In order to achieve controlled nuclear fusion, we have to produce high temperature and high density plasma with long duration. In nuclear fusion plasmas, the electromagnetic waves of various wavelengths are emitted. The electromagnetic waves emitted by the plasma take energy, which are called the radiation loss. In addition, a plasma particle and energy run away to the plasma outside from the inside of the plasma. The various fluctuations generate the energy loss mechanisms in plasma. Therefore, it is very important for us to study the plasma conditions with measuring various plasma parameters and to improve the plasma diagnostic methods.
2. Plasma spectroscopic diagnosis in the nuclear fusion plasma
In the light emitted from plasma, there are Bremsstrahlung by the coulomb collision between the electrons and ions, recombination radiation by the ions and atoms, or line spectral emissions by the radiative transition of the impurity ions, atoms and molecules. The emission power of the bremsstrahlung from the plasma depends on the square of number of the electric charges Z of the impurity ions. Then we must prevent the pollution of the plasma by impurities in order to improve energy confinement of the plasma. The spectra by the radiative transition from impurity ions are things peculiar to impurities ion. Then we can specify impurity ions by measuring their spectra and can identify the impurity ions. Moreover, we can study the impurity transport mechanisms by measuring the temporal and spatial distributions of impurity emissions. Furthermore, we can measure the transportation phenomenon of the impurities ion, and they become important when we study electric potential structure in plasmas. In addition, we can measure a rotation speed of the plasma and ion temperatures by measuring the Doppler shift and Doppler broadening of the impurity ion spectra, respectively. We can derive the spatial distribution of the electric field strength in the plasma by the measurement of the plasma rotation speeds. Moreover, it is possible to obtain the spatial distribution of the neutral atomic density by the measurement of the absolute emission intensities of neutral atoms. We can obtain the plasma parameters such as the electron temperature, the plasma densities by putting the measured spectroscopic data into the spectroscopic model, such as a collisional-radiative model.
2-1. Spectroscopy in GAMMA 10
Spectroscopic measurements are important study for fusion plasmas. They have a lot of important information of the fusion plasmas, such as plasma particle confinements, impurity transport, plasma density, and plasma temperature, etc. We have studied impurity ions and neutral hydrogen radiation intensities in GAMMA 10 for plasma diagnostics. In these days, a collisional-radiative model (CR-model) is an important model for the plasma spectroscopy. However, it is needed to compare the calculation results of CR-model for impurity ions and atoms to the spectroscopic measurement results in order to evaluate the CR-model for plasma diagnostics. We have studied impurity ion radiation intensities in the GAMMA 10 plasma with comparing the CR-model calculation results to the measured impurity ion spectra using spectrograph systems of ultraviolet and visible (UV/V) spectrographs, vacuum ultraviolet (VUV) spectrograph, and soft X-ray (SX) spectrograph for plasma diagnostics. These spectrograph systems are absolutely calibrated by using suitable light sources for their wavelength range. The CR-models for hydrogen atom, carbon, and oxygen ions has been developed to use impurity ion densities and their transport.
Ha emissions are measured with Ha line emission detectors which consist of interference filters, focusing lenses, apertures, optical fibers and photomultiplier tubes (PMTs). Near the mid-plane of the central cell, vertical and horizontal arrays of Ha detectors are installed to measure the spatial profiles of Ha line-integrated brightness. Each array has 12 channel detectors. The output signals from the PMTs are amplified and led to a CAMAC system. This system is absolutely calibrated by a standard lamp.
The UV/V spectrograph system views the plasma column vertically. The spectrograph covers the range from x = -20 cm to x = 20 cm. The observable wavelength of the spectrograph is in the range of 250 nm to 700 nm. The output images of the spectrograph are recorded by the CCD camera whose frame rate is 30 frames/s.
We have a newly constructed wide-wavelength-range UV/visible spectroscopic system by using two spectrometers (Ocean Optics USB2000). One of the spectrometers can measure the wavelength range of 200-500 nm with wavelength resolution of 0.944 nm at 365 nm. The other is 400-700 nm with wavelength resolution of 0.467 nm at 546 nm. This system provides the advantage of measuring an entire wavelength range of the UV/visible radiation spectra in single plasma shot with frame rate of 18 frames/s. These spectrograph and spectrometer were absolutely calibrated by using tungsten ribbon filament lump.
The VUV spectrograph can provide spatial and spectral distributions of plasma radiation in the wavelength range 15-105 nm. It consists of an entrance slit of limited height (100 µm × 6 mm), an aberration-corrected concave grating with varied spacing grooves (Hitachi P/N001-0464), and an image-intensified two-dimensional detector system. One can observe the upper half of the plasma with a field of view of about 25-cm diameter.
