According to my understanding the
thermionic emission will occur regardless of how the material is heated - AC - DC - Solar or any means you choose provided the material has reached an emissive state - which usually coincides with exceeding the
Draper Point.
The problem with nonelectrical emission is that it leaves the filament charged and theoretically would result in a condition where the charged filament would be devoid of any new carriers and current would cease to flow. But when supplied with new carriers via an electrical connection the filament can allow current flow across a vacuum for many years. Such is exactly the case in CRT type Televisions and monitors where the screen is the HV Anode and the filaments are called the Electron Guns (where generally there are three - one each for Red, Green and Blue in color sets and only one in single trace sets like our analog oscilloscopes or black and white TVs and monitors).
Grids exist in the vacuum path near the Guns to affect the flow rate. By altering the voltage on the grid we alter the intensity of the electron beam that passes through the grid and on to the Anode. The more intense the beam, the brighter the dot produced by the phosphor coating on the screen.
Is it possible to encourage electrons to emerge from a vacuum to keep the filament full of new carriers? I think it is. But my hypothesis would need to be proven and following is a description of the apparatus setup to accomplish that goal. It is based on the theory that metals only have a finite quantity of free electrons and once they are siphoned off no new carriers can be produced within the material.
Construct a small generator with a magnetic coupling on the shaft to drive the generator. The generator must supply enough energy to power the filament under test. Encase the generator in an ampule with a vacuum so no external charges can interfere with its operation directly (hence the magnetic coupling). Attach the generator to the filament (Gun) of a CRT and seal the connections so no external charges may be introduced to the connections. Apply the necessary HV to the anode of the CRT.
Now drive the generator with an external mover that is coupled magnetically. The system is a closed system electronically. The generator cannot provide any more electrons than are available in the metal materials. Therefore, as the electrons are siphoned off to the anode one of two conditions will emerge in the test. Either the thermionic filament will become positively charged and no new electrons will be found so that current stops flowing, or nature will balance the deficit by extracting new electrons from the vacuum via the magnetic field within the generator and the CRT will continue to function.
The Kick:
When an electrical current flows - regardless of the medium, vacuum or otherwise - a magnetic field exists around that current. Each carrier has it's own field, but if there is a sufficient line of carriers in close proximity, the fields will merge into a single field. Such is the case within tubes. Just as a
straight wire has inductance so the path of the vacuum also will exhibit some value of inductance. When a sizable current is flowing in the vacuum and it is abruptly stopped, the collapsing magnetic field will 'Kick' to keep it going and not necessarily at the same consistent flow rate. Instead it could come crashing in as a major surge of current at the end of the cycle - depending of course on the amount of electrons available and the energy density of the field.
If charges can be extracted from the vacuum by deep, sharp magnetic spikes, then perhaps the 'Kick' is doing just that - maybe.