Characterization of QSPA plasma streams in plasma - surface interaction experiments: simulation of ITER ELMs
Experimental simulations of ITER ELMs with relevant surface heat loads (energy density up to 2.4 MJ/m² ) have been performed with a quasi-steady-state plasma accelerator QSPA Kh-50. Additional shielding effect has been registered during irradiation of the combined carbon–tungsten (C–W) surfaces. Cor...
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Makhlaj, V.A. 2016-11-21T20:57:39Z 2016-11-21T20:57:39Z 2013 2013 Characterization of QSPA plasma streams in plasma - surface interaction experiments: simulation of ITER ELMs / V.A. Makhlaj // Вопросы атомной науки и техники. — 2013. — № 1. — С. 73-75. — Бібліогр.: 17 назв. — англ. 1562-6016 PACS: 52.40 Hf https://nasplib.isofts.kiev.ua/handle/123456789/109236 Experimental simulations of ITER ELMs with relevant surface heat loads (energy density up to 2.4 MJ/m² ) have been performed with a quasi-steady-state plasma accelerator QSPA Kh-50. Additional shielding effect has been registered during irradiation of the combined carbon–tungsten (C–W) surfaces. Correlation between mass losses and thresholds of erosion products ejection has been analyzed depending on the surface heat loads. For heat load below cracking threshold mass losses are caused by physical spattering. Splashing of liquid material is registered for heat load exceeding the melting threshold. Экспериментальное моделирование ELMs в ИТЭРе с соответствующими тепловыми нагрузками на поверхности (плотность энергии до 2.4 МДж/м²) было выполнено в квазистационарном плазменном ускорителе КСПУ Х-50. Установлено наличие дополнительного экранирования поверхности при облучении комбинированных углеродно-вольфрамовых поверхностей. Корреляция между потерями массы и порогами инжекции продуктов эрозии была проанализирована в зависимости от величины тепловых нагрузок. При тепловой нагрузке выше порога образования трещин потери массы обусловлены физическим распылением. Разбрызгивание расплавленного материала регистрируется при тепловых нагрузках, превышающих порог плавления. Експериментальне моделювання ELMs в ІТЕРі з відповідними тепловими навантаженнями на поверхні (густина енергії до 2.4 МДж/м²) було виконано в квазістаціонарному плазмовому прискорювачі КСПП Х-50. Встановлено наявність додаткового екранування поверхні при опромінюванні комбінованої вуглецево- вольфрамової поверхні. Кореляція між втратами маси і порогами інжекції продуктів ерозії була проаналізована в залежності від величин теплових навантажень. При тепловому навантаженні вище порогу утворення тріщин втрати маси обумовлені фізичним розпорошуванням. Розбризкування розплавленого матеріалу реєструється при теплових навантаженнях, що перевищують поріг плавлення. This work has been supported in part by IAEA’s CRP F1.30.13. The author would like to acknowledge I.E. Garkusha for discussions and interpretation of experimental results as well as the QSPA Kh-50 team for assisting in the experiments. en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники ИТЭР и приложения для термоядерного реактора Characterization of QSPA plasma streams in plasma - surface interaction experiments: simulation of ITER ELMs Определение характеристик потоков плазмы КСПУ в экспериментах по взаимодействию плазмы с поверхностью: моделирование ELMs в ИТЭРе Визначення характеристик потоків плазми КСПП в експериментах по взаємодії плазми з поверхнею: моделювання ELMs в ІТЕРі Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
Characterization of QSPA plasma streams in plasma - surface interaction experiments: simulation of ITER ELMs |
| spellingShingle |
Characterization of QSPA plasma streams in plasma - surface interaction experiments: simulation of ITER ELMs Makhlaj, V.A. ИТЭР и приложения для термоядерного реактора |
| title_short |
Characterization of QSPA plasma streams in plasma - surface interaction experiments: simulation of ITER ELMs |
| title_full |
Characterization of QSPA plasma streams in plasma - surface interaction experiments: simulation of ITER ELMs |
| title_fullStr |
Characterization of QSPA plasma streams in plasma - surface interaction experiments: simulation of ITER ELMs |
