Hanbit Mirror Research Articles

A Hybrid Method for Solving the Heating Problem in Hanbit

Solutions for the RF heating problems in HANBIT mirror machine have been obtained by analytically under the assumptions of uniform plasma and magnetic field [1]. For the case of non-uniform plasma and magnetic field, a numerical calculation for entire region requires considerable computing times and delicate considerations of the antenna current. In this work, we developed a hybrid method in solving the RF heating problem in which the outer region of the plasma limiter including the antenna is treated by an analytic technique and the plasma region is solved by a numerical method. Solutions for two regions are matched on the plasma-vacuum interface self-consistently without loss of generality. Using this method, we can reduce the calculation time and required computer memories and present some results for HANBIT case.

Study of Wall Recycling and Conditioning on the Hanbit Mirror Device

The wall recycling effect dominantly appears in the ICRH discharge with ω < ωci in the HANBIT plasma. The methods and evaluation of wall conditioning are described. The progress of wall conditioning is monitored with neutral pressure and plasma parameters. Electron cyclotron resonance–discharge cleaning(ECR-DC) is applied to improve wall conditioning, and then electron impact desorption(EID) by filament heating is utilized in order to desorb the impurities from the wall. The impurities are analyzed quantitatively by quadrupole mass spectrometer(QMA). We also install new baking system by Halogen lamp radiation with 2 kW in the HANBIT central cell. It is also observed that Hα emission reduces after lamp heating. The evolution of neutral pressure profiles are carefully evaluated during discharge and monitored discharge cleaning effect after several hundred of radio frequency(rf) shots. The partial pressure of light impurities much reduced after rf discharges The line integrated density and edge density much decreased after rf shots, while edge temperature increases. After ECR-DC, also line density decreases, but edge temperature much increases. Plasma beta goes up more than three times after 250 rf shots.

Plasma Transport and Confinement Studies in Hanbit Mirror Device

A brief overview is presented on initial study results of plasma transport and confinement in HANBIT mirror device. The parallel confinement is calculated using a generalized Pastukhov's formula, and compared with some experimental estimates. It is shown that the confinement time is less than 1 ms in typical HANBIT discharges. Analysis and simulation study are also presented on HANBIT discharges, particularly, ting to clarify the plasma density jump phenomena, which was observed in HANBIT when the RF frequency ω becomes smaller than the ion cyclotron frequency ωci. It is shown that the jump in plasma density (and beta, as shown from recent measurements) might be explained mainly as due to the increase in the parallel confinement time by the onset of ICRH ion heating at ω < ωci. The long-pulse operation with high-density plasma, even with a small initial fueling, can be also explained as due to the strong wall-recycling by fast neutrals generated from the ICRH heated hot ion at ω< ωci.

The Measurements of Ion Currents and Ion Energy Distributions in the Hanbit Mirror Plasma Heated by ICRF

Ion cyclotron resonant heating (ICRH) experiments have been carried out on the central cell of the HANBIT magnetic mirror. In these experiments were tested startup, heating, electrostatic ion confinement and electric potential modification which makes use of the electron cyclotron resonant heating (ECRH).The ions heated by the wave are generally either reflected near magnetic field or are eventually disappeared into the loss cone. The small Faraday cup is used to measure the ions heated by the RF wave. End loss analyzers, types of electrostatic potential multi-grid Faraday cup have been used to measure the current and the axial energy distribution of ions escaping along magnetic field lines in the magnetic mirror machine. The change of the end loss ion current was shown to be consistent with the measured changes in the plasma potential when the ECRH occurred at the Plug region.

Recent Results of the Hanbit Mirror Device

The HANBIT device is a non axi-symmetric mirror being operated as a national users’ facility. Plasmas are routinely produced by ICRF at 3.5 MHz with a slot antenna with gas puffing and the line-integrated densities are in the range between 2×1012 and 1×1014 cm–2. The pulse length is normally 250 msec, but higher wall recycling happened usually after 100 msec into the discharge. Characterization and application of various methods of wall conditioning have been performed. Ion heating had been tried by RF with a double half-turn antenna, however, the heating effects were vaguely seen. Optimum heating schemes have been actively pursued with different heating method and antenna types. RF-induced electric fields have been known to affect the plasma stability. This effect of RF on stability seems important in HANBIT because of lacking of stabilizing mechanisms such as the minimum-B effect and the line-tying effect. In addition, stabilization by a hot electron ring generation and by other methods is being pursued. Detailed experimental results on these topics will be presented.

Effects of Heating on the Hanbit Mirror Plasma

The Hanbit plasmas with densities in the 1012 cm-3 range, are produced by applying 200 kW of pulsed RF power at 3.5 MHz to a slot antenna and 85 kW at 3.75 MHz to a double half turn (DHT) antenna. Experiments have been performed in the Hanbit device to determine the effect of ion cyclotron resonance frequency (ICRF) heating on the ion pitch angle distribution, end loss ion currents, and the total temperature. The ion velocity distribution function is approximated by a Bi-Maxwell distribution with two effective temperatures, parallel and perpendicular to the magnetic field. The ICRF heating affects the pitch angle distribution directly and is a main cause of the anisotropy of the ion distribution. The ions heated by the ICRF are generally either trapped and reflected near the resonant magnetic field or are untrapped and eventually disappear into the loss cone. For ICRF in resonant fields the ion loss into the end regions is lower than that with non-resonant fields even in the absence of plugging. The pitch angle distribution in the central cell is measured using a small Faraday cup and the end loss ions in both end regions using end loss energy analyzer (ELA).

