update on various target issues presented by ron petzoldt d. goodin, e. valmianski, n. alexander, j....
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Update on Various Target Issues
Presented by Ron Petzoldt
D. Goodin, E. Valmianski, N. Alexander, J. Hoffer
Livermore HAPL meetingJune 20-21, 2005
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IFT\P2005-071
Accomplishments
1) We demonstrated improved tracking with 1st generation system
2) Evaluated impurity effects on target reflectivity
3) Modeled the impact of foam shell non-concentricity on DT ice non-concentricity
4) Calculated time limits for “handoff” of layered targets to an injector
5) Completed cryogenic coil resistance testing
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IFT\P2005-071
1)Improved tracking
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IFT\P2005-071
The “Gen-I” system is tracking targets full length for position prediction calculations • Improved laser beam collimation reduced cross-talk
between horizontal and vertical position measurements
Laser
D2 measurements taken in two horizontal positions 20 mm apart
Targetheight
0 mm
25 mm
-15
-10
-5
0
5
10
15
20
0 10 20 30
Vertical position
Measurement pixel change
D2 Old Optics
D2 New Optics
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IFT\P2005-071
-5.00
0.00
5.00
10.00
15.00
20.00
25.00
1 2 3 4 5 6 7 8 9 10
Shot number
Target Height (mm)
DCC Measured Pos
DCC Predicted Pos
Predict error
-5
0
5
10
15
20
25
1 2 3 4 5 6 7
Shot number
Target height (mm)
DCC Measured Pos
DCC Predicted Pos
Prediction error
Target position prediction improved from 2.0 mm to 0.49 mm (1 )
• Measured position in flight at two stations, predicted position at DCC, measured position at DCC, and compared measurement/prediction
• “Gen-II” tracking system is under evaluation (Graham Flint talk)
Gun D1 (4.1 m) D2 (8.7 m) DCC (17.7 m)
Shots fromOctober 2004
Shots from3 June 2005
Air rifle shotsAir rifle shots
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IFT\P2005-071
2)Impurity effects on target reflectivity- Impurities in DT supply- Transfer to the layering system- Impurities in the cryogenic fluidized bed- Transfer to the injector
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IFT\P2005-071
Impurity gases can freeze on target surface and reduce target reflectivity
• <~1 m of air deposit is required for target reflectivity (water thickness must be even less)
• This could increase in-chamber target heating
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IFT\P2005-071
Deposits during cool down in permeation cell are small
• Example: Assume 99.999% pure DT in permeation cell with 600 m DT layer with equal DT outside a 2.4 mm radius target
€
Impurity volume = V = 2 0.00001( ) 4π /3( ) 2.4 mm( )3
− 1.8 mm( )3
[ ] = 6.7 ×10−4 mm3
€
Impurity thickness = Δr =V
4πr2=
6.7 ×10−4 mm3
4 3.14( ) 2.4 mm( )2 = 9.26 nm
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IFT\P2005-071
Maximum deposition rate at 10-6 Torr and 20 K is ~40 nm/min
• Example: N2 at 10-6 Torr = 1.310-4 Pa
€
Mass flux = Φm =ρ gv g
4=
2.25 ×10−8kg/m3( ) 123 m/s( )
4= 6.9 ×10−7kg/m2s
€
dx
dt=
Φm
ρ s
=6.9 ×10-7kg/m2s
1026 kg/m3= 6.7 ×10−10m/s = 40 nm/min
€
ρ =PM
RT=
1.33×10-4 Pa( ) 0.028 kg/mole( )
8.31 J/mole ⋅K( ) 20 K( )= 2.25 ×10-8kg/m3
• This would mean ~ 1 micron buildup would occur in 25 minutes
• Thus << 10-6 Torr is needed for the transfer to fluidized bed
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IFT\P2005-071
Transferring targets in cryogenic vacuum should prevent significant cryo-deposits
• Cryogenic chamber in vacuum keeps vapor pressure low
Heat exchangers ~14 K
Fluidized bed ~19 K
Blower
Gas flow direction
Cryogenicchamber
Permeation Cell
Vacuum chamber ~10-6 Torr impurity gases
<<10-6 Torrimpurity gases
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IFT\P2005-071
Most gases have extremely low vapor pressure in a cryogenic environment
• Design concepts allow << 10-6 Torr and negligible impurity buildup• Similar - negligible buildup in fluidized bed loop or in transfer to the injector
