Presented at:

COFE 2006

Advances in Dense Plasma Focus

R&D for Space Power and Propulsion

George H. Miley

NPRE, University of Illinois, Urbana, Illinois, 61821

Acknowledgements

Largely based on:An Investigation of Bremsstrahlung Reflection in a Dense Plasma Focus Propulsion Device G.H. Miley, Robert Thomas, F.B. MeadPresented at STAIF 2006

andOn Use of D-He3 in Fusion Space PropulsionG. H. Miley, H. Momota, J. Shrestha, S. Krupakar Murali and John SantariusPresented at ANS Summer Meeting 2006

andPropulsion and Power Generation Capabilities of a Dense Plasma Focus (DPF) Fusion System for Future Military Aerospace VehiclesSean D. Knecht, Robert E. Thomas, Franklin B. Mead, George H. Miley, and H. David FroningPresented at STAIF 2006Prospects for Fusion propulsionFrancis Thio Report to FSAC on non-electrical uses of fusion, 2003.

OutlineWhy Fusion Propulsion?

Prior Fusion Propulsion Design Studies

Dense Plasma Focus Background

Applicability for Space Propulsion

DPF System Studies

Study of a key issue, Bremsstrahlung

Conclusions

Equations of Rocket Dynamics – two issues = exhaust velocity and jet power

Constant power - Rocket on full blast

Variable exhaust velocity to match the acceleration profile

exvmdt

dvm

Rocket momentum equation

Rocket energy equation

exexjet vamvmP2

1

2

1 2

3/1

1

0

20

1

2

2

3

m

mP

smt

jet

fFlight time

m0 - m1 = propellant mass burnt on the outbound2

2

0

1

22

DD qq

m

m

F

FDD mm

mq

m

m

00

,

Jet power

Exhaust velocity

v

t

Specific jet power = Pjet / m (kW/kg)

Fusion Will Provide Capabilities Not Available from Other Propulsion Options

10-5 10-410-3 10-2 10-1 101

103

104

105

106

107E

xh

au

st

ve

loc

ity

(m

/s)

Thrust-to-weight ratio

Gas-core fission

Nuclear(fission)electric

Fusion

Chemical

Nuclear thermal

1 kW/kg

10 kW/kg

0.1 kW/kg

JFS 2005 Fusion Technology Institute

Fusion PropulsionWould Enable Attractive Solar-System Travel

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

JFS 1999

Fast human transportFast human transport Efficient cargo transportEfficient cargo transport

JFS 2005 Fusion Technology Institute

The Challenges of Human Interplanetary Travel

Nearest approach to Earth(in 106 km)

Mercury 92Venus 41Mars 77Jupiter 629Saturn 1279Uranus 2725Neptune 4,354Pluto 5,750

Enormous distancesPhysiological hazardsCostsZero-g

Muscle and skeleton deterioration set in after about 100 days

Cosmic RadiationLeukemia and other cancer risks become significant after about one year in-orbit

The Challenges of Human Interplanetary Travel

Propellant exhaust vel > 500 km/sSpecific jet power > 10 kW/kg

Specific Flight Peak Peak Accel- TotalJet Power time exhaust velocity eration jet power(kW/kg) (days) velocity (km/s) (g) (GW)

0.1 710 173 15 0.00008 0.016 1 330 334 33 0.0003 0.16 10 153 806 71 0.0017 1.6 100 70 1740 154 0.008 16

Mission to Jupiter: IMLEO = 640 tonnes; Outbound payload = 200 tonnes; Return payload = 80 tonnes; Mass of propulsion system = 160 tonnes

Vehicle Trajcetory

0

100

200

300

400

500

600

700

800

-200 -100 0 100 200 300 400

xxx (Gm)

yyy

(Gm

)

Robotic Mission to the Outer Planets – Requires less power, but still MWs

Power System Spec. Mass (kg / kW)

Acceleration (g’s)

Payload Mass (kg)

Propellant Mass (kg)

Final Mass (kg)

IMLEO (kg)

Total of Flight (days)

Power (MW)

Isp (sec)

Thrust (N)

0.010.010.010.010.010.01

0.00310.00370.00360.00450.00520.0069

69,99869,96669,99469,99569,98469,962

186,740138,924144,32798,32668,78627,590

100,000100,000100,000100,000100,000100,000

286,740238,924244,327198,326168,786127,590

2312291731097227

3,0003,0033,0013,0013,0023,004

70,50070,50070,50070,50070,50070,500

8,6768,6858,6778,6778,6808,686

PlutoNeptuneUranusSaturnJupiterMars

Propulsion May Offer an Earlier Opportunity for Application of Fusion

The technical priorities for applying fusion to propulsion are somewhat different from those for terrestrial power generation, though the underlying plasma science and technologies have considerable overlapA qualitatively different, if not wider, window of technical options may be available to fusion for propulsion

