IFE Chamber Walls: Requirements, Design Options, and Synergy with MFE Plasma Facing Components

1
IFE Chamber Walls Requirements, Design Options,
and Synergy with MFE Plasma Facing Components

• A. R. Raffray1, D. Haynes2 and F. Najmabadi1
• 1University of California, San Diego, 458 EBU-II,
  La Jolla, CA 92093-0417, USA
• 2Fusion Technol. Inst., Univ. of Wisconsin, 1500
  Eng. Dr., Madison, WI 53706-1687, USA
• PSI-15
• Gifu, JapanMay 27, 2002

2
Outline

• IFE chamber operating conditions
• Comparison with MFE
• Dry Walls (major focus of presentation)
• Design operating windows
• Critical issues and required RD
• Synergy with MFE
• Wetted Walls
• Example analysis and critical issues
• Concluding Remarks

3
IFE Operating Conditions

• Cyclic with repetition rate of 1-10 Hz
• Target injection (direct drive or indirect
  drive)
• Driver firing (laser or heavy ion beam)
• Microexplosion
• Large fluxes of photons, neutrons, fast ions,
  debris ions toward the wall
• - possible attenuation by chamber gas

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Energy Partitioning and Photon Spectra for
Example Direct Drive and Indirect Drive Targets
Energy Partitions for Example Direct Drive and
Indirect Drive Targets
Photon Spectra for Example Direct Drive and
Indirect Drive Targets
(25)
(1)

• Much higher X-ray energy for indirect drive
  target case (but with softer spectrum)
• More details on target spectra available on ARIES
  Web site http//aries.ucsd.edu/ARIES/

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Example IFE Ion Spectra
154 MJ NRL Direct Drive Target
458 MJ Indirect Drive Target
Fast Ions (2)
Fast Ions (12)
Debris Ions (4)
Debris Ions (16)
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There are Similarities Between IFE and MFE Armor
Operating Conditions e.g. ITER Divertor and 154
MJ NRL Direct Drive Target Spectra Case
Although base operating conditions of IFE
(cyclic) and MFE (steady state goal) are
fundamentally different, there is an interesting
commonality between IFE operating conditions and
MFE off-normal operating conditions, in
particular ELMs - Frequency, energy
density and particle fluxes are within
about one order of magnitude Assess
performance of chamber dry wall option under
these direct-drive target conditions
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Candidate Dry Chamber Armor Materials Must Have
High Temperature Capability and Good Thermal
Properties for Accommodating Energy Deposition
and Providing Required Lifetime
Carbon and refractory metals (e.g. tungsten)
considered - Reasonably high thermal
conductivity at high temperature (100-200
W/m-K) - Sublimation temperature of carbon
3370C - Melting point of tungsten 3410C
In addition, possibility of an engineered
surface to provide better accommodation of high
energy deposition is considered - e.g. ESLI
carbon fiber carpet showed good performance
under ion beam testing at SNL (5 J/cm2 with
no visible damage)
Example analysis results for C and W armor for
NRL 154 MJ direct drive target case
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Characteristics of the Target Spectra Strongly
Impact Chamber Wall Thermo-Mechanical Response
Penetration range in armor dependent on ion
energy level - Debris ions (20-400 kev) deposit
most of their energies within mms - Fast ions
(1-14 Mev) within 10s mm Important to
consider time of flight effects (spreading energy
deposition over time) - Photons in sub
ns - Fast ions between 0.2-0.8 ms - Debris
ions between 1-3 ms - Much lower maximum
temperature than for instantaneous energy
deposition case
Energy Deposition as a Function of Penetration
Depth for 154 MJ NRL DD Target
Ion Power Deposition as a Function of Time for
154 MJ NRL DD Target
Chamber Radius 6 m
9
Temperature History of C and W Armor Subject to
154MJ Direct Drive Target Spectra with No
Protective Gas
For a case without protective gas - Tungsten
Tmax lt 3000C (MP3410C) - Some margin for
adjustment of parameters such as target
yield, Rchambe, Tcoolant, Pgas - Similar
results for C (Tmax lt 2000C) All the action
takes place within lt100mm - Separate
functions high energy accommodation in thin
armor, structural function in chamber wall
behind - Focus IFE effort on armor can use
MFE blanket
10
Target Injection Requirements Impose Constraints
on Pre-Shot Chamber Gas Conditions
Total qmax on injected target is limited to
avoid D-T reaching triple point and possibly
causing local micro-explosion instability For
a direct drive target injected at 400 m/s in a 6
m chamber, qmax lt6000 W/m2 - Max. qrad from
the wall 6000 W/m2 for Twall 545 K - Example
combinations of TXe and Pxe resulting in a max.
qcondens. 6000 W/m2 - Tgas1000 K and PXe
8 mtorr - TXe 4000 K and PXe 2.5
mtorr - Narrow design window for direct drive
target - Need more thermally robust target No
major constraint for indirect drive targets (well
insulated by hohlraum)
11
Example Design Window for Direct-Drive Dry-Wall
Chambers
12
In addition to Vaporization, Other Erosion
Processes are of Concern in Particular for Carbon
Chemical Sputtering Radiation Enhanced
Sublimation - Increases with temperature
Physical sputtering - Not temperature-dependent
- Peaks with ion energies of 1kev (from J.
Roth, et al., Erosion of Graphite due to
Particle Impact, Nuclear Fusion, 1991)
13
Tritium Inventory in Carbon is a Major Concern

