February 5-6, 2004 HAPL meeting, G.Tech. 1 Survivable Target Strategy and Analysis Presented by A.R. Raffray Other Contributors: B. Christensen, M. S. Tillack UCSD D. Goodin, R. Petzoldt General Atomics HAPL Meeting Georgia Institute of Technology Atlanta, GA February 5-6, 2004
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February 5-6, 2004 HAPL meeting, G.Tech. 1 Survivable Target Strategy and Analysis Presented by A.R. Raffray Other Contributors: B. Christensen, M. S.
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February 5-6, 2004HAPL meeting, G.Tech.
1
Survivable Target Strategy and Analysis
Presented by A.R. Raffray
Other Contributors: B. Christensen, M. S. Tillack
UCSD
D. Goodin, R. PetzoldtGeneral Atomics
HAPL MeetingGeorgia Institute of Technology
Atlanta, GAFebruary 5-6, 2004
February 5-6, 2004HAPL meeting, G.Tech.
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Outline
• Survivable Target Strategy
• Accommodation and Sticking Coefficients
• Phase Change
• Summary
February 5-6, 2004HAPL meeting, G.Tech.
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Overall Strategy to Develop a Survivable Target
• Uncertainty in chamber gas requirements and resulting heat flux on target
- Min. gas density set by chamber wall protection
- Max. gas density set by target placement and tracking accuracy
- Uncertainty in accommodation and sticking coefficients for high temp. chamber gas on cryogenic target
• Prudent to consider dual target approach and address key issues- Basic target
- Thermally robust target with insulated foam coating
- Increase target heat flux accommodation through low temp. target and possible allowance of phase change
• Once sufficient information available down-select “best”target design
Several Factors Influence the Heat Flux on the Target from the Chamber Gas
• The condensation or ‘sticking’ coefficient
• The accommodation coefficient (≈ “fraction of energy transfer”)
• Target shielding by cryogenic particles leaving the surface of the target
• Evaporation/sublimation of condensed background gas due to radiation heat transfer
Incoming High Temperature Background Gas (T ~ 4000 K)
Condensed Material
Outgoing Cryogenic Gas
Radiation From Chamber Walls
IFE
TARGET
February 5-6, 2004HAPL meeting, G.Tech.
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Condensation (Sticking) Coefficient of High Temperature Gas on Cryogenic Target(Very Little Data Found, Applicable to our Prototypical Conditions)
2 x 1014 s-1cm-2
4 x 1015 s-1cm-2
4 x 1016 s-1cm-2
CO2 Beam on Cu Target
Ar Beam on Cu Target
1400 K
300 K
• Condensation coefficient is a function of several parameters, including:- Ttarget, Tgas, flux, angle of incidence...
• Condensation coefficient decreases rapidly with increasing Ttarget past a certain point (Brown, et al., 1969) - No obvious mechanisms causing
the threshold (i.e melting or boiling point of gas species)
- MP (Ar) = 83.8 K- BP (Ar) = 87.3- MP (CO2) = 194.6 K
- BP (CO2) = 217.5 K
• For an insulated target the surface temperature will increase rapidly; thus the condensation coefficient will decrease rapidly
Con
den
sati
on C
oeff
icie
nt
Con
den
sati
on C
oeff
icie
nt
Target Temperature (K)
Target Temperature (K)
February 5-6, 2004HAPL meeting, G.Tech.
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DSMC Results of Heat Flux for No Sticking and Complete Accommodation
• Results shown in Frost (1975) indicates accommodation close to unity for 1400K Ar over a wide range of Cu target temperature and surface conditions (77-280 K)
• Effect of shielding from no sticking for accommodation of unity show a slight reduction in heat flux due to shielding effect
• Bubble size for which bending theory approaches membrane theory is independent of pressure, ~ 37 m in this case
• Would need much smaller bubble size in target to avoid large “membrane-like” deflections
February 5-6, 2004HAPL meeting, G.Tech.
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Pre-existing Vapor Bubbles Could Close if Initial Bubble is Below a Critical Size and
the Heat Flux Above a Critical ValuePlastic Shell
Local Vapor Bubble
Rigid DT Solid
tv,o
ro
• Encouraging results for self-healing• Need verification with 2-D model + experimental data• Physics requirements (bubble has close but are solid+liquid layers ok?)
0.00E+00
1.00E-06
2.00E-06
3.00E-06
4.00E-06
5.00E-06
6.00E-06
7.00E-06
8.00E-06
0 1 2 3 4 5 6 7 8 9 10
Heat Flux (W/cm2)
Vapor Thickness (m)
Rigid, tv_o = 1e-6 m
Rigid, tv_o = 3e-6 m
Bending, tv_o = 1e-6 m, ro = 5e-6 m
Bending, tv_o = 3e-6 m, ro= 5e-6m
Bending, tv_o = 1e-6 m, ro = 7e-6 m
t = 0.015 s
Tinit = 18 K
+
February 5-6, 2004HAPL meeting, G.Tech.
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Summary• A dual-target strategy is proposed: basic target + thermally
robust target
• Converge on final target design once sufficient information is obtained on:- Target fabrication and behavior- Heat loads on target (chamber gas density, sticking + accommodation coefficients)- Physics requirements
• Small pre-existing vapor bubbles (defects) could be eliminated by solid to liquid phase change (self-healing)- Depends on heat flux and size of bubble- Based on 1-D model and assumptions such as rigid solid DT- Need experimental data and 2-D model to better understand- Is this acceptable based on target physics requirements?