R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Plasma interactions with Be surfaces R....
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Transcript of R. P. Doerner, 2 nd PMIF Meeting, Juelich, Sept. 19-21, 2011 Plasma interactions with Be surfaces R....
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Plasma interactions with Be surfaces
R. P. Doerner, D. Nishijima, T. Schwarz-Selinger and members of the PISCES Team
Center for Energy Research, University of California – San Diego, USA
Work performed as part of: Plasma-Surface Interaction Science Center (MIT and UTenn)
US-EU Collaboration on Mixed-Material PMI Effects for ITER
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
The PISCES-B divertor plasma simulator is used to investigate ITER mixed materials PSI.
PISCES ITER (edge)
Ion flux (cm2s–1) 1017–1019 ~1019 - 1020
Ion energy (eV) 20–300 (bias) 10–300 (thermal)
Te (eV) 4–40 1–100
ne (cm–3) 1012–1013 ~1013
Be Imp. fraction (%) Up to a few % 1–10 (ITER)
Pulse length (s) Steady state 1000
PSI materials C, W, Be C, W, Be ..
Plasma species H, D, He H, D, T, He
• PISCES-B is contained within an isolated safety enclosure to prevent the release of Be dust.
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
PISCES-B has been modified to allow exposure of samples to Be seeded plasma
Radial transport guard
102 mm
153 mm
76 mm
195 mm
45 o
Cooled target holder
Heatable deposition probe assembly
Thermocouple
Thermocouple
Water cooled Mo heat dump
Resistive heating coils
High temperature MBE effusion cell used to seed plasma with evaporated Be
12 °
PISCES-B PlasmaTarget
Depositionprobesample
Axial spectroscopic field of view
Berylliumimpurityseeding
Radial transport guard
102 mm
153 mm
76 mm
195 mm
45 o
Cooled target holder
Heatable deposition probe assembly
Thermocouple
Thermocouple
Water cooled Mo heat dump
Resistive heating coils
High temperature MBE effusion cell used to seed plasma with evaporated Be
12 °
PISCES-B PlasmaTarget
Depositionprobesample
Axial spectroscopic field of view
Berylliumimpurityseeding
P-B experiments simulateBe erosion from ITER wall,subsequent sol transport and interaction with W bafflesor C dump plates, as well asinvestigation of codepositedmaterials using witness plates
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Outline of Presentation
• Erosion in the plasma environment– Comparison to TRIM and ion beam data– Surface characterization– Role of morphology
• Redeposition/sticking efficiency
• Summary
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Be erosion yield measurements in PISCES
Two techniques are used to measure physical sputtering yield
• Weight loss measurements, use low density plasma to reduce redeposition (i.e. long ionization mean free path).
• Line emission spectroscopy, uses high density plasma to minimize geometrical loss terms (i.e. short ionization mean free path).
These presentation will focus on the weight loss technique and changes to spectroscopic measurements that are normalized to weight loss measurements. [He plasma on Be, weight loss are 5-10 times lower than TRIM calculations, He plasma on C, weight loss agree with TRIM calculations].
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Significant variations in the Be sputtering yield are measured
