Fusion Propulsion

Opening the Solar System Frontier

John F Santarius

Lecture 28 Resources from Space NEEP 533/ Geology 533 / Astronomy 533 / EMA 601 University of Wisconsin

March 31, 2004 Key Points

• D-3He appears to be the fusion fuel of choice for space applications. • D-3He fusion will provide capabilities not available from other propulsion options. • Several configurations appear promising for space propulsion, particularly the field-reversed configuration (FRC), magnetized-target fusion (MTF), spheromak, and spherical torus. • Successful development of D-3He fusion would provide attractive propulsion, power, and materials processing capabilities.

JFS 2004 Fusion Technology Institute 2 A Prophecy Whose Time Will Come

“The short-lived Uranium Age will see the dawn of space flight; the succeeding era of fusion power will witness its fulfillment.”

From the essay “The Planets Are Not Enough” (1961).

JFS 2004 Fusion Technology Institute 3 D-3He Fusion Will Provide Capabilities Not Available from Other Propulsion Options

107 Fusion )

s 6 / 10 10 kW/kg m (

y 1 kW/kg t i c o l 5 0.1 kW/kg e 10 v t Nuclear

us (fission) Ga s-core fission

ha electric x 4 E 10 Nuclear thermal

Chemical 103 10-5 10-4 10-3 10-2 10-1 1 10 Thrust-to-weight ratio

JFS 2004 Fusion Technology Institute 4 At the Predicted Specific Power, α=1-10 kW/kg, Fusion Propulsion Would Enable Attractive Solar-System Travel

• Comparison of trip times and payload fractions for chemical and fusion rockets:

Fast human transport Efficient cargo transport

1000 1 900 0.9 )

s 800 0.8 on y Chemical i t a 700 0.7 c d 600 Fusion (1 kW/kg) 0.6 ( Fusion (10 kW/kg) 500 fra 0.5 d me i 400 0.4 t oa l p 300 y 0.3 a

Tri 200 P 0.2 100 0.1 0 0 Earth-Mars Earth-Jupiter Earth-Mars Earth-Jupiter (33% payload) (7% payload) (260 days) (1000 days)

JFS 2004 Fusion Technology Institute 5 Key Fusion Fuel Cycles for Space Applications

D + 3He → p (14.68 MeV) + 4He (3.67 MeV) D + T → n (14.07 MeV) + 4He (3.52 MeV) D + D → n (2.45 MeV) + 3He (0.82 MeV){50%} → p (3.02 MeV) + T (1.01 MeV) {50%} 10-21 L 1 - s 3 -22 m

H 10 D-3He t a

r D-T p-11B n -23

o 10 i t 3 c He- a 3 e D-D He Re 10-24 1 5 10 50 100 500 Ion temperature keV JFS 2004 Fusion Technology Institute 6

H L Physics Viewpoint: D-3He Fuel Requires High β, nτ, and T†

Confinement Power density

1000 1 y t

500 i s n

e D-T d

r -1 100 e 10

3 w L

He:D=1:1 o V

50 p e k n H

i 3 o

i He:D=1:1

T D-T s

u -2 10 f 10 e v

5 i D-D t a l e R -3 1 10 1019 1020 1021 1022 1 10 100 nt m-3s Ion temperature keV † β = plasma pressure/magnetic field pressure τ = energy confinementH L time H L JFS 2004 Fusion Technology Institute 7 D-3He Fuel Could Make Good Use of the High Power Density Capability of Some Innovative Fusion Concepts

• D-T fueled innovative concepts become limited by neutron wall loads or surface heat loads well before they reach β or B-field limits. • D-T fueled FRC’s (β~85%) optimize at B ≤ 3 T. • D-3He needs a factor of ~80 above D-T fusion power densities.

