Hanbit Magnetic Mirror Device Research Articles

Mirror stabilization experiments in the Hanbit device

The Hanbit magnetic mirror device has been involved in a series of experiments on stabilization of the MHD flute type mode. We undertook investigations to see whether a divertor and the kinetic stabilizer (KS) of Post et al can stabilize the MHD instability. The Hanbit divertor configuration used one of the central cell coils with reversed current as the divertor coil and two other coils with increased current to compensate the field droop. The divertor eliminated the m = −1 instability when the null point was inside the vacuum tank. The KS used microwave produced plasmas on field lines in the cusp tank region. Two coils on the cusp tank were configured to produce expanding field lines with a large positive radius of curvature. A 5 kW 2.45 GHz magnetron was used to produce an electron cyclotron resonance heated plasma pressure in this region. A reduction in instability duration was observed for high power plasmas. However, for low power plasmas that terminate violently with an m = −1 instability, the KS action made the duration of the instability longer.

Characterization of the Electron Drift Wave and the Interchange Mode in the Hanbit Magnetic Mirror

The operational windows of the low-frequency dissipative instabilities in the Hanbit magnetic mirror device are investigated experimentally in a simple mirror con guration. Both the electron drift wave and the interchange mode are stabilized by nonlinear interactions with the sideband waves that originate from nonlinear beating between the plasma modes and texternally driven radio-frequency (rf) waves. Typically, both modes are stabilized near the fundamental ion cyclotron resonance condition as the external rf power and magnetic eld increase, which is in good agreement with theoretical prediction based on twouid plasma dispersion. The onset characteristics of the two modes are also found to depend on the neutral gas pressure; the drift waves are observed at lower pressure, in contrast to the interchange mode, which is found at higher neutral pressure.

Fast Reciprocating Probe Assembly for the KSTAR

AbstractA fast reciprocating probe assembly (FRPA) that can scan a distance of up to 32.5 cm with a maximum speed of 1.7 m/s was fabricated to obtain the spatial profile of basic plasma parameters in the edge region of the KSTAR plasma during a plasma discharge of 20 s. The probe driving mechanism consists of two parts; first a slow movement driven by a servo motor system and then a fast reciprocating movement actuated by a pneumatic system. A performance test of the drive system, which is remotely controlled by a PC, was carried out. The prototype of the probe head for the KSTAR FRPA was fabricated as a modular type for easy replacement. It was installed in the fast injection probe assembly (FIPA) in the Hanbit magnetic mirror device, and a radial plasma parameter profile measurement was carried out. The performance test results of the drive system and the profile measurements using the probe head are presented. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reconstruction of Neutral Hydrogen Density Profiles in HANBIT Magnetic Mirror Device Using Bayesian Probability Theory

Hydrogen is the main constitute of plasmas in HANBIT magnetic mirror device, therefore, measurement of the emission from excited levels of hydrogen atoms is an important diagnostic tool. From the emissivity of Hα radiation one can derive quantities such as the neutral hydrogen density and the source rate. An unbiased and consistent probability theory based approach within the framework of Bayesian inference is applied to the reconstruction of Hα emissivity profiles and hydrogen neutral density profiles in HANBIT magnetic mirror device.

Neutral Transport and Particle Balance in Hanbit Magnetic Mirror Device

According to the recent low gas-puff experiments in Hanbit magnetic mirror device, the achievable ion temperature is limited largely by neutral content. For discharges with the low gas-puff with the pre-ionization technique, higher ion temperature is estimated compared to the high gas-puff case. Neutral transport and corresponding particle balance in this low gas-puff discharges in Hanbit are analyzed with the two dimensional Monte-Carlo simulation code coupled with a simple parallel plasma confinement formula. The radial ion temperature and power loss profiles are also derived. The global particle balance is between the plasma pumping and the recycling processes, hence the initially puffed amount of gas has negligible contribution to the total particle source.

Tomographic reconstruction of Hα emissivity profiles in HANBIT magnetic mirror device

Hydrogen is the main constitute of plasmas in HANBIT magnetic mirror device, therefore, measurement of the emission from excited levels of hydrogen atoms is an important diagnostic tool. From the emissivity of Hα radiation one can derive quantities such as the neutral hydrogen density and the source rate, i.e., the rate at which the plasma protons are replenished by ionization of neutral atoms diffusing into the plasma. The reconstruction of emissivity profiles from Hα radiation measurement is a highly undetermined and ill-posed inversion problem, due to the restricted viewing access, number of chords and the increased noise level. An unbiased and consistent probability theory based approach within the framework of Bayesian inference is provided by the maximum entropy method which is independent of model assumptions, but allows any prior knowledge available to be incorporated. The formalism is applied to the reconstruction of Hα emissivity profiles in HANBIT magnetic mirror device.

