Tandem Mirror Research Articles

Visible Light and Hα Imaging of Mirror Confined Plasmas in the Central Cell of GAMMA 10

Imaging of mirror confined plasmas in the central cell of the GAMMA 10 tandem mirror was performed for the first time with visible light and Ha line. A mirror polished stainless steel was used to improve the line of sight. The interaction between the plasma and the central cell limiter was studied with the aid of electrostatic probes.

CO2 Laser Collective Thomson Scattering Diagnostic of α-Particles in Burning Plasmas

Collective Thomson scattering (CTS) diagnostic based on a pulsed CO2 laser (wavelength 10.6 μm) has been developed to establish a diagnostic method of confined α-particles in burning plasmas. A high-repetition and high-energy transversely excited atmospheric (TEA) laser has been developed as a source of the CTS diagnostic. In order to obtain single-mode output, which is needed for CTS diagnostic, seed laser is injected into the cavity with unstable resonator. Pulse energy of 17 J with a repetition rate of 15 Hz has been achieved in a single-mode operation. This result gives a prospect for the CTS diagnostic on International Thermonuclear Experimental Reactor (ITER), which requires energy of 20 J with repetition rate of 40 Hz. Proof-of-principle test will be carried out in the JT-60U tokamak by using the newly developed laser. Preliminary consideration of the CTS diagnostic in the tandem mirror GAMMA 10 shows that axial profiles of ion temperature will be obtained using a circumferential collection mirror of scattered power.

Study of Efficient Electron Cyclotron Wave Coupling with Grooved Mirror Polarizer in the GAMMA 10 Tandem Mirror

An improvement of electron cyclotron heating (ECH) system in the central cell of the GAMMA 10 tandem mirror is carried out by the use of two grooved mirrors. Two grooved mirrors consist of a twister and a circular polarizer. In case of the ECH system in the central cell region, fundamental ECH is examined and launched microwave beam from the antenna propagates through the two parabolic mirrors. The incident microwave is radiated from the second mirror at a position with magnetic field intensity B=2.0 T and with an incident angle of 35.1 degrees. Two grooved mirrors can generate suitable elliptical polarizations for both of ordinary mode (O-mode) and extraordinary mode (X-mode) in oblique launch with a slanted magnetic field line. The first results of the polarization control of the incident wave for high power ECH experiments are reported.

Effects of Plasma Confining Potentials and the Associated Radially Sheared Electric Fields on the Plasma Energy Confinement

The effects of the plasma-confining potentials and the associated radially sheared electric fields 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 with electron-confining potentials is studied on the basis of the local energy-balance equation. The obtained theoretical scaling of electron temperatures with electron-confining potentials is then compared with the experimentally observed relation between these two parameters.Recently, by the use of new 0.5-MW level gyrotrons in the plug region, four-time progress in the formation of the ion-confining potential c including a new record of 3 kV has been achieved in a hot-ion mode having bulk-ion temperature Ti = several keV. In the hot-ion mode, intermittent vortex-like turbulent structures are observed in the case without the gyrotron injections; in this case, radially produced weak shear of electric fields dEr/dr and appreciable transverse losses are observed. However, during the application of electron-cyclotron heatings, the associated potential rise produces a stronger shear in the central cell (dEr/dr = several 10 kV/m2) resulting in the disappearance of such intermittent turbulent vortices with plasma confinement improvement.In order to investigate the effect of the radially sheared electric fields on the electron energy confinement, the radial profiles of the thermal diffusivity are derived from the local power-balance analysis by the use of the data from the following various diagnostics in the above-described hot-ion mode. The obtained radial profiles of radial electric field and thermal diffusivity imply that the reduction of the thermal diffusivity is associated with the radially produced strong shear of electric fields.

