National Aeronautics and Space Administration

Glenn Research Center

Hollow Cathode Development for the High Voltage Hall Thruster

By: Ephraim Chen

Principle Investigator: Dr. Hani Kamhawi

1. Background

Electric propulsion offers profound advantages for spacecraft propulsion in reduced-gravity applications because of its ability to achieve high specific impulses—relative to chemical propulsion—which translates to higher payload fractions and smaller spacecraft size. One example of such a propulsion system is the High Voltage Hall Accelerator (HiVHAC), developed by the NASA Glenn Research Center in partnership with Aerojet. Much work is now being done to sufficiently improve the operational lifetime of HiVHAC, while meeting performance requirements, to allow for its use on NASA space science missions.

The HiVHAC thruster has a cylindrical configuration, with the discharge chamber forming a circular channel around the centerline. An axial electric field is applied between a hollow cathode electron emitter mounted on top of the cylinder, just downstream of the “exit plane,” and a circular anode plate placed in the back of the discharge chamber, upstream of the exit plane. A Hall current develops perpendicular to this axial electric field and an applied radial magnetic field, trapping electrons in an azimuthal flow. This Hall current “swirl” serves to ionize the propellant and interacts with the radial magnetic field to transmit the electrostatic/electromagnetic body force that accelerates the ionized propellant downstream. Hall thruster advantages, when compared to an ion engine, include a high ionization rate, due to the trapped electron spiral, and the lack of a space-charge limitation on the current, since the azimuthal electron flow and axial ion current create a quasi-neutral plasma.

It is generally desirable for HiVHAC’s hollow cathode to operate in “spot mode”—characterized by a more steady constricted current flow and small voltage oscillations—which can be achieved through higher cathode propellant flow rates and higher operating cathode currents. Operating the cathode in spot mode will reduce cathode deterioration and electromagnetic interference (EMI), improving cathode lifetime. However, a higher mass flow rate from the cathode will increase erosion of other thruster components and decrease overall thruster efficiency and specific impulse. Finally, operating the cathode at a higher current (and hence power level) also lowers HiVHAC’s thruster efficiency, in addition to resulting in decreased cathode emitter life.

2. Objectives and Methodology

To address this trade-off challenge between thruster lifetime and performance, the HiVHAC team is currently developing a hollow cathode for HiVHAC that will operate in spot mode while minimizing the required propellant flow rate and cathode power usage. The process involves using different keeper materials and varying cathode dimensional parameters such as emitter inner diameter, cathode plate orifice throat length, keeper plate orifice diameter, and cathode-keeper gap against a baseline configuration in order to arrive at some optimal cathode configuration. Several cathode configurations have been fabricated and are ready for testing.

My objectives for this summer project are listed below with each followed by preliminary methodology plans:

• Conduct baseline characterizations of the new cathode configurations by:

  •  Assembling experimental setups for new hollow cathodes in Vacuum Facility 56 (VF-56) 
  •  Collecting Langmuir probe data to provide insights into the plasma properties (electron number density and temperature) as they elate to cathode operation and to transitioning between spot and plume mode operation
  •  Collecting cathode plate orifice temperature magnitudes for the different cathode operating conditions

• Perform overall thruster wear tests with new cathode configurations, which will involve:

  •  Formally documenting the overall procedure for cathode and thruster installation and calibration of data acquisition equipment
  •  Setting up and executing wear tests of the thruster with each new cathode configuration
  •  Monitoring thrust, current and voltage measurements to evaluate thruster performance

• Maximize my exposure work being done in aeronautics and astronautics