Approximate vapor pressure in Torr
4K 20 K 77K 150 K Triple PointTemperature
Water <10–13 <10–13 <10–1310-7 273 K
Carbondioxide
<10–13 <10–1310-8 10 217 K
Argon <10–1310-13 160 Above
critical temp84 K
Oxygen <10–1310-13 150 Above
critical temp54 K
Nitrogen <10–1310-11 730 >1 atm 63 K
Neon <10–1330 >1 atm >1 atm 25 K
Hydrogen 10-7 760 >1 atm >1 atm 14 K
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IFT\P2005-071
3)Impact of foam shell non-concentricity on DT ice non-
concentricity
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IFT\P2005-071
Calculated total DT layer thickness is insensitive to foam non-concentricity (#1)
• We calculated DT temperature difference by initially assuming uniform DT layer thickness inside a non-concentric foam with a uniform outer surface temperature
T1
T2
€
kDT + f = ks1−δ kDT
δ
= 0.25 W/m ⋅K
ks = Thermal conductivity of foam solid = 0.065 W/mKkDT = Thermal conductivity of solid DT = 0.29 W/mK = Volume fraction DT = 90%
DT/foam
DT Offset of DT icelayer from center
2 m 10 m
T1 ( )K 19.6063 19.605683T2 ( )K 19.606609 19.607231ΔT ( )K 3.09E-4 1.548E-3
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IFT\P2005-071
Calculated total DT layer thickness is insensitive to foam non-concentricity (#2)
• We then found the shift in inner DT center that leads to a uniform inner DT temperature (equilibrium)
T1
T2
Offset of DT iceouter layer fromcenter
2 m 10 m
Offset o f DT iceinner laye r fromcenter
-0.08m -0.4m
T1 ( )K 19.606454 19.606453T2 ( )K 19.606454 19.606454ΔT ( )K 0 1E-6
• Thus the total variation in ice thickness is estimated to be more than an order of magnitude less than the variation in the foam thickness
DT/foam
DT
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IFT\P2005-071
Thermal conductivity model needs verification for solid DT in foam• Model has been tested for liquid DT in foam*
• Smaller crystals and possible void spaces in foam may cause reduced thermal conductivity
• LLE plans to measure thermal conductivity of D2 in foam • Results are insensitive to small changes in conductivity
D. E. Daney and E. Mapoles, Thermal conductivity of liquid hydrogen filled foam,Cryogenics, Vol. 27 (Aug. 1987) 427.
*
ks = 0.5*k (PS)= 0.0325 W/m•KkDT+Foam = 0.233
k(PS)= 0.065 W/m•KkDT+Foam = 0.250
ks = 2*K (PS)= 0.13 W/m•KkDT+Foam = 0.268
Offset DT+foam 10m 10m 10mOffset DT -0.84m -0.4m 0.03 mT1 ( )K 19.607766 19.606453 19.605243
T2 ( )K 19.607767 19.606454 19.605244ΔT ( )K 1E-6 1E-6 1E-6
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IFT\P2005-071
Layer thickness in a layering sphere was less sensitive to DT/foam conductivity
Layering sphere (17.8 K) 1” diameter
He gas
Target
ks = 0.5*k (PS) k (PS)= 0.065 W/m•K
ks = 2*K (PS)
Offset DT+foam 10 m 10 m 10 mOffset DT 1.2 m 1.3 m 1.4 mT1 ( K) 19.7568 19.7557 19.7545
T2 ( )K 19.7568 19.7557 19.7545ΔT ( )K 0 0 0
With this assumption, the DT offset is still nearly an order of magnitude less than the foam offset
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IFT\P2005-071
4)Time limits for “handoff” of layered targets to an injector
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IFT\P2005-071
We investigated layer degradation after target removal from fluidized bed
• Low dnsv/dT for DT and high He-3 build up time (t) increase beta layering time constant
€
τ =ρs
KDT
Dn
t
E
Vs
Vv
h
hs
dnsv
dT T1
⎛
⎝ ⎜
⎞
⎠ ⎟−1
+s
G
⎡
⎣ ⎢ ⎢
⎤
⎦ ⎥ ⎥
ρs is solid DT density in molecules per unit volume, KDT is the thermal conductivity ofsolid DT, D is the diffusion coefficient, n is the total number density (He+DT), t is thetime since purifying the DT, E is the average energy released per beta decay, Vs is thevolume of solid DT, Vv is the volume of the vapor space, h is the diameter of the vaporspace, hs is the total thickness of the solid DT (sum of both sides), nsv is the density of thesaturated DT vapor, T is the temperature, s is latent energy of sublimation per DTmolecule, and G is the beta decay power per unit volume.
