magnetic collector for traveling wave direct energy...

© 2015 Electric Power Research Institute, Inc. All rights reserved.

A. G. Tarditi1, J. H. Scott2

1Electric Power Research Institute,, Knoxville, TN 2NASA Lyndon B. Johnson Space Center, Code EP3, Houston, TX

Magnetic Collector for Traveling

Wave Direct Energy Conversion

of Fission Reaction Fragments

NETS 2015, Albuquerque, NM, February 2015

2© 2015 Electric Power Research Institute, Inc. All rights reserved.

Acknowledgement

Work performed under contract from NASA Johnson Space

Center, Propulsion and Power , Energy Conversion branch (EP3)


Summary

Fission fragment direct energy conversion (FFDEC) into electricity can dramatically improve the specific mass of fission-based electric propulsion rocket

This study is focused on the conversion via traveling wave DEC, that has the advantage of being able to generate high frequency power (MHz range) and does not require high voltage technology, unlike electrostatic energy conversion.

The FFDEC is considered as a best fit to an accelerator-driven fission core to improve the efficiency and the overall specific mass

Future work:

– Fragment extraction

– Neutralization issues


Introduction

Goal: improve drastically the fission-electric rocket specific

mass to reach

The problem of fragment extraction: need a thin core

Fission criticality: presence of a moderator impacts system mass

Solution:

– Subcritical fission core (thin, light), neutron source driver, electrical power

re-circulating from FFDEC

– Interesting analogy: for a fusion reactor DEC requires aneutronic fusion,

that also requires a driven system


Fission Fragment Direct Energy

Conversion is not a New idea


1966: Fission Fragment Direct Energy Conversion Experiment

Early JPL work: http://archive.org/details/nasa_techdoc_19670002490

http://archive.org/details/nasa_techdoc_19670002490


Previous Work on Fission DEC

Schematic of proposed Fission Fragment Rocket.

Fissile dusty plasma fuel is confined to dust chamber,

where RF induction coils heat the plasma.

Fission fragments are collimated by the magnetic field

either to collection electrodes for power, or exit the re-actor

for thrust.


• Fission fragments direct energy conversion has been

considered in the past for increasing power plant efficiency

[1-3] and for space propulsion [4-6]

• These concepts are focused on the direct conversion of the

charged fragments utilizing high-voltage DC electrodes.

__________________________________________[1] S. A. Slutz et al. ,Phys. Plasmas 10, 2983 (2003)

[2] P. V. Tsvetkov, et al., Trans. American Nucl. Soc., 91, 927 (2004)

[3] http://www.ne.doe.gov: 2003 and 2004 annual reports

[4] R. Clark and R. Sheldon, AIAA 2005-4460 (2005)

[5] G. Chapline and Y. Matsuda, Fusion Technology 20, 719 (1991)

[6] P. V. Tsvetkov, et al., AIP Conference Proceedings 813.1, 803, (2006)



The Big Picture: Where FFDEC Fits


Motivation

Making Nuclear (Fission) Electric Rocket a More Appealing

Option for Space Propulsion

Direct energy conversion instead of steam cycle

– Less heat: less radiators

– No pumps, pipes, generators: lowering mass

– Lowering mass + Improving efficiency

Reactor with Lower Specific Mass a (kg/kW)


Fitting in the Big Picture of Space Propulsion

Typical Electric Propulsion Concept: separate electric power generation and propulsion systems

Electric Propulsion

Thruster

Power Conditioning

Electric

Power

Exhaust

Power Conversion

Primary Energy

Source


What about Direct Fragment Utilization?

Acknowledgement: fission fragment direct energy conversion into

propulsion by R.A. Clark and R. B. Sheldon

– This proposed approach is complementary to the fission fragment direct

utilization for propulsion since it can provide a more versatile scenario

where a lower Isp is provided by plasma acceleration while at the same

time some of the extremely high Isp provided by the fission fragment

beam is being reduced.


Technology Issues and R&D Needs


Exploring of DEC configurations that could be implemented

within a nuclear fission core

Requires collecting and collimating a beam of charged fission

fragments

Options:

- thin solid core for optimal fragment extraction, [1]

- dust core [4]

- gas core (e.g. vortex confinement, [7])

___________________________[7] Sedwick, AIAA Journal of Propulsion and Power, Vol 23, No. 1, Jan-Feb

2007.

