Tandem Mirror Plasma Research Articles

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.

Energetic Particle Effects on MHD Stability of a Tandem Mirror Plasma

Energetic particle effects on MHD stability of a tandem mirror plasma is studied theoretically. An algebraic dispersion equation with respect to eigenfrequency is derived from a radial eigenmode equation for flute-mode perturbations with an appropriate boundary condition, in which the effects of energetically drifting ions are taken into account. The dispersion equation describes interactions between curvature-driven interchange modes and drift modes driven by energetically drifting ions. In the absence of wave-particle interactions for energetically drifting ions, the dispersion equation is solved numerically, and a stable region formed due to the charge uncovering effects of energetically drifting ions is shown in a parameter space related with the number density and magnetic drift frequency of energetically drifting ions.

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.

Flute Stability Analysis of a Quadrupole-Anchored Tandem Mirror Plasma

We study flute mode stability of a quadrupole-anchored tandem mirror plasma. The present analysis is based on a Newcomb's Lagrangian density with the assumption of small Larmor radius of the ion in the paraxial approximation. A radial eigenmode equation for flute perturbations is derived without the eikonal approximation in the flux coordinates, where the effects of E×B plasma rotation due to an ambipolar electric field are taken into account. The obtained eigenmode equation is applicable to a mode with arbitrary azimuthal mode number, and can describe interchange modes, E×B rotational modes and also Kelvin -Helmholtz modes driven by the shear effect of E×B plasma rotation.

A diagnostic method for both plasma ion and electron temperatures under simultaneous incidence of charge-exchange particles and x rays into a semiconductor detector array

An idea for using semiconductor detectors to simultaneously observe both plasma ion Ti and electron Te temperatures is proposed. The idea is also experimentally verified in tandem-mirror plasma shots. This method is developed 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 of a thin (on the order of nm thick) SiO2 layer is employed for analyzing Ti in the range from hundreds to thousands of eV. Even under the conditions of simultaneous incidence of such particles and x rays into semiconductor detectors, the different dependence on their penetration lengths and deposition depths in semiconductor materials makes it possible to distinguish particles (for Ti) from x rays (for Te). In this letter, proof-of-principle plasma experiments for the proposed idea are carried out to verify the availability of this concept of distinguishing and identifying each value of Ti and Te by the use of various thin filtering materials prior to the use of thinner dead layers.

Observation of Gentle Hump Structure on Energy Distribution Functions of End-Loss Ions in the rf-Driven Tandem Mirror Plasma

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.

Wavelet analysis of plasma fluctuations in microwave reflectometry

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 Alfvén range of frequencies sometimes propagate from the center towards the magnetic plugs with velocities close to 1000 km/s. In general, the wavelet spectra of plasma turbulence in the Alfvén range of frequencies, viewed by means of the microwave reflectometer, appear to be much more irregular than the corresponding spectra of magnetic field fluctuations obtained using magnetic probes.

Ion Transport in Velocity Space Due to Alfvén-Ion-Cyclotron Fluctuations Excited in the RF Driven Tandem Mirror Plasma

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 Alfvén 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.

Investigations of Electron Behavior in the Gamma 10 Tandem Mirror on the Basis of X-Ray Analyses Using a Novel Theory on Semiconductor Detector Response

