Sensitivity improvement of infrared imaging video bolometer for divertor plasma measurement.

The sensitivity of an infrared imaging video bolometer (IRVB) was improved for the measurement of relatively low energy plasma radiation from the viewpoint of the metal foil absorber material. The photon energy of the radiation was considered up to 1 keV for the divertor plasma measurement. The thickness of the foil absorber was evaluated not only for conventional heavy elements, e.g., platinum, but also for light elements by the relation between the photon energy and attenuation length and by mechanical strength. A heat-transfer calculation using ANSYS suggested that light elements with practical foil thickness provide a higher temperature rise of the foil absorber compared with heavier elements with practical foil thickness. The maximum of the temperature rise was evaluated using He-Ne laser irradiation onto absorber samples. The material dependence of the temperature rise has a similar tendency between calculations and experiments. Experimentally, the sensitivity of the IRVB improved from 280 to 110 µW/cm2 using titanium with 1 µm thickness compared with conventional platinum with 2.5 µm thickness. Consequently, the signal-to-noise ratio of the IRVB could be improved from 2.8 to 9.1.

Improvement in multipass Thomson scattering system comprising laser amplification system developed in GAMMA 10/PDX.

The multipass Thomson scattering (MPTS) technique is one of the most useful methods for measuring low-electron-density plasmas. The MPTS system increases Thomson scattering (TS) signal intensities by integrating all multipass (MP) signals and improving the TS time resolution by analyzing each pass signal. The fully coaxial MPTS system developed in GAMMA 10/potential-control and diverter-simulator experiments has a polarization-based configuration with image-relaying optics. The MPTS system can enhance Thomson scattered signals for improving the measurement accuracy and megahertz-order time resolution. In this study, we develop a new MPTS system comprising a laser amplification system to obtain continuous MP signals. The laser amplification system can improve degraded laser power and return an amplified laser to the MP system. We obtain continuous MP signals from the laser amplification system by improving the laser beam profile adjuster in gas scattering experiments. Moreover, we demonstrate that more MP signals and stronger amplified MP signals can be achieved via multiple laser injections to the laser amplification system in the developed MP system comprising a laser amplification system.

Measurement of axial phase difference of density fluctuations owing to spontaneously excited waves by using microwave reflectometer on GAMMA 10/PDX.

In the GAMMA 10/PDX tandem mirror, plasma with strong ion-temperature anisotropy is produced by using the ion cyclotron range of frequency waves. This anisotropy of ion temperature causes several Alfvén-Ion-Cyclotron (AIC) waves to spontaneously excite in the frequency range just below the ion cyclotron frequency. In addition, difference-frequency (DF) waves are excited in the radial inner region of the plasma by wave-wave coupling among the AIC waves. The radial density profiles were measured at multi-axial positions using a frequency-modulation reflectometer with an axial array of microwave antennas, and an axial variation of the density was found to be significant. In addition, a relative phase difference of the DF wave between axially separated two points was first obtained by finely choosing the probing frequency of the reflectometers with a maximum coherence used as a measure, indicating that the DF wave is a propagating wave, while the pump AIC waves are standing waves in the axial region of measurement.

Development of a laser amplification system for the multi-pass Thomson scattering system for GAMMA 10/PDX.

The multi-pass Thomson scattering (MPTS) system is a useful technique for increasing the Thomson scattering (TS) signal intensities and improving the TS diagnostic time resolution. The MPTS system developed in GAMMA 10/PDX has a polarization-based configuration with an image relaying system. The MPTS system has been constructed for enhancing the Thomson scattered signals for the improvement of measurement accuracy and the megahertz sampling time resolution. However, in the normal MPTS system, the MPTS signal intensities decrease with the pass number because of the damping due to the optical components. Subsequently, we have developed a new MPTS system with the laser amplification system. The laser amplification system can improve the degraded laser power after six passes in the multi-pass system to the initial laser power. For the first time worldwide, we successfully obtained the continued multi-pass signals after the laser amplification system in the gas scattering experiments.

Measurement of neutral gas pressure in the D-module of GAMMA 10/PDX by using ASDEX type fast ionization gauge.

In the divertor simulation experiments in the GAMMA 10/PDX tandem mirror, pressure of the neutral gas was investigated by using a fast ionization gauge. The gauge was absolutely calibrated for hydrogen gas by using a capacitance manometer. Change of the gauge sensitivity due to the magnetic field of GAMMA 10/PDX was also evaluated. The typical gas pressure measured in detached plasma experiments was 0.1-10 Pa. The degree of plasma detachment determined from the reduction of heat flux was enhanced as the gas pressure increases. Rapid increase of the gas pressure under the plasma flow was also observed.

Measurement of ion and electron temperatures in plasma blobs by using an improved ion sensitive probe system and statistical analysis methods.

We have measured ion temperature as well as electron temperature in plasma blobs observed in a linear plasma device by using an improved ion sensitive probe. Current-voltage characteristics of the ion sensitive probe inside and outside plasma blobs were re-constructed with a conditional sampling method. It is clearly found that both ion and electron temperatures in plasma blobs decrease more slowly in a cross-field direction than those in a bulk plasma without plasma blobs.