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 Massachusetts Institute of Technology Cambridge, Massachusetts, USA Background - “Tungsten Fuzz”
Why and How i. Motivation for the development of in situ Elastic Recoil Detection Analysis ( is ERDA) ii. Implementation of is ERDA in the Dynamics of ION Implantation and Sputtering Of Surfaces (DIONISOS) experiment iii. Evolution of the Helium concentration depth profile in Tungsten under ITER-like conditions June 2, 2015 K. Woller, 26th SOFE, Austin, TX 2
Why and How i. Motivation for the development of in situ Elastic Recoil Detection Analysis ( is ERDA) ii. Implementation of is ERDA in the Dynamics of ION Implantation and Sputtering Of Surfaces (DIONISOS) experiment iii. Evolution of the Helium concentration depth profile in Tungsten under ITER-like conditions June 2, 2015 K. Woller, 26th SOFE, Austin, TX 3
Helium bubbles modify materials in intriguing ways 1. Tungsten is desirable as a plasma-facing material T s = 1100K 2. Tungsten surface morphology becomes nanometer-sized Γ He ~ 10 22 He m -2 s -1 filiform structure under ITER-like Helium irradiation (900- E He ~ 60 eV 1500K, 10 22 He m -2 s -1 , 30-90 eV) commonly referred to as W fuzz Start Helium irradiation Increasing Helium Fluence 10 24 He ions/m 2 *Helium plasma on tungsten target in DIONISOS *transition from shadowed region into fuzzy region on same target June 2, 2015 K. Woller, 26th SOFE, Austin, TX 4
Tungsten fuzz is both interesting and terrifying 1. Tungsten is desirable as a plasma-facing material T s = 1100K 2. Tungsten surface morphology becomes nanometer-sized Γ He ~ 10 22 He m -2 s -1 filiform structure under ITER-like Helium irradiation (900- E He ~ 60 eV 1500K, 10 22 He m -2 s -1 , 30-90 eV) commonly referred to as W fuzz 3. Modified surface properties (emissivity, sputtering, conductivity, etc.) inch June 2, 2015 K. Woller, 26th SOFE, Austin, TX 5
Helium Perspective: A view from inside W fuzz 1. Tungsten is desirable as a plasma-facing material 2. Tungsten surface morphology becomes nanometer-sized filiform structure under ITER-like Helium irradiation (900- 1500K, 10 22 He m -2 s -1 , 30-90 eV) commonly referred to as W fuzz 3. Modified surface properties (emissivity, sputtering, conductivity, etc.) 4. He concentration depth profile measured after exposure shows a uniform distribution of He through the W fuzz Expected layer 5. Concentrations are an order of magnitude lower than expected for modeling of He bubbles Measured* *Samples from PILOT-PSI, PISCES-A, Alcator C-MOD June 2, 2015 K. Woller, 26th SOFE, Austin, TX 6
What is the helium doing during the growth process? View of helium irradiation of tungsten from 90° 1. Tungsten is desirable as a plasma-facing material viewport of DIONISOS 2. Tungsten surface morphology becomes nanometer-sized filiform structure under ITER-like Helium irradiation (900- 1500K, 10 22 He m -2 s -1 , 30-90 eV) commonly referred to as W fuzz Interaction region 3. Modified surface properties (emissivity, sputtering, conductivity, etc.) Charged particle detector 4. He concentration depth profile measured after exposure shows a uniform distribution of He through the W fuzz layer 5. Concentrations an order of magnitude higher were expected for modeling the He bubbles Measurement during exposure needed to test discrepancy June 2, 2015 K. Woller, 26th SOFE, Austin, TX 7
Why and How i. Motivation for the development of in situ Elastic Recoil Detection Analysis ( is ERDA) ii. Implementation of is ERDA in the Dynamics of ION Implantation and Sputtering Of Surfaces (DIONISOS) experiment iii. Evolution of the Helium concentration depth profile in Tungsten under ITER-like conditions June 2, 2015 K. Woller, 26th SOFE, Austin, TX 8
Use high energy ion beams to interrogate a material surface General IONex tandem ion accelerator N 2 Gas 1.7 MV Solid state Stripper Power Supply Elastic Recoil Detection (ERD) Analysis Grazing incidence high-Z ion forward scatters low-Z atoms (e.g. H, He) in substrate. Detector Positive High energy Low energy Terminal Output injection Potential Continuous operation Target Energy up to 10.5 MeV Current up to 3 μ A on target Modus operandi: High vacuum < 10 -4 Pa (10 -6 torr) Minimize electromagnetic and thermal noise Flexibility in geometry June 2, 2015 K. Woller, 26th SOFE, Austin, TX 9
Side note: Fix high energy ion beam machine General IONex tandem ion accelerator 1.7 MV Solid state Power Supply High energy Low energy Output injection Continuous operation Energy up to 10.5 MeV Current up to 3 μ A on target The terminal power supply was limited to half its full potential We desired full operational capabilities for improved measurement June 2, 2015 K. Woller, 26th SOFE, Austin, TX 10
Many areas to check… 25 year old components, Capacitor-diode voltage Limited documentation, multiplier network «Proceed with caution» Bleed resistor SF 6 gas chains insulation Control Circuit 6 kV plate 40 kV step up 40 kHz Electron power supply transformer tube oscillator June 2, 2015 K. Woller, 26th SOFE, Austin, TX 11
Start at ground and meet in the middle Start Checked and replaced many components along the way, but still could not reach full potential, until… June 2, 2015 K. Woller, 26th SOFE, Austin, TX 12
Insulator rings allowed outer conductor to slip vertically 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” June 2, 2015 K. Woller, 26th SOFE, Austin, TX 13
Share in our tiny triumph Machine nominal terminal potential = 1.7 MV 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 Thank you to: been 1” Dave Terry, Pete Stahle, Dennis Whyte, Regina Sullivan, Graham Wright, Harold Barnard, and Alcator C-MOD electrical and RF engineering team June 2, 2015 K. Woller, 26th SOFE, Austin, TX 14
DIONISOS - Dynamics of ION Implantation and Sputtering Of Surfaces 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 10 20 - 10 23 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 K. Woller, 26th SOFE, Austin, TX 15
Plasma exposure chamber is non- ideal for Elastic Recoil Detection 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 K. Woller, 26th SOFE, Austin, TX 16
Plasma exposure chamber is non- ideal for Elastic Recoil Detection Challenges: 1. RF plasma generates electromagnetic noise in exposure chamber (unstable grounding introduces noise into Solution: detection circuit) Add shielding to protect 2. Heat from target and plasma get to detector 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 K. Woller, 26th SOFE, Austin, TX 17
Plasma exposure chamber is non- ideal for Elastic Recoil Detection Challenges: 1. RF plasma generates electromagnetic noise in exposure chamber (unstable grounding introduces noise into Solution: detection circuit) Add shielding to protect 2. Heat from target and plasma get to detector 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 K. Woller, 26th SOFE, Austin, TX 18
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