Future of Antiproton Triggered Fusion Propulsion

Brice Cassenti & Terry KammashUniversity of Connecticut & University of Michigan

Future of Antiproton Triggered Fusion Propulsion

• Propulsion Concepts

• Nuclear Reactions

• Challenges– Lithium-6 fuel– Ablation radiation shield– Antiproton trigger scattering

Inertial Confinement Fusion Propulsion Concepts

• Critical Mass Systems

• External Compression Systems

• Antiproton Triggered Systems

• MICF Hybrid Pellets

• Hybrid Fission-Fusion Pellets

Orion

From Martin and Bond, JBIS

Courtesy of G. Smith

Nuclear Reactions

• DT Fusion Reaction

• Uranium Fission

• Lithium Fission

10

42

31

21 nHeHH

10

23892

10 2

21

1nXXUn N

kN

k

31

42

63

10 HHeLin

Fusion Reactions

• The DT reaction

• And Lithium fission reaction

• Are equivalent to

10

42

31

21 nHeHH

31

42

63

10 HHeLin

42

42

63

2HeHeLiH1

Thermonuclear Weapon

Antiproton Annihilation Reactions

• Antiproton-Proton Annihilation

• Antiproton-Neutron Annihilation

• Antiproton-Uranium Annihilation

nnmpp 0

)1(0 nnmnp

mnnPaUp 23791

23892

10

2

2

1

1nXX N

kN

k

Some Technical Challenges

• Compression Driver

• Cryogenic Storage

• Neutron Radiation Absorption & Heat Rejection

• Ignition

MICF Laser Pellet Ignition

MICF Antiproton Pellet Ignition

Antiproton Triggered MICF Ignition

Antiproton Triggered MICF Pellet

MICF Transient Magnetic Fields

sin

10

1sec10/1

2

d

m

keV

T

pMG

B e

sin

1.1

2/1)1(2/1102/1

16.3

10

ZA

d

m

keVeT

MG

B

teBtB 10)(

2/1

2/1)1(1.110

1sec36

1

A

Z

d

m

keV

T

pB

B e

Magnetic field intensities dependcritically on spot size.

Antiproton Dispersion

Antiproton Dispersion

Antiproton Dispersion Effects

• Annihilation

• Scattering

• Energy deposition

Annihilation Approximations

Scattering

• Particle physics approximations

–

–

–

• Monte-Carlo simulations

Energy Deposition

Monte-Carlo Simulations

• Two layers: fusion fuel & uranium• Each layer divided into 50 intervals• Updated antiproton direction, coordinates

and energy• Ten thousand simulations per case• Final beam radius set to final antiproton

position standard deviation• Spread angle set to 90 degrees.

Simulation Results

Antiproton Dispersion Conclusions

• Antiprotons:-Are a high energy density storage mechanism. -Can be used to initiate a fission reaction -Magnetic field strength depends on scattering-Beam energy at minimum of fusion fuel spectrum

• Need experiments to measure transmission spectra for antiprotons for low energy antiproton beams.

• Specific impulse well in excess of 50,000 seconds and high thrust-to-mass ratios are possible.

Tritium Fuel Considerations

• Tritium is naturally radioactive– Beta decay– Half-life ~12 years

• Tritium requires cryogenic storage

• Lithium-6 is not radioactive

• Lithium-6 does not require cryogenic storage

Deuterium-Tritium Pellet Construction

Lithium-Deuteride Pellet Construction

Pellet Discretization

Compression Simulation

Momentum Conservation

0)()(3

1 21

221

211

331 iiiiiiiiiii rprrprpurr

Mass Conservation.)( 33

1 constrr iii

Constitutive Lawp=p()

Initial & Boundary Conditions

• No initial displacements or velocities

• Center velocity is zero

• Outer pressure is zero

• Explosive temperature found from energy

• Explosive pressure from gas law

Neutron Interactions

• Scattering

• Fission – Uranium and Lithium

• Cross sections

• Mean free path– =1/n

Pellet Geometry

Internal External Both

Material Radiuscm

Material Radiuscm

Material Radiuscm

Am 0.01 Am 0.01 Am 0.01

LiH 1.00 LiH 1.00 LiH 1.00

U 1.25 TNT 1.75 U 1.25

TNT 2.00 U 2.00 TNT 2.00

- - - - U 2.25

Material Properties

Mat'l Mg/mole

f 0

g/cm3T0

degKK0

GPa

TNT 10.8 3 1.65 3500 -U238 238 3 18.9 300 100LiH 8 3 0.93 300 6.0H2 2 3 0.185 20 0.2

Nuclear Properties Determine Pellet Size

Molecule sc - cm

f - cmDT 16 -

Li6H2 6 14U238 3 20Pu239 4 8

Molecule n/1024 - 1/cm3

sc -

barns

f -

barnsDT 0.025 2.5 -

Li6H2 0.070 2.5 1.0U238 0.048 7.5 1.0Pu239 0.050 5.0 2.5

Internal Tamper

0

10

20

30

0.E+00 1.E-05 2.E-05

Time - s

Den

sity

- g

/cm

3

U

Am

TNT LiH

Lithium Fuel Conclusions

• Advantages:– Produces charged particles– Is not radioactive– Is solid at room temperature

