A. Nagashima

YAG laser Thomson scattering diagnostic on the JT-60U

A YAG laser Thomson scattering system has been developed to measure the time evolution of electron temperature (Te) and density (ne) profiles in the JT-60U. Presently this system provides Te and ne periodically every 100 ms (20 ms at the minimum) at 15 spatial channels. A six spectrum channel polychromator, which is a successive interference filter type, was designed to measure a wide Te range (20 eV–20 keV) by using five channels. The sixth channel is used for calibration by Rayleigh scattering. However, we also used the Rayleigh channel to measure Te and ne since the stray light level was very low. As a result, we were able to measure temperatures below 20 eV. The temperature of the silicon avalanche photodiode (APD) is stabilized by a Peltier element so that the gain is kept constant within ±0.5%. The stable performance of the APD enables accurate Te and ne measurement. During plasma operation, the electron temperature is measured over a wide range from the low temperature plasmas in the runaway plasma...

Dual CO2 laser interferometer with a wavelength combination of 10.6 and 9.27 μm for electron density measurement on large tokamaks

A new CO2 laser interferometer has been developed to measure the electron density of tokamak plasmas. Two different wavelength oscillators of 10.6 and 9.27 μm are utilized for simultaneous measurement of the density component and the optical path length change. A new technique using a common frequency shifter for two color lasers improves the stability of the system by a complete matching of both beat frequencies. The system provides advantages for practical problems of large tokmaks related to window darkening and large mechanical vibrations of reflection mirrors and offers improved laser beam monitoring and a simplified optical layout by using closer wavelengths. The electron density of JT‐60U is successfully measured for a plasma current of up to 3 MA. The density behavior during a fast major disruption is also diagnosed without a fringe loss. An effective density resolution is observed to be 2×1019 m−2, which corresponds to about 1/10 of a fringe. A very high resolution (1/104 fringe) phase comparator...

A twin optically-pumped far-infrared CH3OH laser for plasma diagnostics

A twin optically-pumped far-infrared CH3OH laser has been constructed for use in plasma diagnostics. The antisymmetric doublet due to the Raman-type resonant two-photon transition is reproducibly observed at 118.8 μm. With the 118.8-μm line, it is obtained from the frequency separation of the anti-symmetric doublet that CH3OH absorption line center is 16±1 MHz higher than the pump 9.7-μm P(36) CO2 laser line center. It is shown that the Raman-type resonant two-photon transition is useful in order to get several-MHz phase modulation for the far-infrared laser interferometer. Some preliminary performances of this twin laser for the modulated interferometer are described.

Frequency‐stabilized single‐mode cw 118.8‐μm CH3OH waveguide laser for large tokamak diagnostics

A frequency‐stabilized single‐mode cw 118.8‐μm CH3OH waveguide laser was newly developed for polari‐interferometry on the JT‐60 tokamak. Interferometric plasma diagnostics on large fusion devices requires a mode purity of a source laser for the long‐distance propagation and frequency stability to obtain an adequate fringe resolution. The accomplished performances are Δf/f<1×10−8, power stability <0.03%, and single EH11 mode to yield a Gaussian‐shaped profile in the far field. Homogeneous pumping of laser media with a Ge etalon beam coupler and feedback stability control are the keys to the source far‐infrared laser performances required for large tokamak diagnostics. The developed laser was totally adjustment free for more than 2 weeks and reliably in operation for 2 years, including the real‐time electron density feed‐back control on the JT‐60 tokamak.

Development of a pumping laser system for x-ray laser research

A two-beam chirped-pulse-amplification Nd:glass laser system dedicated to x-ray laser research is described. Each beam provides an output energy of 20 J with a typical pulse duration of 1.3 ps. A prepulse of variable duration is generated by use of a novel, to our knowledge, optical system. A reflection optical system, comprised of an off-axis parabolic mirror and a spherical mirror, produces a line focus with 6-mm length and 165-microm width without chromatic aberration. By use of this pumping laser system, the nickel-like silver x-ray laser at a wavelength of 13.9 nm has been demonstrated.