| title_full_unstemmed |
Characterization of QSPA plasma streams in plasma - surface interaction experiments: simulation of ITER ELMs |
| title_sort |
characterization of qspa plasma streams in plasma - surface interaction experiments: simulation of iter elms |
| author |
Makhlaj, V.A. |
| author_facet |
Makhlaj, V.A. |
| topic |
ИТЭР и приложения для термоядерного реактора |
| topic_facet |
ИТЭР и приложения для термоядерного реактора |
| publishDate |
2013 |
| language |
English |
| container_title |
Вопросы атомной науки и техники |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| format |
Article |
| title_alt |
Определение характеристик потоков плазмы КСПУ в экспериментах по взаимодействию плазмы с поверхностью: моделирование ELMs в ИТЭРе Визначення характеристик потоків плазми КСПП в експериментах по взаємодії плазми з поверхнею: моделювання ELMs в ІТЕРі |
| description |
Experimental simulations of ITER ELMs with relevant surface heat loads (energy density up to 2.4 MJ/m² ) have been performed with a quasi-steady-state plasma accelerator QSPA Kh-50. Additional shielding effect has been registered during irradiation of the combined carbon–tungsten (C–W) surfaces. Correlation between mass losses and thresholds of erosion products ejection has been analyzed depending on the surface heat loads. For heat load below cracking threshold mass losses are caused by physical spattering. Splashing of liquid material is registered for heat load exceeding the melting threshold.
Экспериментальное моделирование ELMs в ИТЭРе с соответствующими тепловыми нагрузками на поверхности (плотность энергии до 2.4 МДж/м²) было выполнено в квазистационарном плазменном ускорителе КСПУ Х-50. Установлено наличие дополнительного экранирования поверхности при облучении комбинированных углеродно-вольфрамовых поверхностей. Корреляция между потерями массы и порогами инжекции продуктов эрозии была проанализирована в зависимости от величины тепловых нагрузок. При тепловой нагрузке выше порога образования трещин потери массы обусловлены физическим распылением. Разбрызгивание расплавленного материала регистрируется при тепловых нагрузках, превышающих порог плавления.
Експериментальне моделювання ELMs в ІТЕРі з відповідними тепловими навантаженнями на поверхні (густина енергії до 2.4 МДж/м²) було виконано в квазістаціонарному плазмовому прискорювачі КСПП Х-50. Встановлено наявність додаткового екранування поверхні при опромінюванні комбінованої вуглецево- вольфрамової поверхні. Кореляція між втратами маси і порогами інжекції продуктів ерозії була проаналізована в залежності від величин теплових навантажень. При тепловому навантаженні вище порогу утворення тріщин втрати маси обумовлені фізичним розпорошуванням. Розбризкування розплавленого матеріалу реєструється при теплових навантаженнях, що перевищують поріг плавлення.
|
| issn |
1562-6016 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/109236 |
| citation_txt |
Characterization of QSPA plasma streams in plasma - surface interaction experiments: simulation of ITER ELMs / V.A. Makhlaj // Вопросы атомной науки и техники. — 2013. — № 1. — С. 73-75. — Бібліогр.: 17 назв. — англ. |
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ISSN 1562-6016. ВАНТ. 2013. №1(83) 73
CHARACTERIZATION OF QSPA PLASMA STREAMS IN PLASMA -
SURFACE INTERACTION EXPERIMENTS: SIMULATION OF ITER ELMS
V.A. Makhlaj
Institute of Plasma Physics NSC “Kharkov Institute of Physics and Technology”, Kharkov, Ukraine
E-mail: makhlay@ipp.kharkov.ua
Experimental simulations of ITER ELMs with relevant surface heat loads (energy density up to 2.4 MJ/m2 ) have
been performed with a quasi-steady-state plasma accelerator QSPA Kh-50. Additional shielding effect has been
registered during irradiation of the combined carbon–tungsten (C–W) surfaces. Correlation between mass losses and
thresholds of erosion products ejection has been analyzed depending on the surface heat loads. For heat load below
cracking threshold mass losses are caused by physical spattering. Splashing of liquid material is registered for heat
load exceeding the melting threshold.
