Effect of thermionic cathode heating current self-magnetic field on gaseous plasma generator characteristics

Abstract

The performance capabilities of the PINK, a plasma generator with a thermionic cathode mounted in the cavity of a hollow cathode, depending for its operation on a non-self-sustained low-pressure gas discharge have been investigated. It has been shown that when a single-filament tungsten cathode 2 mm in diameter is used and the peak filament current is equal to or higher than 100 A, the self-magnetic field of the filament current significantly affects the discharge current and voltage waveforms. This effect is due to changes in the time and space distributions of the emission current density from the hot cathode. When the electron mean free path is close to the characteristic dimensions of the thermionic cathode, the synthesized plasma density distribution is nonuniform and the cathode is etched nonuniformly. The cathode lifetime in this case is 8-12 h. Using a cathode consisting of several parallel-connected tungsten filaments ∼0.8 mm in diameter moderates the effect of the self-magnetic field of the filament current and nearly doubles the cathode lifetime. The use of this type of cathode together with a discharge igniting electrode reduces the minimum operating pressure in the plasma generator to about one third of that required for the generator operation with a single-filament cathode (to 0.04 Pa).

Alternative model of space-charge-limited thermionic current flow through a plasma

“…Generally, the cathode sheath will be thin relative to the gap and Finv limited to a few Temit. Although (3) and (5) suggest the cathode sheath may become large when Np << Nemit, the logarithms limit the sheath growth and the inverse mode breaks down at very low Np for a reason explained below.…”

“…Since ne(x) > Np at all x within the cathode sheath, a useful upper limit on its size Lcsh can be derived by supposing ne were flat with density Np. Solving for the parabolic j(x) with the sheath potential difference Finv and zero electric field at the sheath edge used as boundary conditions leads to, (5) Generally, the cathode sheath will be thin relative to the gap and Finv limited to a few Temit. Although (3) and ( 5) suggest the cathode sheath may become large when Np << Nemit, the logarithms limit the sheath growth and the inverse mode breaks down at very low Np for a reason explained below.…”

“…That estimate is only an upper limit since collisions make electrons spend a longer time in the anode sheath, leading to a higher ne and a thinner sheath. Putting Ncse in place of Np in (5) gives an upper bound on Lcsh.…”

“…Hot cathode sputtering by ion impacts limits the lifetime of many devices [5,58]. Sputtering is often thought to be an inescapable problem, since acceleration of ions into the cathode is inevitable under the prevailing assumption that the cathode sheath must be classical or SCL.…”

“…This is crucial for understanding the current flow, power dissipation and electrode erosion for any plasma application that relies on hot cathodes. Examples include thermionic discharges [5,6], thermionic converters [7], thermionic tethers [8], emissive probes [9], arc cutting [10], torches [11] and electron transpiration cooling [12].…”