• A long layering time constant slows layer movement in a non-uniform temperature environment
0.01
0.10
1.00
10.00
100.00
10 12 14 16 18 20
Temperature (K)
dn/dT (moles/m^3•K)
T. P. Bernat, E. R. Mapoles, and J. J. Sanchez, Temperature- and Age-Dependence ofRedistribution Rates of Frozen Deuterium-Tritium, ICF Quarterly Report, January –March 1991, Vo l. 1, No. 2, UCRL-LR-105821-91-2, LLNL, Livermore, CA.
*
*
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IFT\P2005-071
Layering time constant increases with decreased temperature
• Long layering time constant increases layer survival time in a temperature gradient
10
100
1000
10000
100000
10 12 14 16 18 20
Temperature (K)
Beta layering time constant (minutes)
Assumes baseline NRL target and 1 day He-3 buildup
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IFT\P2005-071
Time to offset DT by 1% (after 1 day)
0.1
1.0
10.0
100.0
10 12 14 16 18 20
Temperature (K)
Time (minutes)
100 mK
200 mK
400 mK
Time to change layer uniformity depends on T and T
• Example: time available to transfer target is < 18 s
• Lower temperature would greatly increase time
18 s at 16 K and 100 mK across target
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IFT\P2005-071
5)Cryogenic coil resistance testing
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IFT\P2005-071
Coil resistance dropped substantially when annealed
• Recall L/R>>25 ms is required to sustain coil current in an attractive force EM accelerator
• Previous results showed increased conductivity with welded annealed coil than soldered and not annealed
• New testing shows annealing is the major contributor
• L/R at 15 K and 0.9 Tesla annealed is 80 ms
59 Turn 5N Al e-beam welded Coil (lot Q3756)
0.001
0.01
0.1
1
0 20 40 60 80 100
Temperature
L/R time constant (s)
Annealed (B=0)
Annealed (B=0.9 T)
Not Annealed (B=0.9 T)
Not Annealed (B=0)
Accelerating CoilSabot Coil
Fr
Fr
Fz
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IFT\P2005-071
Composition variations between lots significantly affect coil resistance
• Much higher low-temperature resistance!• Coil purity must be controlled to achieve
consistent results
57 Turn 5N Al e-beam welded Coil (lot Q115)Apparently less pure than lot Q3756
0.001
0.01
0.1
0 20 40 60 80 100
Temperature (K)
Time constant(s)
Not annealed B=0.9 T
Not annealed (B=0 T)
Annealed (B=0 T)
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IFT\P2005-071
Summary
• External tracking position prediction accuracy improved by a factor of 4
• Impurity buildup on targets must be controlled
• Model indicates that total DT layer thickness is relatively insensitive to target foam non-concentricity– Experimental measurement of conductivity needed
• Low target temperature greatly increases DT layer shift time in temperature gradient– Sufficient time is available for target transfer with
low T
• Coil resistance was improved by annealing but varied with lot number on 5N Al wire