Spectroscopic Measurement Using Wide Range UV/Visible Spectroscopic System GAMMA 10

Impurity spectra have been measured to evaluate impurity behavior and plasma parameter in the tandem mirror GAMMA 10. An ultraviolet and visible (UV/visible) spectroscopic system has been designed to measure the impurity emission intensity in detail. It consists of two spectrometers to obtain an entire wavelength range of UV/visible impurity spectra in one plasma shot. The time-varying emission intensity of radiation spectra have been measured successfully with electron cyclotron resonance heating (ECRH) or pellet injection plasmas. We evaluate radiation loss with ECRH from the GAMMA 10 plasma in the UV/visible range; further we estimate the electron density and temperature after applying the spectral intensity data measured from the pellet injection experiment to a collisional-radiative model.

Behavior of Fueled Particles and Its Effects on Plasma Parameters in the GAMMA 10 Tandem Mirror

The development and optimization of the fueling method enable us to increase and sustain the plasma density. In the present plasma experiments, three major ways, which are gas puff, NBI and pellet injection, are used for particle supply to keep or increase the plasma density. In the GAMMA 10 tandem mirror, the study of particle fueling has been carried out in term of high performance plasma production. In the central cell of GAMMA 10, a pellet injector, a neutral beam injector and some gas puffers have been installed. To clarify the feature of particle fueling method and compare with each other, the calculation with simple slab model is applied to the experimental results, and the effects of the particle fueling method on the plasma parameters are compared experimentally. As the results, it is found that the change of the electron density and Hα line emission profiles caused by pellet injection has the peak near the center of the plasma and the change of the density and temperature occurs quickly. The change of the plasma parameters caused by the gas puff mainly occurs in the peripheral region of the injection side. And the change of the plasma profiles occurs in core region of the plasma and the plasma temperature is sustained during several ms when the particles are fueled by the neutral beam injection.

Upgrade Program of ECRH System for GAMMA 10

In GAMMA 10, the upgrade program of high power plug and central ECRH have started for the study of the physics of plasma potential and the demonstration of high plasma performance proving the tandem mirror advantage. A new high power (upgrade) gyrotron has been developed in collaboration with JAEA from 2003 for this purpose and now three upgrade gyrotrons were installed in GAMMA10. The first gyrotorn in 2004 outputted about 500 kW power at the gyrotron window, corresponding to 400kW injection power to plasma. In 2005, the power of 570 kW at the window was obtained from the second one, which was improved mainly in mode converter. These gyrotron were applied to plug cells and a new record value of ion confining potential Φc = 3kV was obtained at 470 kW injection, which is four times higher than the previous value before 2003. For the central ECRH system, the upgrade of DC power supply was conducted using the DC power supply from JFT-2M ECH system and the system outputted 400kW at MOU out with the upgrade gyrotron. The central ECRH has been conducted with the combination of the new efficient antenna, transmission line and the upgrade gyrotron, and the initial experiment resulted Te ≥ 500eV.

Behavior of High Energy Ions During the Drift Type Instability in the GAMMA 10 Tandem Mirror

To measure the behavior of high energy ions, a semiconductor detector (named: ccHED) is installed at the central cell mid-plane of GAMMA 10. When ccHED is located at the radius of R=25cm from the center of plasma, the burst like signal is observed. To investigate this burst like signal, FFT method is applied to the signal of ccHED and signal of electrostatic probes (ESPs) which is used to measure fluctuations. As a result, it is clearly observed that the frequency component of the burst like signal has the same peak as one of the frequency components of the fluctuations. The amplitude of the signal has the pitch angle dependence. These observations suggest the existence of the radial transport of high energy ions due to the drift-type fluctuations.

