Initial experiments with liquid target materials in PSI-2 and TEXTOR

Member of the Helmholtz Association 

Initial experiments with liquid target 

materials in PSI-2 and TEXTOR 

B. Unterberg, J.W. Coenen, A. Kreter, V. Philipps, M. Reinhart, G. 

Sergienko, A. Terra and T. Wegener 

Institut für Energie- und Klimaforschung – Plasmaphysik 

Forschungszentrum Jülich, Ass. EURATOM- Forschungszentrum Jülich, 

Trilateral Euregio Cluster, D- 52425 Jülich, Germany 

4th IEA International Workshop on Plasma Material Interaction Facilities for 

Fusion Research (PMIF 2013), Oak Ridge, TN, USA, September 9th – 13th 2013

Program on liquid targets in Jülich 

§ After assessment of 

alternative target 

concepts, FZJ 

concentrates on 

alternatives to Li in CPS 

configuration, in the 

frame of the co-ordinated 

EU fusion program 

(EFDA-PEX) and in TEC 

collaboration with FOM- 

DIFFER 

§ Development of samples 

for exposure in PSI-2 and 

TEXTOR 

Ø Report on initial 

experiments with tin 

September 9th, 2013 

B. Unterberg | Institut für Energie- und Klimaforschung – Plasmaphysik, Forschungszentrum Jülich Nr. 2

Optimization of temperature window 

motivates work on alternative materials 

Temperature limit 

§ Lower limit 

§ Melting 

§ Wetting 

§ Upper limit 

§ Evaporation flux: impact on 

plasma performance, redeposition, 

migration to 

remote areas 

§ Chemistry (e.g. LiH 

formation), corrosion / alloy 

formation (large window for 

Li, show stopper for Ga) 

Evaporation rates 

[R. Majeski, "Liquid metal walls, lithium, 

and low recycling boundary conditions in 

tokamaks" AlP Conf. Proc. vol. 1237, 122.] 



Re-Deposition 

Factor 10 increase in redepostion 

enhances T 

window by

Material compatibility of tin with mesh metals 

SST/Ga SST/Sn Mo/Sn W/Sn 

SST show a strong ability to form 

alloys with all investigated liquid metal 

candidates. 

Mo alloys with liquid Sn but in very 

small amounts. EDX analysis shows 

small inclusions (

Wetting 

950 °C 

10 -6 mbar 

Wetting - contact angle < 90° 

Adhesion > Cohesion 

Mo/Sn 

“Clean” metal surfaces are 

normaly wettable with liquid 

metals 

BUT they nearly always have 

oxide layers which reduce the 

wettability of the surface! 

No Wetting - contact angle >90° 

990 °C 

10 -6 mbar 

W/Sn 

1mbar H 2 

950°C 

W/Sn 

Adhesion < Cohesion 



Overview of wetting characteristics 

Contact angle 



Linear plasma device PSI-2 

Plasma 

source 

Coils 

TEAC 

Target 

manipulator 

Side-fed 

manipulator 

Periphery 

level 

3 m 

§ Plasma conditions (deuterium 

plasmas), with target biasing 

q = 0.1 -2 MW m -2 , simulation of 

transients by laser irradiation 

(120 J / 4 ms) 

n e = 10 17 - 10 19 m -3 

T e up to 20 eV (T i ~ 0.5 T e ) 

E ion = 10-300 eV (biasing) 

Γ ion = 10 21 - 10 23 m -2 s -1 

F = 10 27 m -2 in 4 h 

Δ flow channel ~ 6 cm 



Target holder in PSI-2 



Setup 

Tungsten Mesh: d=0.1-0.2mm 

molybdenum disk & mounting plate 



Exposure 

Infra-red 

visible 

IR-Camera Pyrometer Pyrometer 



Exposed samples 

Empty 

Before 

After 



Material Cuts 



Exposure characteristics 

Plasma characteristics 

§ Total exposure time (D2- 

plasma): 64 min. 

§ Plasma parameters in 

front of target: n e = 

8x10 17 m -3 /T e = 9 eV 

§ Plasma flux density: 

5x10 21 m -2 s -1 

Surface temperature 

§ Heat flux density: 60 

kWm -2 (no biasing) Equilibrium reached after ~900s 

IR-cam: temperature variation across 

sample ≤ ±100° C 



Sn mass loss in during exposure 

§ 

§ 

§ 

§ 

Total loss: 240 mg (of 1139mg) 

Surface temperature needed 

for evaporation of 240 mg: 

1120°C (full area) 

“effective erosion yield”: 

Y=0.15 >> Y sputter 

Penetration depth of Sn atoms: 

λ= 1.7 cm, E kin =1.5 eV >> E kin 

=0.15 eV (evaporated Sn) 

Ø Indicating sputtered particles 

§ 

SnI intensity rising during 

increase of target temperature, 

indication of temperature 

enhanced erosion? 

Intensity distribution SnI (380 nm) 

target 



TEXTOR experiment: Aims 

A: 

Tin Evaporation 

Temperature Evolution 

Tin Spectroscopy 

Liquid Metal Stability 

under quiescent Plasmas 

B: 

Stability of Liquid Tin Layers 

under Disruptive Events 

DMV triggered disruptions 

And be prepared for suprises 



Setup 

Tungsten Mesh filed with Tin 

TZM Holder 

filled 

empty 

Heatable Limiter-Setup 



Conditions 

Tin surface 

The Limiter is preheated to 

300˚C, to allow liquid Tin to 

be present at all times 

Position at the Bottom of 

TEXTOR 

TZM 

Graphite 

49cm, LCFS ~ 47cm 

1MW NBI heated (1-4.5s) BT=2.25 T, Ip=350 kA 



Exposure 

Before 



Wetted surface 

After 



Wetting Loss 

After 



Camera View 



3 frames 

#119916 

Strong droplet mission 

#119919 



Fast Camera 

Single Frames 

3000fps 

#119916 

25 frames 

50 frames 500 frames 



Droplets 



Disruptions 

14 frames before 6 frames during 

Droplets do not originate from mesh during disruption 



Mass Loss in TEXTOR 

Droplets sizes 

seem to be in the 

µm range in sized 

r=10µm--> 30ng 

Sn Mass 

available 2.12g 

Mass Loss: 

5.3159*10 -3 g 

~180000 Droplets 

Consistent with numbers 

extrapolated from fast CCD 

Mechanism not yet 

understood 

Differences PSI-2 – TEXTOR to be assessed: 

§ Plasma temperature / ion energies 

§ Carbon background in TEXTOR (coating of Sn surface) 

§ Magnetic field strength 



Summary and outlook 

§ Tin has been studied as liquid target material alternative to 

Lithium within the concept of a the capillary porous system. 

§ Tungsten has been found as optimum mesh material, wetting 

improved under hydrogen atmosphere. 

§ Exposure in PSI-2: mass loss could not be explained by 

physical sputtering / evaporation – temperature enhanced 

sputtering? 

§ Exposure in TEXTOR: mass loss dominated by strong droplet 

emission, mechanism unclear to date 

§ Next exposure in PSI-2: target biasing to increase ion energy 

to come close to TEXTOR conditions and to assess 

temperature enhanced erosion 

§ Next exposure in TEXTOR: positioning of targets in erosion 

dominated zone to prevent carbon deposition

Initial experiments with liquid target materials in PSI-2 and TEXTOR

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