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