Overview of FRC Propulsion and Materials Research

David Kirtley and George VotroubekMSNW LLC, Redmond, WA 98052, USA

John Slough, Samuel Andreason, and Chris PihlPlasma Dynamics Laboratory, University of Washington

Aydin Tankut and Fumio OhichiMaterials Science Department, University of Washington

Richard Milroy, Brian Nelson and Eric MeierPlasma Science and Innovation Center, University of Washington

Alan Hoffman, Kenneth Miller and Daniel LotzRedmond Plasma Physics Laboratory, University of Washington

AFOSR Propulsion Materials WorkshopNovember 4, 2010

Discussion Index

1. Basic of FRC Physics

2. Summary of Propulsion Activities

3. Summary of Related DOE Activities

4. Related Materials Interests

5. Current Materials Research Program

6. Needs and Discussion

EXTERNALFIELD

FRC CLOSED POLOIDAL FIELD

R – null radius

rs – separatrix radius

rc – coil radius

xs – rs/rc

Equilibrium Relations:

2s

r

020 x1drB

P2

2

1s

2svac

ext x1

BB

0

2ext

00 2

BkTnP

Radial Pressure Balance

Axial Pressure Balance

Flux conservation

Physics of Pulsed PlasmoidPropulsion - The FRC

Rotating Magnetic Field Formation

Synchronous electron motion j(r) = e ne r

m=1, “saddle” coils positioned radially external to axial field coils. Two oscillators phased at 90 produce constant amplitude B.

– Decreases circuit requirements (100 % solid state)

– Decreases radiation losses (operation on heavy gases)

– Increases plasma currents and acceleration force

– Minimizes wall interaction

ELF

(1) Rotating Magnetic Fields (RMF) form high-density, FRC plasmoid

(2) FRC grows and accelerates driven by RMF generated currents & steady field (3) FRC expands as ejected, converting any thermal to directed energy

J

rz BjF

RMF generated plasma current from

synchronous electronsSteady magnetic field in conical

geometry

12

3

The Generic RMF-Based Thruster

Trim Coil RMF Antenna

Steady Bias Field Coils

Advantages• Vast Operating Range- 10-100 kW, 1000-6000 s Isp in a single thruster• Technology Scalable- 0.1-1000’s of kWs, 1018-1020 operating densities• Low Mass Thruster and PPU- 1-2 kW/kg including PPU• Efficient Ionization- Rapid, high-temperature , and magnetically isolated• COTS Electronics- Low voltage, solid-state switching• Long Life and Any Propellant- Electrodeless and magnetically isolated

Electrodeless Lorentz Force (ELF) Thruster RMF Generation of the Field Reversed Configuration (FRC) *

*Started October 2010

Status Basic operation and performance demonstrated

with earlier ELF program Current program aims to develop from 6.1 to 6.2

and prove a steady state thruster Initial design and advanced antenna geometry

demonstrated Neutral entrainment chamber constructed and

passive operation underway Modeling effort contracting is underway

Dynamic formation and acceleration of an FRC, shown with 4 field coils

Thrust

Fundamental Science

*Started October 2010

Isp

Neutral Entrainment *

ISSUE: Plasma formation is the primary loss mechanism for ALL electrostatic or electromagnetic propulsion systems. It prevents operation at lower specific impulse, lowers efficiency at all exit velocities, and requires high mass propellants.

Solution: Entrain neutrals in an acceleration field after formation.

ELF

ElectroMagnetic Plasmoid Thruster (EMPT)*A 1 kW-scale FRC thruster for deep space missions

EMPT Thruster:

• 200-2000 Watt FRC thruster (3”diameter, 4” long, 0.2-2 Joule)

• Very long lifetime, throttle-able power, deep space

propulsion

• Dramatically lower mass than existing EP

• In-situ operation on ambient propellants

First Demonstration of FRC formation at <1 Joule

• Plasmoid Achieved 1000-6000s Isp in Xenon, Hydrazine (simulant)• Revolutionary step in both scale and performance• First demonstration of steady state operation (50 plasmoids at 2 kHz)• Demonstrated technology scaling and circuit efficiencies (10 nH stray )

