermittent turbulent vortex-like fluctuations and reducing transverse losses

Following the 19th IAEA Fusion Energy Conference (Lyon, 2002), (1) three-time progress in the formation of ion-confining potential heights ϕc including a record of 2.1 kV in comparison to those attained 1992–2002 is achieved for tandem-mirror plasmas in the hot-ion mode with ion temperatures of several kiloelectronvolts. (2) The advance in the potential formation gives the bases for finding the remarkable effects of radially produced shear of electric fields Er, or non-uniform sheared plasma rotation on the suppression of intermittent vortex-like turbulent fluctuations. (i) Such a shear effect is visually highlighted by x-ray tomography diagnostics; that is, spatially and temporally intermittent vortex-like fluctuated structures are clearly observed as two-dimensionally reconstructed visual structures for the first time in kiloelectronvolt order ion-cyclotron heated plasmas having a weak shear in GAMMA 10. (ii) However, during the application of plug electron-cyclotron heatings (ECH), the associated potential rise produces a stronger shear (dEr/dr = several 10 kV m−2) resulting in the disappearance of such intermittent turbulent vortices with plasma confinement improvement. X-ray observations also show elongation of a vortex structure from a circular into an ellipsoidal shape, as depicted in H-mode theories, with an outward shift. (3) For the physics interpretations and control of such potential and the associated shear formation, the validity of our proposed theory of the potential formation is extensionally tested under the conditions with auxiliary heating. The data described above fit well to the extended surfaces calculated from our proposed consolidated theory of the strong ECH theory (plateau formation) with Pastukhov's theory on energy confinement.

Neutral Beam Injection Experiments and Related Behavior of Neutral Particles in the GAMMA 10 Tandem Mirror

Results of neutral beam injection (NBI) experiments in the GAMMA 10 tandem mirror plasmas are presented together with the neutral particle behavior observed in the experiments. A hydrogen neural beam was injected into the hot-ion-mode plasmas by using the injector installed in the central-cell for the plasma heating and fueling. High-energy ions produced by NBI were observed and its energy distribution was measured for the first time with a neutral particle analyzer installed in the central-cell. The temporal and spatial behavior of hydrogen was observed with axially aligned Hα detectors installed from the central midplane to anchor-cell. Enhancement of hydrogen recycling due to the beam injection and the cause of the observed decrease in plasma diamagnetism are discussed. The Monte-Carlo code DEGAS for neutral transport simulation was applied to the GAMMA 10 central-cell and a 3-dimensional simulation was performed in the NBI experiment. Localization of neutral particle during the beam injection is investigated based on the simulation and it was found that the increased recycling due to the beam injection was dominant near the injection port.

Modeling of three-dimensional neutral transport in tandem mirror plasmas using a Monte-Carlo code

Modeling of neutral transport in the tandem mirror plasmas has been performed based on simulation studies using a Monte-Carlo code. A three-dimensional neutral transport simulation in both the central-cell (an axisymmetric simple mirror) and anchor-cell (a non-axisymmetric minimum-B configuration) was successfully carried out using the DEGAS version 63 code. In order to realize the geometrical structure in the simulation space, a ‘second wall’ is introduced in the code, which enabled us to perform a more detailed simulation. The simulation results agreed well with the Hα measurements of the experiment. The effect of cold gas influx from the beamline of the neutral beam injection (NBI) system on the particle fueling is also investigated and it is found that the azimuthally uniform particle source due to the recycling on the central vacuum chamber wall prevails over the localized fueling from cold gas influx accompanying the NBI.

Measurements of the relaxation of ion anisotropic distribution functions in tandem-mirror plasmas

Our proposed “matrix-type” semiconductor detectors are applied for studying the relaxation of ion anisotropic distribution functions having several keV in the central cell of the tandem mirror GAMMA 10. The matrix-type detector array consists of compactly produced six “rows” having different thicknesses of thin dead layers (SiO2) on its surface. Each row has seven channel units (“columns”) for measuring radiation profiles in the radial direction of plasmas. These various SiO2 layers are, thus, employed as “unbreakable ultrathin radiation–absorption filters” having various thicknesses to analyze x-ray energies and distinguish x rays from neutral particles simultaneously. Development of these detectors along with high power gyrotrons allow us to investigate energy transport analyses between ion-cyclotron heated hot ions and electron-cyclotron heated electrons. An application of the matrix-type detectors reveals detailed behavior of electron and ion relaxation from the perpendicular to parallel ion energies in the central cell.

