Caruso, Natalie R. S.

Dankanich, John W.

Liu, Thomas M.

Polzin, Kurt A.

Scogin, Tyler

Walker, Mitchell L. R.

NASA Technical Reports Server

Initial Thrust Measurements of 
Marshall’s Ion-ioN Thruster
Presented by: Kurt Polzin
Presented  at the 51st AIAA/ASME/SAE/ASEE Joint Propulsion Conference July 27-29, 2015
Natalie R.S. Caruso, Tyler Scogin, Thomas M. Liu, and Mitchell L.R. Walker
High-Power Electric Propulsion Laboratory - Georgia Institute of Technology, Atlanta, Ga 30332
Kurt A. Polzin and John W. Dankanich
NASA George C. Marshall Space Flight Center, Huntsville, AL 35811 
https://ntrs.nasa.gov/search.jsp?R=20150016574 2019-08-31T06:25:18+00:00Z
Overview
Introduction to Electronegative Ion Thrusters
Project Motivation & Goals
• Design of the MINT
• Calculated Performance
Marshall’s Ion-ioN Thruster
• Thruster Operating Conditions
• Facility & Diagnostics
Experimental Set-Up
• Configuration C1
• Configuration C2
Results & Analysis
Conclusions
Future Work
References
MSFC
Classic RF Gridded Ion Thruster Diagram:
Ion Thruster Basics
Ionization of noble gas 
yields electrons and 
positive ions.
Positive ions accelerate 
through grid assembly.
Electrons ejected from 
neutralizer cathode into 
positive ion beam.
Stage 1: Stage 2: Stage 3:
MSFC
• Lifetime Limiting Components:
– Acceleration Grids.
– Neutralizer Cathode.
• Constraints:
– High purity source (often xenon) 
required for cathode operation.
Drawbacks of Ion Thrusters MSFC
Electronegative Ion Thruster Diagram:
Electronegative Ion Thruster
Ionization of an 
electronegative propellant.
Ion-ion plasma formation.
Positive and negative ion 
acceleration.
Stage 1: Stage 2: Stage 3:
REF. [2-5]
MSFC
• Benefits:
– Elimination of neutralizer cathode.
– Faster recombination in plume.
– Thrust generation by both charge species.
Project Motivation
Marshall’s Ion-ioN Thruster
1st domestic investment in electronegative thruster concept.
MSFC
• Determine feasibility of electronegative ion thrusters 
through direct thrust measurement enables:
– Assessment of key design drivers impacting thruster operations.
– System level analysis and comparison to classic gridded ion thrusters 
and Hall thrusters.
– Elevation of Technology Readiness Level from TRL2 to TRL3.
Project Goals
Advised by:
NASA George C. Marshall 
Space Flight Center
MSFC
• Electronegative ion thruster concept patent by École
Polytechnique accepted in 2007. [Ref. 4]
• PEGASES: Plasma Propulsion with Electronegative GASES.
• Previous focus on diagnostics required to characterize 
quasineutral plume.
Advancements to Date
[Ref. 5] [Ref. 8]
MSFC
Marshall’s Ion-ioN Thruster (MINT)
Thruster Design
Ionization of propellant 
using double-helix, half-
turn Nagoya antenna.
Electron filtering using 250 
Gauss magnetic filter with 
Neodymium magnets.
Positive and negative ion 
acceleration through 
alternating bias grids.
Stage 1: Stage 2: Stage 3:
1 2 3
MSFC
Calculated Performance
MINT Performance Estimates:
MSFC
Property Value Units Description
𝜸 0.958 - Thrust Correction Factor1
𝑽𝒔 350 V Screen Grid Bias
𝑽𝒂 0 V Acceleration Grid Bias
𝑽𝒃 315 V Beam Voltage
𝑷𝒊𝒏 700 W Total Input Power
𝑻𝒐𝒑𝒕 0.65 - Physical Grid Transparency
𝑱𝒊𝒐𝒏𝒔 ~1.5 mA/cm2 Ion Current Density
𝑻𝑴𝑨𝑿 ~1.2 mN Maximum Possible Thrust
• Assumptions:
– Child-Langmuir Law for round apertures.
– Nitrogen:argon volumetric propellant mixture 5:1.
– Classical grid design techniques as described in [Ref. 1].
Acceleration Grid Assembly.
Thruster Configurations:
• Configuration C1: Complete thruster including all 3 electronegative ion thruster 
stages.
Thruster Operating Conditions MSFC
Thruster Configurations:
• Configuration C2: Magnetic filter removed enabling thruster to operate as a 
cathode-less, traditional gridded ion engine.
Thruster Operating Conditions MSFC
Operating Conditions:
• Volumetric Flow Rates: 5:1 Nitrogen to Argon ratio at 6, 12, and 24 sccm.
• 150 and 350 Watts forward RF power.
• 13.56 MHz RF with a Standing Wave Ratio (SWR) < 1.05.
Thruster Operating Conditions MSFC
Acceleration Grid Biasing Schemes:
• An Agilent 33220A 20MHz Function/Arbitrary Waveform Generator sends a 
sinusoidal or square waveform at a frequency of 4, 10, 25, 125, or 225 kHz.
• A Trek Model PZD350A M/S bi-polar power amplifier with a current limit of 
400 mA that biases the upstream screen grid (±350 V) relative to the 
downstream acceleration grid.
Thruster Operating Conditions MSFC
Grid Biasing Schemes Waveform Frequency (kHz)
1 Sinusoidal 25
2 Sinusoidal 125
3 Sinusoidal 225
4 Square 4
5 Square 10
6 Square 25
7 Square 125
8 Square 225
Vacuum Test Facility-1:
• Effective pumping speed of 125,000 
L/s on argon.
