In situ Elastic Recoil Detection Analysis of Tungsten Surfaces - - PowerPoint PPT Presentation
In situ Elastic Recoil Detection Analysis of Tungsten Surfaces during ITER-like Helium Irradiation Kevin Woller , D. Whyte, G. Wright Center for Science and Technology with Accelerators and Radiation Plasma Science and Fusion Center
Background - “Tungsten Fuzz”
Kevin Woller, D. Whyte, G. Wright
Center for Science and Technology with Accelerators and Radiation
Plasma Science and Fusion Center
Massachusetts Institute of Technology
Cambridge, Massachusetts, USA
June 2, 2015
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filiform structure under ITER-like Helium irradiation (900- 1500K, 1022 He m-2 s-1, 30-90 eV) commonly referred to as W fuzz Start Helium irradiation Increasing Helium Fluence 1024 He ions/m2
*transition from shadowed region into fuzzy region on same target *Helium plasma on tungsten target in DIONISOS
Ts = 1100K ΓHe ~ 1022 He m-2 s-1 EHe ~ 60 eV
June 2, 2015
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filiform structure under ITER-like Helium irradiation (900- 1500K, 1022 He m-2 s-1, 30-90 eV) commonly referred to as W fuzz
conductivity, etc.)
Ts = 1100K ΓHe ~ 1022 He m-2 s-1 EHe ~ 60 eV
inch
June 2, 2015
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filiform structure under ITER-like Helium irradiation (900- 1500K, 1022 He m-2 s-1, 30-90 eV) commonly referred to as W fuzz
conductivity, etc.)
shows a uniform distribution of He through the W fuzz layer
expected for modeling of He bubbles Measured* Expected
*Samples from PILOT-PSI, PISCES-A, Alcator C-MOD
June 2, 2015
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filiform structure under ITER-like Helium irradiation (900- 1500K, 1022 He m-2 s-1, 30-90 eV) commonly referred to as W fuzz
conductivity, etc.)
shows a uniform distribution of He through the W fuzz layer
expected for modeling the He bubbles
View of helium irradiation of tungsten from 90° viewport of DIONISOS Charged particle detector Interaction region
June 2, 2015
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June 2, 2015
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1.7 MV Solid state Power Supply
General IONex tandem ion accelerator Modus operandi:
Target Detector Continuous operation Energy up to 10.5 MeV Current up to 3 μA on target
High energy Output
Elastic Recoil Detection (ERD) Analysis
Grazing incidence high-Z ion forward scatters low-Z atoms (e.g. H, He) in substrate.
N2 Gas Stripper Positive Terminal Potential
June 2, 2015
10 Low energy injection
1.7 MV Solid state Power Supply
General IONex tandem ion accelerator Continuous operation Energy up to 10.5 MeV Current up to 3 μA on target The terminal power supply was limited to half its full potential
capabilities for improved measurement
High energy Output
June 2, 2015
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25 year old components, Limited documentation, «Proceed with caution» SF6 gas insulation Capacitor-diode voltage multiplier network Bleed resistor chains Control Circuit 6 kV plate power supply 40 kHz Electron tube oscillator 40 kV step up transformer
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Checked and replaced many components along the way, but still could not reach full potential, until…
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Final Diagnosis: Push-Pull oscillator feedback capacitor made of 2.5” diameter copper cylinder and 3/8” copper tubing was arcing across a 1/16” gap that should have been 1”
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Final Diagnosis: Push-Pull oscillator feedback capacitor made of 2.5” diameter copper cylinder and 3/8” copper tubing was arcing across a 1/16” gap that should have been 1”
Thank you to: Dave Terry, Pete Stahle, Dennis Whyte, Regina Sullivan, Graham Wright, Harold Barnard, and Alcator C-MOD electrical and RF engineering team Machine nominal terminal potential = 1.7 MV
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Helicon linear plasma device
Continuous operation Differential pumping allows up to 5 Pa with accelerator at 10-4 Pa Up to 0.1 T Flux densities 1020 - 1023 He m-2 s-1 He Ion Energies up to 300 eV by target biasing Target temperature controlled from RT to 1400K by ceramic heater
June 2, 2015
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Challenges: 1. RF plasma generates electromagnetic noise in exposure chamber (unstable grounding introduces noise into detection circuit) 2. Heat from target and plasma get to detector, which loses stability at elevated temperatures (leakage current through detector is bad for detector resolution) 3. Lorentz force bends ion trajectories (.5-1.5 degree change) plus magnetic field limits material choices for fabrication 4. Background neutral gas pressure introduces additional component to ERD Spectra (2-5 Pa)
Challenges: 1. RF plasma generates electromagnetic noise in exposure chamber (unstable grounding introduces noise into detection circuit) 2. Heat from target and plasma get to detector, which loses stability at elevated temperatures (leakage current through detector is bad for detector resolution) 3. Lorentz force bends ion trajectories (.5-1.5 degree change) plus magnetic field limits material choices for fabrication 4. Background neutral gas pressure introduces additional component to ERD Spectra (2-5 Pa)