• Operation experience in todays tokamaks strongly
  indicates that both MFE and IFE devices with
  carbon armor will accumulate tritium by
  co-deposition with the eroded carbon in
  relatively cold areas (e.g. R. Causeys ISFNT-6
  presentation) - H/C ratio of up to 1
• - Temperature lower than 800 K
• Source of carbon in IFE
• - From armor C dry wall (even one molecular
  layer lost per shot results in cms of C lost
  per year)
• - From target (but much smaller amount)
• Redeposition area in IFE
• - C armor at high temperature (2000C)
• - However, penetration lines for driver and
  target injection would be much colder
• If C is to be used, techniques must be
  developed for removal of co-deposited T
• - Baking, mechanical, local discharges

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Major Issues for Dry Wall Armor Include
MFE IFE P P P P
P P P P
P P P
P P
P P
Commonality of Key Armor Issues for IFE and MFE
Allows for Substantial RD Synergy
15
Major Issues for Wetted Wall Chambers

• Chamber clearing requirements
• Vapor pressure and temperature
• Aerosol concentration and size
• Condensation trap in pumping line

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Processes Leading to Vapor/Liquid Ejection
Following High Energy Deposition Over Short Time
Scale
Surface Vaporization
Liquid Film
X-Rays
Impulse
Spall Fractures
Impulse
Phase Explosion Liquid/Vapor Mixture
17
High Photon Heating Rate Could Lead to Explosive
Boiling

• Effect of free surface vaporization is reduced
  for very high for heating rate (photon-like)
• Vaporization into heterogeneous nuclei is also
  very low for high heating rate
• Rapid boiling involving homogeneous nucleation
  leads to superheating to a metastable liquid
  state
• The metastable liquid has an excess free
  energy, so it decomposes explosively into liquid
  and vapor phases.
• - As T/Ttc increases past 0.9, Becker- Döhring
  theory of nucleation indicate an avalanche-like
  and explosive growth of nucleation rate (by
  20-30 orders of magnitude)

Ion-like heating rate
Photon-like heating rate
From K. Song and X. Xu, Applied Surface Science
127-129 (1998) 111-116
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Phase Explosion from Photon Energy Deposition
Would Provide a Source Term for Aerosol Formation
in Chamber
Assumed ablated Pb vapor pressure 1000 torr
Example Results from Volumetric Model with Phase
Explosion in Pb Film Liquid and vapor mixture
evolved by phase explosion shown by shaded area
- 0.5 mm with quality gt0.8 Could be higher
depending on behavior of 2-phase region
behind Initial source for aerosol formation
Esensible Energy density required for the
material to reach the saturation temperature E (
0.9 Ttc ) Energy density required heat the
material to 0.9 Tcritical Et Total evaporation
energy ( Esensible E Evaporation)
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Analysis of Aerosol Formation and Behavior

• Spherical chamber with a radius of 6.5 m
• Surrounded by liquid Pb wall
• Spectra from 458 MJ Indirect Drive Target

From P. Sharpes calculations, INEEL
Region 1
Region 4

• From this example calculations, significant
  aerosol particles present after 0.1 s
• 109 droplets/m3 with sizes of 1-10 mm in Region
  1
• This could significantly affect target injection
  (approximate limits 50 nm limit for direct drive
  and about 1 mm for tracking) and driver firing
  and necessitate additional chamber clearance
  actions
• More detailed analysis under way (aerosol
  behavior target and driver requirements)

20
Film Condensation Rate Would Affect the Pre-Shot
Chamber Conditions for a Thin Liquid Film
Configuration
Example Analysis of Pb Vapor Film Condensation in
a 10-m Diameter Chamber
Characteristic time to clear chamber, tchar,
based on condensation rates and Pb inventory
for given conditions For higher Pvap (gt10 Pa
for assumed conditions), tchar is independent
of Pvap As Pvap decreases and approaches Psat,
tchar increases substantially
Typically, IFE rep rate 110 Time between
shots 0.11 s Pvap prior to next shot could
be up to 10 x Psat
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Analysis Experiments of Liquid Film Dynamics
and Thick Liquid Wall Hydraulics Are On-going

• 2-D 3-D Simulations of liquid lead injection
  normal to the chamber first wall using an
  immersed-boundary method (Georgia Tech.)
• Onset of the first droplet formation
• Whether the film "drips" before the next fusion
  event
• Lead film thicknesses of 0.1 - 0.5 mm
  injection velocities of 0.01 - 1 cm/s
• Inverted surfaces inclined from 0 to 45 with
  respect to the horizontal
• Experiments on high-speed water films on
  downward-facing surfaces, representing liquid
  injection tangential to the first wall (Georgia
  Tech.)
• Reattachment of liquid films around cylindrical
  penetrations typical of beam and injection port
• Experiments and modeling of thick liquid jet
  formation and behavior (UCB, UCLA)
• Understand behavior of thick liquid jet and
  formation of pocket and required penetration
  space
• Preferred fluid candidate is FLiBe
• These issues and activities are relevant to both
  IFE and MFE

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Concluding Remarks

• Very challenging conditions for chamber wall
  armor in IFE
• Different armor materials and configurations
  are being developed
• - Dry wall option
• - Wetted wall options
• - Similarity between MFE and IFE materials
• Some key issues remain and are being addressed
  by ongoing RD effort
• - Many common issues between MFE and IFE chamber
  armor

Very beneficial to - develop and pursue
healthy interaction between IFE and MFE
communities - make the most of synergy
between MFE and IFE chamber armor RD