First Demonstration of FRC formation at <1 Joule

• Plasmoid Achieved 1000-6000s Isp in Xenon, Hydrazine (simulant)• Revolutionary step in both scale and performance• First demonstration of steady state operation (50 plasmoids at 2 kHz)• Demonstrated technology scaling and circuit efficiencies (10 nH stray )

*Phase II awarded November 2010

High Energy FRC Programs

500 Joule Xenon RMF FRC - PLC

•Plasma Liner Compression (PLC)• High energy magnetic compression• Xenon plasma ions at > 2 keV at end regions

• Pulsed High Density (PHD)• FRC formation and collision, >10 MW plasma• > 10 GW/m2 transient energy loading

•Foil Liner Compression (FLC)• Metallic liner implosion• Intense transient neutron and UV radiation pulse

•Translation Compression and Sustainment (TCSU)• Steady State RMF-Formed FRCs at 10 MW energies• Low density, requires low recycle/impurity rates

1m diameter colliding FRC- PHD

High speed photography of Foil Liner Compression

Study Formation & Sustainment of RMF driven FRCs.

Either form FRCs directly using RMF alone, or translate and expand theta-pinch formed FRCs from LSX/mod.

LSX/mod(formation & acceleration)

TCS Chamber(confinement & RMF drive)

RMF Antennas

RMF Translation, Compression, and RMF Translation, Compression, and Sustainment, Experiment (TCS)Sustainment, Experiment (TCS)

FRC/RMF Materials IssuesFundamentally Non-Equilibrium

Transient loading • Pulsed devices have transient wall loading of optical radiation, electric fields, and perhaps

some ions. • What are the equilibrium temperature and sputtering effects of pulsed wall loading? • Sputtering rates are non-linear, how is this affected?• Gas deposition and recycling are key to fusion plasmas.

Wall chemistry for reactive gases• FRC thrusters and fusion devices operate on chemically reactive gases.• What effects do high-temperature ion-wall interactions (even if they are very reduced)

create if the ions are Oxygen, Nitrogen, or Hydrogen? In fusion plasmas the chemical sputtering rate can be more important than the purely kinetic energy sputtering rates.

Effects of magnetized plasma• FRC thrusters run with magnetically confined ions. This means pulsed, large magnetic

fields (300-3000 Gauss), large gyroradii (possibly greater than the device), and very large electric fields at the wall (measured up to kV).

• How does pulsed magnetic fields this change the wall interaction and lifetime picture?Optical and Nuclear Radiation

• A pulsed, high-temperature device will deliver pulsed optical radiation to the wall. This has effects for both contamination as well as the sputtering.

• What are the effects of pulsed optical radiation on a thruster wall, both in terms of thermal loading and interaction with a neutral gas at the wall boundary.

FRC Propulsion Specific InterestsMagnetic Isolation dramatically limits plasma-wall interaction –

To what degree?

RMF – FRC Propulsion Materials Interests1. Very long lifetimes must be demonstrated

• ELF, EMPT development programs• Erosion of insulator. How small? Is an insulator needed?

2. RF coupling to high-temperature insulators (dissipation)• Quartz is great, but low thermal conductivity• BN, AlO, SiN FRC wall materials are unknown

3. Transient thermal and electromagnetic radiation loading is new for propulsion, must be studied

4. Fundamentally non-equilibrium

FRC Thruster Elements

1. Wall : Quartz, SiN, Other Insulators

2. Magnet: Aluminum Flux Conservers

3. RMF: Copper Antenna

Quartz Insulator

Backplate

Aluminum Flux Conservers Reflectors, and Heatsinks

Current DOE Plasma-Wall Research 1

Materials Concerns1.Impurities from Wall Materials2.Impurities from Gaseous Wall Loading3.Chemical and Kinetic Sputtering4.Gas Implantation – Recycling, Embrittlement5.Neutron Activation, Embrittlement

1. First Wall Material Coatings and Preparation• Ta, SiO2, coating for surface gas loading

2. Steel and Quartz Chemistry during reversals• Siliconization, atomic oxygen loading

3. Ti-Gettering4. Diagnostics (SAS)

• Cylindrical Mirror Analyzer (CMA)• Energy Dispersive X-Ray Spectroscopy (XPS)• Plasma Chemical Vapor Deposition (PCVD, RGA)• Aurger Electron Spectroscopy (AES)• Vacuum materials handling capabilities

