Update on Various Target Issues

1
Update on Various Target Issues

• Presented by Ron Petzoldt
• D. Goodin, E. Valmianski, N. Alexander, J. Hoffer
• Livermore HAPL meeting
• June 20-21, 2005

2
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

3

1. Improved tracking

4
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

25 mm
Target height
Laser
0 mm
D2 measurements taken in two horizontal positions
20 mm apart
5
Target position prediction improved from 2.0 mm
to 0.49 mm (1 ?)
Gun
D1 (4.1 m)
D2 (8.7 m)
DCC (17.7 m)
Air rifle shots
Air rifle shots
Shots from October 2004
Shots from 3 June 2005

• 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)

6

• Impurity effects on target reflectivity
• Impurities in DT supply
• Transfer to the layering system
• Impurities in the cryogenic fluidized bed
• Transfer to the injector

7
Impurity gases can freeze on target surface and
reduce target reflectivity

• This could increase in-chamber target heating

• lt1 ?m of air deposit is required for target
  reflectivity (water thickness must be even less)

8
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

9
Maximum deposition rate at 10-6 Torr and 20 K is
40 nm/min

• Example N2 at 10-6 Torr 1.3?10-4 Pa

• This would mean 1 micron buildup would occur in
  25 minutes
• Thus ltlt 10-6 Torr is needed for the transfer to
  fluidized bed

10
Transferring targets in cryogenic vacuum should
prevent significant cryo-deposits

• Cryogenic chamber in vacuum keeps vapor pressure
  low

11
Most gases have extremely low vapor pressure in a
cryogenic environment
Approximate vapor pressure in Torr

• Design concepts allow ltlt 10-6 Torr and negligible
  impurity buildup
• Similar - negligible buildup in fluidized bed
  loop or in transfer to the injector

12

1. Impact of foam shell non-concentricity on DT ice
   non-concentricity

13
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

DT
T1
T2
DT/foam

• ks Thermal conductivity of foam solid 0.065
  W/m?K
• kDT Thermal conductivity of solid DT 0.29
  W/m?K
• Volume fraction DT 90

14
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)

DT
T1
T2
DT/foam

• 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

15
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

16
Layer thickness in a layering sphere was less
sensitive to DT/foam conductivity
With this assumption, the DT offset is still
nearly an order of magnitude less than the foam
offset
17

• Time limits for handoff
• of layered targets to an injector

18
We investigated layer degradation after target
removal from fluidized bed

• A long layering time constant slows layer
  movement in a non-uniform temperature environment

• Low dnsv/dT for DT and high He-3 build up time
  (t) increase beta layering time constant

19
Layering time constant increases with decreased
temperature
Assumes baseline NRL target and 1 day He-3
buildup

• Long layering time constant increases layer
  survival time in a temperature gradient

20
Time to change layer uniformity depends on ?T and
T
18 s at 16 K and 100 mK across target

• Example time available to transfer target is lt
  18 s
• Lower temperature would greatly increase time

21

1. Cryogenic coil resistance testing

22
Coil resistance dropped substantially when
annealed

• Recall L/Rgtgt25 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

Fr
Fz
Fr
Accelerating Coil
Sabot Coil

• L/R at 15 K and 0.9 Tesla annealed is 80 ms

23
Composition variations between lots significantly
affect coil resistance

• Much higher low-temperature resistance!
• Coil purity must be controlled to achieve
  consistent results

24
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