The SX spectrograph system is the same as the VUV spectrograph system. The grating (Hitachi P/N 001-0266) is designed to cover the 2-35 nm wavelength range. This spectrograph observes the upper half of the plasma about 20 cm in radial direction. The spectral images of both VUV and SX spectrographs are recorded by MOS type cameras whose frame rate is 50 frames/s in normal use. Absolute calibration experiments for VUV and SX spectrographs have been performed on the overall sensitivities for the wavelength of synchrotron radiation produced at the Photon Factory in the High Energy Accelerator Research Organization.
3. Microwave diagnostics
The tandem mirror GAMMA 10 utilizes an electron cyclotron heating (ECH) for forming a confinement potential. Fluctuation in the plasma is important to be measured for studying the improvement of the plasma confinement by the formation of the plasma confinement potential. Density fluctuation is observed using microwaves, such as interferometer, reflectometry and Fraunhofer diffraction (FD) method, and electrostatic probes. Ultrashort-pulse reflectometry has an advantage of detecting fluctuation locally. The wave number can be obtained by the FD method. We have constructed a new multi-channel microwave interferometer to measure the plasma density profile and density fluctuation profile in a single plasma shot.
3-1. Interferometer systems in GAMMA 10.
There are eight interferometer systems in GAMMA 10. They are the heterodyne interferometer systems in the central, anchor, barrier and plug cells and the homodyne interferometer system in a central throat in order to measure the line integrated densities in each cell. In the central cell, the single channel microwave interferometer with movable horns (z = 0.6 m) was installed and operational. It can measure the radial line density profile by changing the measuring positions shot-by-shot. The multi-channel interferometer system is installed in the central cell mid-plane. It is designed using Gaussian-beam propagation theory and ray tracing code. The transmission horn is set at the x = 1.15 m and y = -0.10 m though a Teflon lens system (the radius, the radius of curvature and the thickness are 0.0545 m, 0.0536 m, and 0.0236 m, respectively) from the upper port of GAMMA 10. We could measure the radial line density profile between -0.12 m < y < 0.05 m. The spatial resolution of the system is about 0.03 m. We observed the phase change of received signal due to the plasma density compared with the reference signal by using the phase detection circuit. The phase change is given by the electron density and then the line-integrated electron density of each position is calculated numerically. After using Abel inversion technique, we obtain the electron density radial profiles.
4. Gold neutral beam probe system
We use the heavy ion beam probe (HIBP) for potential measurement of core plasma at central cell in GAMMA 10. We measured the potential and its fluctuations by using the gold neutral beam probe (GNBP) in the central cell. In the case of a vertically directed beam sweep for radial potential profile observations in a single discharge, ionization points move in the vertical direction due to the incident angle changes. The features of GNBP are using the neutral primary beam and the negative gold ion by Cs sputtering. The energy and the incident angle of the primary beam passing the plasma center are about 12 keV and 40 degrees, respectively. Typical primary beam current is obtained 2 mA using the Faraday Cup. The GNBP system has two incident angle electrostatic deflectors of the vertical and the horizontal directions. A parallel plate type electrostatic energy analyzer with the incident angle of 45 degrees is installed on the x-y plane. In the analyzer, the micro-channel plate detector of 32 anodes which is mounted along y direction is utilized for the beam detector. The detected positive secondary beam is derived from the neutral primary beam ionized at the ionization position. The electron-impact ionization process is dominant in a case of the ionization of the primary beam. The secondary beam current depends on the electron distribution function at the ionization position. The density fluctuation is obtained from the perturbation of the detected beam intensity, and the potential fluctuation is obtained from the perturbation of the plasma potential. It is possible to measure the potential, density fluctuations and their phase difference at the arbitrary point simultaneously by GNBP. GNBP can measure the radial potential profile from R ~ 0 cm to R ~ 14 cm in the error of ±10 V. The path integral effect is low in the density and potential fluctuations measured in the density range of GAMMA10 experiment and low fluctuation levels.
5. Thomson scattering system in GAMMA 10.
Electron temperature is one of the most important plasma parameters to describe the plasma condition. In GAMMA 10, with applying an electron cyclotron heating (ECH), electrons in central cell are heated directly by the central ECH (C-ECH) and ion and electron confinement potentials are formed by plug/barrier ECH (P/B-ECH). In order to obtain the electron temperature, we installed Thomson scattering system which is highly reliable measurement of electron temperature. We can successfully measure the electron temperature of GAMMA 10 plasma shot by shot. The electron temperatures of 30 ~ 50 eV during only ion cyclotron range of frequency (ICRF) heating period, 60 ~ 80 eV during P/B-ECH period, were obtained by using the G10-Thomson scattering system for the first time. We will add the radial observing position with making the optical fibers and polychromators, and construct the new data acquisition system for multichannel charge to voltage converter system.