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Two color laser diagnostics using terahertz laser sources are under development for a high performance operation of the Large Helical Device and for future fusion devices such as ITER. So far, we have achieved high power laser oscillation lines simultaneously oscillating at 57.2 and 47.7 μm by using a twin optically pumped CH3OD laser, and confirmed the original function, compensation of mechanical vibration, of the two color laser interferometer. In this article, application of the two color laser diagnostics to the ITER poloidal polarimeter and recent hardware developments will be described. [ABSTRACT FROM AUTHOR]

In order to measure the internal structure of density fluctuations using a microwave reflectometer, the broadband frequency tunable system, which has the ability of fast and stable hopping operation, has been improved in the Large Helical Device. Simultaneous multipoint measurement is the key issue of this development. For accurate phase measurement, the system utilizes a single sideband modulation technique. Currently, a dual channel heterodyne frequency hopping reflectometer system has been constructed and applied to the Alfvén eigenmode measurements. [ABSTRACT FROM AUTHOR]

Microwave to millimeter-wave diagnostic techniques such as interferometry, reflectometry, scattering and radiometry have been powerful tools for diagnosing magnetically confined plasmas. Recent advances in electronic devices and components together with computer technology have enabled the development of advanced diagnostics in microwave to millimeter-wave region. The two-dimensional and three-dimensional profiles of plasma density/temperature and dynamic behaviors of their fluctuations are measured by using the advanced diagnostic systems. The measurements have clarified the physics issues such as stability, wave phenomena and fluctuation-induced transport. We report here on the representative microwave diagnostics and their contribution to plasma confinement physics. [ABSTRACT FROM AUTHOR]

A four channel pulsed radar reflectometer system has been installed on the Large Helical Device (LHD). The complicated magnetic structure in LHD causes mode conversion and/or polarization rotation of the microwaves. Pulsed radar reflectometry is a suitable reflectometric technique, because it measures the delay time of the reflected wave, not the phase, and X-mode and O-mode polarized waves can be distinguished. By using X-mode operation of the pulsed radar reflectometer so that each pulse width is about 2 ns, and the repetition rate is up to 200 kHz, the critical density where the microwave is reflected is about 1 × 10[sup 16] m[sup -3]. Also it is found that the static natural island affects the X-mode reflectometric measurements. [ABSTRACT FROM AUTHOR]

We report here on the application of an ultrashort-pulse reflectometer (USPR) to Large Helical Device in National Institute for Fusion Science. An impulse with picosecond pulse width is used as a source in an USPR. Since the bandwidth of a source is inversely related to the pulse width, we can utilize the frequency range of microwave to millimeter-wave by using wide band transmission lines. The density profiles can be reconstructed by collecting time-of-flight signal of each frequency component of an impulse reflected from each cutoff layer. Remote control system using super science information network has been introduced to the present USPR system. [ABSTRACT FROM AUTHOR]

The two-color CO2/YAG laser imaging heterodyne interferometer was recently equipped with a precise digital phase counter (eight channels, 0.5 mrad resolution, 1 MHz carrier frequency and sampling rate, 500 kHz bandwidth, ±250 rad recording range, and a maximum data record of 8.4×106 samples/channel). This modification enables detailed measurements of plasma density dynamics during pellet ablation and, in addition, observations of density fluctuations. Examples of fast rising density peaks in pellet discharges as well as spatial and spectral characteristics of plasma density fluctuations obtained with the heterodyne interferometer are presented. [ABSTRACT FROM AUTHOR]

CO2 laser based phase contrast interferometer was used previously on LHD to study plasma density microfluctuations. New two-dimensional 6×8 elements array of CdHgTe photoconductors was employed for detection. In addition to commonly measured intensity, frequency spectrum, and two-point correlation two-dimensional detector array allows determining the direction of propagation of waves in plasma. From the angular distribution it is possible to obtain localized along the viewing line properties of fluctuations using strong magnetic shear in LHD. As a result, the radial profiles of density microfluctuations can be obtained with 1 μs temporal resolution determined by detector bandwidth. The described scheme was tested using ultrasonic wave in air. New imaging diagnostics has been installed on LHD and first experimental results are reported. [ABSTRACT FROM AUTHOR]

CO2 laser-based phase contrast imaging (PCI) equipped with a 6 by 8 element two-dimensional (2-D) array of photoconductors is used on the large helical device (LHD) for studying plasma density microfluctuations. Observing 2-D structures of turbulence and strong magnetic shear along the viewing line on LHD permit conversion of a 2-D-PCI image to a 2-D picture of the radial distribution of the turbulence k-spectrum. The PCI images are sampled at 1 MHz so the images or associated spectral distributions, when assembled in a movie, present the dynamic radial structure of turbulence. [ABSTRACT FROM AUTHOR]

Abstract: Phase counters, which are used with heterodyne interferometers for plasma density measurements, frequently suffer from phase jumping and cause difficulties for data interpretation. An automatic fringe jump corrector (AFJC) circuit has been developed to compensate for fringe jumps. The AFJC can correct phase jumps automatically in real-time. The AFJC, which is integrated on one chip, is installed on the presently working phase counter circuit. As for the specification of this phase counter the intermediate beat signal is 1MHz, the phase detection range is 31 fringes with phase resolution of 1/80 of a fringe and the response time of 10μs. The circuit has been tested on the far infrared (FIR) laser interferometer on LHD. The AFJC works fine to correct fringe jumps, when fringe jumps occurred due to the strong density gradient produced by the hydrogen pellet injection. [Copyright &y& Elsevier]