5. Diagnostics (TCS and UW)• He-GDC• Scanning Electron Microscopy (SEM)• Multi-point Thompson Scattering (MTS)• In-situ Optical First Wall Diagnostic • Scrape-off layer Langmuir

Current DOE Research Program 2• UW Experimental Efforts

• Wall Materials Selection• First Wall Processing• High Energy Diverter Studies• Neutron Studies, DPA limits

• PSI-Center Modeling Efforts• NIMROD FRC modeling of scrape-off and wall

layer• NIMROD modeling of diverter flow• First wall interaction *

• A. Tankut, G. Vlases, K.E. Miller, et al. “Wall conditioning in TCSU and its effect on plasma performance”, Journal of Fusion Materials, 2010• A. Tankut, K.E. Miller, et al. “An XPS study on the evolution of type 304 stainless steel surface during routine TCSU operation”, JFM, 2010• Aydin Tankut “Surface Analysis Studies in the Translation, Confinement, and Sustainment Upgrade (TCSU) Experiment”, Doctoral Thesis, 2009• A. Tankut, F. S. Ohuchi . “Surface Analysis Studies of TCS-U Components”, Innovative Confinement Concepts Meeting, 2008• A. Tankut, F.S. Ohuchi.” Surface Analysis Studies on the Wall Conditioning of TCSU”, APS 2008

Needs and Discussion

1. Experimental effort to demonstrate FRC lifetimes and identify erosion concerns2. Empirical quantification of erosion, deposition, chemistry in pulsed, magnetically

confined plasmas

3. Modeling effort to determine transient plasma radiation and thermal transport

4. Modeling effort to assess optimal geometries and materials

Fundamental Science Question: What are the average temperature and sputtering effects of, non-equilibrium, chemically-reactive, transient plasma-wall interaction in a highly magnetized plasma?

Proposed Effort Extension

1. Add interface to the Neutral Entrainment experiment to utilize existing SAS Hardware2. MSE post doc runs investigation (Dr. Tankut), supervised by Shumlak, Nelson3. MSE diagnostic package

1. Erosion2. Deposition3. Surface Chemistry 4. Materials Selection

4. PSI-Center Boundary Condition and Geometry Group modeling effort1. NIMROD runs for ELF2. Implement UEDGE Tokamak code

• Leverage University of Washington DOE programs for propulsion efforts• UW MSE, RPPL experience, diagnostics, and hardware • UW Personnel*• PSI-Center Modeling

Questions to Answer1. What and where is the erosion on a steady state ELF thruster2. What is the erosion, deposition, and chemical interaction for NE and reactive propellants3. What are the steady state temperature and erosion rates of non-equilibrium,

chemically-reactive pulsed plasma-wall interaction in a highly magnetized plasma?

Equipped With:- Analysis Chamber: X-Ray Photoelectron

Spectroscopy (XPS), Auger Electron Spectroscopy (AES)

- Glow Discharge Test Chamber: Ta-electrode, Residual Gas Analyzer (RGA)

- Sample Transfer Device

Electron EnergyAnalyzerX-ray

Source

Ta electrode

Glow Gas Inlets

RGA

IonPump

Transportable P-supply

Sample Transfer DevicePbase: low 10-8 Torrs

Analysis Chamber

Pbase: low 10-9 Torrs

Glow Discharge Test ChamberPbase: high 10-8 - low 10-7 Torrs

Turbomolecular

Pump

TCSU Surface Analysis System

Additional (Campus) FacilityScanning Electron Microscopy (SEM)

+Energy Dispersive X-ray Spectroscopy (EDS)

Impurity problem in TCS

Significant fraction of the input power was radiated

Radiated Power vs. Total Input Power

Impurities prevented the study of FRC physics in TCS

Summary of the evolution of SS in TCSUAfter He-GDC After Plasma

Bulk: Fe0, Cr0, Ni0

Fe-O , Cr-O

After Standard Cleaning

CxHy, H2O

He+

After Up to atmospheric N2

H+

e-

Fe0, Cr0, Ni0

H2O

N2O2

Fe0, Cr0, Ni0

Up to N2 atm.

