Performance improvement of vacuum arc thrusters
Date
2016-03-16
Authors
Lun, Jonathan
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Abstract
Vacuum arc thrusters (VATs) hold great promise as an attractive pulsed plasma micropropulsion
technology for satellites. To reach their full potential as practical devices,
substantial efforts must be made to explore and better understand the intricacies of VAT
design and operation. This body of work contributes to this effort by studying a number of
new thruster designs, operational modes and cathode fuels that may lead to performance
improvement of the VAT. These include (1) the use of millisecond-long arc pulses lengths as
a new form of thruster operation, (2) the use of conical cathode shapes, (3) the novel use of
carbon and graphite-based materials as cathode materials, and (4) novel heat management
using the discrete anode switching technique. Detailed and extensive experimental
measurements of thrust, cathode erosion rate, ion current and plasma jet ICDD profiles were
undertaken in order to characterise the behaviour and performance of VAT prototypes. Testing
was accomplished with the development of a μN-level direct thrust measurement stand, a
Faraday Cup probe, a two-axis thruster rotation system, an ion collector and various data
collection and processing systems.
In a number of cases, new thruster designs and operations were found to (1) enhance ion
production rates, (2) reduce the cathode erosion rate by improving cathode spot motion, and
(3) tailor the plasma jet shape towards favourable performance. As a result, several VAT
prototypes demonstrated significant increases in thrust production and specific impulse
values of up to 100% (∼400 μN/A) and 250% (∼1000 s) respectively over typical baseline
VAT designs (∼200 μN/A, 400 s). Several previously unexplored relationships and
interactions between VAT performance and thruster parameters such as arc current, arc pulse
length, cathode surface geometry, cathode microstructure and thermal properties, plasma jet
shape, and cathode spot dynamics were recognised and characterised. The role of the arc
pulse length, spot motion and erosion coverage were highlighted as key elements which
profoundly impacted on VAT design and performance. This work opens up new areas of VAT
design, whilst posing new questions about the fascinating nature and utility of vacuum arc
spots for efficient and effective plasma jet production.
Description
A thesis submitted to the Faculty of Engineering and the Built Environment at theUniversity
of theWitwatersrand, Johannesburg, in fulfilment of the requirements for the degree
of Doctor of Philosophy.
School ofMechanical, Industrial and Aeronautical Engineering
Johannesburg, 2015