I-V traces of strongly emitting emissive probes are investigated in a multidiople filament discharge. It is found that at sufficiently high neutral pressure and emitting current, the variation of the I-V traces and their associated inflection points no longer follow the previous predictions of space charge limited (SCL) models. A new, steep slope region of the I-V trace appears near the plasma potential when the probe is strongly emitting, causing the inflection point and the floating potential to increase towards the plasma potential as emission current increases, rather than staying constant. This is, to our knowledge, the first experimental evidence that the effects predicted by Campanell et al's inverse sheath theory (2017 Physics of Plasmas 24 057101) not only affect the floating potential but also a region in the I-V trace of an emissive probe. It is also found that the double inflection point structure when the probe is biased below the ionization energy of the working gas is highly likely to be an emission retardation effect from enhanced virtual cathode formation due to the increased local electron density. The implications of these findings on hot cathode sources are briefly discussed.

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.

Hot cathode discharges are common plasma sources for fundamental plasma physics studies and other applications due to their quiescent and relatively simple properties, and tungsten filaments are commonly used for the ease of heating them. Recently, tungsten filaments are increasingly being replaced by less luminous alternatives, such as barium oxide or lanthanum hexaboride. These materials can emit electrons at temperatures close to 1000 K lower than tungsten, greatly reducing their blackbody radiations. This results in significant improvement in signal recovery for active spectral diagnostic, such as laser-induced fluorescence. However, these less luminous cathodes often come in vastly more complicated designs than those of tungsten hot cathodes and are much more expensive to procure and difficult to operate. In this paper, we present a simple, low cost direct current heated design of a LaB6 cathode that is manufactured at suitable dimensions and make a comparison of the laser-induced fluorescence (LIF) signal-to-noise ratio of this LaB6 hot cathode discharge with that of a typical tungsten filament discharge, revealing that LaB6 has, indeed, an improved LIF signal-to-noise ratio compared with the tungsten filament.