Fe0, Cr0, Ni0

PlasmaGDC

OC

Fe

OC

Fe

O

Fe

O

Fe

e-

OHO

A. Preliminary Ti-gettering tests:- Basic understanding of processing parameters; SEM/EDS system was used.- Morphology was similar for Ti-film deposited on SS and quartz.- Increasing the substrate temperature from RT to 100oC did not result in an observable change in morphology or deposition rate.- Reducing the filament current b y ~15% reduced the deposition rate significantly.

High temp (100oC)

Ti/SST: 100oC, I: 16.5A, D: 60 min.

Low current (~15% lower)

Ti/SiO2

T: RT, I: 14.2A, D: 60 min.

Ti/SST: RT, I: 16.5A, D: 60 min.

ON SS ON Quartz

Ti/SiO2

T: RT, I: 16.5A, D: 60 min.

C

TiO

Fe

ONiC

O

Si

C

Ti

OC

Surface analysis: Ti-gettering in TCSU (Phase I):

Reduction of Fe by Ti on Ti-coated sample surface

Plasma shots lead to oxidation of Ti and reduction of Fe:- Volatiles from SS were trapped in Ti

Bulk: Fe0, Cr0, Ni0

Bulk: Fe0, Cr0, Ni0

Fe-O , Cr-O

Ti filmH2OH+

H+

X

H2O

H+

X

+16 Plasma shots

20% Ti0

Binding Energy (eV)

Inte

nsi

ty (

au

)

54% Fe0

Binding Energy (eV)

Inte

nsi

ty (

au

)

SS 2

SS 1

Ti (f

ew n

m)

hν

Inte

nsi

ty (

au

)In

ten

sity

(a

u)

Binding Energy (eV)

Pre Ti-gettering 1st Ti-gettering

24% Ti0

30% Fe0 38% Fe0

Binding Energy (eV) Binding Energy (eV)

Inte

nsi

ty (

au

)

Fe-O was reduced by Ti:-∆Go

TiO2 > -∆GoFe2O3

XPS on Ti-coated surface (SS1)

e-

e-

- Preliminary Tests: Deposition rate can be controlled by filament current

- Ti-gettering in TCSU:

1st Ti-gettering: Improved vacuum, pumping H-species, reduced Fe-O

Plasma shots: Inhibited release of H2O

- Plasma Performance:

Drastic reduction in impurity radiation (Ti on N and S bellows)

Drastic reduction in H-recycling

3. Ti-gettering - Summary

RMF in R- plane generated by two sets of axial conductors (Helmholtz-like pair) placed orthogonal to each other.

Oscillating currents, phased 90 apart creates a rotating field of constant magnitude

FRC Generation Employing aRotating Magnetic Field (RMF)

SeparatrixL ~ 5rs

Rotating MagneticField B

rs rw

Field ReversedConfiguration (FRC)

I0cos(t)

I0sin(t)

m=1, “saddle” antenna coils are positioned radially external to the axial field coils. Two oscillators phased at 90 produce a constant amplitude rotating field B.

Synchronous electron motion j(r) = e ne r

-3Ba- Ba

Ba

rw r

3Ba

rw r

Ba

ww

t = 0 t > 0

Initial axial magnetic field Ba is reversed by synchronous J current

driven by RMF

RF Antenna Configuration for RMF FRC (ce >> ei) {Helicon (ce < ei)}

(a) Instantaneous RMF coil voltage with floating, resonant ring-up in Nitrogen. (b) Instantaneous coil current for a pulsed-discharge in Xenon.

0 0.05 0.1 0.15 0.20

0.5

1

1.5

2x 10

4

Time [ms]

Co

il V

olta

ge

[Vo

lts]

VacuumNitrogen

Steady State Operation1. RF-capacitor circuit is charged through pulsed-charging or steady DC

current2. Bias fields are steady or fed through similar pulsed-inductive network3. Gas flow is steady or synchronously chopped for high-throttled flow, no

puff4. RMF discharge is timed to power flow and neutral density distribution5. Thermal/Ionization energy is added to plasmoid through current drive

(Ohmic)6. Kinetic energy is transferred to plasmoid through inductive transfer

(reactive)

Downstream plasma density on centerline for a 2700 s Isp, 32 Joule Xenon discharge.