¾ Superconducting magnets can Power Density Relative to a reach at least 20 T. D-TFRC with b=85%and B=3T ¾ Fusion power density scales as β2 B4. 2000 1000 x ¾ Potential power-density 1000 100 x 10 x improvement by increasing 1 x 0 β and B-field appears at right. 1 20 0.75 15 0.5 10 b 0.25 5 B T 0

JFS 2004 Fusion Technology Institute 8 H L Engineering Viewpoint: D-3He Fuel and High β Relax Constraints

1 • Reduced neutron flux allows D-T e s D-D ¾Smaller radiation shields a e m l s e r a ¾Smaller magnets l p g r

e ¾Less activation n o 3 r ed

fa He:D -1 10 =1:2 ¾Easier maintenance o i n s o u r Increased charged-particle fy t • u

e Tritium burn on

nm flux allows direct energy n fraction = 50% 3 s o He:D i as t conversion to thrust or c =1:1 a r 3He:D=3:1 Ff electricity 10-2 1 10 100 Ion temperature keV

H L

JFS 2004 Fusion Technology Institute 9 Spacecraft Mass Gains Nonlinearly with Thrust Efficiency

• High efficiency increases thrust power and reduces radiator mass.

4 a e L h s t a s

m Direct a 3 wt o

t conversion η a te i d a g rs r 1−η e 2 n tr

eo Thermal e u f w cycle e o s po

uy 1 or ol o i o t i t a rf a H

Rf 0 0 0.2 0.4 0.6 0.8 1 Thrust efficiency • Doubling efficiency from a thermal cycle’s ~1/3 to direct conversion’s ~2/3 gives 4 times better power per unit waste heat.

JFS 2004 Fusion Technology Institute 10 Predicted Specific Power of D-3He Magnetic Fusion Rockets is 1-10 kW/kg

• Prediction based on reasonably detailed magnetic fusion rocket studies.

Specific Power First Author Year Configuration (kW/kg)

Borowski 1987 Spheromak 10.5

Borowski 1987 Spherical torus 5.8

Santarius 1988 Tandem mirror 1.2

Bussard 1990 Riggatron 3.9

Teller 1991 Dipole 1.0

Nakashima 1994 Field-reversed configuration 1.0

Williams 2003 Spherical torus 8.7

Thio 2002 Magnetized-target fusion 50

Emrich 2000 Gasdynamic mirror 130

Wessel 2000 Colliding-beam FRC 1.5

JFS 2004 Fusion Technology Institute 11 Generic Fusion Rocket Model Supports 1-10 kW/kg

• Rationale for this performance published by J.F. Santarius and B.G. Logan, “Generic Magnetic Fusion Rocket,” Journal of Propulsion and Power 14, 519 (1998). Features of the model are:

¾ Cylindrical geometry

¾ Main mass contributors: radiation shields, magnets, refrigerators, and radiators

¾ Heat flux limit of 5-10 MW/m2

¾ Neutron wall load limit of 20 MW/m2

¾ Radiators reject 5 kW/kg

¾ Low temperature superconducting magnet He refrigerators require 1000 kg/kWrejected ¾ Low-mass radiation shield (LiH with 10% Al structure)

¾ Magnet mass calculated by virial theorem and by winding-pack current density limit (50 MA/m2); larger value used

¾ Development of high-temperature superconductors should reduce the power-plant mass.

‰ Reduced refrigerator mass for magnet coolant

‰ Reduced shielding, because more magnet heating can potentially be tolerated before quenching

JFS 2004 Fusion Technology Institute 12 Earliest D-3He Reactor Design Was a Fusion Rocket

G.W. Englert, NASA Glenn Research Center New Scientist (1962)

“If controlled thermonuclear fusion can be used to power spacecraft for interplanetary flight it will give important advantages over chemical or nuclear fission rockets. The application of superconducting magnets and a mixture of deuterium and helium-3 as fuel appears to be the most promising arrangement.”

JFS 2004 Fusion Technology Institute 13 Conventional Tokamaks Have Large Mass

JFS 2004 Fusion Technology Institute 14 EFBT Toroidal Fusion Rocket J. Reece Roth, NASA Lewis, 1972

JFS 2004 Fusion Technology Institute 15 Spherical Torus Space Propulsion

• ST’s give high β, implying Princeton Plasma Physics Lab NSTX experiment high power density. • Crucial problems are recirculating power and providing thrust from a toroidal configuration. • Martin Peng has suggested helicity ejection, and the concept will be tried on NSTX.

