Page 3 - Fast-framing camera based observations ...ing two magnetized coaxial plasma guns

10E108-3
Matsumoto et al.
Rev. Sci. Instrum. 89, 10E108 (2018)
FIG. 2.
muir probe at the mid-plane of the confinement vessel. A magnetic probe array inserted from r = 0 to vessel wall, and a triple Langmuir probe was installed and scanned over the radius.
In order to measure the electron density and temperature, a triple Langmuir probe which consists of single and double Langmuir probes is used.
III. EXPERIMENTAL RESULTS AND DISCUSSION A. Camera raw images
Figures 3(a) and 3(b) show the raw color images at t=38μsandt=52μsaftermaintriggeroftheMCPG, respectively. The frame rate of these figures was 500 kHz, and the shutter speed was 1 μs. No filter was applied to these images. To decide on these camera settings, one has to con- sider the CT’s velocity inside the glass chamber. Typically, CT’s translation velocity inside the magnetic field is 70–100 km/s measured by the time-of-flight from the signals of the magnetic probes and flux loops along the z-axis.
In our experiment, the deuterium gas is used as a work- ing gas. Thus, the visible light of the camera image has a Balmer series spectrum: D↵ (656 nm), D (486 nm), and D (434 nm). Figure 3(a) shows the two CT structures, and the merging process has just started around the center of the image. As CTs collide, plasma light is strongly emitted;
therefore, we can presume that the density and temperature were increased at that point. Figure 3(b) shows the merged CT at t = 52 μs. The magnetic field lines appeared around the CT after merging. The collided CTs have fully formed into a single plasmoid, and visible light is emitted inside the core plasma region. Outside of the plasmoid has a purple- like color. It means that the outside of the plasma is a colder region.
After CTs merge, the camera image captured the equilib- rium time. The lifetime of the CT from camera images can be estimated as approximately 50 μs.
B. Camera RGB images
As one of the techniques to understand images, RGB image can be used; the color images can be split into RGB images. Atomic deuterium emits not only the Balmer lines but also the Fulcher-↵ band (580–640 nm). To esti- mate a ratio between those wavelengths, an Ocean Optics HR2000+ is also adopted/mounted to verify the spectrum of the emission in the confinement vessel. The integration time of this spectrometer is set to 1 ms. The output signal is integrated for all times of the CT’s existence along the line of sight. Figure 4 shows the result of the measurement. The principle components of the emission are the Balmer lines, D↵ and D , which are the dominant wavelengths in the confine- ment vessel. Additionally, other wavelengths, such as stainless steel derived from metal, did not show up in the confinement vessel.
For this reason, the images can be split into RGB to provide a rough estimation of plasma emission characteristics. From the above results, the red color means D↵ and the blue color means D,. Figures 5(a)–5(c) are a split image to red, green, and blue images, respectively. These images are the decomposition of Fig. 3(b). The red line and blue line can be compared as cold and hot plasma images. Therefore, Figs. 5(a) and 5(c) can be used to evaluate the plasma’s behavior. The region of the merged CT disappeared from the mid-plane of Fig. 5(a). The magnetic field lines also appeared clearly around the glass chamber. While Fig. 5(c), blue color image, captured the core plasma region and erased the outside object of the core plasma. It means that the outside of the core plasma is a cold region. The blue color region in Fig. 5(c) shows the hot
FIG.3. (a)PictureofthetwoCTsrightbeforethecolli- sion and merging at t = 38 μs. Two CTs collided around the center of the glass chamber. (b) Picture of the merged CT after CTs merged at t = 52 μs. The plasmoid shape appeared around the mid-plane of the glass chamber.
Axial view of an installation of the magnetic probe and triple Lang-