Downstream ion saturation current on a double Langmuir probe for a low velocity 38 Joule xenon FRC. Shown are the fast-moving, leading

edge jet and the slower, bulk FRC.

0 0.05 0.1 0.15 0.20

2

4

6

8

10x 10

18

Time [ms]

Ele

ctro

n D

ensi

ty [m

-3]

10 cm90 cm

0 0.1 0.2 0.3 0.4 0.50

0.02

0.04

0.06

0.08

0.1

0.12

Time [ms]Io

n S

atur

atio

n C

urre

nt [A

]

10 cm50 cm

Downstream FRC Characteristics

.A typical plot of Impulse and plasmoid velocity versus puff timing for a resonant amplifier-driven ELF. 60 psig nitrogen discharge.

Density and velocity for a high-power, 15-35 Joule Pulsed Discharge FRC at various fill pressures. Xenon, 10 psi.

Thrust impulse and velocity for a low-power,5-10 Joule Pulsed Discharge FRC. Nitrogen, 50 psig.

General Scaling Laws• Increasing neutral fill leads to

larger, slower plasmoids• Decreasing neutral fill leads to

hotter, faster plasmoids• Below 1E19 m-3 current drive suffers

and both thrust and velocity decrease• 5-10 Joule FRCs yield 20-30 km/s• 25-50 Joule FRCs yield 30-50 km/s

Increasing ← Neutral Fill → Decreasing

Increasing ← Neutral Fill → Decreasing Increasing ← Neutral Fill → Decreasing

Task 1: ELF Thruster Program Plan

Technology Development

Year 1 - Initial thruster designDevelop a thruster prototype that can demonstrate several pulse operation in a

representative testing environment.

• Full thruster design using existing hardware and infrastructure•Operate in a pseudo-steady state mode with extended operation

•Investigate chamber effects•Validate performance goals and identify thruster design issues•Multiple Discharge, single gas puff Testing•Investigate chamber effects•Initial testing at AFRL to validate MSNW LLC results•High-Q power processing and circuit design

Year 2 - Demonstration thruster and electronics packageDevelop a nose-to-tail thruster and electronics package for wide testing

•Implement upgrades from Year 1 into a complete thruster package•Complete in-situ performance validation effort•Full thruster testing at AFRL•Validation testing at MSNW, UM

Nitrogen and Air Results• Initial testing of multiple FRC discharges• No noticeable erosion or thruster damage

Xenon Results• Demonstrated Kinetic >>Thermal Energy (impulse and magnetic pressure balance)

Hydrazine (Simulant), Nitrous Oxide Results• Demonstrated ionization and electromagnetic acceleration of a monopropellant

First Demonstration of Non-Inductive Formation, Acceleration, and Ejection of FRC Plasmoids

• Plasmoid Achieved 1000-6000s Isp in Air, Xenon, Monopropellant Isp Measurement- Langmuir, B-probes track FRC acceleration, ejection• Produced > 1 mN-s /pulse (Air propellant) Direct Thrust Measurement- via ballistic impulse pendulum developed at MSNW, calibrated at NASA GRC• Average Power 50 kW, 5-50 J discharges

First Demonstration of Non-Inductive Formation, Acceleration, and Ejection of FRC Plasmoids

• Plasmoid Achieved 1000-6000s Isp in Air, Xenon, Monopropellant Isp Measurement- Langmuir, B-probes track FRC acceleration, ejection• Produced > 1 mN-s /pulse (Air propellant) Direct Thrust Measurement- via ballistic impulse pendulum developed at MSNW, calibrated at NASA GRC• Average Power 50 kW, 5-50 J discharges

Key Results to Date

Dynamic formation and acceleration of an FRC, shown with 4 field coils

Thrust

Fundamental Science

[1] Matsuzawa, Y., et. al, “Effects of background neutral particles on a field-reversed configuration plasma in the translation process”. Phys. Plasmas 15, (2008).

Isp

Task 2: Neutral Entrainment

ISSUE: Plasma formation is the primary loss mechanism for ALL electrostatic or electromagnetic propulsion systems. It prevents operation at lower specific impulse, lowers efficiency at all exit velocities, and requires high mass propellants.