Propulsion Terrestrial Power Generation

Conversion of fusion energy to thrust

Conversion of fusion energy to electricity

Mass per unit jet power Cost per unit electrical energy

Lower Q may be acceptable Q is a driver for COE

Vacuum without boundary is freely available in space

Vacuum with material boundary is a necessary part of the engineering

Fusion PropulsionConcepts : Past R&D Efforts (Prior to 1999)

1958 - 1978 Fusion Program NASA Rocket Propulsion,” J Lewis Research CenterRoth, J. R., “A Preliminary Study of Thermonuclear. British Planetary Society, 18, 99, (1961)Norman R. Schulze, “Fusion Energy for Space Missions in the 21st Century,” NASA Technical Memorandum 4298, Aug 1991.Hyde, Wood and Nuckolls, Laser fusion propulsion, (1972)Borowski, Spherical torus: 1000 tonne (1987)Santarius, Tandem mirror: 1200 tonne (1988)Orth, Laser fusion propulsion, VISTA: 1800 tonne (1987)Teller, et al., Dipole: 1300 tonne (1992)Carpenter, et al., Thermal barrier tandem mirror: 700 tonne (1993)Nakashima, et al, Field reversed configuration: 1000 tonne (1994)Smith, et al: Antiproton catalysed fusion, ICAN: 700 tonne (1996)Kammash/Emrich, Gasdynamic mirrors: 1000 mT (1995), 400 tonne (1998)Williams et al., Spherical Torus: 1300 tonne (1998)

Fusion Propulsion Concepts Presented at NASA Fusion Propulsion Workshop 2000

1 Spherical torus 10 Z-pinch 2 Electric tokamak 11 Field reversed configuration 3 Levitated dipole 12 Magnetokinetic compression

of compact toroid 4 Electric field bumpy torus 13 Spheromak 5 Laser driven ICF (with fast

ignition) 14 Colliding-beams FRC

6 Antimatter catalysed fusion 15 Tandem mirror 7 Dense plasma focus 16 Gasdynamic mirrors 8 Magnetized target fusion 17 Inertial Electrostatic Fusion 9 Magnetically compressed

compact toroid fusion

Physics Design “Drivers” for Fusion Propulsion

Fusion driver and fuelConversion of fusion energy into thrust

Example: Magnetic nozzle, Direct Energy conversion

Remote re-start capability becomes key issueRadiation shielding of crew and critical components – with a-neutronic fuels, space radiation sets limits for shielding

Enabling fusion technologies – confinement sys for a-neutronic fuels must be developedThermal management – must capitalize on high rejection temperature to minimize weight.

Costs at Initial Orbit in Space (IOS) – set bottom line

Costs at Initial Orbit in SpaceMission cost = Propulsion cost + Costs to achieve mission

objectivesPropulsion cost = Costs at IOS* + In-space Cost + Cost at DestinationCosts at IOS = Launch cost + Cost of producing the

propulsion unitThis must be reasonable ($5 B ~ $10 B?)Launch cost ~ $10,000/kg - today’s cost ~ $1,000/kg in 2025Cost of producing the propulsion unit ~ $20 K - $100K/kgFor a propulsion cost < $5 B Mass of propulsion system

< 250 tonnes

(*IOS – Initial Orbit in Space)

Examples of concepts for some fusion propulsion engines weighing less than 80 MT Dry studies prior to 2000

Flowing Liquid Metal Heat Exchanger/ Breeder

~ 20 m

BURN CHAMBER (Rc ~ 13 mm)

5 m

MagneticExpansion Nozzle

1 m

Accelerator Source

IMPAC

MKCCT: 20 MT, 300 MW (UW)

Colliding-beams FRC: 13 MT, 68 MW (UCI)

MTF: 80 MT, 4 GW

FIGURE 1. Image of 100 MWe IEC Fusion Powered Spacecraft with Ion Thruster Propulsion.

IEC: ? MT, ? MW (jet power)

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

JFS 2005 Fusion Technology Institute

A-neutronic fusion fuels are essential

D+T = n + He4 = std DOE fuelProblems Neutrons – radiation effects and

materail damage Tritium – radiactive and must breed Direct conversion of energy to thrust

low

Aneutronic – all charged particles D-He3; p-B11;He3-He3

Fusion cross sections illustrate the issues for going to a-neutronic fuels.

UIUC Design study of D-He3 IEC propulsion unit

Illustrates promises and issues Intended for relative near tern – uses DEC and proven ion thruster designIEC shares many features in common with DPF

Image of Fusion Ship II, 750 MWe IEC Fusion Powered Manned Spacecraft with Ion Thruster Propulsion

Orbital path leaving earth showing earth as the central circle.Initial orbit is at Geosynchronous orbit with spacecraft spiraling from that orbit outward to an escape velocity of 2.1 km/s at 29 earth radii.

