Hollow cathode electron properties are consistent with marginally stable turbulence

The scaling of the electron Mach number in a 20 A class hollow cathode plume is characterized experimentally as a function of the local plasma properties. These local properties are inferred from measurements with an incoherent Thomson scattering diagnostic, configured to measure the axial projection of the electron velocity distribution on the cathode centerline. The time-averaged electron temperatures are found to be 1–2 eV for xenon flow rates between 1.35 and 2.25 mg/s and increase above 5 eV at a lower flow rate of 0.45 mg/s. This transition in temperature corresponds to the cathode's transition from the so-called spot mode to the plume mode. The electron Mach number is found to be between 0.2 and 0.8 for all flow rates. The scaling of the Mach number with the ratio of electron temperature to ion temperature is examined, which reveals a non-monotonic relationship that can be approximately described by the assumption of marginally stable wave growth. The possibility of leveraging this assumption as a zero-equation closure for the electron fluid equations is discussed in the context of past experiments.