IMPACT OF SEEDING IMPURITIES ON ITER PLASMA-FACING MATERIALS

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IMPACT OF SEEDING IMPURITIES ON ITER PLASMA-FACING MATERIALS

B. Unterberg ^1* , S. Brezinsek ¹ , T. Dittmar ¹ , L. Gao ² , W. Jacob ² , A.

Kreter ¹ , Ch. Linsmeier ¹ , G. Meisl ² , S. Möller ¹ , M. Rasinski ¹ , M.

Reinhart ¹ , T. Schwarz- Selinger ²

1 Forschungszentrum Jülich GmbH, Institut für Energie und Klimaforschung, D-52425 Jülich, Germany

2 Max-Planck Institut für Plasmaphysik, Boltzmannstr. 2, 85748 Garching, Germany

ICFRM-17, Aachen, Germany, 12-16 October 2015

Impact of impurities on

plasma material Interaction processes

 Physical sputtering by impurities

 Larger energy gain within Debye sheath (∆E = 3 Z T _e )

 Larger energy transfer to lattice atoms during binary collisions → larger yield, lower sputtering threshold

 Formation of nano-structures within plasma facing materials (inert gases)

 Surface modifications (roughness, 3D structures), defect formation

 sputtering yields (incident angle), re-deposition of eroded materials

Fuel retention

 Formation of mixed surface layers for chemically active

impurities

Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Nr. 3

Plasma facing materials and impurities in ITER

Plasma facing materials

 Tungsten (divertor)

 Beryllium (first wall armour)

 Stainless steel (plasma facing surfaces of port plugs)

 Mixed W – B layer systems

 Tungsten beryllide systems (800-1200°C)

October 14^th 2015

Plasma facing materials and impurities in ITER

(Seeding) impurities

 Helium (product of DT

fusion, during low activation face) – inert gas, 5-10%

 Nitrogen (divertor radiation, replacement for carbon) – chemically active, < 3%

 Neon (edge / divertor

radiation) – inert gas, < 2 %

 Argon (edge / main

chamber radiation) – inert gas → DEMO, <1%

A. Kallenbach et al., Nucl. Fusion 2009

Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Nr. 5

Plasma facing materials and impurities in ITER - this contribution

Plasma facing materials

 Tungsten (divertor)

 Beryllium (first wall armour)

 Stainless steel (plasma facing surfaces of port plugs)

 Mixed W – B layer systems

 Tungsten beryllide systems (800-1200°C)

(Seeding) impurities

 Helium (product of DT

fusion, during low activation face) – inert gas

 Nitrogen (divertor radiation, replacement for carbon) – chemically active

 Neon (edge / divertor radiation) – inert gas

 Argon (edge / main

chamber radiation) – inert gas → DEMO

October 14^th 2015

Experiments

Toroidal confinement devices

 JET ITER-like wall (Be first wall, W divertor)

S. Brezinsek, JET-EFDA contributors, JNM 463 (2015) 11–21

 ASDEX-Upgrade with full tungsten wall

A.Kallenbach et al., Plasma Phys. Control.

Fusion 55 (2013) 124041

 EAST, WEST

Linear plasma devices

 PISCES-B (UCSD)

 MAGNUM-PSI / Pilot-PSI (DIFFER)

 PSI-2 (FZ Jülich)

 NAGDIS-II (U Nagoya)

 Linear plasma generator (JAERI)

Ion beam experiments Laboratory plasma

experiments

 PLAQ (IPP Garching)

Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Nr. 7

Experiments

Toroidal confinement devices

 JET ITER-like wall (Be first wall, W divertor)

S. Brezinsek, JET-EFDA contributors, JNM 463 (2015) 11–21

 ASDEX-Upgrade with full tungsten wall

A.Kallenbach et al., Plasma Phys. Control.

Fusion 55 (2013) 124041

 EAST, WEST

Linear plasma devices

 PISCES-B (UCSD)

 MAGNUM-PSI / Pilot-PSI (DIFFER)

 PSI-2 (FZ Jülich)

 NAGDIS-II (U Nagoya)

 Linear plasma generator (JAERI)

October 14^th 2015

Ion beam experiments Laboratory plasma

experiments

 PLAQ (IPP Garching)

Linear plasma device PSI-2

target positions plasma source

target exchange &

analysis chamber

linear manipulator

Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Nr. 9

Linear plasma device PSI-2

Coils

Side-fed manipulator Plasma

source

Target station

TEAC

Periphery level

October 14^th 2015

Plasma exposure parameters in PSI-2

Magnetic field in

exposure chamber 0.1 T steady-state Plasma species D, H, N, Ar, He, Ne etc.

Electron temperature 1 - 25 eV (for D) El. density ~10

- 10

m

Particle flux ~10

- 10

m

s

Particle fluence up to ~10

m

per

exposure

Incident ion energy ~10 - 300 eV (negative bias)

Sample temperature RT - 2000°C Diameter of plasma

column ≈ 6 cm

Langmuir probe measurements for deuterium plasma

Plasma parameters

Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Nr. 11

Plasma exposure parameters in these studies

Incident ion flux ~ 10 ²² m ^-2 s ^-1 Incident ion fluence ~ 10 ²⁶ m ^-2 Incident ion energy ≈40 eV Sample temperature 380 K Fraction of seeded

Helium and Argon ions 0 – 8%, controlled by spectroscopy Sample surface mechanically polished, annealed at 1000°C for 2 h

October 14^th 2015

Deuterium retention in tungsten

under influence of helium and argon

0 200 400 600 800

0 1 2 3 4 5 6 7 8

D r el eas e r at e [ x10

m

s

]

