Experimental study of PFCs erosion under ITERlike transient loads at plasma gun facility QSPA

1
Experimental study of PFCs erosion under
ITER-like transient loads at plasma gun facility
QSPA

• N. Klimov1, V. Podkovyrov1, A. Zhitlukhin1,
  D. Kovalenko1, B. Bazylev2, G. Janeschitz3,
  I. Landman2, S. Pestchanyi2, G. Federici4,
  A. Loarte5, M. Merola4, J. Linke6, T. Hirai6,
  J. Compan6

1 SRC RF TRINITI, Troitsk, Russia 2
Forschungszentrum Karlsruhe, IHM, P.O. Box 3640,
76021 Karlsruhe, Germany 3 Forschungszentrum
Karlsruhe, Fusion, P.O. Box 3640, 76021
Karlsruhe, Germany 4 ITER JWS Garching,
Boltzmannstr. 2, 85748 Garching, Germany 5 EFDA,
Boltzmannstr. 2, 85748 Garching, Germany 6
Forschungszentrum Jülich, Julich, Germany
2
Outline

• Experimental facilities and diagnostics
• ELMs simulation experiments
• Disruption simulation experiments
• Tungsten erosion products investigation
• Conclusion
• Future work

3
Experimental facilitiesQuasistationary plasma
accelerator
QSPA-T facility
QSPA-Be facility

• QSPA plasma parameters (ELMs)
• Heat load 0.5 2.5 MJ/m2
• Pulse duration 0.1 0.6 ms
• Plasma stream diameter 5 cm
• Ion impact energy 0.1 1.0 keV
• Electron temperature lt 10 eV
• Plasma density 1022 1023 m-3

QSPA plasma gun
1 coil of pulse electromagnetic gas valve 2
valve disk 3 volume of pulse valve 4
isolator 5 gas supply tube 6 cathode 7
anode.
QSPA facility provides adequate pulse durations
and energy densities. It is applied for erosion
measurement in conditions relevant to ITER ELMs
and disruptions
4
Experimental facilitiesQSPA facility. Scheme of
PFCs testing
Exposed sample
Diagnostic window
Plasma gun
Vacuum chamber (receiver)
Plasma flow
a 0
60 cm
Target chamber
Sample heater
5
Experimental conditionsTarget design

• Special 15 ITER-like targets (6 CFC, 3 pure W and
  6 W-1La2O3)
• were designed and manufactured by Plansee AG
  (Austria),
• were pre-characterized by Forschungszentrum
  Jülich (Germany) by optical / electron microscopy
  and laser-profilometry

6
Experimental conditions Target orientation and
temperature, energy density, pulse duration
View of the target on a heater

• Value of surface energy density in the axis of
  plasma stream (central part of the sample) was
• 0.51.5 MJ/m2 in ELMs experiments
• gt2.2 MJ/m2 in disruption experiments

• Target was preheated up to 500O C by radiate
  heater
• Plasma pulse duration was t 0.5 ms
• Total number of pulses was
• 100 for ELMs experiments
• 5 for disruptions experiments

• The samples was observed after each 10-20 pulses
  by SCR RF TRINITI
• The samples was observed before exposures and
  after full plasma pulse series by FZJ.

7
Diagnostics
Diagnostics
Plasma parameters measurements
Products erosion study
PFCs testing
1. Pressure probe
1. Calorimeters
1. Online diagnostics for dust particle ejection
study
2.System of plasma stream velocity measurements
2. Microbalance
2. Dust collectors
3. Profilometer
4. SEM
Velocities and sizes of dust particles, onset
conditions parameters of films, porosity and
fractal structure
Plasma pressure, plasma flow duration, plasma
stream velocity, ion impact energy, plasma flow
energy density
Absorbed energy density, mass losses, erosion
value, surface modification
8
Experimental resultsAbsorbed energy density
distribution
The energy density distribution on CFC surface,
9
Experimental resultsPlasma flow parameters and
heat loads
10
Experimental resultsExposed samples
11
ELM experimentsPure tungsten and lanthanum
tungsten (comparison)
12
ELM experiments W-1La2O3, local energy density
Q 0.9 MJ/m2
13
ELM experiments Comparison between pure W and
W-1La2O3 (20 exposures)
W(gt99,96)
W-1La2O3
14
ELM experiments Comparison between pure W and
W-1La2O3 , Q 0.8 MJ/m2
15
ELM experiments Comparison between pure W and
W-1La2O3 , Q 1.0 MJ/m2
16
ELM experiments Cracks formation, Q 1 MJ/m2,
W-1La2O3, 100 pulses
17
ELM experiments Gas outlet, W-1La2O3 , Q 0.5
MJ/m2,
53 exposures
53 exposures
53 exposures
18
Disruption experimentsSurface development,
W-1La2O3, Q gt 2.2 MJ/m2
Before exposure
After 5 pulses
Plasma stream direction
19
Disruption experiments Melt layer splashing,
W-1La2O3, Q gt 2.2 MJ/m2
20
Summary (1)Erosion of pure tungsten and
lanthanum tungsten
Various erosion processes of lanthanum tungsten
start at lower energy density as compare to pure
tungsten
21
ELM experiments Mass loss, specific erosion
Mass loss of lanthanum tungsten sample
(W-1La2O3), Q 1 MJ/m2
Mass loss of pure tungsten sample (Wgt99,96), Q
1.5 ???/?2