Incident ion energy ~100 eV
J. Roth et al., FED 37(1997)465.
PISCES
discrepancy between - JET - PISCES-B - ion beam – TRIM - sputter yields(< 45%) (< 0.7%) (< 8%) (< 3.5%)
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Uncertainties in sputtering yield measurements
Uncertainties in weight loss measurements:• Surface contamination• Incident ion energy• Ion flux measurement• Ion species mix (molecular ions)• Redeposition fraction• Surface morphology changes• Atomic D adsorption on the surface (secondary ion yield)
Uncertainties in absolute spectroscopic measurements:• Atomic physics database• Electron energy distribution (non-Maxwellian)• Angular distribution of sputtered particles• Geometrical loss fraction
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Native beryllium oxide surface is removed early during the plasma exposure
• Distinctive oxygen lines near 777 nm can monitor erosion of O from surface• Background helium plasma does not change (second order He I line)• Larger ion energy will remove oxide layer quicker• BeO (0,0) molecular band emission (@ 470.86 nm) is not detectable
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 200 400 600 800 1000
OI@7771/HeI@3888
Time[s]
He plasma (30 eV ions)
Helium/oxygenplasma
(30 eV ions)
0
500
1000
1500
2000
776.5 777 777.5 778 778.5
t = 265 st = 641 s
Wavelength (nm)
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
AES reveals a relatively ‘clean’ Be surface after sputtering yield measurements
Time from plasma shut-off (s)
0 1000 2000 3000 4000 5000
Sur
face
com
posi
tion
(%
)
020
4060
8010
0
Be CO
(7.0)
(0.9)
(92.1)
PISCES
500 1000 1500
before
dN(E
)
1.25·1022 Dx
+/cm2 , -100V bias
FeB O
Be
after
kinetic energy (eV)
C Cu
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Incident ion energy is corrected for plasma potential. Ion flux is uniform over sample surface.
From B. LaBombard et al, JNM 162-164 (1989) 314.
PISCES-A space potential measurements show Vpl ~ 1-2 Te
Eion = |Vbias | - 1.5Te
Target
From D. Whyte et al, NF 41 (2001) 47.
Ion flux calculated from upstream Langmuir probe agrees with total current collected on the sample manipulator when biased into Isat (with ~10%)
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Molecular ion fractions are calculated from zero-d rate balanced model, based on measurements
• Model uses the atomic and molecular processes to the right
• Rate equations predict molecular ion fractions based on ne, Te, B, NH2
• Turbulent radial transport is assumed (1/B scaling)
• Model is verified with molecular ion species measurements
From E. Hollmann et al., Phys. Plasmas 9 (2002) 4330.
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Molecular ion effects change both the shape and magnitude of sputtering yield curve
PISCES
10-5
10-4
10-3
10-2
10-1
0 20 40 60 80 100 120 140
Measured Yield in D plasma (*5)Y_D(Ei)2*Y_D(Ei/2)3*Y_D(Ei/3)total_Y(0.25,0.47,0.28)
Ei [eV]
10-5
10-4
10-3
10-2
10-1
0 20 40 60 80 100 120 140
Measured Yield in D plasmaY_D(Ei)2*Y_D(Ei/2)3*Y_D(Ei/3)total_Y(0.25,0.47,0.28)
Ei [eV]
Shape of measured yield agrees with molecular ion model predictions.Magnitude is a factor of ~5 too low.
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
ERO calculates 10-20% ionization of sputtered Be in the PISCES-B plasma
• Shape of Be I (457 nm) axial profile agrees well with experimental profiles
• Magnitude of profiles are normalized
• Molecular ion fractions provided by previous model
• No Be deposition observed on W or C targets exposed to D plasma (no wall source)
PISCES
From D. Borodin et al, Phys. Scr. T128 (2007) 127.
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Be surfacebefore plasma exposure
after
Surface morphology evolution with time / fluence
spectroscopy:
mass loss:
0 1 2 3 4 50
2
4
6
8
10
12
spu
tte
r yi
eld
(1
0-3 B
e p
er
ion
)
ion fluence (1022/(cm2s))
D2, <330K, -100V
morphology change can account for a factor of 2 in reduction of the yield
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Similar yield evolution with time/fluence is documented in the
literature
morphology change can account for a factor of 2 reduction of the yield
1keV, H2+
7.3E21 ions/cm2
Mattox and Sharp, J. Nucl. Mater. 1979:
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
1) “maximum” – static TRIM + MD2) “minimum” – SDTrimSP with 50% of D (reasonable limit)
Plasma atoms remaining in the near surface also can reduce the sputtering yield by a factor of 2-3
From C. Björkas
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Gross erosion is expected to remain constant with increasing Be redeposition/influx
• N = G * (1-R), where N = net erosion, G = gross erosion, R = redeposited fraction
• In PISCES-B, Gross = net erosion with no Be seeding (λion is large compared to rplasma, so redeposition is small)
• Ion influx from Be oven is identical to sputtered atoms that are ionized in the plasma and redeposited on the target, this allow a controlled and independent variation of the Be influx to the target
PISCES
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1
Current theoretical picture
Gross (theory)Net (theory)
Redeposited fraction or influx (normalized units)
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Erosion/deposition balance in Be seeded high flux D discharges
• Use Be oven seeding to balance surface erosion to test input parameters of material migration models
• Mass loss measures net erosion• Spectroscopy measures gross
erosion (Be I line)• Y Be→Be ≈ Y D→Be, and low
concentration of Be• When incident/seeded Be ion
flux = sputtered flux of Be, net erosion should = 0.
