Extinguishment of hot cathode discharges by space-charge and surface magnetic effects

Abstract

Emission characteristics of a directly heated, rod-shaped LaB6 hot cathode has been studied in a multi-dipole confinement device. It is found that contrary to predictions from the conventional space-charge limited emission theory, emission current is not simply limited to a maximum value but eventually extinguished by space-charge related effects. In addition, directional preference of electron emission towards the positive lead of the applied heating voltage can be visibly observed. Results suggest that thermionically emitted electrons trapped in the virtual cathode are pumped towards the high voltage lead of the applied heating, and that the magnetic effects for filament heating currents exceeding 50 A may be the underlying mechanism of extinguishment; that is, the gyromotion of surface emitted electrons for sufficiently great heating currents may prevent electrons from escaping the virtual cathode. Geometrical effects are also briefly discussed.

Improving discharge performance of space-charge limited hot cathodes via local neutral collisionality

“…Beyond the maximum I Dis , additional heating no longer increase I Dis , unless the cathode bias increases. Particularly with the LaB 6 cathode being 2 mm thick, increase of heating power is generally realized by a higher heating current, which eventually forms a strong enough magnetic field near the rod to prevent electrons from escaping the virtual cathode, extinguishing the discharge, as illustrated in figure 1 [15]. Note that any hot cathode can be TL or SCL depends on plasma parameters, just as emissive probes always experience a TL regime and a SCL one depending on the probe bias and the parameters of the plasma in which the probe is immersed in [23][24][25][26].…”

“…However, with hot cathode discharges, controlling the energy of the energetic primary electrons is often desirable because formation of sheath effects and metastable ions and neutrals are often related to the energetic primary electrons [9,30,31]. Increasing the neutral pressure is also known to increase SCL cathode emission [15]. This increase is due to two distinct effects.…”

“…With recent development of LaB 6 manufacturing capabilities, a simple DC heated LaB 6 hot cathode has been developed to directly replace tungsten filaments [14,15]. These rod shaped LaB 6 hot cathodes can be manufactured as thin as 2 mm, which makes them compatible with DC heating simply using copper filament holders installed onto standard electrical vacuum feedthroughs, and can be heated by a low voltage (∼5V) and high current (∼100A) DC heating power supply.…”

“…However, compared to tungsten and tantalum wires, the larger dimensions of LaB 6 cathodes remains also much larger than the plasma Debye length along any direction, making them spacecharge limited (SCL) hot cathodes [12,16,17] which limited their emission characteristics, as opposed to temperature limited (TL) hot cathode from with which emission is expected to be limited by the cathode temperature. Studies also discovered that these DC heated hot cathodes can be extinguished by the surface magnetic field created by their own heating current, often larger than 50A [15], complicating the usage of these cathodes.…”

Recent development of LIF diagnostics in ASIPP

“…Using a quiescent multi-dipole confined plasma removes most source related plasma disturbance, making it fitting for first principle demonstrations needed in the early phases of diagnostics design. The use of the LaB 6 cathode instead of tungsten filaments, although being space charge limited [49], facilitates smaller blackbody radiation, longer cathode life, and higher plasma densities. At a discharge current currently limited to 0.6 A restricted by chamber heating issues, plasmas with densities ranging from 10 8 to 10 10 cm −3 and electron temperatures in the range of 1-5 eV can be produced at an operating pressure range spanning from 10 −2 Pa to 10 −1 Pa [50].…”