Construction and Commissioning of a 100 KW RF Amplifier System for Hanbit Central Cell Plasma Heating

To complete basic configuration for the RF heating scenario in Central Cell of the Hanbit mirror device, a RF amplifier system with a power of 100 kW and frequency range of 1.4 – 4.0 MHz, and a high power RF matching network have been developed in KBSI. The output pulse width of the amplifier ranges from 10 msec to 100 msec and the duty cycle is 100 msec/300 sec. The developed amplifier will be used for plasma production and heating by utilizing the slot antenna and the double-half-turn antenna, respectively.In this paper, we present the engineering design of the amplifier system and matching network, and experimental results such as power test of amplifier on dummy load, impedance matching test on the double-half-turn antenna in vacuum condition. Also, the first RF launching experiment result with the double-half-turn antenna on the target plasma which is produced by the slot antenna is presented.

Overview of Hanbit Experimental Program

The first phase of the HANBIT project that started in 1996, is ending this year. The goal of the first phase is to establish a decent experimental facility based on the HANBIT mirror device and to set up a framework for operating this machine as a National Joint Users’ Facility. A five-year plan has been being undertaken to improve the performance of HANBIT, refurbished from the old TARA machine, by adding diagnostics and heating capacity. As the goal of the first phase has been achieved, the second phase is being planned, in which basic mirror confinement physics will be pursued as well as plasma applications and training of people. Most of new works are possible via close collaboration with external experts. We are exercising a new form of collaboration between universities and the institute and between industries and the institute through the HANBIT User Program, which is a key ingredient for successful operation of a National Users’ facility like the HANBIT device.

Estimate of Local Ion Temperature Using Multichord Spectrum on Hanbit Mirror Device

Five line integrated spectra are obtained from Hanbit mirror plasma using optical chords installed in the horizontal direction of central cell chamber.In this work, we describe the spectroscopic method to obtain the local spectra in the radial direction by applying the deconvolution method to the five line integrated spectra. Among several fitting methods, the deconvolution fitting is well known as the most effective one.[1]From these, the local intensity profiles and ion temperature are obtained. In order to compensate for the instrumental broadening of spectrometer, a slit function obtained from the low pressure mercury line 253.65 nm is used. The C III 229.68 nm line from the Hanbit central cell plasma is selected as sample data. We applied the deconvolution fitting method using a slit function to eliminate the instrumental broadening.

Magnetohydrodynamic Stability of the Hanbit Device: Ponderomotive Force Effects on the Interchange Mode

Investigation is carried out on the effect of the ponderomotive force on the interchange mode, which determines the basic operation regime of the Hanbit mirror machine. This effect is calculated using the electrical field profile obtained by an RF heating code with a real antenna geometry. In the calculation of the ponderomotive force, the effects of the trapped particles in the mirror geometry and the plasma temperature anisotropy are also considered.

A Self-Consistent Global Discharge Simulation for the Reference Operation Mode in Hanbit Mirror Device

A self-consistent global discharge simulation code, which combines a global transport module and a radiofrequency (RF) wave heating module in a self-consistent manner, has been developed for the Hanbit-device discharge modeling. Global fluid equations of ions and electrons are solved with oxygen impurity recycling equations in the global transport module, while Maxwell-Boltzmann equations are solved by the mode analysis technique in the RF heating module. Using the code, the global transport dynamics of ions, electrons, neutrals, and oxygen impurities can be studied as a function of external parameters, in the self-consistent calculation of the RF power deposition into the plasma from a model antenna system. Here, a simulation study is presented for the reference operation mode of the Hanbit mirror device to predict its performance.

The Measurement of end Loss Ion Energy Distribution in Cusp of Hanbit Mirror Device

The end loss ion energy distributions are measured in Hanbit1 mirror plasma produced by a 3.75-MHz, 500 kW° amplifiers with the slot antenna. The measurement of the ion energy distribution in the end region gives us the important information on the loss cone region of mirror plasmas.The ion energy analyzer, mounted on the end region of cusp in the Hanbit, is versatile diagnostics designed to measure the parallel ion energy distributions. We can obtain the ion energy distribution from the current-voltage curve by sweeping the voltage on the ion repeller while keeping the other grids properly biased to repel the unwanted species. We show that the end loss ion currents, involved the magnetic confinement, vary according as the strength of magnetic fieldWe compared an end loss ion distribution obtained from the Hanbit with ion energy from DC discharge to test the analyzer.

A Versatile End Tank System for Hanbit Magnetic Mirror

A versatile end tank system called “Cusp” has been constructed to comply with the main objectives of Hanbit mirror machine - a) control of neutral reflux, b) plasma application by using the end loss plasma flow, c) providing axi-symmetric minimum-B configuration for MHD study. Cusp system consists of a vacuum vessel, a liquide-helium-cooled cryopump, a set of magnets to provide axi-symmetric semi-cusp shaped magnetic field configurationThe pumping speed of the cryopump has been designed to be 2×105 liter/sec and tested to be 2.3×105 liters/sec. To observe the effect of neutral reflux, dynamic neutral pressure and end loss ion's energy distribution has been measured at Cusp during plasma shot. It has been shown that the energy confinement at the Central Cell has been improvedFor plasma application, a gas target divertor3 simulation experiment has been performed as a feasibility study. A target has been mounted at the end of Cusp system and dynamic surface temperature has been measured by IR camera. End loss ion current also has been measured. The system configuration and experimental result will be presented

X-ray crystal spectrometer for the HANBIT mirror machine

Design study of an x-ray crystal spectrometer for the HANBIT mirror machine is presented. The x-ray emissivity of HANBIT plasmas is rather low as a result of the low values of the electron temperature and density. The design difficulty from the low x-ray emissivity can be overcome by utilizing a vacuum spectrometer sightline parallel to the long axial length of HANBIT and using an x-ray sensitive vacuum charge-coupled device camera as a detector.