Page 4 - Development of a magnetized coaxial plasma gun for compact toroid injection into the C-2 field-reversed configuration device

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field and the magnetic probe signal decreases because the glass chamber is not a flux conserver. At t = 32 μs the CT penetrates the field and then the probes on z = 20-30 cm decrease. The collimated fiber array measured also an enhanced emission of light around z = 42, 60 cm, as seen on the contour map of Fig. 6 at t = 30 and 32 μs. At these times, the magnetic probes at 40–60 cm were not decreased. We can assume that this emission is from initial plasma, which was generated by the initial high-voltage breakdown between electrodes. Due to injected plasma, the transverse magnetic field is frozen and the magnetic field increases locally. Thus the magnetic field increases at emission time before CT penetration. Af- ter CT penetration, the probe signal decreases further at t = 34 μs, and the upper magnetic probe signal changed from negative to positive (t = 38 μs) in order to be frozen by CT’s plasma.
On the other hand, if the CT is injected into the glass tube without the transverse magnetic field, it expands with Alfven speed so that it is not compressed by magnetic field.7 Therefore, the magnetic probes will measure the CT’s mag- netic field. In fact, the magnetic probes measured magnetic field. However this picked up magnetic field was smaller than fluctuation of the magnetic field when the CT is penetrated. Conversely, the axial velocity from magnetic probes was same speed in the both case.
E. Fast-framing camera
To measure the trajectory and movement of CT, we usu- ally use the magnetic probe and collimated fiber arrays. How- ever, this diagnostic system cannot monitor position clearly due to limited views, i.e., local measurement at each probe/ fiber position. Thus we needed a qualitatively di↵erent diag- nostic for global motion’s measurement. Accordingly, we adopted the fast-framing camera for trajectory measurement of CT inside the magnetic field.
The fast-framing camera used was a ULTRA Cam HS- 106E developed by NAC Image Technology. It has ISIS (In situ Storage Image Sensor)-CCD image sensor and we can take real color images. The camera performance characteristics are as follows: shutter speed, exposure time, number of frames, and pixels of this camera are 60–1 250 000 fps, 0.1 μs—open, 120 (fixed), and (H) 360 ⇥ (V) 410, respectively. The distinc- tive feature of this camera is the world’s fastest shutter speed with color. Therefore it is possible to take a movie of the CT trajectory/movement.
Figure 7 shows a time sequence of contour map of emis- sion of light inside the glass tube by the fast-framing camera. Shutter speed and exposure in this shot are 600 kHz and 1.5 μs, respectively. When the CT’s velocity is 100 km/s, the integrated length of CT along the z-axis is 15 cm. The camera is mounted such that is has a side-view of the glass tube. The CT enters from right-hand side of picture and moves to the left. The fiber array is visible on the left side on each figure. The camera image shows similar behavior as the fiber signal shown in Fig. 6 (t = 30 and 32 μs). After the initial plasma, the CT (core plasma) penetrates the transverse magnetic field at t = 36 μs and moves downward. This motion seems like Kink
FIG. 7. Time series of emission of light from side-viewing photographs for CT penetration. Shutter speed and exposure are 600 kHz and 1.5 μs, respectively. Black line shows the displacement of CT trajectory.
instability generated by the transverse magnetic field and the current that flows through the center of CT.8
III. CONCLUSION
We have constructed a test stand to measure and charac- terize CT plasma parameters and trajectory inside a transverse magnetic field region. In the drift tube, we measured the CT velocity and density. In the glass tube, we measured the speed and the trajectory of CT using the fluctuation of magnetic field along the x-axis and a collimated fiber array. The CT trajectory was also monitored by the magnetic probe arrays, fiber arrays, and side-on fast-framing camera. The CT velocity of ⇠100 km/s was not decelerated even with a strong transverse magnetic field of ⇠1 kG, which confirms that the current CT injector system is su cient to penetrate into C-2U external magnetic field to reach the core FRC plasma region for particle refueling.
ACKNOWLEDGMENTS
We thank our shareholders for their support and trust, and all fellow TAE sta↵ for their dedication, excellent work, and extra e↵orts. This work is supported in part by the MOU as a part of research cooperation between the University of California at Irvine (School of Physical Science, Department of Physics and Astronomy) and Nihon University (College of Science and Technology, Department of Physics).
1T. Matsumoto, J. Sekiguchi, T. Asai, H. Gota, E. Garate, I. Allfrey, T. Valentine, M. Morehouse, T. Roche, J. Kinley et al., Rev. Sci. Instrum. 87,
2053512 (2016).
M. Binderbauer, T. Tajima, L. Steinhauer, E. Garate, M. Tuszewski, L. Schmitz, H. Guo, A. Smirnov, H. Gota, D. Barnes et al., Phys. Plasmas 22, 056110 (2015).
3R. Raman, F. Martin, B. Quirion, M. St-Onge, J. Lachmbre, D. Michaud, B. Sawatzky, J. Thomas, A. Hirose, D. Hwang et al., Phys. Rev. Lett. 73, 3101
4(1994).
5C. Xiao, A. Hirose, and S. Sen, Phys. Plasmas 11, 4041 (2004).
T. Ogawa, N. Fukumoto, M. Nagata, H. Ogawa, M. Maeno, K. Hasegawa, T.
Shibata, T. Uyama, J. Miyazaki, S. Kasai et al., Nucl. Fusion 39, 1911 (1999). 6M. Nagata, N. Fukumoto, H. Ogawa, T. Ogawa, K. Uehara, H. Niimi, T. Shi- bata, Y. Suzuki, Y. Miura, N. Kayukawa et al., Nucl. Fusion 41, 1687–1694
7(2001).
J. Yee and P. M. Bellan, Phys. Plasmas 7, 3625 (2000).
8A. Moser and P. Bellan, Nature 482, 379–381 (2012).