JFS 2004 Fusion Technology Institute 16 Plasma-Jet Magnetized-Target Fusion Allows Liner Standoff from Target

Plasma gun Based on NASA MSFC figures

Plasma jet

Arrows indicate flow direction Plasma liner Magnetized target plasma

• An approximately spherical • The jets merge to form a distribution of jets is launched spherical shell (liner), towards the compact toroid at imploding towards the center. the center of a spherical vessel.

JFS 2004 Fusion Technology Institute 17 The MTF Explosion/Implosion Process Involves a Complicated Mixture of Shock Waves

• Red=target; Purple=plasma jet; Green=buffer plasma 0.05

Case 030313_02 mt=4.38 mg; Dt=5.0 cm 0.04 mj=0.2 g; Dj=2.4 cm mb=2.0 g; Db=22.1 cm vj=125 km s; vb=125 km s

L 0.03 m

H ê ê u i d a r e n

o 0.02 Zs

0.01

0 0 0.5 1 1.5 2 Time ms

JFS 2004 Fusion Technology Institute 18 H L Conversion of Fusion Energy into Thrust -4 -1 -5 -3 -2 4 5 0 1 2 3 -4

Reversed Conical

-2 Theta Compressed Main Pinch magnetic flux Coils

0 Expanding plasma Fusion front burst 2

• Fusion produces a high-temperature plasma, which can be used to push against a magnetic field to produce thrust directly. • Direct conversion of the fusion energy into thrust is important in realizing the benefits of fusion for propulsion. NASA MSFC RevolutionaryRevolutionary AerospaceAerospace SystemsSystems ConceptsConcepts (RASC)(RASC) FY02FY02 StudyStudy ProposalProposal

GroupGroup 22 –– HHumanuman ExplorationExploration OctoberOctober 2,2, 20012001 BeyondBeyond MarsMars RASC/HOPE MTF Engine Configuration

Reversed Conical Plasma Gun Theta Pinch (Approx. 56 in. long x 9.5 in. dia.) Theta Pinch Gun (48 plcs) (Located 180 deg. Apart) (2 plcs) Structural Ring Stiffener (12 in. thk.) (4 plcs)

Thrust Coil (7.87 in. dia.) Structural Tapered Spline (8 plcs) (12 plcs) NASA MSFC TD30/Fincher 7/31/02 NASA Produced a Conceptual MTF Rocket Design

¾ Fusion power = 4 GW ¾ Total power-system mass = 80 Mg α = 50 kW/kg

JFS 2004 Fusion Technology Institute 22 Plasma Power Flows in Linear Devices Differ Significantly from Flows in Toroidal Devices

• Power density can be very high due to β2B4 scaling, but first-wall heat fluxes would remain manageable. ¾ Charged-particle power transports from internal plasmoid to edge region and then out ends of fusion core. ¾ Magnetic flux tube can be “pinched” on one end by increasing the magnetic field on that side, giving primarily single-ended flow.

Expanded Not to scale FRC core region flux tube to reduce heat

flux

Neutrons Bremsstrahlung ThrustThrust Charged particles

JFS 2004 Fusion Technology Institute 23 Linear Geometry Greatly Facilitates Engineering

• Steady-state heat flux is broadly spread and due almost exclusively to bremsstrahlung radiation power. ¾ Relatively small peaking factor along axis for bremsstrahlung and neutrons. • Maintenance of single-unit modules containing blanket, shield, and magnet should be relatively easy, improving reliability and availability. • Considerable flexibility and space exist for placement of pipes, manifolds, etc. • Direct conversion of transport power to thrust by a magnetic nozzle can increase efficiency.

