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

11D406-2 Matsumoto et al. Rev. Sci. Instrum. 87, 11D406 (2016)
FIG. 1. Schematic view of the test stand with MCPG, drift tube, and glass tube region.
current typically reaches ⇠200 kA on every shot. The gun voltage is measured by a Pearson current probe placed around a 4.5 k⌦ chain-resister across the electrodes.
B. Typical CT parameters
In order to properly penetrate the CT into the transverse field, the velocity and density of CT/plasmoid are the most important parameters. Kinetic energy density Ek is used as the CT’s energy to evaluate the field penetration. This en- ergy density must be higher than the magnetic energy density produced by the transverse field Em, i.e., 1/2⇢v2 1/2B2/μ0, where ⇢, v, and B are mass density of CT, CT velocity, and magnetic field, respectively. We use the drift-tube region to measure/estimate the typical CT parameters before entering to the transverse magnetic field. The radius of this drift tube is 5 cm, which is almost the same size as the MCPG outer electrode, as illustrated in Fig. 1. For diagnostic purpose there are 6 ports attached on the tube at 3 di↵erent axial locations with 2 opposing ports; the distance between the diagnostic ports is 26.7 cm (⇠10.5 in.).
As shown in Fig. 1, there is a magnetic probe, two colli- mated fibers, a dispersion interferometer, and a triple Lang- muir probe mounted on the drift-tube to measure and char- acterize CT performance. The magnetic probe and the first collimated fiber are installed on opposite sides of the first port from the MCPG side. The purpose of this magnetic probe is to estimate the trapped flux in the CT as well as to assess the CT’s timing and length by picking up the magnetic field. The collimated fibers measure radial optimal emission which is mainly D↵ and Bremsstrahlung. The CT is accelerated by the Lorentz-force before leaving the injector. So, we deter- mine the CT’s velocity, after the acceleration phase, by the di↵erence in arrival time of the peaks of the optical and mag- netic measurements. The dispersion interferometer measures the line-integrated electron density on the second port. The triple probe is located on the third port and can measure local electron density and temperature. Figure 2 shows the typical waveforms of (a) gun current and voltage between electrodes, (b) magnetic probe signal, and (c) and (d) signals from colli- mated fibers. In case of Fig. 2, the centroid of the CT is around the peak of the magnetic probe signal. Then this is used for the evaluation of the velocity as the first timing. From those measurements, the CT was ejected at around 15 μs, and the CT length can be estimated using the CT’s velocity and the magnetic probe’s pulse width. Typical CT parameters are as follows: average velocity ⇠100 km/s, average electron density ⇠1.9 ⇥ 1015 cm 3, electron temperature 30–40 eV, length of the CT (⇡v · t)⇠50 cm, and mass of the CT⇠12 μg.
C. Evaluation of CT energy
To penetrate the transverse magnetic field, the energy density of CT must be higher than 4 kJ/m3, which is the C-2U external magnetic field energy density. The mass density and velocity are estimated from dispersion interferometry and fiber array, respectively. To vary the CT velocity, we control the gas pressure, gas pu↵ duration, and gas pu↵ timing. Figure 3 shows the relationship between electron density, kinetic energy, and velocity. In the case of slower velocity, gas volume is higher than faster speed case. The CT’s kinetic energy density is over the required energy so that CT can penetrate to transverse magnetic field to fuel the FRC.
FIG. 2. Time evolutions of the typical waveforms on the MCPG and drift tube: (a) gun current through the inner electrode and voltage between the electrodes; (b) magnetic field measurement at the wall inside vacuum; (c) and (d) the first and the second collimated fiber signals, respectively. Filled region indicates the CT signal, which means the CT length.