# SITAEI HC1 Low-Current Hollow Cathode
## 1 Introduction  
The low-current cathodes find their principal use in electric thrusters for nano- and microsatellites, whose growth has been boosted by advancements in microelectronics and miniaturized systems, allowing for efficient implementation in scientific research; Earth observation; remote sensing; astronomy; as well as technological, educational, and military applications [1]. The target characteristics of the propulsion subsystems for this market are low cost, low power consumption, low mass, high thrust controllability, and manufacturing capability. The latter aspect is particularly important for satellite constellations, which will mainly use electric propulsion for end-of-life de-orbiting. Since 2011, the number of launches of small-scale satellites has increased at an approximate annual rate of 40%, a trend predicted to continue in future years [2]. The propulsion systems for the new generation of satellites still present challenges to be overcome. One of the critical aspects to be addressed is the hollow cathode, which coupled with the micro- and nano-scale Hall and ion thrusters, must be able to provide a discharge current below 1 A [3].  
Among the family of hollow cathodes developed at SITAEI [4], HC1 is the smallest one, specifically conceived to couple with HT100, a 100-W-class Hall thruster [5], and designed to provide a discharge current in the 0.3-1 A range. The cathode operates in steady-state conditions at mass flow rates between 0.08 and 0.2 mg/s of xenon.  
The cathode design follows the architecture of a typical orifice hollow cathode, schematically represented in Figure 1. The cathode assembly includes an active thermionic electron emitter placed inside a tube wrapped by a heating filament (electrically insulated from the surrounding metal components by means of a ceramic material) and enclosed in an electrode, called a keeper, used to help the cathode ignition by applying a positive potential with respect to the inner tube. The keeper also protects the internal components from ion bombardment damage. The heater is used during the ignition phase to ease the discharge initiation by increasing the emitter temperature to thermionic emission values. Once the discharge is ignited, a plasma gap forms between the cathode and keeperorifices. The emitter surface is then maintained at the operating temperature by means of heating from the plasma and conduction from the orifice walls.  
There are two main types of electron emitters used in space applications--the dispenser emitter and lanthanum hexaboride (LaB\({}_{6}\)). The dispenser emitter relies on chemically activated metallic surfaces to produce the required electrons. It consists of a porous tungsten matrix impregnated with barium aluminate compounds under hydrogen atmosphere, providing a relatively low work function of about 2.1 eV [6], which implies an operating temperature of about 1300 K to provide a current density in the order of 10 A/cm\({}^{2}\), as compared with up to 1900 K for LaB\({}_{6}\), the work function for which is 2.4\(-\)2.7 eV [7].  
The following section illustrates the development of HC1, starting from the first prototypes and arriving at the recent achievements in the framework of the \(\mu\)HETSat programme, an in-orbit validation mission jointly supported by the Italian Space Agency (ASI) and the European Space Agency (ESA) [8], during which SITAEL completed the full ground qualification of the propulsion system.