Update on Various Target Issues Presented by Ron Petzoldt D. Goodin, E. Valmianski, N. Alexander, J. Hoffer Livermore HAPL meeting June 20-21, 2005

IFT\P 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\P )Improved tracking

IFT\P 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 Target height 0 mm 25 mm

IFT\P 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) GunD1 (4.1 m)D2 (8.7 m)DCC (17.7 m) Shots from October 2004 Shots from 3 June 2005 Air rifle shots

IFT\P )Impurity effects on target reflectivity - Impurities in DT supply - Transfer to the layering system - Impurities in the cryogenic fluidized bed - Transfer to the injector

IFT\P 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\P Deposits during cool down in permeation cell are small Example: Assume % pure DT in permeation cell with 600  m DT layer with equal DT outside a 2.4 mm radius target

IFT\P Maximum deposition rate at Torr and 20 K is ~40 nm/min Example: N 2 at Torr = 1.3  Pa This would mean ~ 1 micron buildup would occur in 25 minutes Thus << Torr is needed for the transfer to fluidized bed

IFT\P 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 Cryogenic chamber Permeation Cell Vacuum chamber~10 -6 Torr impurity gases <<10 -6 Torr impurity gases

IFT\P Most gases have extremely low vapor pressure in a cryogenic environment Design concepts allow << Torr and negligible impurity buildup Similar - negligible buildup in fluidized bed loop or in transfer to the injector Approximate vapor pressure in Torr

IFT\P )Impact of foam shell non-concentricity on DT ice non-concentricity

IFT\P 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 k s = Thermal conductivity of foam solid = W/m  K k DT = Thermal conductivity of solid DT = 0.29 W/m  K  = Volume fraction DT = 90% DT/foam DT

IFT\P 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 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

IFT\P 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 D 2 in foam Results are insensitive to small changes in conductivity *

IFT\P Layer thickness in a layering sphere was less sensitive to DT/foam conductivity Layering sphere (17.8 K) 1” diameter He gas Target With this assumption, the DT offset is still nearly an order of magnitude less than the foam offset

IFT\P )Time limits for “handoff” of layered targets to an injector

IFT\P We investigated layer degradation after target removal from fluidized bed Low dn sv /dT for DT and high He-3 build up time (t) increase beta layering time constant A long layering time constant slows layer movement in a non-uniform temperature environment * *

IFT\P Layering time constant increases with decreased temperature Long layering time constant increases layer survival time in a temperature gradient Assumes baseline NRL target and 1 day He-3 buildup

IFT\P 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

IFT\P )Cryogenic coil resistance testing

IFT\P 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 Accelerating CoilSabot Coil Fr Fz

IFT\P Composition variations between lots significantly affect coil resistance Much higher low-temperature resistance! Coil purity must be controlled to achieve consistent results

IFT\P 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