Magnetic Collector for Traveling Wave Direct Energy...
Transcript of Magnetic Collector for Traveling Wave Direct Energy...
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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
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Acknowledgement
Work performed under contract from NASA Johnson Space
Center, Propulsion and Power , Energy Conversion branch (EP3)
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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
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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
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Fission Fragment Direct Energy
Conversion is not a New idea
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1966: Fission Fragment Direct Energy Conversion Experiment
Early JPL work: http://archive.org/details/nasa_techdoc_19670002490
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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.
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• 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)
Previous Work on Fission DEC
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The Big Picture: Where FFDEC Fits
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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)
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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
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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.
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Technology Issues and R&D Needs
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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
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Fission Fragments Extraction
Concept: a thin fissile layer is needed to extract most of the
fragments
Fragments
Structural SupportFissile Layer
Neutrons
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Accelerator Driven Subcritical Reactors
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Fragment Energy Direct Conversion
into Electricity
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• 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
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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.
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• 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
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TWDEC Beam Modulation
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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
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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.
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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
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Controlling Velocity Spread for TWDEC
Injection
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Particle Trajectory Studies
Region with retarding electric fieldCases with 25% Velocity Spread
No electric field
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Particle Trajectories Studies
Fragments velocity spread can be utilized for forming a bunched
beam
Region with retarding electric field10% Velocity SpreadNo electric field
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Particle Trajectories Studies
Doubled electric field25% Velocity SpreadRegion with retarding electric field
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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
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• 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
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Backup
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• 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
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Previous Work on Fission DEC
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Previous Work on Fission DEC