Langmuir probe diagnostics in a multi-dipole confined hot cathode discharge has been revisited with an improved sheath expansion model with fitted expansion parameters. The results show a three-Maxwellian EEDF composition also observed in previous studies using a very similar system. The dependence on neutral pressure and discharge current of each electron species are consistent with known physical processes in a multi-dipole confined hot cathode discharge. This shows that the employment of the new ion saturation fitting method does not eliminate the contribution of the high energy degraded primaries to the Langmuir probe I-V trace, as sheath expansion and high energy electron current contributes differently on an I-V trace. The result remains supportive to the existence of a very hot (Tdp >15eV) degraded primary electron species separate from the typical hot electron (Thot ~ 3eV) species. Thus it is recommended to sweep the Langmuir probe beyond the bias voltage of the hot cathode, whenever possible, in order to properly resolve the contribution to the I-V trace of the degraded primaries.

A recent theory suggest that electrons enter electron sheaths at an electron Bohm velocity given by (Te/me)1/2 instead of the electron thermal velocity as conventionally assumed. To test this theory, the flux density ratio Γe,se/Γi,se of electrons and ions entering their respective sheaths was directly measured via an almost continuous Ai/Ae area ratio scanning. The measured value agrees with the predictions assuming electrons entering the electron sheaths at their thermal velocity. The predictions associated with the electron Bohm criterion has not been found. If the predictions of such theories are true, the electron or ion presheath density drops will be very different from conventionally expected values.

The effects of neutral gas inlet location to the performance of SCL hot cathodes is investigated. It is found that placing the neutral gas inlet very close (~5mm) to the hot cathode can significantly improve the emission characteristics via immersing the hot cathode in a volume of elevated neutral density. Such placement of the neutral gas inlet was found to make no observable effect on global plasma parameters when the discharge conditions are otherwise identical, facilitating the implementation of this measure onto existing plasma devices. Even with only a small part of the hot cathode is immersed in a volume of an elevated neutral density, the improvement of electron emission is found to be very significant. Thus, one can envision a design of a hot cathode assembly with an integrated neutral gas inlet to significantly improve its emission performance. The underlying mechanism of this improvement of hot cathode performance is the reduction of SCL effects, thus applicable to SCL hot cathodes regardless of heating configurations. This will be particularly useful in low pressure applications where a widened range of operable plasma parameters are often desirable.

Hot cathode discharges are common plasma sources for fundamental plasma physics studies and other applications due to their capability to produce quiescent plasma. This work presents experimental observations of presheath-associated non-thermal metastable ion velocity distributions in a miniaturized multi-dipole confined plasma device measured by laser-induced fluorescence. The intensity of this non-Maxwellian component is related to the collisions of these metastable ions with background particles. Additionally, the flow velocity of this component is lower than the Bohm velocity; thus, its energy is lower than the presheath potential drop kTe/2. This implies that these non-thermal metastable ion velocity distributions are formed via presheath acceleration and are associated with the source asymmetries of the miniaturized device. The strength decreases as the neutral pressure increases once the neutral pressure is adjusted, suggesting that the presheath length is the critical condition that determines whether these components can be observed.