Launching - Geosynchronous Orbit

Orbital Path Entering Jupiter’s Orbit and Reverse Thrust Braking

Orbital path entering Jupiter’s orbit showing location of stages of the transfer, achieved at full thrust to minimize time

Size Comparison of Several Current Spacecraft Designs & Two Fusion Spacecraft Designs

While fusion ship II deminsions are much large than for a Saturn rocket it must be remembered that Fusion ship II is for a much more demanding Jupiter round trip.

S p a c e S h u t t l e 2 , 0 4 1 M T I s p = 3 5 0 T h r u s t = 3 1 , 0 5 4 k N

S a t u r n V 2 , 7 6 6 M T I s p = 3 0 0 T h r u s t = 3 3 , 3 6 2 k N

I E C F u s i o n S h i p I 5 0 0 M T I s p = 1 6 , 0 0 0 T h r u s t = 1 0 2 8 N

I E C F u s i o n S h i p I I 5 0 0 M T I s p = 3 5 , 0 0 0 T h r u s t = 4 3 6 9 N

Dense Plasma Focus (DPF) – a key approach to p-B11

One of first fusion concepts - originally developed in the mid 1950’s

Prior to ITER funded by US Government (NASA, DOE).

Development still in early stages ; to date experiments with units < 1 MJ have shown the primary feasibility of the concept

Operational Phases of DPF

1. Breakdown phase – Capacitor bank discharged across electrodes ionizing gas and forming a plasma sheath

2. Rundown Phase – J x B force accelerates plasma sheath down length of anode

3. Pinch Phase – Collapsing sheath focuses towards the central axis of the anode forming a plasma where fusion reactions take place

DPF Suitability for Space Propulsion

.

Ideally suited for p-B11 – High density pinch plasma and no B field induced radiation loses

Can provide the necessary exhaust velocity; Specific Impulse from 2000 s to 106 s, trading off lower

values with higher thrust

Can provide the necessary specific energy: ~ 100 times higher than conventional chemical systems

DPF Rocket Schematic – Advantages = simple and low mass structures; efficient thrust

development

DPF Model Assumptions set requirements

Fine structure fusion dominates giving high Ti/Te ratio

Pinch lifetimes, can be extended an order of magnitude longer than present experimental values

Fusion fuel and charged fusion products are confined during entire pinch

Refection of Bremsstrahlung above 50%

Using these assumptions we obtain:

For 500 kN, 2000 sec Isp p-B11 DPF, the required pulse power, energy, and voltage are:

3.07 Q

kV 400

MJ 80

MW 800

0

V

W

Power

Physics Issues Ascertained From Study:

Investigation of achieving a high Ti/Te

Methods of increasing pinch lifetime

Reflection of Bremsstrahlung

Direct energy conversion of plasma; e.g. B field penetration

A Recent Study of Key p-B11 Issue by R. Thomas EAFB- Bremsstrahlung Control

Investigate reflection physics for high energy Bremsstrahlung radiation emission during p-11B fusion Identifies 2 potential approaches - Hohlraum Cavities and Super Multilayers

For 500 kN Thrust Level, ~ 10 m of Reflector Material Ablated per Day (10 Hz Pulsed Continuously)

Classical Heat Transfer Analysis formulated under conditions of thermonuclear interest (Kammash) used to estimate wall ablation and temperatures

Most severe thermal loading at end of discharge when plasma “dumped” on wall

For T > 105 K, plasma forms at wall- it becomes an intense radiator itself

Plasma wall temperatures greatly exceed 106 K--- hence plasma forms at wall- Favorable to prevent ablation and use Holhraum physics for reflectivity

Use extensive data from Inertial Confinement Fusion

(ICF) Hohlraum target studies

Haan, S., “On Target Designing for Ignition,” http://www.llnl.gov/str/Haan.html

Cylindrical gold-plated cavities.

Laser used to implode fuel pellet

Confinement Arises because cavity walls heat up and becomes strong emitter of soft x-ray radiation

For numerical modeling laser replaced by fictitious source of x-rays inside cavity- in our case this is the p-11B reaction

X-Ray Reemission Model

Pakula, R., and Sigel, R., Phys. Fluids 28, 232 (1985).

