Mirror Throat Research Articles

Measurement of Plasma Density Using a Gold Film Detector in the Inner Mirror Throat of the Gamma10 Tandem Mirror

In the tandem mirror GAMMA10, confining potential is formed at the plug region in order to decrease the loss region which exists in the velocity space of ion. Furthermore to increase the confining potential effectively, the electron which flows into the plug cell from the central cell is decreased by forming a potential dip (thermal barrier potential) between the central cell and the plug cell. The electrostatic potential at the inner mirror throat (IMT) of the plug/barrier cell may decrease and act as effective thermal barrier potential because of the effects of the strongest magnetic field and the anisotropy of ion temperature in the central cell. Simultaneous measurements of both the potential and the density in the IMT region are important to investigate the potential formation mechanism.

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.

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.

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.

The characteristics of the newly designed microchannel plate detector in a strong magnetic field

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

Characteristics of a novel ion energy spectrum f(E∥,E⊥) analyzer in measurements of plasma flow in a magnetic mirror throat

The new ion energy analyzer is developed to measure the energy distribution function f(E∥,E⊥) of ions lost along the confinement magnetic field [M. Okubo et al., J. Plasma Fusion Res. 70, 1102 (1994)]. The end-loss ions have larger E⊥ components compared with their E∥ components in the magnetic mirror throat. In this case, the ion energy distribution function is distorted because of the incident angle dependency of the retarding grid transmission. We try to compensate a measured energy distribution function by calculated grid transmission functions.

Effects of Anisotropy of End Loss Electrons on Axial Potential Profile and Electron Transport in the End Region of a Tandem Mirror

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

Current Balance at an Endplate of the GAMMA10 Tandem Mirror

Mesh bias experiments, in which secondary electron emission from an endplate is suppressed, suggest that simple balance between an end loss electron current and an end loss ion current does not hold at the endplate. An ion current flowing into the endplate from its back side is observed, and a theoretical model is constructed which takes account of this current and secondary electrons emitted from the mesh wires. Mirror reflection of secondary electrons off an outer mirror throat is also considered in this model. The model includes the current balance and the endplate potential self-consistently. The current balance at the endplate is well accounted for by this model.

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.

A new method of radial potential profile measurement utilizing magnetic field gradient near a mirror throat

A gold neutral beam is perpendicularly injected into a plasma column along a main chord near an inner mirror throat of a plug/barrier cell in the tandem mirror GAMMA 10, in order to directly measure time evolution of the electrostatic radial potential profile during one shot. The new method pays attention to the fact that trajectories of the ions, which are ionized at each point on the path of the neutral beam in the plasma column, are shifted due to the magnetic field gradient and the electrostatic potential. After traveling about a half period of Larmor motion, ions are detected by a plate type of ion detector. The radial profiles of the plasma potential are determined analyzing positions of the ion beam spots on the detector plane. The main advantage of this method is that radial potential profiles in a strong magnetic field region can be measured as a function of time during one shot without sweeping incident beam angles.

The new 1.2 T–14.5 GHz Caprice source of multicharged ions: Results with metallic elements

The 1.2 T–14.5 GHz ECR Caprice source is an upgrade version of previous Caprice sources where the three main ingredients of an ECR ion source of multicharged ions have been optimized: (i) the magnetic configuration has higher axial fields (1.4–1.45 T at the mirror throats) and radial field (1.05 T at the wall inside the plasma chamber) as well as higher mirror ratios, (ii) the 14.5 GHz rf frequency is convenient to higher magnetic fields, and (iii) more efficient electron sources allow the electron density to reach higher values, and thus high extracted ion currents (both first stage and wall coating). Emphasis is given to the metallic elements, the source is able to produce 10 eμA of Ca14+, 3 eμA of Fe17+ and Ni20+, 1 eμA of U37+, while being able to deliver 1130 eμA of O6+, 190 eμA of O7+, and 100 eμA of Ar12+.

On the Electrostatic Potential Profile Determined from Ion Endloss Spectrum

The electrostatic potential profile in the outer region of a tandem mirror is calculated by analyzing the ion endloss data which were obtained experimentally. The potential profiles depend on the boundary condition of the position where the potential is zero on the axis. Comparing the calculation with experimental data it is found that the point of maximum potential (plug) does not exist at the outer mirror throat.

Continuity of Electrostatic Potential around Magnetic Mirror Throat

The electrostatic potential profile around a magnetic mirror throat is studied. Bohm's criterion expresses the necessary condition for the formation of a stationary sheath in front of the absorbing wall. This condition, when applied to the mirror throat of a magnetic mirror, requires that the ion drift velocity flowing into the mirror throat from the confining region must exceed the ion sound velocity for the continuity of the potential. It is found, however, that the ionization around the mirror throat renders the potential continuous even with zero ion drift velocity.