Low-Frequency Turbulence and Non-Diffusive Cross-Field Plasma Transport in Mirror Systems

Low-frequency turbulence and the resultant cross-field plasma transport in mirror-based systems are studied by means of direct computer simulations of nonlinear plasma dynamics. Under the low-beta assumption the nonlinear dynamics are simulated in a frame of adiabatically reduced one-fluid MHD model. Simulations of self-consistent plasma evolutions have shown formation of large-scale flute-like stochastic vortex structures, which have broad-band frequency and wave-number spectra and are similar to the intermittent vortex-like structures observed in GAMMA 10 experiments. Simulations were performed both for the conventional tandem mirror configuration and for axisymmetric non-paraxial configuration with divertor-like separatrix. Various regimes of plasma confinement with sheared plasma rotation have been modeled and analyzed.

Numerical Analysis of the Second Harmonic Resonance Ion Heating in the HIEI Tandem Mirror

Recently a magnetic divertor was introduced in the HIEI tandem mirror as an effective stabilization method, and the second harmonic resonance ion heating was examined. The purpose of the paper is to clarify the physical mechanism of the ion heating. RF field with large wave number was confirmed in the divertor configuration which could not be found out in the nondivertor configuration. Power absorption in the vicinity of the second harmonic resonance regions was also confirmed.

Optimization of 28 GHz Gyrotron Output Performance for ECRH Experiment of the GAMMA 10

A new 28GHz 500kW gyrotron was developed for the electron cyclotron resonance heating system of tandem mirror GAMMA10. The maximum output power of 570 kW was obtained by experimental optimization. The efficiency of the gyrotron is 40˜50% in the power range of 300˜500 kW output with the optimization in spite of the restriction of the present magnet. The non-desirable influences caused by the leakage field of the magnet system of GAMM10 were minimized by the adjustment of gyrotron coi l parameters and the minor change of the gyrotron and magnetic shield design.

Analysis of Neutral Transport in Fully 3-Dimensional Geometry in the GAMMA 10 Tandem Mirror

Fully 3-dimensional model for Monte-Carlo simulation code (DEGAS) was newly applied in the GAMMA 10 central cell. Based on the measurement and DEGAS simulation, neutral transport during the gas puffing and neutral beam injection (NBI) in the central cell is investigated. The predicted Hα profile using DEGAS well explained the experimental result. It is clarified that the localized neutral source has much influence on the behavior of neutral particles in the plasma. In this paper, detailed discussion is shown from the view point of both the experiment and the simulation.

Direct Energy Conversion Experiment on the GAMMA 10 Tandem Mirror

A direct energy converter (DEC) designed for thermal ions escaping from a fusion reactor consists of a cusp magnetic field and one or two stage decelerating electrodes. The electrons are deflected along the field lines and consequently separated from thermal ions that are not fully magnetized. The ions are led to the electrodes to produce DC power. For basic investigation of this type of DEC, the CUSPDEC, a small-scale experimental device is constructed and its characteristics have been investigated by using a low-energy plasma source. In this paper, the device 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 in much more reactor-relevant environment. The separation of electrons and ions with energies of the order of keV is achieved by using a slanted cusp magnetic field for the first time. It is also found 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 DC power.

Observation of Radial Particle Transport Induced by the Fluctuation Measured with a Gold Neutral Beam Probe

The radial particle flux induced by the fluctuation is measured by a Gold Neutral Beam Probe. Then the transient transport phenomenon induced by the fluctuation is investigated in the tandem mirror GAMMA 10. When the drift wave is excited at the central cell, the density near the center is reduced and the divergence of the flux becomes similar to the time derivative of the electron density. It shows that the density reduction is caused by the flux induced by the drift wave. After the density reduction, the drift wave is saturated and comes to the steady state because the density reduction accompanies the reduction of density gradient. Therefore, the transport phenomenon accompanying the growth and saturation of the drift wave is observed experimentally. In the steady state, the phase difference obeys the boltzman relation including the electron non-adiabatic term.

Concept of the Magnetic Divertor in GAMMA 10

The magnetic divertor in the GAMMA10 tandem mirror is presented. Here non-axisymmetric base-ball coils and race-track coils in anchor region are replaced by the axisymmetric divertor coils. The analysis of the flute interchange mode stability and plasma equilibrium to the divertor is carried out.