JFS 2004 Fusion Technology Institute 24 Radioactivity Will Most Likely Lead to a Requirement for Remote Maintenance

MaintenanceMaintenance Scheme forScheme a Terrestria for FRC l-Electric FRC Using a aTelescopic Telescopic Vacuum Vacuum Vessel Vessel

Feed to Module # N Telescopic Vacuum Vessel

Return From Module # N Module # N

Seal

LIFT

• Design by E.A. Mogahed

JFS 2004 Fusion Technology Institute 25 Several Concepts with Linear External Magnetic Fields Have Been Investigated for Space Propulsion

Spheromak FRC

Tandem mirror

JFS 2004 Fusion Technology Institute 26 D-3He Space-Propulsion Tandem Mirror

Tandem mirror engine

Tandem mirror rocket design by UW EMA 569 students

Specific power 1.2 kW/kg Thrust power 1500 MW Length 113 m Ave. outer radius 1 m Core B field 6.4 T

JFS 2004 Fusion Technology Institute 27 Field-Reversed Configurations (FRC) Would Be Attractive for Space Applications

Azimuthal • High β≡Pplasma/PB-field current • Linear external B field • Cylindrical geometry • Rotating B field current drive

From Univ. of Washington web page for the Star Thrust Experiment (STX): www.aa.washington.edu/AERP/RPPL/STX.html JFS 2004 Fusion Technology Institute 28 ARTEMIS Field-Reversed Configuration (D-3He, Momota, et al., NIFS, 1992)

JFS 2004 Fusion Technology Institute 29 Colliding-Beam FRC Conceptual Design Exists for Space Propulsion

• Variant of “classic” FRC. • Invokes p-11B fusion fuel.

• 51 MWthrust, 33 Mg mass ⇒α= 1.5 kW/kg

JFS 2004 Fusion Technology Institute 30 The Dipole Configuration Offers a Relatively Simple Design That an MIT/Columbia Team Is Testing

Io plasma torus around Jupiter LDX experiment (under construction at MIT)

12 m Dipole space propulsion design (Teller, et al., 1992)

JFS 2004 Fusion Technology Institute 31 Inertial-Electrostatic Confinement (IEC) May Be Attractive for Space Propulsion

•Key principle: spherical or UW IEC experiment cylindrical electrostatic focussing.

10 cm

100 kV Outer grid RF feedthru bias feedthru

D2 3He

Outer grid Gas Flow +- 100V ac Controllers @245 kHz RGA product 45 cm isotope out

10 cm 7" Proton Observation Detector 65 cm Window inner tungsten cathode grid -25 to -70 kV target isotope in

Ion source Ion source Filaments Filaments Neutron Detector 91 cm

8" gate valve

To 550 L/s turbo pump JFS 2004 Fusion Technology Institute 32 Other IEC Concepts Potentially Attractive for Space Propulsion

Barnes-Nebel-Turner, Bussard, Penning Trap Polywell

1 cm

1 m

JFS 2004 Fusion Technology Institute 33 VISTA: Fusion Propulsion Using Inertial-Confinement Fusion (ICF)

Charles Orth, et al., “The VISTA Spacecraft--Advantages of ICF for Interplanetary Fusion Propulsion Applications,” IEEE 12th SOFE (1987).

JFS 2004 Fusion Technology Institute 34 D-3He Fusion Propulsion Could Provide Flexible Thrust Modes

Fuel Mass- Thermal plasma augmented exhaust exhaust exhaust

Pellet injection

JFS 2004 Fusion Technology Institute 35 Direct Conversion to Electricity Could Take Advantage of the Natural Vacuum in Space

Direct converter grids and collectors Core plasma

Barr-Moir experiment, LLNL (Fusion Technology, 1973) Magnetic field lines

Faraday cage

JFS 2004 Fusion Technology Institute 36 Plasmas Provide Many Materials Processing Capabilities

• B.J. Eastlund and W.C. Gough, “The Fusion Torch--Closing the Cycle from Use to Reuse,” WASH-1132 (US AEC, 1969).

JFS 2004 Fusion Technology Institute 37 Summary

• D-3He fusion requires continued physics progress. • D-3He engineering appears manageable. • Several configurations appear promising for space propulsion, particularly the field-reversed configuration (FRC), magnetized-target fusion (MTF), spheromak, and spherical torus. • Successful development of D-3He fusion would provide attractive propulsion, power, and materials processing capabilities.

JFS 2004 Fusion Technology Institute 38