Page 2 - Plasma and Fusion Research,ISSN 1880-6821

Plasma and Fusion Research: Regular Articles
Volume 13, 3405062 (2018)
the MCPG can break down reproducibly under even lower gas pressure; the PI system can reduce the amount of neu- tral gas and generate somewhat hotter CTs without cooling target plasmas by the interaction with retained neutral gas.
In this paper, we report the design of these new tech- niques and the effects of iron-core bias coil and the PI sys- tem. The combination of these systems can fulfill require- ments for repetitive CT injection and expands the operating range of the CT injector.
2. Experimental Apparatus
Figure 1 shows a schematic drawing of the developed MCPG. It consists of a set of coaxial cylindrical electrodes, iron-core bias coil, gas injection ports, and a miniature gun [4] as a PI system. The outer diameter of the in- ner electrode is φ 54.0 mm, and the inner diameter of the outer electrode is φ 83.1 mm. The inner electrode is coated with tungsten to reduce the impurity influx. Energy stor- age for the main discharge is a capacitor bank of 125 μF with 9.5 kV charging voltage; the inner electrode is neg- atively biased. The peak value of gun current is approxi- mately 150 kA, its rise time is ∼ 10 μs, and the half width is ∼ 15 μs.
To evaluate CT performance, the MCPG is mounted onto the drift tube, which has the following diagnostics; collimated fibers, magnetic probes, triple Langmuir probe, and spectrometer. The CT velocity is estimated using time-of-flight (TOF) method from the signals of collimated fibers; the distance between fibers is 9.5cm. The axial magnetic field Bz and azimuthal magnetic field Bt are mea- sured by the magnetic probes located at just inside of the drift-tube inner wall. The electron density and temperature on the edge of the CT is measured by the triple Langmuir probe located by the inner wall as well. The ion tempera- ture is estimated from Doppler broadening of the divalent carbon (C III) line at 464.7 nm using the spectrometer.
2.1 Iron-core bias coil
An iron-core bias coil has been developed to gener- ate a stationary bias field. The solenoid coil is wound on an acrylic tube (φ 31.8 mm O.D.), which has 1000 turns across two layers and the coil length of 565 mm. It is cov- ered by another acrylic tube (φ 44.5 mm O.D.) to be iso- lated from the inner electrode. The iron-core (φ 24.5 mm, magnetic permeability: 5000) made of cast iron is inserted into the solenoid coil. A complete set of iron-core bias coil is inserted inside the inner electrode. The solenoid coil and iron-core are also movable independently in the axial direction. By inserting iron-core, an equivalent magnetic field is generated with a much smaller current; even a cur- rent of a few amperes produces the required bias field of a few milli-Teslas. The designed bias coil generates approx- imately 0.25 mWb of magnetic flux at 1 A of bias current. As mentioned above, DC power supply is employed for the bias coil so that the discharge system can be more compact
Fig. 1
Schematic drawing of the developed MCPG.
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Fig. 2
Axial distribution of radial magnetic field Br measured by a hall probe at the middle of the gap between electrodes.
and created easily.
Figure 2 shows the axial distribution of the radial mag-
netic field Br measured by a hall probe. The edge of the bias coil is fixed at 10.2 cm in z-axis, and only the iron-core is varied from 5.08 cm to 30.5 cm. The maximum Br is dif- ferent in both cases. By making a short length between the end of iron-core and coil, Br is increased compared to the long iron-core case. On the other hand, the distribution of Br is spread over a spatially wide range by the iron- core length protruding from the edge of the bias coil, and this result is similar to a calculation result by COMSOL Multiphysics modeling software [5]. From this result, the bias magnetic field can be easily controlled by changing the iron-core length and coil position.
2.2 Pre-Ionization system (miniature gun)
A miniature gun [4] has been developed as a new PI system to reduce excess neutral gas. Figure 3 shows the diagram of the experimental setup of the miniature gun with the discharge circuit. It has coaxial cylindrical elec- trodes and bias coil. The outer diameter of the inner elec- trode is φ 8.0 mm, and the inner diameter of the outer elec-