Desorption temperature [°C]

D

D

+ 1% He D

+ 5% He

0 200 400 600 800

0 1 2 3 4 5 6 7 8

D r el eas e r at e [ x10

m

s

]

Desorption temperature [°C]

D

D

+ 4% Ar D

+ 8% Ar Thermal desorption spectra (TDS) of tungsten exposed to mixed plasmas

0.4 K/s ramp

0.4 K/s ramp

M. Reinhart et al., JNM, 463 (2015) 48639, 1021-1024

Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Nr. 13

Deuterium retention in tungsten

under influence of helium and argon

Effect of helium:

 Total deuterium retention is reduced by a factor of 3

Effect of argon:

 Total deuterium retention slightly increased

 TDS spectra show different shapes

→ Change in trapping sites due to material damage by argon

[M. Reinhart et al.,

JNM, 463 (2015) 48639, 1021-1024]

Total amount of deuterium retained in exposed tungsten

0 1 2 3 4 5 6 7 8 9 10 0

1 2 3 4 5

deut er ium r et ent ion [ x10 20 m -2 ]

impurity ion fraction [%]

D + He D + Ar

October 14^th 2015

TEM cross-section images

for D, D+He and D+Ar exposure

a) platinum coating

b) damaged surface layer/

He nano-bubbles c) bulk tungsten

D, D+Ar exposures:

• damaged layer depth is in the ion penetration range (2 nm) D+He exposure:

• damaged layer depth is beyond the ion penetration range

→ formation and growth of helium nano-bubble layer

• Layer thickness constant at fluencies 10 ²⁴ -10 ²⁶ m ^-2 but increases with

Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Nr. 15

Deuterium retention in tungsten for variation of incident fluences

Influence of He develops at low fluences (He

fluence <10

m

)

Reduction in retention of factor of 3-4 remains constant for the range of fluences

10 ²⁴ 10 ²⁵ 10 ²⁶

0.1 1 10

deut er ium r et ent ion / 10 20 m -2

deuterium fluence / m ^-2 D

D + He

D + 5% He: ~Φ ^0.4±0.1 pure D: ~Φ ^0.35±0.1

October 14^th 2015

PlaQ (versatile plasma implantation source)

[1] A Manhard, et. al. 2011 Plasma Sources Sci. T. 20 015010

 D implantation: PlaQ ^[1]

• 1.0 Pa: D ₃ ⁺ (94%) + D ₂ ⁺ (3%) + D ⁺ (3%)

• Flux: 9.9×10 ¹⁹ D∙m ^-2 ∙s ^-1 (at 200 V) 1.07 ×10 ²⁰ D∙m ^-2 ∙s ^-1 (at 600 V)

• Fluence: 1×10 ²³ - 6×10 ²⁴ D∙m ^-2

• Ion energy: 10 to 600 V

• Temperature: 230 to 800 K

IPP Garching

Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Nr. 17

W sputtering by impurity ions

October 14^th 2015

A.Kallenbach et al., Plasma Phys.

Control. Fusion 55 (2013) 124041

Cold divertor

plasma required

ELMs govern W

erosion

W sputtering by impurity ions –

Dynamics of WN formation reduces W sputtering

K. Schmid et al., Nucl. Fusion 50 (2010) 025006

Co-bombardment of D and N:

Preferential sputtering of N by D out of WN layer can undo W shielding effect

Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Nr. 19

Formation of WN layers by N impact

October 14^th 2015

Numbers: fluence of 10 keV N ₂ ⁺ beam

K. Schmid et al., Nucl. Fusion 50 (2010) 025006

 N content in W surface saturates at stoichiometry of W-nitride (50% N).

 Nitride formation within ion

implantation range

 WN unstable for T> 600 K

(decomposition by

N outgassing)

Fuel retention in WN model system (produced via magnetron sputtering)

 Exposure of WN model system in PlaQ

 Analysis of fuel retention by NRA

 Implantation of deuterium within implantation zone, no diffusion across WN layer at 300K

 Diffusion at 600 K much slower than in W reference samples

WN

W

Si

70 nm

Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Nr. 21

Influence of nitrogen pre-implantation on deuterium retention in tungsten

 Exposure of bulk W to N or D plasmas in PlaQ

 N pre-implantation at a fluence of

1.5x10 ²² N m ^-2 (N ₂ ⁺ dominating)

 D exposure at a fluence of 10 ²⁴ D m ^-

2 (D ₃ ⁺ dominating)

October 14^th 2015

L. Gao, et al, Phys. Scr.

T159 (2014), 014023

300 K

500 K

No N pre-implantation With N pre-implantation

Influence of nitrogen pre-implantation on deuterium retention in tungsten

L. Gao, et al, Phys. Scr.

Thin WN layer acts as barrier for diffusion to surface

 Exposure of bulk W to N or D plasmas in PlaQ

 N pre-implantation at a fluence of

1.5x10 ²² N m ^-2 (N ₂ ⁺ dominating)

 D exposure at a fluence of 10 ²⁴ D m ^-

2 (D ₃ ⁺ dominating)

Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich, Nr. 23

Conclusions

Impurities govern PMI in fusion devices to a large extent and will do so in ITER

 Low temperature operation in the divertor required because of sputtering thresholds of impurities

 Strong surface modifications by inert gases, structure formation influences fuel retention decisively.

 Chemically active impurities such as N form layers which might reduce erosion of bulk material but act as diffusion barriers to enhance deuterium retention.

October 14^th 2015