Technique of Neutral Transport Simulation Using the DEGAS Monte-Carlo Code-Application to GAMMA 10-

Techniques for the neutral transport simulation using a Monte-Carlo code are briefly reviewed by explaining the examples of the simulation with the DEGAS Monte-Carlo code. Tandem mirror plasma produced in GAMMA 10 is employed as a target plasma for the simulation. An axisymmetric simulation using the ver.35 code is applied for the central-region and a three-dimensional simulation in non-axisymmetric anchor region is performed with the ver.63 code. In this review brief introduction of both versions, newly introduced processes and mesh models designed for axisymmetric and non-axisymmetric structures in the GAMMA 10 are described.

Study of Action of Unstable AIC Waves on Electrons in a Tandem Mirror Plasma from Measurement of End Loss Electrons

An Alfven ion cyclotron (AIC) wave spontaneously excited in the central cell of the GAMMA 10 tandem mirror has strong effects on electrons. The end loss electron flux, the electron temperature and the field aligned loss power carried by these electrons increase with excitation of the AIC wave. Since electrons escape to the machine end with a good memory of the processes having in the plasma, the action of the AIC wave on electrons has been studied through measurement of end loss electrons. Response of electrons to the AIC wave is dominated by Landau damping. Electrons are accelerated by the parallel electric field of the AIC wave only when the electron thermal velocity is nearly equal to the parallel phase velocity of the excited wave. No strong action of the AIC wave on electrons is observed if this condition does not hold. A simplified calculation shows that the temporal variation of the electron temperature is the result of balance between the absorbed power through Landau damping and the field-aligned...

Progress in Millimeter-Wave Imaging Diagnostics

Significant advances in microwave and millimeter wave technology have enabled the development of a new generation of imaging diagnostics as a visualization tool in this frequency region. Millimeter-wave imaging diagnostic system is expected to be one of the most promising diagnostic methods that measure profiles and fluctuations of magnetically confined plasmas. It successfully measures the time evolutions of both radial and axial profiles of density and electron temperature and their fluctuation components in a tandem mirror plasma as well as tokamak plasmas. This paper presents the representative results obtained via imaging systems such as phase imaging interferometry, electron cyclotron emission imaging, and microwave imaging reflectometry.

Radial Potential Control for Plasma Confinement in the Tandem Mirror GAMMA 10

Existence of the bouncing ion between the plug potentials improves the axial confinement of the tandem mirror plasma. Trajectories of the bounce ion passed through the anchor cells with nonaxisymmetric magnetic configuration was calculated 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. It was found that 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 indicated that flattened radial potential profile was effective for the decrease of the radial drift.

Effects of Thermal-Barrier Potentials on the Central-Cell Electron Energy Confinement

The effects of the thermal-barrier potentials ɸb on the central-cell electron energy confinement are theoretically and experimentally investigated in the GAMMA 10 tandem mirror. In particular, the scaling of the central-cell electron temperatures Te with “the central-cell electron-confining potentials” ɸb is studied on the basis of the electron energy-balance equation and the generalized Pastukhov theory. The obtained theoretical scaling of Te with ɸb is then compared with the experimentally observed relation between these two parameters. In GAMMA 10, the main tandem-mirror operations 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 report, the scaling of Te with ɸb covering over these two representative operational modes is investigated, since the scalings of Te or the dominant parameters which determine Te have been remained for a long time as an unresolved important issue for tandem-mirror plasmas. It is found that the data in the two representative operational modes of the high-potential and hot-ion modes in the GAMMA 10 tandem mirror are in good agreement with the theoretically derived scaling formula, though the heating-source parameter dependence in the electron energy-balance equation is quite different in the two modes.

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.