Nuclear Materials and Energy - Hanyang · 2019. 11. 29. · These ELM filaments, with higher...
Transcript of Nuclear Materials and Energy - Hanyang · 2019. 11. 29. · These ELM filaments, with higher...
Nuclear Materials and Energy 12 (2017) 1259–1264
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Nuclear Materials and Energy
journal homepage: www.elsevier.com/locate/nme
Type I ELM filament heat fluxes on the KSTAR main chamber wall
M.-K. Bae
a , R.A. Pitts b , J.G. Bak
c , S.-H. Hong
c , H.S. Kim
c , H.H. Lee
c , I.J. Kang
a , K.-S. Chung
a , ∗
a Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul, Republic of Korea b ITER Organization, Route de Vinon-sur-Verdon, CS 9 046 13067 St. Paul-lez-Durance, France c National Fusion Research Institute(NFRI), 169-148 Gwahak-ro, Yuseong-gu, Daejeon, Republic of Korea
a r t i c l e i n f o
Article history:
Received 16 July 2016
Revised 14 March 2017
Accepted 16 April 2017
Available online 3 May 2017
a b s t r a c t
Heat loads deposited on the first wall by mitigated Type I ELMs are expected to be the dominant con-
tributor to the total thermal plasma wall load of the International Thermonuclear Experimental Reactor
(ITER), particularly in the upper main chamber regions during the baseline H-mode magnetic equilib-
rium, due to the fast radial convective heat propagation of ELM filaments before complete loss to the
divertor. Specific Type I ELMing H-mode discharges have been performed with a lower single null mag-
netic geometry, where the outboard separatrix position is slowly ( ∼7 s) scanned over a radial distance
of 7 cm, reducing the wall probe–separatrix distance to a minimum of ∼9 cm, and allowing the ELM fila-
ment heat loss to the wall to be analyzed as a function of radial propagation distance. A fast reciprocating
probe (FRP) head is separately held at fixed position toroidally close and 4.7 cm radially in front of the
wall probe. This FRP monitors the ELM ion fluxes, allowing an average filament radial propagation speed,
found to be independent of ELM energy, of 80–100 ms −1 to be extracted. Radial dependence of the peak
filament wall parallel heat flux is observed to be exponential, with the decay length of λq, ELM ∼25 ± 4 mm
and with the heat flux of q ‖ , ELM = 0.05 MWm
−2 at the wall, corresponding to q ‖ ∼ 7.5 MWm
−2 at the sec-
ond separatrix. Along with the measured radial propagation speed and the calculated radial profile of the
magnetic connection lengths across the SOL, these data could be utilized to analyze filament energy loss
model for the future machines.
© 2017 The Authors. Published by Elsevier Ltd.
This is an open access article under the CC BY-NC-ND license.
( http://creativecommons.org/licenses/by-nc-nd/4.0/ )
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. Introduction
Edge localized modes (ELMs) are periodic relaxation phenom-
na due to the steep edge gradients of plasma pressure and lead
o ejection of high energy and particles. In recent years, which
re the intermittent filamentary structure of ELM filaments during
LMs has been observed in the edge plasma and scrape-off layer
SOL) [1–7] . These ELM filaments, with higher density than am-
ient plasma, are mostly extended along the magnetic field lines,
nd partially propagate outward to the wall due to negative curva-
ure and gradient related with E ×B drift [8,9] .
Since Type I ELM heat flux mainly reaches along the field lines
o the divertor plates, heat fluxes on the first walls of present toka-
aks would be negligible. However, if the heat flux of ELM fila-
ent exceed the thermal limitation of divertor ( ∼20 MW/m
2 for W
nd CFC) and first wall ( ∼0.5 MW/m
2 for Be mockups) [10,11] due
∗ Corresponding author.
E-mail addresses: [email protected] (M.-K. Bae), [email protected]
K.-S. Chung).
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ttp://dx.doi.org/10.1016/j.nme.2017.04.006
352-1791/© 2017 The Authors. Published by Elsevier Ltd. This is an open access article u
o fast radial convective transport of ELM filament [12] , it would
ause severe damage to the first wall in advanced tokamaks such
s ITER and DEMO reactors. The specific parameters of ELM fil-
ments such as density and temperature would be essential not
nly to understand the basic mechanisms of ELM filaments and
heir radial transport but also to estimate the damage to the first
all for the extrapolation to future devices.
Propagation of ELM filament is analyzed by the fast camera and
RP in MAST [1] . A comparison between ELM filament transport at
igh and low field side SOL is carried out in JT-60U [13] . In DIII-
, far SOL transport and plasma interaction with the main wall
ave been investigated [6] . Characteristics of ELM filament and far
OL transport have been investigated by electric probes in ASDEX
pgrade [3–5] . Recently, particle and heat fluxes in the far SOL
ave been studied using fast reciprocating probe (FRP) and Thom-
on scattering in ASDEX Upgrade [14] . So far, a number of studies
ave been reported ELM filament transport in SOL and far SOL, but
nderstanding of effect of ELM filament on the first wall is still
imited.
In this paper, the results of first measurements of heat fluxes
oward the first wall of KSTAR device by electric probes are
nder the CC BY-NC-ND license. ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )
1260 M.-K. Bae et al. / Nuclear Materials and Energy 12 (2017) 1259–1264
Fig. 1. Horizontal and vertical cross section of KSTAR and position of fast reciprocating probe (FRP), toroidal limiter (TL) and poloidal electric probe (PEP) are presented. The
dark green dashed line indicates the D α view direction (DVD). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of
this article.)
