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 1
What is tungsten fuzz? 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. Baldwin, R.P. Doerner, Nucl. Fusion 48 (2008) 035001 S. Takamura et al. Plasma Fusion Res. 1 (2006) 051. 2
Growth conditions for W fuzz are welldefined from experiments on linear plasma devices 1. Bare W or Mo surface 2. 1000 K < T surface < 2000 K 3. Flux of He-ions with E He 20 ev 4. Layer thickness depends on t 1/2 Surface Temperature (K) 2500 2000 1500 1000 500 0 Bubble Formation Fuzz Formation 0 20 40 60 80 100 120 Incident Ion energy (ev) S. Kajita et al. Nucl. Fusion 49 (2009) 095005 S. Kajita et al. Nucl. Fusion 49 (2009) 095005 3
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/m 2 è 830 K, 15 MW/m 2 è 1500 K Ø Reactor W will operate above ~800 K for efficiency and below W recrystallization temperature (~1600 K) 2500 Bubble Formation W re-crystallization Surface Temperature (K) 2000 1500 1000 500 0 Fuzz Formation 0 20 40 60 80 100 120 Incident Ion energy (ev) ITER divertor with 15 MW/m 2 ITER divertor with 10 MW/m 2 ITER divertor with 5 MW/m 2 Minimum T wall for efficient reactor 4
Will surface morphology of tungsten modify into fuzz under helium bombardment in ITER and reactors? 3. Flux of He ions with E 20 ev? Ø He ions born from D-T reactions (or pure He plasma in ITER) Ø Growth rate saturates at fluxes >10 22 He/m 2 s [2] For total ion fluxes of ~10 24 m -2 s -1, only 1 % He content needed for maximum fuzz growth rate 4. Growth time? Ø ITER has 300 s pulse length Ø Fusion reactors operate in steadystate All basic growth conditions can be met in the ITER divertor and a fusion reactor. M.J. Baldwin et al. J. Nucl. Mater. 390-391 (2009) 886 5
Will inherent differences between tokamak plasmas and linear device plasmas prevent fuzz growth in a tokamak? B-field C-Mod Tokamak 5.4 T, Grazing incidence Parallel Heat flux ~200 MW/m 2 ~1 MW/m 2 Exposure stability Transient, Disruptions Ionization MFP, Re-deposition Short, prompt redeposition Recycling flux High Low Generic Linear Device ~0.1 T, Typically normal incidence Steady-state Typically > plasma column radius, little or no redeposition Ion incidence Oblique Closer to normal Exploit ITER/reactor-like densities in the C-Mod divertor to find the answer 6
Alcator C-Mod helium plasmas produced necessary plasma conditions for fuzz growth at the outer strike point He plasma discharges Strike point run above vertical divertor face to reduce flux expansion allowing for higher local surface temperatures. W Langmuir probe 14 repeated L-mode discharges T e,divertor 20-25 ev, q > 0.2 GW/m 2 è ~22 s of total plasma exposure Heat flux instrumented tiles 7
Tungsten Langmuir probe and nearby Mo surfaces reached surface temperatures required for fuzz growth Bubble regime Nano-tendril growth regime W Langmuir probe ramped ~11 o 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, T surf is determined from 1-D finite element heat flux modeling. Mo surface temperatures determined by IR and calorimetry. 8
Nano-tendrils are fully formed on surface of the tungsten probe exposed to heat fluxes of 30-40 MW/m 2 After exposure W Langmuir Probe Mo ramped tiles Mo surfaces are unchanged Diameter of individual tendril is ~100 nm. 9
Is the empirical growth rate determined on PISCES applicable to the fuzz grown in Alcator C-Mod? 715 nm 550 nm Bulk 750 nm Fuzz layer The measured fuzz layer thickness was 600 ± 150 nm from FIB crosssectioning. 900 ka disruptions 1 shot Baldwin formula Sputtering of bulk W Growth is estimated through t 1/2 -dependence: layer depth = δ G(T surf ) t 1/2 where G exp(-e act /kt surf ), E act = 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) 10
High rate of physical sputtering is inhibiting growth of fuzz on Mo surfaces Sputtering of bulk Mo Baldwin formula 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 overestimate 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. 11
Pilot-PSI is capable of re-creating many of the plasma and surface conditions in the Alcator C-Mod divertor Similarities to Alcator C-Mod: Ø High plasma fluxes and densities Ø Plasma heated surfaces/high heat flux Ø Able to operate in short pulses Central flux density ~3x10 24 m -2 s -1, off-center (10 mm) still >10 23 m -2 s -1 Exposure consists of 14 x 1.5 s shots = 22.5 s of total plasma exposure time. 12
Pilot-PSI exposure allows for the examination of the surface morphology at several different temperatures 1 µm T surf, max = 2150 K 13
Pilot-PSI exposure allows for the examination of the surface morphology at several different temperatures 1 µm T surf, max = 2000 K 14
Pilot-PSI exposure allows for the examination of the surface morphology at several different temperatures 1 µm T surf, max = 1850 K 15
Pilot-PSI exposure allows for the examination of the surface morphology at several different temperatures 1 µm T surf, max = 1670 K 16
Pilot-PSI exposure allows for the examination of the surface morphology at several different temperatures 1 µm T surf,max = 1420 K 17
Pilot-PSI exposure allows for the examination of the surface morphology at several different temperatures 1 µm T surf, max = 1250 K 18
Morphology of C-Mod fuzz is reproduced almost exactly at very similar surface temperatures in Pilot-PSI Alcator C-Mod (tokamak) Tsurf, max = 2200 ± 300 K Tendril diameter = 100 ± 10 nm Layer depth = 600 ± 150 nm Pilot-PSI (linear device) Tsurf, max = 1850 ± 200 K Tendril diameter = 95 ± 20 nm Layer depth = 550 ± 200 nm G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012 19
Fuzz growth in tokamak environment and linear environment are near-identical Legitimizes fuzz growth data on linear devices for predictions and estimates of fuzz growth in future tokamaks. Continued synergy between linear devices and tokamaks is needed to answer open questions about fuzz. Linear Plasma Devices Physics questions How does n-irradiation affect fuzz growth and morphology? What are the mechanisms determining growth rates? Helium content in surface? Tokamaks Operations questions How does large surface area of fuzz impact operations? What is the maximum fuzz layer thickness that can survive in a tokamak? Dust production in a tokamak? 20
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. 21
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. 22
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. 23