PACS: 52.40 Hf
INTRODUCTION
The energy load to plasma facing components
(PFCs) during disruptions in ITER are expected in the
range of tens of GW/m2 with time duration of about
1 ms [1, 2]. In contrast to disruptions that are relatively
rare events, type I Edge Localized Modes (ELMs) are
anticipated to occur with a frequency of the order of
1 Hz (i.e., few hundred ELMs per ITER pulse). Plasma
loads at the tungsten surface for type I ELMs are
foreseen to be of q = (0.2…2) M/Jm2 and the ELM
duration of t = (0.1…0.5) ms [2].
The power loads on current tokamaks associated
with the type I ELMs generally do not affect the lifetime
of divertor elements. However, the ITER ELMs may
lead to unacceptable lifetime [2-5]. Special
investigations on material behavior at the ELM relevant
loads are thus important. To estimate the range of
tolerable loads the effects of ELMs on the lifetime of
plasma facing components should be experimentally
simulated for large numbers of impacts with varying
energy density.
A number of powerful test facilities are available to
provide examination of different materials under
transient ITER-like loads [5-8]. Among most important
tasks for simulation experiments with plasma
accelerators are studies of the threshold conditions for
cracking of different tungsten grades and issues related
to volumetric damages in the course of a large number
of exposures, dynamics of evaporated tungsten and
penetration of W impurities into plasma (especially for
the combination of disruption events with edge-
localized modes (ELMs)).
This paper presents the characteristics of QSPA Kh-
50 plasma streams and analysis of contribution of
different erosion mechanisms to the material damage
under plasma heat loads expected for ITER ELMs.
1. EXPERIMENTAL SETUP AND
DIAGNOSTICS
Experimental simulations of ITER transient events
with relevant surface heat load parameters (energy
density and the pulse duration) as well as particle loads
have been performed with the QSPA Kh-50 quasi-
steady-state plasma accelerator, which is the largest and
most powerful device of this kind [7, 9].The main
parameters of QSPA plasma streams were as follows:
ion impact energy about 0.4…0.6 keV, the maximum
plasma pressure 3.2 bars, and the plasma stream
diameter about 18 cm. The surface energy loads were
varied in the range of (0.1…1.1) MJm2 [10, 11]. The
plasma pulse shape was approximately triangular, and
the pulse duration was about 0.25 ms.
Values of plasma stream energy density were
determined on the basis of time resolved measurements
of the plasma stream density and its velocity [7, 10, 11].
The energy density in the shielding layer was measured
by displacing the calorimeter through a hole in the center
of the exposed targets made of tungsten, graphite or
combined W-C samples [12]. The calorimeter could be
moved into the near-surface plasma up to the distance of
5 cm from the target surface. Observations of plasma
interactions with exposed samples, the dust particles
dynamics and the droplets monitoring were performed
with a high-speed 10 bit CMOS pco.1200 s digital
camera PCO AG (exposure time from 1 µs to 1 s,
spectral range from 290 to 1100 nm) [10]. A surface
analysis was carried out with an MMR-4 optical
microscope equipped with a CCD camera.
2. EXPERIMENTAL RESULTS
2.1. ENERGY DENSITY IN SHIELDING LAYER
The energy density measured in shielding layer by
movable calorimeter has been increased with an
increasing distance from the target surface (Fig. 1).
Afterwards, the energy density reaches saturation at
some distance from the surface (2…4) cm, depending
on the energy density in the plasma stream) achieving
the value of energy density in the incident plasma
stream [12]. This allows conclusion that only a part of
the plasma jet energy is transferred to the target through
the shielding plasma layer.
The energy density absorbed by the tungsten target
surface is below 60 % of the QSPA plasma energy
density q ≥ 0.5 MJ/m2. This reduction in the energy
density is caused by the formation of a dense layer by
stopped head part of impacting plasma stream. Such
shielding layer protecting the target surface even
without evaporation effects is formed during the first
instants of the plasma-surface interaction. The plasma
density near the target surface achieves 2×1017 cm-3.
