1 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Comparison of tungsten fuzz growth in Alcator C-Mod and linear plasma

devices!G.M. Wright1, D. Brunner1, M.J. Baldwin2, K. Bystrov3, R. Doerner2, B.

LaBombard1, B. Lipschultz1, G. de Temmerman3, J.L. Terry1, !and D.G. Whyte1!

1 Plasma Science & Fusion Center, MIT, Cambridge USA!2 Center for Energy Research, UCSD, San Diego, USA!

3DIFFER, Nieuwegein, NL!!!

U C S DUniversity of California San Diego

2 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

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. B

aldwin, R

.P. Doerner, N

ucl. Fusion 48 (2008) 035001 !

S. Takamura et al. Plasma Fusion Res. 1 (2006) 051.!

3 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Growth conditions for W fuzz are well-defined from experiments on linear

plasma devices!

1.  Bare W or Mo surface!2.  1000 K < Tsurface < 2000 K!3.  Flux of He-ions with EHe ≥ 20 eV!4.  Layer thickness depends on t1/2!!

0

500

1000

1500

2000

2500

0 20 40 60 80 100 120

Sur

face

Tem

pera

ture

(K)

Incident Ion energy (eV)

S. Kajita et al. Nucl. Fusion 49 (2009) 095005!

Bubble Formation!

Fuzz Formation!

S. Kajita et al. Nucl. Fusion 49 (2009) 095005!

4 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

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-

crystallization temperature (~1600 K)!!

0

500

1000

1500

2000

2500

0 20 40 60 80 100 120

Sur

face

Tem

pera

ture

(K)

Incident Ion energy (eV)

Bubble Formation!

Fuzz Formation!

W re-crystallization!

Minimum Twall for efficient reactor!

ITER divertor with 5 MW/m2!

ITER divertor with 10 MW/m2!

ITER divertor with 15 MW/m2!

5 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

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 >1022 He/m2s [2]!

§  For total ion fluxes of ~1024 m-2s-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 steady-

state!

M.J. Baldwin et al. J. Nucl. Mater. 390-391 (2009) 886!

All basic growth conditions can be met in the ITER divertor and a fusion reactor.!

6 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Will inherent differences between tokamak plasmas and linear device plasmas prevent

fuzz growth in a tokamak?!C-Mod Tokamak! Generic Linear Device!

B-field! 5.4 T, Grazing incidence!

~0.1 T, Typically normal incidence!

Parallel Heat flux! ~200 MW/m2! ~1 MW/m2!Exposure stability! Transient,

Disruptions!Steady-state!

Ionization MFP, Re-deposition!

Short, prompt re-deposition!

Typically > plasma column radius, little or no redeposition!

Recycling flux! High! Low!Ion incidence! Oblique! Closer to normal!

•  Exploit ITER/reactor-like densities in the C-Mod divertor to find the answer!!

7 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Alcator C-Mod helium plasmas produced necessary plasma conditions for fuzz

growth at the outer strike point !

•  14 repeated L-mode discharges •  Te,divertor 20-25 eV, q|| > 0.2 GW/m2 è~22 s of total plasma exposure

He plasma discharges!

Strike point run above vertical divertor face to reduce flux expansion allowing for higher local surface temperatures.!

W Langmuir probe!

Heat flux instrumented tiles!

8 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

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

required for fuzz growth!

Nano-tendril!growth regime!

Bubble regime!

9 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Nano-tendrils are fully formed on surface of the tungsten probe exposed to heat

fluxes of 30-40 MW/m2!After exposure!

W Langmuir Probe!

Mo ramped tiles!!

Diameter of individual !tendril is ~100 nm.!

Mo surfaces are unchanged!

10 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Baldwin formula!

Sputtering of bulk W!

•  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)!!

715 nm!

750 nm!

550 nm!

1 shot!900 kA disruptions!

11 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

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.

Baldwin formula!

Sputtering of bulk Mo!

12 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

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 ~3x1024 m-2s-1, off-center (10 mm) still >1023 m-2s-1!

•  Exposure consists of 14 x 1.5 s shots = 22.5 s of total plasma exposure time.!

13 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Pilot-PSI exposure allows for the examination of the surface morphology at

several different temperatures!

1 µm!

Tsurf, max = 2150 K!

14 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Pilot-PSI exposure allows for the examination of the surface morphology at

several different temperatures!

1 µm!

Tsurf, max = 2000 K!

15 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Pilot-PSI exposure allows for the examination of the surface morphology at

several different temperatures!

1 µm!

Tsurf, max = 1850 K!

16 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Pilot-PSI exposure allows for the examination of the surface morphology at

several different temperatures!

1 µm!

Tsurf, max = 1670 K!

17 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Pilot-PSI exposure allows for the examination of the surface morphology at

several different temperatures!

1 µm!

Tsurf,max = 1420 K!

18 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Pilot-PSI exposure allows for the examination of the surface morphology at

several different temperatures!

1 µm!

Tsurf, max = 1250 K!

19 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

Morphology of C-Mod fuzz is reproduced almost exactly at very similar surface

temperatures in Pilot-PSI!Alcator C-Mod (tokamak)! Pilot-PSI (linear device)!

Tendril diameter = 100 ± 10 nm!Layer depth = 600 ± 150 nm!

Tendril diameter = 95 ± 20 nm!Layer depth = 550 ± 200 nm !

Tsurf, max = 2200 ± 300 K! Tsurf, max = 1850 ± 200 K!

20 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

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?!

21 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

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.!

22 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

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.!

!

23 G.M. Wright, 54th APS-DPP, Providence, RI, 10/30/2012

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.!