(i) A scaling law and its physics mechanism of potential formation for tandem-mirror plasma confinement are investigated. The first result of a generalized scaling covering over two typical plasma operational modes in the GAMMA 10 tandem mirror is presented; that is, a previously obtained potential-formation scaling in a plasma operational mode with a few-kV confinement potentials is found to be extended and generalized to a potential scaling in a hot-ion operational mode with thermal-neutron yield, when we take account of the dependence of potential formation on the ratio of the plug to the central-cell densities as well as the relation of electron temperatures in the central cell to thermal-barrier potentials. The finding of the existence of the same physics basis underlying in these two typical modes may provide the future possibility of simultaneously obtained hot-ion plasmas with high potentials. (ii) For these scaling studies, we have constructed the physics fundamentals of x-ray diagnostics; that is, we proposed a novel theory on the energy response of a widely utilized semiconductor x-ray detector. The theory solves a serious problem of a recent finding of the invalidity of the conventional standard theory on the response of such an x-ray detector; the conventional theory has widely been believed and employed over the last quarter of the century in various research fields including plasma-electron researches in most of plasma-confinement devices. The novel theory on the semiconductor x-ray response is characterized by the inclusion of a three-dimensional diffusion of x-ray-produced minority carriers in the field-free substrate of a detector, while the conventional theory is based only on the charges from an x-ray-sensitive depletion layer (i.e., the region of a p-n junction). Various and serious effects of the novel theory on the determination of electron temperatures and their radial profiles (i.e., the electron-temperature gradient) are also represented.

Two-dimensional imaging X-ray spectrometer for plasma diagnostics

A two-dimensional imaging soft X-ray monochromator is now under development. The main components of this spectrometer are a channel plate collimator (CPC) and multilayer mirrors. We have applied this monochromator to obtain soft X-ray images of GAMMAIO tandem mirror plasmas. In order to obtain clear images in a low energy X-ray regime, we have utilized and tested a focusing effect of the CPC. Ray-trace simulations were carried out under the same conditions as the experiments. After comparison of these results, we have confirmed that the focusing effect of the CPC is useful in practical use. Monochromatized two-dimensional soft X-ray images have been successfully taken on the tandem mirror experiment.

X-ray diagnostics for investigating electron distribution functions and electron potential confinements

X-ray radiation from the GAMMA 10 tandem-mirror plasmas is investigated so as to interpret the electron-velocity-distribution functions as well as the formation mechanism of plasma-confining potentials from the X-ray data. In particular, plasma-electron behavior in the thermal-barrier and the plug regions are highlighted. Two different shapes of the distribution functions (i.e., relativistic Maxwellian electrons and plateau-shaped electrons bounded by an electron-confining potential) characterize the barrier and the plug plasmas, respectively. For the X-ray-data analyses, we employ a new theory on the X-ray response of semiconductor detectors. In particular, it is noted that the use of this theory modifies analysis results from the conventional X-ray-response theory in textbooks, which was employed over this quarter of the century for plasma-electron-temperature analyses.

Behavior of neutral-hydrogen and particle confinement on GAMMA 10 tandem mirror plasmas

Behavior of neutral-hydrogen and particle confinement on GAMMA 10 tandem mirror plasmas

Behavior of neutral-hydrogen and particle confinement on GAMMA 10 tandem mirror plasmas

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

Significance of Tandem Mirror Plasma

Significance of Tandem Mirror Plasma

A laser-induced fluorescence measurement technique for obtaining neutral hydrogen densities in plasmas

The resonance fluorescence of neutral hydrogen illuminated by Hα radiation has been used as a technique for the spatially and temporally resolved density measurements of neutral hydrogen in high temperature plasmas, such as in the tokamak and magnetic mirror plasma fusion devices. The fluorescence signal, usually very weak and buried in the background of stray laser light and Hα emission, is very difficult to extract and its measurements are inaccurate. This paper discusses the improvement of the signal extraction using two optical path laser-induced fluorescence (LIF) methods. One optical path carries the fluorescence signal and the background (the stray laser light and Hα emission), whereas the other path carries only the background signal. By combining these two signals, a clean fluorescence signal can be isolated by subtracting out the background using a differential amplifier. The measurement is obtained instantaneously from these two signals which are taken simultaneously in one pulse rather than being extracted from two separate spectra taken in two sequential pulses (double pulses). This method, therefore, makes a significant improvement on the double pulse technique in terms of the accuracy of the measurement and the time resolution. Using this LIF technique the measurement of the neutral density profile in the exhaust of a tandem mirror plasma propulsion device is obtained and presented.