Development of 180 GHz heterodyne radiometer for electron cyclotron emission measurements in JT‐60U

A 12‐channel heterodyne radiometer system has been developed to obtain electron temperature with high time and spatial resolution from the electron cyclotron emission (ECE) in JT‐60U. Resolutions of a single‐sideband‐type receiver are (1) Δt∼1 μs and (2) Δf=0.5 GHz. The second harmonic extraordinary mode of ECE is utilized for this system and the frequency band is 176–188 GHz. The noise level of the system is determined by unavoidable photon fluctuations, and the signal‐to‐noise ratio was measured to be about 50 with a video bandwidth of 100 kHz for a signal temperature above ∼200 eV. Details of the design, the obtained performance, and typical measurements are described.

Twenty‐channel grating polychromator diagnostic system for electron cyclotron emission measurement in JT‐60

A twenty‐channel grating polychromator diagnostic system has been built to measure the temporal evolution of local electron temperatures in JT‐60. A cross Czerny–Turner diffraction grating spectrometer is utilized for the measurement of second‐harmonic electron cyclotron emission with extraordinary modes in the range 85–300 GHz, in which a grating plate grooved on both faces with different grating periods is applied effectively to yield a wide coverage for the toroidal fields. The grating angle is automatically set up by control of a stepping motor according to the relation of the grating equation. The diffracted light is detected by 20 indium‐antimonide hot‐electron bolometers cooled at 4.3 K in a modified Solvay cycle cryogenic refrigerator. A typical resolving power of the instrument was measured to be λ/Δλ∼130, providing a spatial resolution of 2.3 cm at the plasma center. The transmission line over ∼38 m long is composed of oversized S‐band waveguides. The total transmissivity of this system is estim...

Approach to a window coating problem by in situ transmission monitoring and laser blow-off cleaning developed in the JT-60U Thomson scattering system

For solving a window coating problem, an in situ window transmission monitoring by inferring it precisely from a known attenuation of the deposited film has been developed in the JT-60U Thomson scattering system. Also the practicability of an in situ window cleaning based on a laser blow-off technique has been investigated extensively. The coated film composition could be guessed to be a resin such as polyacrylonitrile with a small amount of metals. The existence of chromatic upper limit has been found in the recovered transmission after the blow-off cleaning, which gives systematic errors only of less than 3% to an apparent measurement of both electron density and temperature at 10 keV or less. The attenuation itself is unchanged before and after the laser blow-off cleaning. A complementary use of both methods will provide the Thomson scattering measurement of high Te plasmas with durable reliability and sufficient precision in the existing tokamaks and the International Thermonuclear Experimental Reactor.

CO2 laser interferometer for electron density measurement in JT‐60U tokamak

A tangential chord CO2 laser interferometer for electron density measurement in the JT‐60U tokamak is described. To compensate for a phase shift due to mirror vibrations, an IR‐HeNe laser interferometer is introduced. Probe beams of the CO2 and the IR‐HeNe lasers travel in a similar form for a distance of 100 m by utilizing relay optics. The effective phase resolution of the system is estimated to be 1/68 fringe, which corresponds to a line integrated density of 3.1×1018 m−2. An ultra‐accurate phase comparator which has a 1/104 fringe resolution is proposed to improve the capability of present and future interferometers.

Electron cyclotron emission measurements in JT-60U

An electron cyclotron emission (ECE) measurement system has been developed to obtain electron temperature with high time and spatial resolution for JT-60U plasmas. The system consists of three different diagnostic systems. These systems have different advantages and have been used complementarily. Several kinds of calibration are periodically carried out in the systems to secure reliable measurements. In order to reduce the transmission loss, a corrugated waveguide transmission line was installed and the reduction of the loss of ca. 3 dB was attained. As preliminary applications toward the operation of future fusion reactors, the feedback control of electron temperature profile and the automatic determination of the sawtooth inversion radius with an adaptive neural network were successfully demonstrated.