D. C. Seo

Development of imaging bolometers for magnetic fusion reactors (invited).

Imaging bolometers utilize an infrared (IR) video camera to measure the change in temperature of a thin foil exposed to the plasma radiation, thereby avoiding the risks of conventional resistive bolometers related to electric cabling and vacuum feedthroughs in a reactor environment. A prototype of the IR imaging video bolometer (IRVB) has been installed and operated on the JT-60U tokamak demonstrating its applicability to a reactor environment and its ability to provide two-dimensional measurements of the radiation emissivity in a poloidal cross section. In this paper we review this development and present the first results of an upgraded version of this IRVB on JT-60U. This upgrade utilizes a state-of-the-art IR camera (FLIR/Indigo Phoenix-InSb) (3-5 microm, 256 x 360 pixels, 345 Hz, 11 mK) mounted in a neutron/gamma/magnetic shield behind a 3.6 m IR periscope consisting of CaF(2) optics and an aluminum mirror. The IRVB foil is 7 cm x 9 cm x 5 microm tantalum. A noise equivalent power density of 300 microW/cm(2) is achieved with 40 x 24 channels and a time response of 10 ms or 23 microW/cm(2) for 16 x 12 channels and a time response of 33 ms, which is 30 times better than the previous version of the IRVB on JT-60U.

Detailed in situ laser calibration of the infrared imaging video bolometer for the JT-60U tokamak

The infrared imaging video bolometer (IRVB) in JT-60U includes a single graphite-coated gold foil with an effective area of 9 × 7 cm 2 and a thickness of 2.5 μ m . The thermal images of the foil resulting from the plasma radiation are provided by an IR camera. The calibration technique of the IRVB gives confidence in the absolute levels of the measured values of the plasma radiation. The in situ calibration is carried out in order to obtain local foil properties such as the thermal diffusivity κ and the product of the thermal conductivity k and the thickness t f of the foil. These quantities are necessary for solving the two-dimensional heat diffusion equation of the foil which is used in the experiments. These parameters are determined by comparing the measured temperature profiles (for k t f ) and their decays (for κ ) with the corresponding results of a finite element model using the measured HeNe laser power profile as a known radiation power source. The infrared camera (Indigo/Omega) is calibrated by...

Charge exchange spectroscopy system calibration for ion temperature measurement in KSTAR.

The charge exchange spectroscopy (CES) system including collection assemblies, lens design, and cassettes for the KSTAR experiment was installed to obtain profiles of the ion temperature and the toroidal rotation velocity from charge exchange emission between plasma ions and beam neutrals near the plasma axis by using a modulated neutral beam and a background system. We can measure the charge exchange spectra of an impurity line such as the 529 nm line of carbon VI to get ion temperature and rotation profiles in KSTAR. The CES and background systems will have absolute intensity and spectral calibrations using a calibrated source and various spectral lamps. The calibration was done inside the tokamak after all CES systems are installed and the optical systems are slid into the cassettes. This requires that the diagnostic systems are installed near the vacuum vessel inside the cryostat maintaining the superconducting state of the superconducting coils. Repeated spectral calibrations of the spectrometer and charge coupled device for CES will be made in the diagnostic room during the experimental campaign. We show a detailed description of the KSTAR CES system, how to calibrate, and the results of calibration.

In-vessel visible inspection system on KSTAR.

To monitor the global formation of the initial plasma and damage to the internal structures of the vacuum vessel, an in-vessel visible inspection system has been installed and operated on the Korean superconducting tokamak advanced research (KSTAR) device. It consists of four inspection illuminators and two visible/H-alpha TV cameras. Each illuminator uses four 150 W metal-halide lamps with separate lamp controllers, and programmable progressive scan charge-coupled device cameras with 1004 x 1004 resolution at 48 framess and a resolution of 640 x 480 at 210 framess are used to capture images. In order to provide vessel inspection capability under any operation condition, the lamps and cameras are fully controlled from the main control room and protected by shutters from deposits during plasma operation. In this paper, we describe the design and operation results of the visible inspection system with the images of the KSTAR Ohmic discharges during the first plasma campaign.

Installation of a fast framing visible camera on KSTAR.

Visible camera technologies have made remarkable progress in recent years, and the fast camera has proven itself to be a capable imaging diagnostic in studies of specific fusion plasma issues such as the start-up physics, plasma wall interactions, edge-localized modes, and disruptions. For the purpose of favorable visible imaging, a fast framing camera has recently been installed on the Korea Superconducting Tokamak Advanced Research (KSTAR) device. The camera uses a complementary metal-oxide semiconductor detector with a maximum resolution of 1280x1024 at 1000 frames/s (fps) and a minimum resolution of 1280x16 at 64 kfps. A 2-m-long viewport having a novel optical rail system was installed on a tangential port to view the tokamak interior. The system is fully controlled from the main control room and protected by a shutter from deposits. To verify that the camera electronics are safe from the high magnetic field and its rapid time variation, possible influences are considered theoretically and experimentally. In this work, we present the design and installation of the fast camera system on the KSTAR device with discussions on the field variation effect issues.