• lanthanum tungsten W-1La2O3
• Q 1.0 MJ/m2 - lt?hgt 0.04 µm/pulse

• pure W(gt99,96)
• Q 1.0 MJ/m2 - mass loss was negligible
• Q 1.5 MJ/m2 - lt?hgt 0.06 µm/pulse

Droplets ejection from tungsten surface is a main
mechanism of sample mass losses under ELM-like
loads on the QSPA facility
22
Disruption experiments CFC fibers erosion
23
Disruption experiments PAN fibers erosion
24
Disruption experiments Fibers erosion
measurements
CFC fibers erosion as a function of pulse number
Number of pulses

• PAN fiber erosion was 6 ? 2 (µm/pulse).

25
Summary (2)CFC erosion
PAN fiber damage is a main mechanism of CFC
erosion under ELM-like and disruption-like plasma
load
26
Disruption experiments Mass loss, specific
erosion
Mass loss of CFC sample (Snecma NB31)
Mass loss of lanthanum tungsten sample (W-1La2O3)

• Average erosion 3 µm/pulse
• Erosion grow with pulse number

• Average erosion 3 µm/pulse

27
Summary (3)Erosion value
28
Droplets ejection studyDroplets ejection from
tungsten surface
W, Qabs 1.2 MJ/m2 , p 1.8 atm
W, Qabs 1.4 MJ/m2 , p 2.5 atm
29
Droplets ejection study Onset conditions and
intensity of droplets ejection
30
Droplets ejection study Droplets velocity
components
Distribution of velocity component VX
W, Qabs 1.6 MJ/m2 , p 2.3 atm
Distribution of velocity component VZ
Distribution of velocity component VY
31
Droplets ejection study Velocity, flight angle,
moment of droplet formation
Distribution of total velocity
Distribution of flight angle
Distribution of radial velocity
Moment of droplet formation
Pulse duration
32
Summary (4)Results of tungsten particle ejection
study

• Onset conditions of droplets ejection from
  tungsten surface Qabs 1.2 MJ/m2 and p- 1.8
  atm.
• The absolute value of velocity lies in a range
  from 0 to 20 m/s.
• The 80 of the droplets are ejected at small
  angles to the target surface (less than 450)
• There are two types of droplets exist
• droplets of the fist type are formed during
  plasma discharge
• droplets of the second type arise within 1.5 ms
  after end of plasma action.
• The droplet diameter lie in a range from 10 to
  100 micrometers.

33
ConclusionPFCs testing under ELMs and disruption
heat loads

• Macroscopic erosion was the main mechanism of
  tungsten target damage and mass losses under ELMs
  plasma loads.
• Tungsten macroscopic erosion includes material
  damage due to brittle destruction and melt layer
  movement. The both processes lead to microscopic
  particle ejection from tungsten surface solid
  particle in a case of brittle distraction and
  liquid droplets in the result of melt layer
  splashing. The particles and droplets ejected
  from the exposed surface were observed in the
  experiments with tungsten macrobrush divertor
  targets.
• Intense droplets ejection may be the result of
  high plasma pressure at the QSPA facility. It is
  possible that in ITER melt layer splashing will
  be less during ELMs.
• The main mechanisms of CFC PFCs erosion under
  ELMs heat loads is a PAN fiber damage and cracks
  formation. Cracks formation can lead to following
  effects thermal conductivity decreasing,
  particles ejection.

34
Future work

• Continue joint EU-RF study of CFC, pure tungsten
  and lanthanum tungsten erosion under ELMs and
  disruption heat loads
• Start joint EU-RF study of the damage of Be-clad
  and Be-coated ITER Plasma Facing Components under
  simulated Type I ELMs, Disruptions and Mitigated
  Disruptions
• Start concurrent investigation of dust formation
  by using online diagnostics and dust collectors

35
QSPA team
36
DiagnosticsOnline diagnostics for droplets and
particles ejection study
The scheme of diagnostics
Pure tungsten, Qabs 1.2 MJ/m2 , p 1.8 atm

• The scheme allows to define
• threshold plasma loads for dust particles
  ejection starting
• components of the dust particle velocity vector
• absolute velocity value and flight angle of the
  particle
• point of time formation and size of the dust
  particle.

x(t), y(t), z(t) droplets coordinates
37
DiagnosticsMeasurements of absorbed energy
density distribution
Shield frame
Cells with thermocouples
Schemes and views of the calorimeters
Absorbed energy density distribution was measured
by means of special CFC and tungsten target-like
calorimeters
38
ELM experiments (4) Varying of plasma incidence
angle, W-1La2O3 , Q 0.5 MJ/m2
50 exposures
20 exposures
100 exposures
a 600
1mm
1mm
1mm
20 exposures
50 exposures
a 800
1mm
1mm
39
Experimental results Droplet ejection, pure
tungsten, energy density Q 1.6 MJ/m2
Typical micrographs of the tungsten droplets
tracks

• During the first shot droplets ejected mainly
  from the edges of the tiles.
• As a result of edge smoothing and bridging of
  gaps the droplet ejection was reduced and mass
  losses were decreased.