40 60 80 100 120 140 160 180
0.01
0.1
1
yie
ld (
% B
e p
er
ion
)
-bias (V)
pure D2
1/6 TRIM
Mass loss
ion fluence: 1022/cm2
target temperature < 320K
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
No change in mass loss is measured when Be seeding flux equals sputtering of Be by D
• ADAS database is used • Be flux from Be II (313.1 nm) and
background plasma flow velocity (E. Hollman JNM, PSI-19)
• Be ion flux is verified during no bias discharges, when weight gain is measured (net deposition)
• Net erosion stays constant, implying gross erosion must increase
• Erosion yield of 0.15% can only be compensated by seeding 2.8% Be
40 60 80 100 120 140 160 180
0.01
0.1
1
0.1<0.03
<0.06<0.01
<0.01
0.26
0.10.9 0.9
eff.
yie
ld (
% B
e p
er
ion
)
-bias (V)
Be seeded pure D
2
Be concentration seeded / non-seeded (%)
0.9
Mass loss
ion fluence: 1022/cm2
D/Be plasmatarget temperature < 320K
2.8%
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Beryllium seeded He discharges
target:bias: < -40V results in E ≈ 30eVHe ion flux: 5·1018 cm-2s-1
Be seeding: nBe/nD = 0 – 4 %
sputter yield He on Be @ 30eV: Y = 0.15 % (measured)
0 20 40 60 80
0.01
0.1
1
10
Be
I (4
57
.3n
m)
inte
nsi
ty (
a.u
.)
z [mm]
4%
1%0.3%
0.06%0.02%
no seeding
target oven
PISCES
: During ‘no Be seeding’ discharge,Gross ≈ Net erosion
Grosserosion
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
Different behavior is observed experimentally
• Net erosion stays constant until influx >> sputtering rate
• Gross erosion increases with increasing Be influx to target
• Reduced sticking of depositing Be, or increased re-erosion could explain observations
• Similar reduced sticking needed to model CD4 and WF6 injection experiments in TEXTOR (from A. Kirschner)
• Possibly similar to C-MOD modeling difficulties reported by J. Brooks (PSI19)
PISCES
0.1
1
10
0 2 4 6 8 10
Artisitc impression of measured results
Gross (theory)Net (theory)Net (exp)Gross (exp)
Influx (normalized units)
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
What PMI issues are still unresolved
• Sputtering Yield – we believe we can explain the observed, low sputtering yields in PISCES-B, due to surface morphology and fuel atoms within the target surface (no Be seeding)– How many gas atoms are in the surface during exposure?
• Gross/Net Erosion – drastic differences in behavior of gross erosion during Be seeding indicates the simple theory regarding the benefits of prompt redeposition may need to be revisited– 10 times more influx is needed to balance erosion and force net
erosion to zero– Low sticking probability or high re-erosion possible explanations
PISCES
U C S DU niversity o f C alifo rn ia S a n D iego
R. P. Doerner, 2nd PMIF Meeting, Juelich, Sept. 19-21, 2011
0 50 100 150 200
1E-4
1E-3
0.01
0.1
part
icle
ref
elce
tion
coef
ficie
nt
energy (eV)
D on Be Be on Be
TRIM(Eckstein 2002)
10 100 1000
10-4
10-3
10-2
10-1
D on Be Be on Be
SP
UT
TE
RIN
G Y
IEL
D (
at/io
n)
ENERGY (eV)
TRIM(Eckstein 2002)
Be seeding fraction (i.e. influx) is at most in the percent range, so Be self-sputtering and Be reflection can be neglected