At t = 0 body is brought into contact with thermal bath

For t > 0, nonlinear wave runs into undisturbed material

Heat wave overtaken by shockwave and ablative heat wave forms

Reemission flux scales by

Sre = 13.0Sint0.46

At a Bremsstrahlung Flux of 1013 W/cm2

the radiation is reflected ~10 times before being lost

Radiation reemission increases with incoming flux

10 reemissions before being lost appear possible

Alternate concept - Reflective Multilayers Provide Reflection over a Wide Energy Range

Layer spacing gradually decreased as a function of depth

Lower energy photons reflected at surface

Tungsten, Lead, Carbides typically used

Joensen, K.D., Nuc. Instr. and Methods in Phys. Research B, 132, pg. 221, 1997.

Multilayer Structures Provide Superior Reflectivities over pure Gold

Tungsten/ Silicon Mirror Used- reflectivities above 30% in entire band

Cutoff at in performance 69.5 keV

Tungsten/ Silicon Carbide successfully reflect over 100 keV (DPF photons > 200 keV)

Limited to very small angles

Joensen, K.D., Nuc. Instr. and Methods in Phys. Research B, 132, pg. 221, 1997.

Limitations of both concepts

Reemission in Hohlraum cavities increase with incoming flux- however high flux leads to higher deterioration of inner walls

Hohlraum physics does not provide reemission over broad energy range

Multilayers currently in use would be destroyed at radiation levels found in p=11B DPF fusion

Multilayers limited to small angles ( < 5 mrad)

Conclusions – Bremsstrahlung

Radiation reemitted 10-18 times depending on Hohlraum size- this may correspond to the 50% re-absorbtion rate previously assumed - further inverse Bremsstrahlung work must be done

Additional multilayer work must be done at high energies (> 150 keV)

Issue #2 – Confinement and low Te/Ti – filament dominated DPF pinches are the key approach

DPF filament formation (Nardi, et al.)

Lerner’s theory for filament dynamics, forming plasmoids

Proposed experiment – controlled filament type of DPF “cage Z-pinch”

Micro-projections anchor filament locations

Filament spacing controllable in Dielectric Barrier Discharges (DBD)

Filament spacing as a function of voltage in the DBD.

Filament DPF simulates the Sandia Labs “Z Machine”, but is much more compact

The Z-pinch principle has been demonstrated with Sandia’s Z accelerator, where very large energy output (1.8 MJ of x-rays) and power levels (up to 230 trillion watts) have been achieved by imploding wire arrays with high load currents (20 MAs).

Propulsion and Power Generation Capabilities of a Dense Plasma Focus (DPF) Fusion System for Future Military Aerospace Vehicles

Presented by Sean D. Knecht for the Space Technology & Applications International Forum (STAIF – 2006), Albuquerque, NM15 February 2006

Evaluation of System Details – assumes reflection and filaments

With system geometry and performance determined and capacitor energy assumed, other system parameters were calculatedMultiplying capacitor energy by specific energy (1.0 – 15.0 kJ/kg) the capacitor mass was found

This mass was assumed to be half of the system mass Thrust-to-weight ratio were then be determined

Capacitor bank volume and system volume were found by assuming a capacitor mass density

Current state-of-the-art is ~ 3.0 MJ/m3

Assuming for advances in the next 20 years, a value of 5.0 MJ/m3 was assumed for this study

Additional power for electricity was found by varying Q and ηprop from their baseline values

Results – Baseline Design Promising

System details that resulted in total system masses between 15 and 25 metric tons were reportedReported baseline parameters Q = 3.0 ηprop = 0.9 Thrust = 500 – 1,000 kN Isp = 1,500 – 2,000 s Capacitor specific energy = 10.0 – 15.0 kJ/kg Thrust-to-weight ratio (T/W) = 20.83 – 44.12 kN/MT System Volume = 25.5 to 54.0 m3

Can we do it??? DPF development, vs. tokomak fusion, has distinct the advantage of allowing small size near-term “products”

Examples -

neutron source for NAA, HS, etc.

Xray source

Light source for semiconductor mfg.

Longer term -- Ultra Hot Fusion Plasmas ProvideMany 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 2005 Fusion Technology Institute

Final Comments

Fusion Propulsion is one of the main options for deep space propulsionOf the various fusion propulsion schemes, the DPF, initially using D-He3, then p-B11 is an outstanding option.Much R&D is needed, but compared to the present DOE terrestrial fusion power programs, the DPF development would be cheaper and faster. Also, there are intermediate uses possible, including as a neutron source and for a light source for semiconductor mfg.

Thank you for your attention

for further information or discussion, contactGeorge H. MileyUniversity of Illinois, UC Campus100 NEL, 103 S. Goodwin Ave.Urbana, Illinois, 61802 USA217-3333772; ghmiley@uiuc.edu

Visit my poster to discuss =The 500-W UIUC/NPL NaBH4/H2O2 Fuel Cell

The active area per cell was 144 cm2 and 15 cells were employed to provide a total stack active area of

2160 cm2.