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A real-time processing system for the two-color CO2 laser interferometer on the JT-60SA has been developed for density feedback control. The system has a novel feature that can detect fringe jumps due to off-normal events, such as loss detection due to displacement of the beam axis and changes in the laser wavelengths. Because a phase change due to the JT-60SA plasma is smaller than π/2, corresponding to the line-integral electron density NL of ∼ 6 × 1 0 19 m−2 in a short interval of 500 ns, the threshold of the fringe jump detection is decided to be π/2. Hence, off-normal events can be detected from a fringe jump, leading to the abort of the real-time feedback control. In the density feedback control of the JT-60SA plasma, the system is employed as a density monitor, with NL being successfully controlled at 16.8% ± 6.6% lower than the reference. [ABSTRACT FROM AUTHOR]

An incoherent Thomson scattering diagnostic will be installed in the JT-60SA tokamak to measure electron temperature and electron density profiles. The target radial spatial resolution is 25 mm with 46 spatial channels. The accuracy in electron temperature and density is a few percent at ne = 7.5 × 1019 m−3, which is the expected value in the plasma core. This paper presents the designs of collection optics, fibers with their alignment system, and polychromators. The collection optics overcomes unique issues for superconducting fusion devices, i.e., limited design space, high-temperature measurements, and harsh radiation condition. When in several years the more performing plasma will generate intense nuclear radiation, the lens materials of the optics can be replaced by radiation resistant glasses without major changes in the lens holder. It will prevent transmission degradation and keep stable measurement accuracy. [ABSTRACT FROM AUTHOR]

Examines electron and ion currents collected at the metal shield and floating shield to study the behavior of high-current vacuum arcs. Behavior of the arcs with the active anode; Property of vacuum arc on small copper electrodes; Difference between the maximum value and the value at the arc-current crest.

A tangential viewing phase contrast imaging system is being designed for the JT-60SA tokamak to investigate microturbulence. In order to obtain localized information on the turbulence, a spatialfiltering technique is applied, based on magnetic shearing. The tangential viewing geometry enhances the radial localization. The probing laser beam is injected tangentially and traverses the entire plasma region including both low and high field sides. The spatial resolution for an Internal Transport Barrier discharge is estimated at 30%-70% of the minor radius at k = 5 cm-1, which is the typical expected wave number of ion scale turbulence such as ion temperature gradient/trapped electron mode. [ABSTRACT FROM AUTHOR]

Magnification of a polarization angle with Littrow layout gratings has been developed. High magnification with a factor of 7.7 using two gratings in Littrow layout was experimentally proved. The magnification range was investigated by calculation at a wavelength of 10.6 µm. The method can be applied for a high magnification factor >30. Larger groove numbers and smaller blaze angles are suitable for the large magnification. Statistical fluctuation of the diffracted polarization angle is compared with that of the incident polarization angle. [ABSTRACT FROM AUTHOR]

A 14-channel grating polychromator for the purpose of measuring the electron cyclotron emission (ECE) spectrum has been developed and installed on the large helical device (LHD). The grating polychromator has a Czerny–Turner setup. The main advantage of the grating polychromator is that it has a broad spectral range. Hence, it can be utilized at different magnetic field strengths. Changing the angle or replacing the grating can do this. Gratings with a grid constant of d=2.3, 3, and 5 mm and a blaze angle of 20° can be applied and the scattered spectrum is directed to 14-liquid He cooled InSb detectors. Standard operation is performed with d=3 mm, f[sup range]=148–105 GHz. This is second harmonic ECE for B=2.75 T. Each channel has a spectral resolution of Δf (full width at half maximum)=2.6 GHz or f/Δf=60 at f=150 GHz. The properties of the instrument and first experimental results of LHD ECE will be described in this article. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A mechanical arcless dc circuit breaker is proposed and has been constructed. A rectifying device is used to block the reverse current of the breaker, allowing the breaker to open under zero current condition created by the discharge of a commutation circuit. A diode and a saturable reactor are used as the rectifying device, with a leakage current of ∼1 and ∼10 A, respectively. The duration of mechanical motion is about 20 ms, and its jitter is 2 ms, which is too long to make an economical system using a commutation capacitor. In order to avoid the dead time, the commutation circuit is started after the breaker kicks in and the final jitter is reduced to 20 μs. Arcless current interruption is achieved at 10 kA and 9.1 kV. Since a strong arc plasma is not generated between the contacts, a long lifetime of the breaker can be expected and a disconnecting switch can be used instead of a circuit breaker, reducing the size of the switch. These characteristics of the new breaker can be expanded for many applications. [ABSTRACT FROM AUTHOR]

