K. Narihara

Design and performance of the Thomson scattering diagnostic on LHD

This article describes the design and performance of a multi-point (200) high repetition rate (4×50 Hz) Thomson scattering diagnostic installed on the Large Helical Device. A unique feature of this system is its oblique back scattering configuration, which enables us to observe the entire plasma region along a major radius on the midplane under a severely restricted port constraint. High throughput collection optics using a mosaic mirror of 1.5 m×1.8 m area yield high quality data even with 0.5 J pulse energy delivered from 50 Hz repetition rate Nd: yttrium–aluminum–garnet lasers. High repetition and high spatial resolution (2–4 cm) of the system enable us to study island evolution in the plasma.

Obliquely backscattered Thomson scattering system on the compact helical system

A Thomson scattering system with an obliquely backscattered configuration with a novel, high throughput collection system was developed and tested on a plasma confinement device, the compact helical system. It can yield the full profiles of electron temperature and density along a major radius with the spatial resolution ranging from 1.5 to 4 cm at a repetition rate of up to 250 Hz.

Active control of laser beam direction for LHD YAG Thomson scattering

We have developed a YAG Thomson scattering (TS) system for the measurements of electron temperature and density profiles on the large helical device (LHD). The LHD-TS has four YAG lasers, and flexible operational modes are possible by using them. For example, (1) high-energy mode: The pulse energy can be increased up to four times by firing the four lasers simultaneously. In this mode, the data quality can be improved for low-density plasmas. (2) High repetition mode: When firing the lasers at intervals of 5 ms, the lasers work as a 200 Hz laser. The laser beams are guided to the LHD by seven steering mirrors. The first mirror is real-time feedback controlled for precise beam transport. The beam pointing stability is improved successfully from 200 μrad to below 4 μrad with the feedback-control system. We describe the details of the laser system for the LHD-TS.

Extension of the measurable temperature range of the LHD Thomson scattering systema)

The large helical device Thomson scattering system was designed for the target electron temperature (T(e)) range, T(e) = 50 eV-10 keV. Above 10 keV, the experimental error becomes rapidly worse. In order to obtain reliable T(e) data in the temperature range above 10 keV, we are planning to extend the measurable T(e) range by following two methods. First we have installed one more wavelength channel that observes shorter wavelength region in polychromators. Next applying forward scattering configuration is another candidate. We estimate the data quality when the two methods are used. Both of the two methods are expected to improve T(e) data quality at T(e) ≥ 10 keV.

Improvements of data quality of the LHD Thomson scattering diagnostics in high-temperature plasma experiments

In Large Helical Device (LHD) experiments, an electron temperature (T(e)) more than 15 keV has been observed by the yttrium-aluminum-garnet (YAG) laser Thomson scattering diagnostic. Since the LHD Thomson scattering system has been optimized for the temperature region, 50u2002eV≤T(e)≤10u2002keV, the data quality becomes worse in the higher T(e) region exceeding 10 keV. In order to accurately determine T(e) in the LHD high-T(e) experiments, we tried to increase the laser pulse energy by simultaneously firing three lasers. The technique enables us to decrease the uncertainties in the measured T(e). Another signal accumulation method was also tested. In addition, we estimated the influence of high-energy electrons on T(e) obtained by the LHD Thomson scattering system.

Radial electron temperature measurements by using newly installed Thomson scattering system in GAMMA 10

An yttrium-aluminium-garnet (YAG) Thomson scattering (TS) system was constructed and applied to the tandem mirror GAMMA 10 device to measure the electron temperature and density. A large solid-angle TS light-collection system was achieved by use of a spherical mirror system and large numerical aperture of bundled optical fiber. A five-channel polychromator with avalanche silicon photo diodes was used. Calibration experiments for TS optical system were performed by Rayleigh and Raman scatterings. An electron temperature increases from 0.04 keV to 0.09 keV was observed with application of electron cyclotron heating (ECH) in the plug/barrier (P/B-) cells. We successfully obtained the radial electron temperature profiles without and with P/B-ECH.

Current status of the LHD Thomson scattering system

The large helical device (LHD) Thomson scattering system measures electron temperature (Te) and density (ne) profiles of LHD plasmas, along the LHD major radius (R). The total length of plasma measured is 3 m (R = 2.325?5.386 m), the number of observation points is 144, and the spatial resolution is 12?25 mm. The sampling frequency is 10?100 msec (10?100 Hz). The measurable temperature and density ranges have been estimated to be 5 eV?20 keV and 1018?1022 m?3, respectively. The LHD Thomson scattering system consists of several subsystems, yttrium-aluminum-garnet (YAG) lasers, light collection optics, polychromators, and data acquisition system. In usual plasma experiments, we use three types of YAG lasers: 2 J/10 Hz, 1.6 J/30 Hz, and newly developed 1.2/50 Hz YAG lasers. Thomson scattering signals are analyzed with the FASTBUS-based data acquisition system. Recently, a hardware technique and three data analysis methods have been tested to improve data quality. By using these methods, the data quality has been increased by more than an order of magnitude in high-Te, low-ne plasma experiments. In the paper, we describe the current status of the LHD Thomson scattering system.

Tangential Thomson scattering diagnostic for the KSTAR tokamak

A Thomson scattering diagnostic system has been developed for electron density and temperature measurements in KSTAR. The KSTAR Thomson scattering diagnostic system has a 90 degree scattering configuration with the tangential laser-beam input optics and the horizontal collection optics. In the KSTAR 4th campaign, measuring spatial points of the Thomson scattering system was 5 ea for core with 120 mm, 60 mm spacing and 12 ea for edge with 20 mm, 10 mm spacing, respectively. For KSTAR Thomson scattering system, we used the commercial 10 Hz, 2 J, 1064 nm Nd:YAG laser that was installed through the Korea-Japan collaboration. To get the Thomson scattering spectrum, we equipped the core and edge polychromators. And the edge polychromators were developed by Korea-Japan collaboration and manufactured by NIFS, Japan. The measurable range of core polychromator was 500 eV to 20 KeV and edge was 10 eV to 1.8 KeV. To evaluate the electron density and electron temperature, we measure the Rayleigh scattering signals by using polychromators 1064 nm filter channels and relative calibration by using a tungsten (W) lamp with monochromator system. The measurement result of Rayleigh scattering signal with nitrogen (N2) gas was clearly proportional to the nitrogen density rate. We use the QDC(Charge-to-Digital Conversion) system with signal amplifier ( × 4) to get Thomson scattering signal. In this paper, we report the first result of electron temperature and density by using the tangential Thomson scattering system on KSTAR 4th campaign.

Design, construction, and performance of a composite mirror for collecting Thomson scattered light from the large helical device plasma

A 1.5u2002m×1.8u2002m rectangular composite mirror composed of 138 segment hexagonal spherical mirrors was constructed for collecting Thomson scattered light from the Large Helical Device plasma. The hexagonal mirrors with side length of 87u2002mm were patched on the surface of a framework made of glass fiber reinforced plastic. The position and angular orientation of each mirror are adjusted with three pairs of push-and-pull screws attached to the back plane of the mirrors so that the image of a tiny light source (0.1u2002mm in diameter) formed by each segment mirror be minimized and coincide with each other on a charge coupled device plate. The optical quality and its long-term stability of the assembled mirror have been monitored and sufficient for the present purpose.