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IFE Ion Threat Spectra Effects Upon Chamber Wall Materials
G E. Lucas, N. WalkerUC Santa Barbara
Threat Spectra• Projected number of ions and
ion energy levels• Depends on the type of target
drive system• Lasers
– NRL Direct Drive Target– High Yield Direct Drive
Target• Heavy Ions
– HI Indirect Drive Target• Threat spectra consists of both
Debris and Burn Product Ions
Threat Spectra
Analytical Procedure• SRIM Software
– Stopping and Range of Ions in Matter
– Uses statistical algorithms to simulate ion and target material collisions and interactions
– Inputs• Ion Type• Ion Energy• Target Material (C or W)• # of Ions• Desired output tables
– Outputs• Displacements per Ion• Ion range in target material• Ion Statistics
Analytical Procedure• Data Interpolation
– Data for threat spectra is only known at a specific set of data points
– Interpolation must be used in order to find data points in-between known data points
– Distribution is divided into small finite number of “bins”
– Each data point falling within each bin is assumed to have the “mid-bin energy”
– Reasonable approximation since bin size is small compared to range of distribution
Analytical Procedure
• Initial Damage Profiles– Utilizing interpolated ion range
data, ion concentration distribution can be found
– Ion concentration distribution and displacement distribution along with physical properties of the chamber wall material allow DPA (displacements per atom) to be determined
– Initial assessment of damage due to displacement can be determined
Analytical Procedure• Blistering/Exfoliation
– Ion concentration distribution can be used to determine the number of ions per chamber wall atom
– The thickness of the blister is determined by the location of maximum concentration (region of maximum gas concentration)
– Once critical ion/atom concentration is achieved blister will exfoliate
• 15 at. % for He ions• 50 at. % for H ions
– All material before the blister is exfoliated from the chamber wall
– Knowing the critical ion/atom concentration, maximum ion concentration as well as the operating conditions of the reactor, the time needed for a blister to exfoliate can be determined
Analytical Procedure• Steady-State Exfoliation
– After blister layer exfoliates the penetrating ion concentration distribution is the same as before
– However, the previous ion concentration distribution is still present in the material
– The two concentration distributions add to give the final ion concentration distribution
– The final ion concentration distribution dictates a new maximum ion concentration and a new location of maximum ion concentration
Analytical Procedure
• Steady-State Exfoliation– The ion concentration distribution continues to change after each
exfoliation until it reaches a steady-state distribution– At steady-state
• The exfoliation thickness is constant (controlled by location of maximum concentration)
• The time to exfoliate is constant (controlled by value of maximum concentration)
– The exfoliation thickness and the time required to exfoliate essentially control the decay rate of the surface material
Results
• Initial Damage Profiles– After the initial fusion reaction the
initial ion concentration distribution and DPA distribution are determined
– As more reactions occur these distributions build up on top of one another
– Blisters form and exfoliation eventually occurs
– Representative distributions are shown in the two figures for Low Yield Direct Drive Debris Ions into C
Results• Exfoliation Rates
– After the first exfoliation occurs a new concentration distribution evolves
– The new concentration distribution will have a• Higher maximum concentration value
– Makes time to exfoliate smaller– Increases exfoliation rate
• Shallower region of maximum concentration– Makes exfoliation thickness smaller– Decreases exfoliation rate
– Exfoliation rate and exfoliation thickness vs. time plotted in figure, Low Yield Direct Drive Debris T into C
– After certain amount of time exfoliation rate and exfoliation thickness become constant
Results
• Steady State Exfoliation RatesDrive System Ion Source Wall Material Exfoliation Rate (cm/year) Exfoliation Thickness (um)
Low Yield Direct Drive Debris-D C 1 0.9Debris-T C 1 1.1Burn-He C 0.018 4.7
High Yield Direct Drive Debris-D C 1.9 0.95Debris-T C 2 1.2Burn-He C 0.037 3.3
HI Indirect Drive Burn-D C 0.005 3Burn-T C 0.007 2.5
Low Yield Direct Drive Debris-D W 1.8 0.6Debris-T W 1.8 0.76Burn-He W 0.037 3.5
High Yield Direct Drive Debris-D W 3.2 0.61Debris-T W 3.4 0.81Burn-He W 0.076 2.5
HI Indirect Drive Burn-D W 0.014 3Burn-T W 0.017 2.5
Diffusion
Check to determinelongevity of concentration profile
H diffusivity from literature
Run finite element diffusionsimulation
Diffusion Profiles at TemperatureH profile rapidly decays in W at temperatures above 150 C
H profile does not decayIn graphite below 1750 C
Conclusions
• Burn Product Ion damage may be survivable
• Debris Ion damage is unacceptably high– Diffusion may help– Still need better diffusivities to complete
analysis