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

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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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

IFT\P2005-071

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.

*

*

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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)

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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