Observation of loss boundaries and ion flux spectra of end-loss ions in the tandem mirror GAMMA 10

Loss boundaries and ion flux spectra of end-loss ions have been observed by means of a new diagnostic device in the tandem mirror GAMMA 10 [Nucl. Fusion Suppl. 1, 429 (1983)]. In consequence of measuring simultaneously both the pitch angle profile and energy of the end-loss ions, the loss boundaries were directly ascertained to be composed of two hyperbolic lines characterized by the electrostatic plug potential in the velocity space, and also high outer mirror throat potential was found. The flux spectra were compared with the two types of theoretical results, and discussed from a viewpoint of Coulomb collision.

ECR ion source using slow wave structures

In this ECR ion source, the possibility of maintaining the plasma by slow wave structures (SWS), helical coil, and slotted line antennas, in the region where wce, wpe ≫ wrf (wce, wpe, and wrf are electron cyclotron, plasma, and microwave frequencies, respectively) is exploited. The plasma parameters, plasma density, and electron temperature are maximized by coupling microwave (frequency: 2.45 GHz; power: 650 W) at two places in a magnetic mirror machine (mirror ratio Rm ≂ 1.45) to obtain higher beam current. Initially, the plasma is produced by coupling microwave to SWS at the mirror throat. The microwave is coupled by exciting the dominant slow wave field component of SWS, using an E-plane horn antenna. Then the plasma is brought to the region wce, wpe ≫ wrf at the mirror throat by increasing the magnetic field. Simultaneously, the ECR region is shifted from mirror throat to the center where second microwave coupling is done at the resonant region using another horn antenna. The characterization of plasma parameters are presented for both helical coil and slotted line antennas. Enhancement of plasma parameters are observed in the present scheme. Also, the SLA is found to produce better plasmas (ne ∼ 7 × 1012 cm−3 and Te ∼ 12 eV) than the helical coil and hence the SLA is chosen for the ion beam characterization. The extracted ion beam current density in the present scheme is ∼25 mA/cm2 at 2-kV extraction voltage.

Electrostatic Potential in the Endcells of a Tandem Mirror under Electron Cyclotron Resonance Heating

The electrostatic potential profile in the endcells of a tandem mirror under electron cyclotron resonance heating is studied by including an ionization process. A plug potential is found to form with a combination of created ions by ionization, wave-heated electrostatically trapped electrons and mirror trapped electrons, but without a help of high energy sloshing ions. In the present model the electrostatic potential has its maximum at the outer mirror throat of end mirror cell.

An experimental study of the anisotropy of the electron distribution function in an ECR plasma by means of a double retarding field analyser

A double retarding field analyser for use in axially inhomogeneous axisymmetric magnetic field configurations is described and applied for measuring anisotropic electron distribution functions in an ECR discharge. The analyser uses the velocity transformation due to the constancy of the magnetic moment at adiabatic motion in an axially varying magnetic field. Its principles of operation and construction are described and discussed. An application to measuring the distribution function of electrons escaping through the mirror throat from an ECR discharge confined by a magnetic mirror trap clearly indicates an anisotropy in the energy distribution. The kinetic temperature of the gyrational motion of the electrons exceeds that of the motion parallel to the field lines.

Preferential electron-cyclotron heating of hot electrons and formation of overdense plasmas

Three electron-cyclotron-heating techniques, which preferentially couple microwave power to different energy segments of the electron distribution function, have been experimentally investigated in the AMPHED facility [C. Bodeldijk, Special Supplement, Nucl. Fusion 26, 184 (1986)]. Whistler waves launched from the high-field mirror throat are strongly absorbed in a single pass across the resonant interaction layer, producing highly overdense cold background plasma but no relativistic hot particles. On the other hand, ordinary waves launched from the system side wall are only weakly damped, giving rise to local cylindrical cavity modes and preferential coupling to hot electrons in the 100 keV region. Low levels (≤5%) of upper-off-resonance heating power were shown to be most effective for preferential hot-electron plasma formation, with ∼100% of the injected power being absorbed by the energetic electrons.

Properties of ECR Plasma in a Simple Mirror Field II–Spatial Structure of the Plasma–

The spatial structure of an ECR plasma in a simple mirror field was experimentally studied. From axial measurements of the cold plasma parameters it was found that a plasma can be efficiently confined between the two resonance zones in the axial direction, rather than between the mirror throats, when the gas pressure is low enough to produce sufficiently hot and/or warm electrons. While measuring the radial distribution of low-energy soft X-ray emissions, it was verified that the 2nd-harmonic, as well as the fundamental, resonance zones contribute remarkably to the production of hot and/or warm electrons.