• Base Pressure: [2.4 x 10-5] torr.
• Operating Pressure: [4.8 to 5.7 x 10-5] 
torr over full range of flow rates.
Thrust Stand:
• Null-type, inverted pendulum.
• LVDT measures position, (2) E.M. 
actuators control assembly motion.
• Recorded null coil current 
corresponds to thrust generation.
Facility & Diagnostics MSFC
Results: Configuration C1 MSFC
Grid Bias Scheme 2:
• Sinusoidal waveform at 125 
kHz.
• Only successful sinusoidal grid 
bias scheme required 24 sccm.
• Initial thrust spike of 3.75 mN
that immediately falls below 
thrust stand noise floor.
Grid Bias 
Scheme
Total Vol. 
Flow Rate
Ar:N
Ratio
RF 
Pwr
Potential 
Thrust 
Thrust 
Error
Description of Thrust 
Behavior
2 24 sccm 5:1 350 W ~3.75 mN +3mN Single spike
5 6 sccm 5:1 150 W ~4.5 mN +3 mN Single spike at grid start
up
5 10 sccm 5:0 350 W ~3 mN +1.75 mN Single spike at grid start
up
5 12 sccm 5:1 350 W ~4.25 mN +3.75 mN Single spike at grid start
up
5 24 sccm 5:1 150 W ~3 mN +2.5 mN Repeated spikes
Results: Configuration C1 MSFC
Grid Bias Scheme 5:
• Square waveform at 10 kHz.
• All cases exhibit single thrust 
spike at start of thruster 
operation with the exception of 
the 24 sccm case.
• 24 sccm case exhibits repeated 
spikes of thrust.
Grid Bias 
Scheme
Total Vol. 
Flow Rate
Ar:N
Ratio
RF 
Pwr
Potential 
Thrust 
Thrust 
Error
Description of Thrust 
Behavior
2 24 sccm 5:1 350 W ~3.75 mN +3mN Single spike
5 6 sccm 5:1 150 W ~4.5 mN +3 mN Single spike at grid start
up
5 10 sccm 5:0 350 W ~3 mN +1.75 mN Single spike at grid start
up
5 12 sccm 5:1 350 W ~4.25 mN +3.75 mN Single spike at grid start
up
5 24 sccm 5:1 150 W ~3 mN +2.5 mN Repeated spikes
Analysis: Configuration C1 MSFC
assembly confirms thermal and electrical loading did not contribute to thrust.
• RF ignition occurs before activation of grids and initial thrust spike.
General Plasma Behavior: Extinction
• Thruster self extinguished at 6 sccm, 150 Watt RF power operating condition –
original design operating condition.
• Additional volumetric flow rate required for steady operation will decrease 
specific impulse and propellant utilization.
Grid Biasing:
• Success of square waveform 
grid biasing consistent with 
results of simulations from 
Reference 2.
Confirmation:
• Thrust data recorded during 
stand alone operation of grid
Analysis: Configuration C1 MSFC
Final Observations on C1
• Continuous thrust no observed in any grid biasing scheme tested
• Scheme 5 (square wave, 10 kHz) was ‘best’, yielding thrust ‘spikes’ at various 
operating parameters
• Perhaps owing to N2 not being electronegative-enough to form sufficient nubers
of negative ions
• A better electronegative gas (SF6 or I2) may produce better results
High Vac Plasma Behavior: 
• 30 sccm minimum flow and 
300W RF required at high vac
• Sinusoidal and square 
waveforms degrade to 
triangular waveforms
Results: Configuration C2 MSFC
Operation as a traditional ion 
thruster (no electron filter)
• No ignition at high vacuum
• Ignition at 1.3x10-4 torr, 40 sccm, 
and 1 kW RF power
• After ignition, testing performed 
at 4.8-5.7x10-5 torr
Observations on C2
• No combination of operating conditions tested yielded discernable, sustained thrust
• Brief thrust spikes in the 
• Scheme 5 (square wave, 10 kHz) was ‘best’, yielding thrust ‘spikes’ at various 
operating parameters
• Perhaps owing to N2 not being electronegative-enough to form sufficient nubers of 
negative ions
• A better electronegative gas (SF6 or I2) may produce better results
• At 125 and 225 kHz, 6 sccm, visible depletion of the plasma upstream of the grids
Upcoming: Direct Thrust Measurements conducted at Marshall 
Space Flight Center with a propellant mixture of argon and sulfur 
hexafluoride focusing on Grid Bias Schemes 2 and 5.
Future Direction MSFC
Dr. Ane Aanesland of École Polytechnique for her input regarding electronegative ion thrusters.
Jason Frieman and Nathan Brown of Georgia Tech for their assistance in experimental set-up and thrust stand operation
_________________________
In addition, Marcellus Parker (HPEPL-Intern), Callen Brooks (NASA-Intern), Michael Paul (K&J Magnetics),  
Keith Chavers (NASA-MSFC), Tommy Reid (NASA-MSFC), Doug Galloway (NASA-MSFC), Adam Martin (NASA-
MSFC), and Stan McDonald (NASA-MSFC) contributed invaluable support of this project in its initial stages.
Jet Propulsion Laboratory for the use of their CEX2D grid modeling program.
Acknowledgments
Advised by:
NASA George C. Marshall 
Space Flight Center
Thank you for your attention.
MSFC
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Initial Thrust Measurements of Marshall's Ion-ioN Thruster

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