Solution: Entrain neutrals in an acceleration field after formation.

Task 2: Neutral Entrainment Program

Year 1 - Feasibility Initial feasibility, interaction, and systems-level study

•Neutral interaction modeling•Manifold and neutral flow design•Dynamic neutral interaction SEL implementation•AFRL numerical support

•Neutral interaction experimental effort•Neutral entrainment chamber construction (ELF chamber mod)•FRC-Neutral drag and ingestion investigations•Neutral flow testing and puff valve modification•Neutral beam diagnostics investigations

Year 2 - Neutral Entrainment DemonstrationDemonstrate and quantify neutral entrainment

•Neutral low modeling•AFRL numerical support

•Neutral entrainment experimental investigation•Dynamic acceleration system design•Couple neutral injection and dynamic acceleration

We have developed an advanced thruster concept that works and has wide-reaching payoffs

• The ELF thruster is a major improvement over traditional electric propulsion, for most power levels and missions

• Neutral entrainment could make it revolutionary – dramatically extending the specific impulse, thrust-to-power, and power ranges.

• Direct innovative application to hypersonic vehicles, air-breathing space propulsion, in-situ propellant utilization, high-altitude recon, propellant sharing (multi mode), and ???.

• The ELF thruster is a major improvement over traditional electric propulsion, for most power levels and missions

• Neutral entrainment could make it revolutionary – dramatically extending the specific impulse, thrust-to-power, and power ranges.

• Direct innovative application to hypersonic vehicles, air-breathing space propulsion, in-situ propellant utilization, high-altitude recon, propellant sharing (multi mode), and ???.

Fundamental Questions:

Are there unforeseen technology development challenges to a pulsed inductive thruster?

What are the power, specific impulse, geometry and density limits of neutral entrainment?

•Field Reverse Configuration used to create Plasmoids in fusion community, combined with Rotating Magnetic Fields promise a breakthrough in high power (1 kW and up) space propulsion

J

rz BjF

Rotating Magnetic Field generated plasma current

from synchronous electrons

Steady magnetic field in conical

geometry

RMF Antenna Steady Field Coil

Single Shot Electrodeless Lorentz Force Thruster Operation Successfully Demonstrated at MSNW

Slough / Kirtley (MSNW), Milroy (University of Washington)

Input Power = 50 J (25-50 kW steady state)Propellant = Air, Argon, Xenon, Nitrous OxideMeasured Thrust impulse = 1mN-s per plasmoid ejectionMeasured Specific Impulse = 1,000-6,000 s depending mass flow rateMeasured Peak Efficiency = ~50% (Xenon), theoretical = 87%

•Full Scale tests and optimization will be conducted at AFRL/RZSS (Haas, Brown ), modeling and Simulation AFRL RZSA / RZSS (Cambier)

Helicon Thruster: With no significant diamagnetic current and negligible magnetic gradient, thruster relies on electrothermal heating and nozzle expansion at exit. Two fluid effects (double layer) enhance Isp, and efficiency . Concerns are efficiency, plasma detachment from thruster fields and beam spread.

High Power Helicon Thruster: Larger RF field amplitude at lower frequency leads to a much larger high density, high plasma. Plasma is lost from stationary plasma through axial JBr as well as electro-thermal expansion. Detachment and beam spread problems greatly reduced.

Electrodeless Lorentz Force (ELF) Thruster: An even larger, lower frequency rotating field imposes synchronous motion of all electrons. The resultant field producing a completely isolated, magnetized plasmoid (FRC). The strong axial JBr force rapidly drives the plasmoid out of the thruster. FRC expansion during ejection converts remnant thermal energy into directed energy. No detachment issues.

Magnetically Accelerated Plasmoid Propulsion: With the FRC formed in ELF, further thrust or Isp can be obtained with peristaltic sequencing of axial array of flux coils. Large JBr force can be maintained throughout FRC passage enabling neutral gas entrainment significantly increasing thruster efficiency at optimal Isp.

zc

Br

FjxB

zc

Br

FjxB

zc

Br

FjxB

zc

Br

FjxB

Br(vac)

a)

b)

c)

d)

Dynamic Behavior with RotatingB - Thruster Configuration