74 ISSN 1562-6016. ВАНТ. 2013. №1(83)
The electron temperature in the near surface layer is
practically constant during all period of plasma-surface
interaction and it does not exceed 4…5 eV [9, 13].
0 2 4 6
0,0
0,2
0,4
0,6
0,8
1,0
q/
q 0
Z, cm
q0= 0,5 MJ/m2
q
0
= 1 MJ/m2
q
0
= 2,4 MJ/m2
Fig. 1. Ratio of energy density (q) in the shielding layer
to energy density (q0) of impacting plasma vs. the
distance from the target surface (Z)
Tungsten evaporation onset under the QSPA Kh-50
exposures appears at 1.1 MJ/m2. The melting threshold
is estimated to be 0.56…0.6 MJ/m2 [10]. The carbon
evaporation threshold is essentially less, being
0.4…0.45 MJ/m2. The surface temperature of the
graphite target grows with plasma load, reaches a peak
value of Ts ≈ 4000 K at q = 0.45 MJ/m2 and then
remains unaltered with further increase of the plasma
load [7]. For combined carbon–tungsten (W–C) targets,
W melting and evaporation was not achieved under
plasma exposures at a fixed energy density. Additional
shielding of the irradiated surface by a C cloud protects
W from evaporation even at an essentially increased
energy density of impacting plasma (Fig. 2). The
fraction of plasma energy, which is absorbed by the
target surface, is rapidly decreased with achieving the
evaporation onset for exposed targets. At this, the value
of heat load to the surface remains practically constant
with further increase of the energy density of impacting
plasma.
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5
0,0
0,2
0,4
0,6
0,8
1,0
Evaporation W
Melting W
W
(W-C)
C
q su
rf
/q
0
q0, MJ/m2
Evaporation C
Fig. 2. Ratio of heat load to the target surfaces (qsurf) to
energy density of impacting plasma (q0) vs. the energy
density of impacting plasma (q0)
2.2. MATERIAL EROSION
The irradiation of tungsten surface in QSPA Kh-50
with plasma heat load below the cracking threshold
(0.3 MJ/m2) does not trigger the generation of erosion
products. At the heat load above the cracking threshold
but below melting threshold only several dust particles
traces have been registered (Fig. 3). Elastic energy
stored in stressed tungsten surface layer should be the motive
force for the cracking process with following acceleration of
separated solid particles in this case [13, 14].
Fig. 3. High speed imaging of plasma interaction with
tungsten target: t = 1.2 ms after the start of plasma-
surface interaction, texp = 1:2 ms; a- qsurf = 0.45 MJ/m2,
b - qsurf = 0.6 MJ/m2, c - qsurf = 0.75 MJ/m2
Further increase of heat load leads to the surface
melting and results in splashing of eroded material.
Number of ejected particles rises with increasing heat
load due to growing thickness of melt layer. The
majority of W particles are ejected from the exposed
surface 0.2 ms after beginning of plasma-surface
interaction. Maximal velocity of tungsten particles may
achieve few tens of m/s [10]. For heat load exceeding
the melting threshold, the flying particles can be
originated from melt surface due to Kelvin-Helmholtz
or Rayleigh-Taylor instabilities [15]. Analysis of
obtained experimental results and comparison with the
results of numerical simulations [13, 15] allows
conclusion that the generation of tungsten particles in
the form of droplets may occur only during the plasma
pulse and (as latest) few tens microseconds after the
pulse end. Other particles may be exclusively solid dust
that is generated due to the cracking process under the
melt resolidification.
The results of QSPA Kh-50 plasma exposures can be
compared with performed experiments on erosion
product monitoring in QSPA-T facility [7, 16]. Similar
velocity of erosion products and the energy threshold of
particles ejection have been were observed.
