Triple Langmuir Probes Measurements of LaB6 Hollow Cathodes Plume

Abstract

Lanthanum hexaboride hollow cathodes represent a viable option for high power Hall effect thruster applications, under development for the next generation of manned and robotic interplanetary missions. In this scenario, SITAEL and the University of Pisa are actively developing high current hollow cathodes capable of providing discharge current in the range 10-100 A to be coupled with high power Hall effect thrusters. The cathode design is based on an in-house theoretical model of the internal sections of the cathode, recently integrated with a simplified model of the cathode plume. Despite the application of hollow cathodes on flight and laboratory model Hall effect thrusters, many questions remain unsolved. In particular, issues related to onset of instabilities, due to plume mode or ion acoustic turbulence, are still unclear, while it is known that they can affect the overall performance of the cathode and thruster unit. This paper focuses on the experimental investigation of the cathode plume by means of measurements of the main plasma parameters, at different operating conditions and for different cathode geometry. Two cathodes were investigated, namely HC20 and HC60, designed to be coupled with SITAEL's HT5k and HT20k (5 kW-and 20 kW-class) Hall effect thrusters. The cathodes were mounted in stand-alone configuration with an auxiliary cylindrical anode. The experimental campaign was performed using triple Langmuir probes as plasma diagnostic system. The probes were mounted on scanning mechanisms to measure the plume parameters at various radial and axial distances from the keeper exit. General trends of electron temperature, plasma potential and plasma density are reported in terms of discharge current, mass flow rate and cathode orifice geometry. The results highlight that the cathode plate orifice selection affects the plume mode onset, giving the possibility to extend the stable mode of cathode operation in the current range required by the thruster.

Parametrization of current–voltage characteristics and operation domains of cylindrical emissive probes in collisionless Maxwellian plasmas at rest

“…For instance, calling ϕ p the normalized probeto-unperturbed plasma bias, the law (a + bϕ p ) c and similar ones were proposed for the current. These fitting laws have been used in many works to interpret experimental measurements from LPs (see for instance [31,33,34] and references therein). A multivariate regression algorithm has been also combined with the theory to interpret the measurements of spherical LPs [35].…”

Resistive MHD modes in hollow cathodes external plasma

“…The temporal fluctuations in the near plume of the hollow cathode were experimentally investigated to show the presence of ion-acoustic turbulence in the plume region [29][30][31][32][33], and its link to the enhancement of keeper wear [34] due to energetic ions bombardment. In addition, dedicated investigations of plasma instabilities and their effect on 25 A cathode size behavior have been performed by research teams [35][36][37][38][39] at other locations.…”

A Review of Low-Power Electric Propulsion Research at the Space Propulsion Centre Singapore

“…Other invasive plasma diagnostics developed at PSAC/SPC include cylindrical Langmuir probes [26,27] for plasma density, electron temperature, plasma potential measurements, and four-grid miniaturized RPAs (0.5 cm × 4.5 cm) employed in Hall thrusters' plumes to map the ion energy distribution functions [22]. Hall effect probes built in-house are extensively employed when characterizing the magnetic field topology and the rotating magnetic field in the rotamak devices.…”