1 G.M. Wright, 54 th APS-DPP, Providence, RI, 10/30/2012 Comparison of tungsten fuzz growth in Alcator C-Mod and linear plasma devices G.M. Wright 1 , D. Brunner 1 , M.J. Baldwin 2 , K. Bystrov 3 , R. Doerner 2 , B. LaBombard 1 , B. Lipschultz 1 , G. de Temmerman 3 , J.L. Terry 1 , and D.G. Whyte 1 1 Plasma Science & Fusion Center, MIT, Cambridge USA 2 Center for Energy Research, UCSD, San Diego, USA 3 DIFFER, Nieuwegein, NL UCSD University of California San Diego
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Comparison of tungsten fuzz growth in Alcator C-Mod … · Comparison of tungsten fuzz growth in Alcator C-Mod and linear plasma devices! G.M. Wright1, ... W fuzz layer depth is in
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• When a tungsten surface is raised in temperature and exposed to He ions, nano-tendrils raise out of the surface.!
• He bubbles that precipitate in the bulk metal are playing an important role in driving the process and determining the exact morphology (e.g. tendril diameter).!
• Should we be worried?!Ø There are some benefits (e.g. lower physical sputtering
yields, reduced cracking from thermal stresses)!
Ø Nano-tendrils reduce thermal conductivity.!
Ø Main concern is the mechanical fragility of the tendrils leading to significant W dust formation.!
5 µm!
600 nm!
M.J. B
aldwin, R
.P. Doerner, N
ucl. Fusion 48 (2008) 035001 !
S. Takamura et al. Plasma Fusion Res. 1 (2006) 051.!
Will surface morphology of tungsten modify into fuzz under helium
bombardment in ITER and reactors?!1. Bare W surfaces?!
Ø Bare W surface in net erosion regions of ITER divertor!Ø Entire first wall is potentially bare W in a fusion reactor!
2. Surface temperatures of 1000-2000 K?!Ø For ITER divertor, 5 MW/m2 è 830 K, 15 MW/m2 è 1500 K!Ø Reactor W will operate above ~800 K for efficiency and below W re-
W Langmuir probe ramped ~11o into parallel heat flux and is electrically/thermally isolated.!è W Langmuir probe intercepts significant parallel heat flux and rapidly reaches high surface temperatures.!
W Langmuir probe surface heat flux is obtained directly from Langmuir probe measurements, Tsurf is determined from 1-D finite element heat flux modeling. Mo surface temperatures determined by IR and calorimetry.!
Tungsten Langmuir probe and nearby Mo surfaces reached surface temperatures
• The measured fuzz layer thickness was 600 ± 150 nm from FIB cross-sectioning.
Bulk!
Fuzz layer!
Is the empirical growth rate determined on PISCES applicable to the fuzz grown in
Alcator C-Mod?!
• Growth is estimated through t1/2-dependence:!layer depth = δ ✕ G(Tsurf) ✕ t1/2!
where G ∝ exp(-Eact/kTsurf), Eact = 0.71 eV !M.J. Baldwin, R.P. Doerner, Nucl. Fusion 48 (2008) 035001 !
!• Calculated cumulative layer depth of ~520 nm for W probe!• Sputtering only a small contribution in W case (~35 nm of calculated bulk W gross sputtering)!!
High rate of physical sputtering is inhibiting growth of fuzz on Mo surfaces!
• Lower surface temperatures leads to slower growth on Mo surfaces.!!• Calculated cumulative layer depth of ~63 nm for Mo surfaces.!
• Gross sputtering dominates in the Mo case (~120 nm of calculated bulk Mo sputtering) indicating if gross sputtering rate > fuzz growth rate than fuzz growth will be inhibited.!
NOTE:!Baldwin formula is based on data set at temperatures of 1120 K and 1320 K. Experimental results from Kajita and de Temmerman show this formula to over-estimate layer thicknesses for T > 1400 K. Further testing is needed to check growth rates between C-Mod and linear devices.!
S. Kajita et al. J. Nucl. Mater. 418 (2011) 152. G. de Temmerman et al. J. Vac. Sci. Technol. A 30 (2012) 041306.
Open physics question: How will n-irradiation impact fuzz growth and morphology?!
!Ø Will irradiation damage (neutrons) have any effect on fuzz
formation or layer growth? !Ø A competition of rates; will damage sites anneal faster than they can be filled
with implanted He?!Ø Due to high temperatures, this can only be studied dynamically!!! DIONISOS experiment allows
us to simultaneously grow W fuzz and irradiate the surface with ~MeV ions to simulate neutron damage.!!Also allows for real-time measurements of He content during fuzz growth.!
Open operations question: How will a fuzzy surface affect tokamak operations?!
Ø How will a large surface area of W fuzz affect wall recycling?!Ø Surface area is increased!Ø Hydrogenic permeation is decreased!
Ø Will W impurity injections increase?!Ø Will broken tendrils cause impurity injections into the core plasma?!
Ø Will W dust production be large enough to cause a major safety concern?!Ø W dust in a nuclear machine will be activated, tritiated and mobile.!!
Ø Planned hot divertor (873 K) in Alcator C-Mod would be greatly beneficial to W fuzz studies in tokamaks. [Soren’s poster]!Ø If you want this project to move forward, please contact your congressman/
woman and senator and request they support the domestic fusion energy program. Visit www.fusionfuture.org for more details.!
Summary: W fuzz can be grown in a tokamak environment and is nearly identical to fuzz
grown in a linear plasma device!
Ø Tokamak plasmas have been shown to be capable of growing W fuzz if the proper growth conditions are met.!
Ø W fuzz layer depth is in good agreement with empirical formula from Baldwin et al. based on data from linear plasma devices.!
Ø Comparison of W fuzz from Pilot-PSI linear plasma device shows nearly identical surface morphology as the C-Mod W fuzz at similar surface temperatures, legitimizing the use of linear plasma devices to predict and analyze W fuzz growth in future fusion devices.!
Ø Future fusion devices meet all fuzz growth conditions as outlined by linear plasma device experiments.!
Ø In order to answer the open questions on both the physics and operations aspects of W fuzz, further experiments in both linear plasma devices and tokamaks are required.!