A double pendulum plasma thrust balance and thrust measurement at a tandem mirror exhaust

For the purpose of measuring the plasma momentum flux in a plasma system, a highly sensitive and precision balance has been developed. It can measure a force, an impulse, or thrust as low as 0.1 mN free of mechanical noise, electrical and magnetic pickups. The double pendulum system consists of two parallel conducting plates. One or both of the plates can be suspended by needles. The needle suspended plate (or plates) can swing freely with negligible friction because of the sharp points of the needles. When one of the plates is impacted by an impulse it will swing relatively to the fixed plate or other movable plate. The capacitance between the plates changes as a result of such a motion. The change of capacitance as a function of time is recorded as an oscillating voltage signal. The amplitude of such a voltage signal is proportional to the impacting force or impulse. The proportional factor can be calibrated. The forces can thus be read out from the recorded value of the voltage. The equation of motion for the pendulum system has been solved analytically. The circuit equation for the electronic measurement system has been formulated and solved numerically. Using this balance the thrust at the exhaust of a Tandem Mirror plasma thruster has been measured. The analytical solution of the overall characteristics agrees greatly with the measurement.

How does the potential get from A to B in a plasma?

Solutions to the question of how the plasma potential varies from point A to point B, at which the values are specified, are discussed. Each answer depends on plasma and boundary conditions. Plasma potential structures measured by emissive probes are considered. Examples include potentials associated with sheaths and presheaths in collisionless and collisional systems, unmagnetized and magnetized double layers with single and multiple step transitions, semiconductor etching and rf self bias, tandem mirror plasma confinement and tokamak antenna impurity production. DC structures, time average structures associated with capacitive and inductive rf and the role of potential well pumping are considered.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Observation of whistler instability in the Phaedrus-B tandem mirror plasma

Experimental results of whistler instability studies in the Phaedrus-B tandem mirror plasma [Phys. Fluids B 2, 787 (1990)] during electron cyclotron resonance heating (ECH) and ion cyclotron range of frequency (ICRF) heating are compared with theoretical models that predict whistler instability. The experimental results suggest that the whistler instability was caused by ‘‘warm’’ anisotropic electrons.

X-ray detector characterization and applications for plasma research

Abstract Using synchrotron radiation, the detection characteristics of microchannel plates, silicon surface barrier detectors and gold photocathodes are investigated; in particular, the detailed structures of the detection efficiency including an extended X-ray absorption fine structure (EXAFS), an X-ray absorption near-edge structure (XANES) and a chemical shift is reported along the with discussion on the physical mechanisms. Also, the applications of these detectors to X-ray tomography diagnostics for the observation of the cross-sections of fusion-oriented tandem-mirror plasmas are described. These data give information on the spatial distribution of the hot electrons which play an important role in the formation of thermal-barrier potentials φb. Also, the tomographic reconstructed X-ray data in the central cell show the effect of φb formation on the improvement in the central-cell electron confinement for the first time.

The design of end magnetic system coils for 'AMBAL-M'

The end of the AMBAL-M tandem mirror plasma trap is an axially symmetric plug with a semicusp MHD (magnetohydrodynamic)-expander. The plug field is formed by a pair of coils, and the semicusp field is formed by another pair of anti-parallel coils. The axial dimension of the end magnet system is 3.2 m, and the middle diameters of the coils are from 0.6 to 1.1 m. The operation mode features a 24-kA current, a 1.5-s effective pulse time, and a repetition rate of 1 pulse per 15 min. The coil design is described. The coil is a packet of parallel modules whose conductor is a copper ribbon with a 500-mm/sup 2/ cross section, and water-cooled copper disks between them for heat removal. The heat calculation principles are given. The characteristics of the smaller semicusp coil with regard to its location in a vacuum are noted. Some manufacturing problems are also described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>