Page 2 - Fast-framing camera based observations ...ing two magnetized coaxial plasma guns

10E108-2 Matsumoto et al. Rev. Sci. Instrum. 89, 10E108 (2018)
FIG. 1. Schematic drawing of CT collision and merging experiment: two MCPGs, drift tube, glass chamber, axial magnetic field coil, and transverse magnetic field coil.
magnetic field inside. Usually, a CT is trapped inside an open magnetic field. The two CT injectors, based on a magnetized coaxial plasma gun (MCPG), are installed and connected to each end of the confinement vessel, which consists of a drift tube section and a glass chamber section in the middle.
An MCPG consists of coaxial electrodes, a bias coil, and
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The MCPG can generate a spheromak- like plasmoid which has toroidal (azimuthal) and poloidal (axial) magnetic fields. Generation of the CT takes place as follows. First, neutral gas is let in between the electrodes and a high voltage is applied between the electrodes to break- down the neutral gas. By applying high voltage, neutral gas is ionized, and current starts to flow through the generated plasma. This current flow makes a toroidal magnetic field, and the plasma is accelerated by self-Lorentz force J ⇥ B, where J is the current and B is a self-generated magnetic field due to the current. Then, the accelerated plasma inter- links with the bias magnetic field. Finally, the captured plasma generates a toroidal current and a spheromak-like magnetized plasmoid is ejected, which has toroidal and poloidal magnetic
a gas inlet system.
fields.
The radius of the glass chamber is ⇠30 cm with ⇠2.1 m
length. Axial and transverse field coils are wound on the glass
chamber to generate the confining magnetic field, as shown
in Fig. 1. The transverse field coil is a square coil, and it is
arranged so as to sandwich the glass chamber. The strength
of the axial field can be controlled by charging voltage. The
diameter of the axial field coil is 80 cm, and the magnetic
field strength on the axis is approximately 300 G with a rise
time of 840 μs. Currently, the transverse field coil has been
used for simulating the CT injection into a transverse field on
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To evaluate the merged CT’s performance, the magnetic probe array and triple Langmuir probe were installed at the mid-plane of the confinement vessel. Figure 2 shows the cross section of the mid-plane of the confinement vessel. A mag- netic probe array uses 50 chip-inductors (manufactured by Coilcraft) that consist of 2 sets of Bz and Bt probes, i.e., 25 Bz probes and 25 Bt probes in the probe array. The Bz probe measures the z component of the magnetic field (in the axial direction of the machine axis), and the Bt probe measures the azimuthal magnetic field. The spacing between each Bz and Bt pair is 2 cm; therefore, the total length of the array is 50 cm. The magnetic probe and magnetic flux loop are also wound around the glass chamber and used to estimate the separatrix radius of the merged CT, which is defined as the closed plasma surface region. A structure of the FRC is formed by an open and closed magnetic field.
C-2 and C-2U.
therefore, the confinement field angle can be adjustable for an experiment, for example, non-straight field, i.e., a field which is neither parallel nor perpendicular to the machine axis. By using the glass chamber, the global motion of the CT inside the magnetic field can be easily captured by using the fast-framing camera. To evaluate the generated CT’s performance, the drift tube has magnetic and triple Langmuir probes, interferometry based on He-Ne laser, and two photomultiplier tubes (PMTs). By using these diagnostics, we can measure typical plasma parameters such as velocity, electron temperature, and density. This CT injector developed for C-2/C-2U has achieved high energy: velocity ⇠100 km/s, electron temperature 30–40 eV,
Those two coils are independently controlled;
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The fast-framing camera is model HS-106E developed by NAC Image Technology. This camera has an ISIS (In situ Stor- age Image Sensor)–CCD image sensor and takes real color images. The number of pixels is fixed at 147,600 [(H) 360 ⇥ (V) 410] for all shutter speeds. The frame rate and shut- ter speed can be adjusted: 6–1,250,000 fps, 0.1 μs—open, respectively.
The camera was used in the experiment of CT injec- tion into the transverse magnetic field in order to cap-
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breakdown and generation of the CTs/plasmoids.
and electron density <1 ⇥ 10 B. Camera setup
m
. Deuterium gas is used for
ture/investigate the penetration depth and behavior.
camera captured the global behavior of plasmoid and insta- bilities. For improvement of the camera image, we focused on the type of lens. A Nikon lens was chosen because it has an aspherical lens: AF-S NIKKOR 50 mm f/1.8G lens. As is well known, an aspherical lens can decrease the optical path difference at the sensor. Therefore, the color image can suppress unevenness in color while secur- ing sharpness of the plasmoid’s shape. To capture the pro- cesses of CT translation and merging, the fast-framing camera was focused at the center of the glass chamber, as shown in Fig. 1.
C. Magnetic and electric probe installation
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The fast