June 2, 2015
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Solution: Add shielding to protect detector
Challenges: 1. RF plasma generates electromagnetic noise in exposure chamber (unstable grounding introduces noise into detection circuit) 2. Heat from target and plasma get to detector, which loses stability at elevated temperatures (leakage current through detector is bad for detector resolution) 3. Lorentz force bends ion trajectories (.5-1.5 degree change) plus magnetic field limits material choices for fabrication 4. Background neutral gas pressure introduces additional component to ERD Spectra (2-5 Pa)
June 2, 2015
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Solution: Add shielding to protect detector
Challenges: 1. RF plasma generates electromagnetic noise in exposure chamber (unstable grounding introduces noise into detection circuit) 2. Heat from target and plasma get to detector, which loses stability at elevated temperatures (leakage current through detector is bad for detector resolution) 3. Lorentz force bends ion trajectories (.5-1.5 degree change) plus magnetic field limits material choices for fabrication 4. Background neutral gas pressure introduces additional component to ERD Spectra (2-5 Pa)
June 2, 2015
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Solution: Add shielding to protect detector
leakage current
hardware inside the exposure chamber.
the detection circuit.
Challenges: 1. RF plasma generates electromagnetic noise in exposure chamber (unstable grounding introduces noise into detection circuit) 2. Heat from target and plasma get to detector, which loses stability at elevated temperatures (leakage current through detector is bad for detector resolution) 3. Lorentz force bends ion trajectories (.5-1.5 degree change) 4. Background neutral gas pressure introduces additional component to ERD Spectra (2-5 Pa)
June 2, 2015
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Solution: Add shielding to protect detector
Challenges: 1. RF plasma generates electromagnetic noise in exposure chamber (unstable grounding introduces noise into detection circuit) 2. Heat from target and plasma get to detector, which loses stability at elevated temperatures (leakage current through detector is bad for detector resolution) 3. Lorentz force bends ion trajectories (.5-1.5 degree change) 4. Background neutral gas pressure introduces additional component to ERD Spectra (2-5 Pa)
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Solution: Add shielding to protect detector Solution: Account for changes during data analysis
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gas feed on for background subtraction Experiment Simulation
x Background
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Uniform layer concentration One He/W layer in simulation gives absolute values for composition of that layer Take data on 30 second time resolution Result:
measurements during plasma exposure Simulated ERD profile Plasma- Surface Interface He and W layer Bulk W
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Sharp rise in He concentration right at start of exposure He concentration reaches some saturation level dependent on exposure conditions (material temperature, flux density, etc.)
concentration measured dynamically
incorporate He influence
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Post Exposure Focused Ion Beam Milling allows us to interpolate the fuzz layer thickness dynamically
isERDA was developed to study the He concentration depth profile in Tungsten under ITER-like conditions i. Detection system can withstand extreme DIONISOS environment with active cooling and insulation from EMI ii. Magnetic field and background neutral gas modify ERDA in a predictable way and are thus be accounted iii. Dynamic He concentrations rise sharply at the beginning of exposure, then level off at a certain saturation value iv. Dynamic W fuzz layer thickness can be inferred and shows similar diffusion-like time dependence as previous static measurements
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Single Tendrils and tendril bundles grown in DIONISOS suggest nanostructure is supplied by surface diffusion
52 degree tilt
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New W fuzz variant is under active investigation, with current measurements indicating growth is sensitive to ion energy and ion energy distribution
52 degree tilt
June 2, 2015
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New W fuzz variant is under active investigation, with current measurements indicating growth is sensitive to ion energy and ion energy distribution
52 degree tilt
June 2, 2015
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New W fuzz variant is under active investigation, with current measurements indicating growth is sensitive to ion energy and ion energy distribution
52 degree tilt