• Disadvantages:– May require external compression– Will produce high energy neutrons

Hybrid Fusion-Fission Nuclear Pulse Propulsion

• Use of Li6 – Reduces tritium handling problems– Decreases specific impulse

• System can be developed in a two step process– Use fusion to boost the specific impulse of a

pulse fission rocket– Evolve to a full hybrid system

Ablative Shield Model

• Heat added from neutron absorption

• Heat transfer by conduction and radiation

• Heat lost through ablation– Moving coordinate system– Ablation velocity used

Ablation Model

q

vs

Q

Heat lost by radiation and ablation

Surface recession velocityNeutron heat added

One Dimensional Ablation Model

•Heat Source:nx

nn eInEdx

dIEQ 0

•Heat Conduction: 002

2

nx

ns eInEx

Tcv

x

T

•Boundary Conditions:

— At x temperature is at ambient

— At x=0 temperature is at sublimation

— At x=0: 0 sdT

Lvdx

Ablation Model Solution

•Surface Recession Velocity:

s

ns cT

IEv

0

•Temperature distribution:

n

ev

ev

n

nIET

nx

s

xv

s

ns

/0

One Sided Radiation Model

•Heat Source:nx

nn eInEdx

dIEQ 0

•Heat Conduction: 002

2

nx

n eInEx

T

•Boundary Conditions:

— At x temperature is at ambient

— At x=0 400 T

dx

dT

One Sided Radiation Solution

•Surface Recession Velocity:

0sv

•Temperature distribution:

nxnn en

IEIET

10

4/1

00

0

Two Sided Radiation Model

•Heat Source:nx

nn eInEdx

dIEQ 0

•Heat Conduction: 002

2

nx

n eInEx

T

•Boundary Conditions:

— At Lx

— At x=0 400 T

dx

dT

400 T

dx

dT

Two Sided Radiation Solution

•Surface Recession Velocity:

0sv

•Temperature distribution:

nxeCCT 121

•Two boundary conditions relate C1 and C2

•Arbitrary constants are solved for iteratively•Solution is checked numerically

Material Properties for Shield

Material Mol. Wt. L c Ts

g/mol kJ/mol KJ/mol-K kg/m3 W/m-K K b

C 12.01 815.9 8.527 2260 5.7 5100 4 W 183.92 859.4 24.27 19300 174 5930 20 U 238.12 432.6 27.665 18950 27.6 4018 10

WC 250.13 815.9 18.096 15630 73.3 6000 14

Neutron Heating andAblation Response Parameters

Material n n vs vs/ dmshield/dt

(1/cm3) (cm2/s) (1/cm) (cm/s) (1/cm) (kg/s)

C 1.13E-01 3.55E-02 4.53E-01 3.92E-01 1.10E+01 6.97E+02 W 6.32E-02 6.83E-01 1.26E+00 2.13E-01 3.11E-01 3.22E+03 U 4.79E-02 1.25E-01 4.79E-01 3.63E-01 2.90E+00 5.40E+03

WC 3.76E-02 6.48E-01 5.27E-01 4.73E-01 7.30E-01 5.81E+03

Carbon Radiation Shield

0

1000

2000

3000

4000

5000

6000

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

Distance from Surface (cm)

Te

mp

era

ture

(K

)

Radiation 1-Side

Radiation 2-Sides

Ablation Only

Ablation Conclusions

• Carbon shield may work without ablation

• Temperature is a maximum between the surfaces

• Ablation will begin at maximum temperature location

• Ablation will not be steady

Typical PelletGeometry

• Core radius 0.05 mm

• Fuel Radius 1.00 cm

• Tungsten Shell Thickness 0.10 mm

• Antiproton Beam Radius 0.10 m

• Uranium Hemisphere Radius 0.30 mm

Typical Pellet Performance

• Antiproton Pulse 2x1013 for 30 ns

• Maximum Field 24 MG

• Pellet Mass 3.5 g

• Specific Impulse– 600,000 s for 100% fusion– 200,000 s for 10% fusion

MICF Propulsion

• Parameters – 200,000 seconds specific impulse– 138 pellets per second– Mass ratio fixed to 1.5 for one-way missions

• Missions– 7 day trip to Mars: acceleration limited– 30 day trip to Jupiter: specific impulse limited– 180 day trip to Pluto: specific impulse limited

Promise of ICF Propulsion

• ICAN-II: 13,500 seconds specific impulse– 30 days to Mars– 90 day trip to Jupiter– 3 year trip to Pluto

• MICF: 200,000 seconds specific impulse– 7 days to Mars– 30 days to Jupiter– 180 days to Pluto

Antiproton Triggered Fusion Propulsion Conclusions

• Technical challenges– DT cryogenic storage– Pellet compression– Neutron radiation damage

• Solutions– Lithium fuel– Tampers and explosives– Non-ablating carbon shield

Future Work

• Complete accurate simulations– Ignition– Fusion propagation– Neutron generation

• Borrow weapon design ideas– Compression using heating and inertia– Fusion boosted fission

• Determine Transmission Spectra