Diagnostics for first plasma and development plan on KSTAR

The first plasma with target values of the plasma current and the pulse duration was finally achieved on June 13, 2008 in the Korea Superconducting Tokamak Advanced Research (KSTAR). The diagnostic systems played an important role in achieving successful first plasma operation for the KSTAR tokamak. The employed plasma diagnostic systems for the KSTAR first plasma including the magnetic diagnostics, millimeter-wave interferometer, inspection illuminator, H(alpha), visible spectrometer, filterscope, and electron cyclotron emission (ECE) radiometer have provided the main plasma parameters, which are essential for the plasma generation, control, and physics understanding. Improvements to the first diagnostic systems and additional diagnostics including an x-ray imaging crystal spectrometer, reflectometer, ECE radiometer, resistive bolometer, and soft x-ray array are scheduled to be added for the next KSTAR experimental campaign in 2009.

Design and fabrication of a multi-purpose soft x-ray array diagnostic system for KSTAR

A multi-purpose soft x-ray array diagnostic system was developed for measuring two-dimensional x-ray emissivity profile, electron temperature, Ar impurity transport, and total radiation power. A remotely controlled filter wheel was designed with three different choices of filters. The electron temperature profile can be determined from the ratio of two channels having different thickness of Be layer, and the Ar impurity concentration transport can be determined from a pair of Ar Ross filters (CaF(2) and NaCl thin films). Without any filters, this diagnostic system can also be used as a bolometer.

Improved calibration technique of the infrared imaging bolometer using ultraviolet light-emitting diodes

The technique used until recently utilizing the Ne-He laser for imaging bolometer foils calibration [B. J. Peterson et al., J. Plasma Fusion Res. 2, S1018 (2007)] has showed several issues. The method was based on irradiation of 1 cm spaced set of points on a foil by the laser beam moved by set of mirrors. Issues were the nonuniformity of laser power due to the vacuum window transmission nonuniformity and high reflection coefficient for the laser. Also, due to the limited infrared (IR) window size, it was very time consuming. The new methodology uses a compact ultraviolet (uv) light-emitting diodes installed inside the vacuum chamber in a fixed position and the foil itself will be moved in the XY directions by two vacuum feedthroughs. These will help to avoid the above mentioned issues due to lack of a vacuum window, fixed emitters, higher uv power absorption, and a fixed IR camera position.In this paper a novel hysteresis current controller for APF(active power filter) with low switching losses is suggested. The controller adjusts the switching frequency with the output current of APF to reduce the switching losses effectively with the same control precision, utilizing the characteristic of APF that the sum of absolute value of three-phase output current of APF would greatly fluctuate in a cycle because it is mainly composed of harmonics currents. The related concept and formulation are presented. Computer simulation is conducted on a testing system with PSCAD/EMTDC program and the results show that the controller can effectively reduce the switching losses with the same current tracking error.

Bolometer Diagnostics on LHD

Abstract Bolometers are a powerful tool for diagnosing plasma radiation in a reactor-relevant environment. Resistive and imaging bolometers have been applied to the Large Helical Device (LHD) to measure radiative phenomena. Installed on LHD are 56 channels of resistive bolometers at four different ports, providing total radiated power measurements and radial profiles with 5-ms temporal resolution. Calibration coefficients are seen to vary slightly year to year. Imaging bolometer foils are installed at four ports. Infrared cameras have been used at some of these ports to provide an image of the foil temperature, which can be analyzed to give an image of the radiated power absorbed by the foil. Upgrades of existing imaging bolometers using platinum foils and more advanced infrared cameras with frame rates of 345 and 420 frames/s (minimum time resolutions of 3 and 2.5 s, respectively) are introduced. Variations of the thermal parameters on thin platinum (2.5-μm) foils are measured in a calibration experiment. The thermal properties of the foil can be quantified experimentally by measuring the responses of the foil temperature in the form of the peak change in temperature and thermal time (average of thermal decay and rise times) to a chopped HeNe laser. These measurements are made in 1-cm increments moving in two dimensions across the foil or at 63 separate locations. The imaging bolometers are intended to give images of complex three-dimensional radiative phenomena and ultimately provide the data for one-, two-, and three-dimensional tomographic inversions.

Descriptions of a linear device developed for research on advanced plasma imaging and dynamics

The research on advanced plasma imaging and dynamics (RAPID) device is a newly developed linear electron cyclotron resonance (ECR) plasma device. It has a variety of axial magnetic field profiles provided by eight water-cooled magnetic coils and two dc power supplies. The positions of the magnetic coils are freely adjustable along the axial direction and the power supplies can be operated with many combinations of electrical wiring to the coils. A 6 kW 2.45 GHz magnetron is used to produce steady-state ECR plasmas with central magnetic fields of 875 and/or 437.5 G (second harmonic). The cylindrical stainless steel vacuum chamber is 300 mm in diameter and 750 mm in length and has eight radial and ten axial ports including 6-in. and 8-in. viewing windows for heating and diagnostics. Experimental observation of ECR plasma heating has been recently carried out during the initial plasma operation. The main diagnostic systems including a 94 GHz heterodyne interferometer, a high-resolution 25 channel one-dimensional array spectrometer, a single channel survey spectrometer, and an electric probe have been also prepared. The RAPID device is a flexible simulator for the understanding of tokamak edge plasma physics and new diagnostic system development. In this work, we describe the RAPID device and initial operation results.