Electrostatic potential measurements and point cusp theories applied to a low beta Polywell fusion device
The magnetic field structure in a Polywell device is studied to understand both the physics underlying the electron confinement properties and its estimated performance compared to other cusped devices. Analytical expressions are presented for the magnetic field in addition to expressions for the point and line cusps as a function of device parameters. It is found that at small coil spacings, it is possible for the point cusp losses to dominate over the line cusp losses, leading to longer overall electron confinement. The types of single particle trajectories that can occur are analysed in the context of the magnetic field structure which results in the ability to define two general classes of trajectories, separated by a critical flux surface. In addition, knowledge of the types of electron trajectories is used to propose a ballistic model for the single particle confinement time and is subsequently compared with simulation results.
Floating potential measurements were carried out on a prototype Teflon Polywell with a cylindrical hollow cathode electron gun. Dynamic floating potentials of up to –250 V were obtained for periods of several milliseconds, suggesting the formation of a virtual cathode stable on at least the millisecond time scale. The dependence of the floating potential on the coil current and background gas pressure was studied. The magnetic field coils were driven by a pulsed current supply and it was found that the virtual cathode could only be established within a narrow range of currents. In addition, it was shown that the magnitude of the floating potential increased with decreasing background gas pressure. It is conjectured that the depth of the virtual cathode and its lifetime are dependent on the magnitude of the injected electron current.
Finally, orbital limited motion theory was been applied to two biased probes in a low beta Polywell approximating a small scale WB6 design, referred to as WB6-mini. The cases studied include electron injection, magnetic field scaling, Polywell bias scaling, and radial position profiles. Langmuir’s original orbital limited motion results for a monoenergetic electron beam are shown to be in excellent agreement for electron injection into the Polywell. A distribution function is proposed for the electron plasma characteristics in the centre of the magnetic null and confirmed with experimental results. A translational stage was used to measure the radial plasma potential profile. In other experiments two probes were used to simultaneously measure the profiles in both the null and a position halfway along a corner cusp. The results confirm a radial potential well created by electron trapping in the device. In addition, we present preliminary results of the potential well scaling with the magnetic field, Polywell bias voltage, and the injected beam current. The electron population was found to maintain non-equilibrium in all cases studied.