Direct Measurement of a Plasma Flow Velocity Using a Mach Probe in Hanbit Magnetic Mirror Device

A plasma flow velocity was measured by using a Much probe in the central cell of Hanbit magnetic mirror device. The Much probe was attached on the fast injection probe system, which can scan the central cell chamber of Hanbit device in the radial direction. The fast injection probe system also has an emissive probe so that the radial profile of the plasma potential is measured simultaneously. Therefore, the flow velocity measured from the Mach probe can be directly compared with Er×B drift calculated from the measured plasma potential profile. The experimental results are analyzed by using existing theories of the Mach probe. The measured flow velocity shows about 3 km/s, and the flow direction and magnitude is approximately the same as the Er×B drift velocity.

Diamagnetic Loop Measurement in the Central Cell of The Hanbit Magnetic Mirror Device

Diamagnetic flux is measured by using diamagnetic loops in the central cell of the Hanbit magnetic mirror device. Diamagnetic loop measurements are carried out during rf discharges with a frequency of 3.5 MHz or 3.75MHz. In the measurement, the rf power is varied from 60 kW to 200kW, and the vacuum magnetic field of 0.130 T – 0.242 T is applied in the central cell. The present status of the diamagnetic loop measurements is described and experimental results from diamagnetic flux measurements under several Hanbit operational conditions are presented. The purpose of this work is preparation of a database (such as the dependence of the diamagnetic flux upon the applied vacuum magnetic field and rf power) for physics studies in the Hanbit device.

Electron Temperature and Density Profile Measurements During Ion Cyclotron Resonance Heating of Hanbit Magnetic Mirror Device

The electron density and temperature profiles were measured in the central cell of Hanbit magnetic mirror device. The measured data were obtained during radio frequency (RF) discharges with frequencies of 3.5 and 3.75 MHz, and the RF heating power up to 200 kW under various experimental conditions. The radial electron density and temperature profiles were directly measured by a fast injection probe (FIP), and the axial and azimuthal ion saturation currents were measured from fixed array probes. The line integrated electron density was measured from a single channel interferometer and used as a reference signal for the FIP. The electron temperature measurements were carried out different diagnostic methods and compared with each other.

Development of a radio-frequency-driven ion source of the diagnostic neutral beam for the Hanbit device

A radio-frequency-driven hydrogen-ion source of the diagnostic neutral beam has been developed for the Hanbit magnetic mirror device at the Korea Basic Science Institute [S. M. Hwang et al., Trans. Fusion Technol. 35, 99 (1999)]. The three-electrode system with 37 apertures extracts the hydrogen-ion beam with 29.8 keV beam energy and 1.2 A beam current at a 9.6 cm diam. The ion source has the capability of varying the beam pulse width from 10 to 100 ms and also is capable of beam modulation at 50 Hz with a 10 ms flattop pulse width. The beam power and the ratio of the mixed ion species were measured by infrared thermography and Doppler shift spectroscopy, respectively. Most recent experimental results will be presented in this article.

Measurements of the electron temperature by the Thomson scattering system on the Hanbit magnetic mirror device

A Thomson scattering system on the Hanbit magnetic mirror device has been installed to measure the electron temperature and density of the plasma in the central cell. The configuration is based on a standard 90° scattering scheme. The optical system consists of a Q-switched Nd:YAG laser, input optics, collection optics, spectrograph optics, detectors, and a data acquisition system. Although the laser beam path is about 50 m long and the background emissions are not low, the electron temperature measurements have been made at a single point on a shot-by-shot basis, in which the stray light was considerably suppressed by using a beam dump, a viewing dump, and baffles. The measured electron temperature is about 50–70 eV in experiments for plasma production and heating by ICRF of 200-kW-rf power using a slot antenna. A description of the installed system and the experimental results are presented.

Diamagnetism measurements in the Hanbit magnetic mirror device

The diamagnetic effect is measured by using the diamagnetic loop with a compensation loop in the central cell of the Hanbit magnetic mirror device. To ensure reliable measurement, the influence of nearby conductors on the diamagnetic loop signal is investigated, and the output voltages during a pulsed vacuum field are measured to calibrate the loops installed inside the central cell. The diamagnetic measurement is carried out during rf discharges with a frequency of 3.75 MHz and heating power of up to 150 kW. In the measurement, the vacuum field of 0.126 T is applied in the central cell. The results from the experimental measurements are presented.