Fission Fragment Extraction


Fission Fragments Extraction

Concept: a thin fissile layer is needed to extract most of the

fragments

Fragments

Structural SupportFissile Layer

Neutrons


Accelerator Driven Subcritical Reactors


Fragment Energy Direct Conversion

into Electricity


• 235U => 140Xe + 94Sr + 2n

• Consider a 100 MeV 140Xe fragment with a +20e charge

• 140Xe fragment speed vXe=1.17∙107 m/s

• For a inter-electrode TWDEC distance of d=1 m the

frequency of the AC power is f0=vXe/2d=5.85 MHz

TWDEC Feed Example


TWDEC Conceptual Scheme

Travelling Wave Direct Energy Converter (TWDEC) [Momota, 1990, 1992]

Conceptual scheme: the energy of the bunched ion beam is collected in the

Decelerator Section producing AC electric output. A small residual energy in the

beam is absorbed at the end.


• No high-voltage electrodes, collects the energy of the beam

through a series of electrode pairs, each at a smaller alternating

potential.

• Electrodes capacitively coupled to a density-modulated

(bunched) beam of charged particles

• Beam bunches travel through of properly spaced electrodes

inducing an alternating potential

• Alternating current has several advantages over DC in terms

power conditioning and distribution

TWDEC Operation


TWDEC Beam Modulation


Fission Fragment Beam Density Modulation

2D PIC code (XOOPIC) in cylindrical geometry

– Beam: 100 MeV, charge 20 e, atomic mass A=100 amu.

– 2 cm radius, 1 m length

– electrodes 5 long, 10 cm apart,AC potential several kV range

4 c

m1 m

t=t0t=t1 >t0

Particles injected Particles leave


Fission Fragment Collimation/Transport Option

Alternating-gradient

beam focusing

Charged fission fragments (positively charged, about 20

electron charges) are magnetically collected and focused

Fission fragment beam of relatively low density, to avoid

significant space charge effects.


Collimated

Fragment

Beam

B

• Side injection can reduce drift speed and TWDEC frequency

• Bunching can provide the non-adiabatic injection required to capture the ions.

Fission Fragment Collimation Example

• Solenoidal magnetic field B0= 0.5 T:

- 140Xe fragment gyroradius= 1.71 m

Fragment at reduced drift

speed into TWDEC


Controlling Velocity Spread for TWDEC

Injection


Particle Trajectory Studies

Region with retarding electric fieldCases with 25% Velocity Spread

No electric field


Particle Trajectories Studies

Fragments velocity spread can be utilized for forming a bunched

beam

Region with retarding electric field10% Velocity SpreadNo electric field


Particle Trajectories Studies

Doubled electric field25% Velocity SpreadRegion with retarding electric field


Conditioning Fragment Beam with Velocity Spread

The beam is injected into a bending magnetic field (velocity filter). Particles with

higher speed will be deflected less. Three cases are shown. Fast (F), medium

(M) and slow (S) particles are co-injected. After the deflection the incident angles

with respect to the solenoidal magnetic field are different: aS > aM > aF.

0i ii

i vf

m vr

q B

unidirectional

beam of

fragments at

different speeds

velocity filter: particles with

larger speed will be deflected

less

injection into a solenoidal magnetic

field: faster particles entering at a

larger angle to the field lines.

faster particles will have a smaller

fraction of their original speed directed

along the solenoidal magnetic field

lines.

beam still made of particles spiraling

with different radii around the field

lines, but characterized by longitudinal

drift speed.

1

2 3

4

5


• Fission fragments need thin fissile fuel

elements

• Fragments carry large positive charge

(≈20 e) and can be collimated with

magnetic fields

• Traveling Wave DEC converts fragment

energy into AC electric power

• Subcritical fission driven systems

combined with fission DEC may become

the most efficient option

Summary


Backup


• In “real life” multiple fragment products (different masses

and energies) must be considered

• Multiple channels may be required for efficiency

• Electron flow must be dealt with

TWDEC Fission Challenges

magnetic collector for traveling wave direct energy...

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