Physics of hollow cathode breakdown and steady-state operation with several inert gas propellants

Abstract

Electron bombardment ion thrusters currently form the cutting-edge of spacecraft propulsion technology.  Offering substantial mass savings in comparison to chemical thrusters and being an enabling technology for some high energy missions, these devices have passed, in recent years, from a high state of development to actual application in space.  This was primarily due to the efforts of such agencies as NASA with its highly successful technology demonstrator Deep Space I, along with their commercial application in NSSK of communication satellites.

Alternative propellants for xenon might be necessary for high specific impulse missions and as cheaper alternatives for reduction of operating costs.  Prior to this, operation with these gases has to be demonstrated and any adverse effects on thruster performance incurred by their use have to be identified.  In particular, characterisation of these gases has to be carried out in the ion thruster’s primary electron production region, the hollow cathode.  The hollow cathode, in spite of its importance to the thruster’s operation, was poorly understood.  This is due to the difficulty of conducting non-intrusive investigations on the small scale required in the hollow cathode cavity.

Experiments have been performed on the T6 hollow cathode to characterise four propellants:  xenon, kerypton, argon and a krypton/xenon mix in the natural occurring ratio.  An extensive study was conducted on the hollow cathode internal plasma dealing with two aspects of hollow cathode operation:  the transient (starting) phase and the steady state phase.  Discharge initiation characteristics were investigated for these selfsame propellants for a wide range of mass flow rates and cathode tip temperatures.  Discharge initiation was found to be repeatable and occurred at low values of keeper potential.  A novel set of experiments was carried out on the breakdown plasma optical emission using photomultipler tubes.  The steady state work involved characterisation of the hollow cathode discharges for all the propellants in diode configuration.  This was then followed by a spectroscopic investigation of the hollow cathode internal plasma under different discharge conditions.  The spectroscopic techniques enabled a non-intrusive comparison and study of the discharge for the different propellant species.

Simple models and theories were developed to describe HC behaviour.  Penning ionisation was found to be active during the breakdown phase, leading to anomalous breakdown behaviour.   A new theory is proposed that relates the plume and spot modes and transition between them to the current collection mechanism at the anode.

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