Triple Langmuir probe measurements in the Hanbit magnetic mirror device

A triple Langmuir probe was used in the initial physics operation phase of the Hanbit magnetic mirror device. The experimental data from the triple Langmuir probe were obtained in the edge of the central cell during radio frequency (rf) discharge plasma operation. Up to 200 kW rf heating power was applied at the frequency of 3.75 MHz. The position of the triple Langmuir probe was controlled by 0.1 mm accuracy. Measurements of the edge electron temperature, electron density, and floating potential were made in a radial scan of the plasma.

RF Field Measurement by Magnetic Probes in the Hanbit Magnetic Mirror Device

The RF-field structures are measured by the magnetic probe arrays in the edge region of the central cell of the Hanbit magnetic mirror device. The probe data was obtained during RF discharges produced by a slot antenna with frequency of 3.75 MHz and heating power of up to 150kW. For further reliable measurements, the calibration of the probe was carried out with a network analyzer for the RF frequency range of 1 – 10 MHz. The experimental results from the measurements are presented.

Loss Cone Effect in a Heating Model of the Hanbit Radio Frequency Heating Discharge Using a Slot Antenna

We inspect the loss cone effect on the power coupling in the previously developed radio frequency(RF)-heating model1 for Hanbit magnetic mirror device. The plasma conductivity tensor is expressed by the mirror ratio and anisotropy factors and the dependence of these loss cone parameters on the plasma resistance is presented as the result of numerical calculations in Hanbit discharge conditions.

Design of a bolometer for Hanbit magnetic mirror device

We designed and constructed a metal-resistor bolometer to measure the total radiation of the plasma in the Hanbit device, a magnetic mirror machine at the Korea Basic Science Institute. The target parameters for the plasma, the duration of which was 100–500 ms, at the central cell were the ion energy of 1 keV, the electron temperature of 200 eV, and the electron density of 5×1012 cm−3. A preliminary test showed that the cooling time constant and the absolute sensitivity of the homemade bolometer were 12 ms and 0.031 V/mW, respectively; the sensitivity was calibrated using an Ar ion laser. A test run of the homemade bolometer in the Hanbit device produced a signal of ∼25 mW/cm2, which was equivalent to the total power loss of about 40 kW, for the input rf power of 200 kW.

Langmuir Probe Measurements in the Central Cell of Hanbit Magnetic Mirror Device

Langmuir probe data were measured in the edge of the central cell plasma of the Hanbit magnetic mirror device. The data were obtained during radio frequency discharge plasma operation. Up to 200 kW heating power was applied at 3.75 MHz. Ion saturation current measurements from three fixed probe arrays were made for intercomparison of the edge plasma density profiles. In addition, measurements of the electron temperature, electron density and floating potential were made in a radial scan of the edge plasma using a movable triple Langmuir probe. The experimental results from these probe measurements are described.

Fast reciprocating probe assembly for the Hanbit magnetic mirror device

A fast reciprocating probe assembly (RPA), which can scan a long length up to 100 cm (with maximum velocity of 136 cm/s), is fabricated to measure basic plasma parameters during a plasma discharge time (∼500 ms) in the Hanbit magnetic mirror device. The probe driving mechanism consists of two steps of movements; first a slow movement to set the probe at a standby position, and then a fast one to measure plasma parameters within an adjustable time interval. Both movements are driven by only a pneumatic system. This is a distinctive feature of the probe drive system that has advantages of a simple driving mechanism and an easy adjustment of the fast stroke length for the wide range. The probe head is fabricated as a modular type for easy replacement. Performance test results and initial measurements from the fabricated RPA will be discussed.

Investigation of a radio frequency-driven multicusp ion source of the diagnostic neutral beam for the Hanbit device at Korea Basic Science Institute

A radio frequency-driven multicusp ion source has been fabricated and tested as a part of the development of a diagnostic neutral beam for the Hanbit magnetic mirror device [S. M. Hwang et al., Trans. Fusion Technol. 35, 99 (1999)]. Hydrogen plasma produced by a three-and-one-half turn antenna and rf power system with 2.5 kW, continuous wave (cw) 1.874 MHz was extracted through a single-aperture accel-decel system. The first results of the ion beam extraction experiments are described in this article.

Plasma Production in the Hanbit Magnetic Mirror Device by Radio Frequency Waves

We report the plasma production experiment in the central cell of the Hanbit device. In the experiment, an RF wave is excited by a slot antenna with a driving frequency of 3.75 MHz, and the RF power is applied up to 200 kW with a flat-top pulse length of 100 ms. The reproducible plasmas are produced without preionization with an averaged density of 2×1012 cm−3. The power absorption characteristics of the slot antenna are investigated by measuring the plasma resistance. The measured value of plasma resistance is in the range of 0.2 to 1.2 Ω. The discharges show transitions of the plasma density as the RF power increases.