Observations of dust/droplets particles ejected from
tungsten are in agreement with measurements of mass
loses during high flux irradiation. The average mass loss
rate for exposures of 1.1 MJ/m2 is about
(36…40) µg cm−2 per pulse (Fig. 4). This result
correlates with profilometry measurements [9, 7]. The
measured value of losses for evaporation onset is 10
times higher than for exposures with a surface load of
0.75 M/Jm2 and 25 to 30 times higher than for
exposures below the melting threshold. The increase in
the mass losses by one order of magnitude is observed
with a rise in the target heat load by no more than 50%
(from 0.75 to 1.1 MJ/m2). Thus, the boiling essentially
adds to the mass losses, possibly due to the
intensification of evaporation and the initiation of
tungsten splashing.
ISSN 1562-6016. ВАНТ. 2013. №1(83) 75
It should be mentioned that, for heat loads above the
melting threshold mass losses periodically fluctuate in
accordance with surface evolution of fine intergranular
cracks. In this case the corrugation damages will appear
for next and next layers, below the surface one. Thus,
the consecutive layer by layer erosion will occur [9].
0,0 0,2 0,4 0,6 0,8 1,0 1,2
0
10
20
30
40
50
Δm
, μ
g/
cm
2 pu
ls
e
q, MJ/m2
Fig. 4. Dependence of mass loss on applied surface heat
load
CONCLUSIONS
Features of plasma surface interaction and tungsten
erosion under repetitive QSPA Kh-50 plasma heat
loads, which are relevant to ITER type I ELMs, have
been studied. The influence of the neighborhood
tungsten and carbon divertor components on the
material response to the repetitive plasma heat loads has
also been analyzed. Distributions of energy density in
shielding layer have been measured as a function of the
energy of the incident plasma streams.
Correlation between tungsten mass loses and
thresholds of erosion product ejection is analyzed and
discussed for different plasma heat loads.
ACKNOWLEDGEMENTS
This work has been supported in part by IAEA’s
CRP F1.30.13. The author would like to acknowledge
I.E. Garkusha for discussions and interpretation of
experimental results as well as the QSPA Kh-50 team
for assisting in the experiments.
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Article received 24.01.13
ОПРЕДЕЛЕНИЕ ХАРАКТЕРИСТИК ПОТОКОВ ПЛАЗМЫ КСПУ В ЭКСПЕРИМЕНТАХ ПО
ВЗАИМОДЕЙСТВИЮ ПЛАЗМЫ С ПОВЕРХНОСТЬЮ: МОДЕЛИРОВАНИЕ ELMs В ИТЭРе
В.А. Махлай
Экспериментальное моделирование ELMs в ИТЭРе с соответствующими тепловыми нагрузками на
поверхности (плотность энергии до 2.4 МДж/м2) было выполнено в квазистационарном плазменном
ускорителе КСПУ Х-50. Установлено наличие дополнительного экранирования поверхности при облучении
комбинированных углеродно-вольфрамовых поверхностей. Корреляция между потерями массы и порогами
инжекции продуктов эрозии была проанализирована в зависимости от величины тепловых нагрузок. При
тепловой нагрузке выше порога образования трещин потери массы обусловлены физическим распылением.
Разбрызгивание расплавленного материала регистрируется при тепловых нагрузках, превышающих порог
плавления.
ВИЗНАЧЕННЯ ХАРАКТЕРИСТИК ПОТОКІВ ПЛАЗМИ КСПП В ЕКСПЕРИМЕНТАХ ПО
ВЗАЄМОДІЇ ПЛАЗМИ З ПОВЕРХНЕЮ: МОДЕЛЮВАННЯ ELMs В ІТЕРі
В.О. Махлай
Експериментальне моделювання ELMs в ІТЕРі з відповідними тепловими навантаженнями на поверхні
(густина енергії до 2.4 МДж/м2) було виконано в квазістаціонарному плазмовому прискорювачі КСПП Х-50.
Встановлено наявність додаткового екранування поверхні при опромінюванні комбінованої вуглецево-
вольфрамової поверхні. Кореляція між втратами маси і порогами інжекції продуктів ерозії була
проаналізована в залежності від величин теплових навантажень. При тепловому навантаженні вище порогу
утворення тріщин втрати маси обумовлені фізичним розпорошуванням. Розбризкування розплавленого
матеріалу реєструється при теплових навантаженнях, що перевищують поріг плавлення.
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