A new method to magnify a small polarization angle with brazed gratings has been developed. In the method, difference in diffraction efficiency for S and P polarization components is used. The magnification dependence on the incident angle can be small by arranging the grating in Littrow layout. A magnification with a factor ~2.7 has been demonstrated for a 10.6 µmCO2 laser beam as expected from a calculation. The method is applicable in many polarimetry fields. [ABSTRACT FROM AUTHOR]

A pair of conventional permanent magnet quadrupoles is used to focus a 2.4 MeV laser-driven proton beam at a 1 Hz repetition rate. The magnetic field strengths are 55 and 60 T/m for the first and second quadrupoles, respectively. The proton beam is focused to a spot with a size of less than ∼3×8 mm2 at a distance of 650 mm from the source. This result is in good agreement with the Monte Carlo particle trajectory simulation. [ABSTRACT FROM AUTHOR]

Electron cyclotron emission (ECE) can be used to determine the electron temperature profile in magnetized plasmas. The complex structure of the magnetic field configuration in the Large Helical Device (LHD) [M. Fujiwara et al., J. Fusion Energy 15, 7 (1996)] complicates the analysis of the ECE spectrum. The magnetic field is a nonmonotonous function of the radius and has a very large shear. The effects of the magnetic-field shear on the polarization of the ECE spectrum have been numerically and experimentally analyzed. For a high enough density (n[sub e]>1.0x10[sup 19] m[sup -3]) the polarization is found to rotate with the sheared magnetic field, yielding only a negligible mode conversion. Experiments on the polarization have been carried out at LHD. X-mode emission was detected under a specific angle, in agreement with the numerical predictions. The elliptically polarized ECE emission showed a very strong O-mode component (70% of X-mode), that can be attributed to the effect of wall reflections. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A real-time processing system for the two-color CO2 laser interferometer on the JT-60SA has been developed for density feedback control. The system has a novel feature that can detect fringe jumps due to off-normal events, such as loss detection due to displacement of the beam axis and changes in the laser wavelengths. Because a phase change due to the JT-60SA plasma is smaller than π/2, corresponding to the line-integral electron density NL of ∼6×1019 m-2 in a short interval of 500 ns, the threshold of the fringe jump detection is decided to be π/2. Hence, off-normal events can be detected from a fringe jump, leading to the abort of the real-time feedback control. In the density feedback control of the JT-60SA plasma, the system is employed as a density monitor, with NL being successfully controlled at 16.8% ± 6.6% lower than the reference., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

A thermal image plate (TIP) using ceramic luminescence materials has been developed in order to visualize a beam spot of a CO 2 laser which is used as a light source of an interferometer developed in JT-60SA. By using CaAlSiN 3 :Eu 2+ (CASN), the TIP was developed, and its property was assessed. A CASN TIP was able to visualize the invisible CO 2 laser beam as a negative image. The beam diameter and the stability of the beam axis measurement obtained with the CASN TIP were approximately the same as that obtained with a commercial TIP. Furthermore, the time response of the CASN TIP at the best was approximately the same as that of the commercial TIP. The CASN TIP which had the highest time response was applied to the beam axis stabilizer, and then the beam axis stabilization was successfully performed in 2 h.

A new method of plasma electron density measurement using interferometric phases (fractional fringes) of an optical frequency comb interferometer is proposed. Using the characteristics of the optical frequency comb laser, high density measurement can be achieved without fringe counting errors. Simulations show that the short wavelength and wide wavelength range of the laser source and low noise in interferometric phases measurements are effective to reduce ambiguity of measured density.