update on various target issues presented by ron petzoldt d. goodin, e. valmianski, n. alexander, j....
TRANSCRIPT
Update on Various Target Issues
Presented by Ron Petzoldt
D. Goodin, E. Valmianski, N. Alexander, J. Hoffer
Livermore HAPL meetingJune 20-21, 2005
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
IFT\P2005-071
1)Improved tracking
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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-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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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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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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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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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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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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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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3)Impact of foam shell non-concentricity on DT ice non-
concentricity
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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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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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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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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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4)Time limits for “handoff” of layered targets to an injector
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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.
*
*
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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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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5)Cryogenic coil resistance testing
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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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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)
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