Wave Excitation in Magnetically Confined Plasmas with an Anisotropic Velocity Distribution

In magnetically confined plasmas, fluctuations in the ion cyclotron range of frequency (ICRF) will be driven by the presence of non-thermal ion energy distributions. In a typical discharge in the GAMMA 10 tandem mirror, Alfvén-ion-cyclotron (AIC) modes are spontaneously excited due to strong temperature anisotropy. On the other hand, in fusion-oriented devices with a toroidal configuration, neutral beam (NB) injection is commonly used to create high performance plasmas. The resultant high-energy ions are trapped in local mirrors and will form velocity distributions with strong anisotropy. Especially in burning plasma experiments on JET and TFTR, fusion-product (FP) ions form non-thermal ion energy distributions in the bulk plasma and ion cyclotron emission (ICE) has been observed. The main purpose of this work is to study the relation between AIC-modes and ICEs in magnetically confined plasmas with non-thermal energy distributions. Recently, fluctuation measurements made by using ICRF antennas as pickup loops on JT-60U have been started. When the deuterium-NBs are injected into deuterium bulk plasma, magnetic fluctuations due to injected beams and FP ions are detected. Wave excitation near the ion cyclotron frequency and its higher harmonics is studied experimentally in plasmas with non-thermal ion energy distributions.

Study of Oxygen Ions Behavior by Using Collisional-Radiative Model in GAMMA 10

In the fusion plasmas, impurity behavior is important issue because of radiation loss and plasma-wall interactions. Spectroscopic measurements were carried out in the GAMMA 10 tandem mirror in order to investigate impurity transfer and plasma diagnostics. Oxygen ions were strongly emitted in the central cell of GAMMA 10. Then we have developed the collisional-radiative model for lower charge state of oxygen ions. The spatial profiles of OII, OIII, OIV and OV spectra were observed by the absolutely calibrated ultraviolet/visible spectrometer. Further more, spatial density profiles of O+, O2+, O3+ and O4+ were evaluated by the comparison of measurements and the CR-model calculation.

Edge plasma behavior in the GAMMA 10 tandem mirror based on gas puff imaging experiments with a high-speed camera

Detailed results of edge plasma and neutral particle behavior are described in the gas puff imaging experiments performed in the GAMMA 10 tandem mirror for the first time. Visible imaging measurement by using a high-speed camera (Ultima-SE, Photron Inc.) was recently performed in the central-cell midplane of GAMMA 10. In the standard plasma discharge heated by ICRF wave, a short gas puff of hydrogen (3ms) was carried out and the time evolution of light emission was visualized precisely. It was found that the time evolution of the emission cloud was similar to that of Hα line intensity measured nearby and was localized close to the gas puff port. Asymmetry in diffusion of the emission cloud is observed radial and axial direction of plasma column in the central-cell. This localization phenomenon of neutrals is qualitatively well explained by a fully 3-dimensional simulation using DEGAS neutral particle transport code.

Alignment of Laboratory-Scale Soft X-ray Microscope Using Laser Plasma

We demonstrate an alignment method for a laboratory-scale soft X-ray microscope, which utilizes a liquid-jet laser-produced plasma source in combination with a tandem ellipsoidal condenser mirror and a diffractive micro zone plate. Because the intensity of the laser-produced plasma source is relatively low, we developed the tandem ellipsoidal condenser mirror to increase the photon density in the objective plane and to obtain a high-resolution image within a short exposure time. But, aligning the tandem ellipsoidal condenser mirror is a difficult task because it has 5 degrees of freedom (DOF) alignment tolerances and there is a connection among these tolerances. We performed its tolerance analysis and developed a new fully automated alignment algorithm via computer control. As a result, a 12.5-µm-pitch gold mesh image was measured using 700× magnification and a 5 min exposure and the resultant edge detection resolution was about 70 nm.

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.