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presented. Poloidal electric probes (PEPs), installed on the outer
main chamber wall of KSTAR, are used to measure the far-SOL
plasma parameters such as plasma density, electron temperature,
ion saturation current, parallel Mach number and floating poten-
tial. From these measurements, ELM filament parameters such as
heat and particle fluxes, radial velocity are calculated, and the
effect of ELM filament on the first wall are analyzed. In addition,
a separate FRP head is held at a fixed position in front of the wall
probe. This FRP monitors the ELM ion fluxes, allowing an average
filament radial propagation speed, found to be independent of
plasma radial position.
2. Experiment
We have installed a poloidal electric probe (PEP) set for the
measurement of far-SOL parameters of KSTAR, which is composed
of 8 electric probes on a rectangular plate with 125 × 280 mm
2 .
The probe tips are made of carbon fiber composite and insulated
by boron nitride covers. The diameter of each probe tip is 4 mm
and the tip is protruded 1 mm from the graphite cover tile surface.
PEPs are fixed on the outboard midplane wall and located 7.5 cm
behind the outboard toroidal limiters, which is also located ∼16 cm
behind from the typical separatrix position of KSTAR diverted plas-
mas as shown in Fig. 1 . Probe tips are arranged poloidally to be
composed of two triple probes and one Mach probe, which al-
lowed direct measurements of electron temperatures, particle and
eat fluxes, and Mach numbers at far SOL region with a fast acqui-
ition rate of 2 MHz. Detail structure of the PEP assembly is shown
n Fig. 1 .
As for the triple probe system, a fixed bias voltage −200 V was
pplied to the two probes ( p 1 and p 3 ), and the other probe ( p 2 )
as for measuring the floating potential ( V f ). Potential differ-
nce between the p 1 and floating probe p 2 is proportional to the
lectron temperature ( T e ), which is given by simple formula as
e = [ e ( V 1 − V f ) ] / ln 2 , where V 1 is positive bias voltage of p 1 . As-
uming T e ≈ T i at the edge plasma, the plasma density ( n e ) is given
y n e = I is ( αe A e f f
√
T e / M i ) , where I is , M i , A eff and α are ion satu-
ation current, ion mass, effective projected area for parallel flow
nd coefficient of I is (for magnetized plasma, α ≈ 0.49), respec-
ively. Parallel particle flux ( �‖ ) and heat flux ( q ‖ ) can be derived
sing following equations: �‖ = I is /e A e f f and q ‖ = γs T e I is /e A e f f ,
here γ s is the sheath transmission coefficient as given in γs = 7 ,
lthough the average value of various tokamaks is to be given as
s = 6 [15] .
. Analysis
.1. Radial propagation of ELM filament
Cross-field transport of heat and particles were experimentally
bserved by an electric probe on outboard midplane wall (PEP).
lthough three limiters exist between A and N bay of KSTAR outer
M.-K. Bae et al. / Nuclear Materials and Energy 12 (2017) 1259–1264 1261
Fig. 2. (a) Typical Single ELM structure observed through change of magnetic signal, D α and probe measurements. From the probe measurements, plasma density, electron
temperature, heat flux and Mach number were calculated. The time difference between the initiation of the filaments (dashed red line: dB/dt) and the arrived (dashed blue
line: V f or I is ) to the poloidal electric probes (PEPs) is about 0.5 ms. D α signal is from the private region of the bottom divertor. (b) Ion saturation current measurements
by poloidal electric probes (PEPs) and fast reciprocating probe (FRP) and position of separatrix during the discharge. (shot #13117). We have not used the data for the time
range of 6 < t < 7 (s) due to sudden current ramp up in (b) FRP measurement. (For interpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)
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idplane (refer to Fig. 1 ), we can know considerable number of
laments do not hit the limiter, by comparison of probe signal at
he wall with ELM peaks in D α signal which came out from the
lasma. ELM filament structures were observed clearly by PEP be-
ause the background plasma density and electron temperature at
ar-SOL are much lower than those of ELM filament.
Since the probes are fixed on the wall to investigate the ra-
ial propagation of ELM filament on the main wall, we have slowly
hanged the outboard separatrix position in the radial direction for
s during an ELMy H-mode discharge with the following condi-
ions of shot 13117: I P = 600 kA, B T = 2.7 T, n e = 1.8 −4 × 10 19 m
−3 ,
NB = 4.5 MW, κ ∼1.6, W Plasma = 30 0–40 0 kJ, H-mode flattop phase
10 s, midplane separation between primary and secondary sepa-
atrices ∼1 cm. The distance between the separatrix and PEP was
aried from 9 to 16 cm, which produced the changed of the sep-
ratrix to be 7 cm. The position of the separatrix was verified
y using the equilibrium reconstruction code (EFIT). The error of
FIT on radial position control of LCFS is within 1 cm. Fig. 2 (a)
hows typical parameters of ELM filament structures based on
ime trace of magnetic fluctuation, D α signal and ion saturation
urrent signal. D α and magnetic signal which was measured by
irnov coil were used to match the filament peaks to those of
robe signals. This Mirnov coil is mounted behind PFC tiles in
utboard divertor region which is near the midplane and probes.
n Fig. 2 (b), ion saturation currents were measured with notice-
ble change during movement of the separatrix. A drastic in-
rease of I is was observed as separatrix approached to the wall
round 4.5 s ( r s ∼ 2270 mm). Peak ELM heat and particle fluxes
hich are calculated by measurement of I is ( Fig. 2 (b)) were an-
lyzed as a function of radial propagation distance as shown in
fiig. 4 . �r s is gap distance between moving separatrix and fixed
EP. Radial dependence of the peak filament heat flux, parallel
ransport to the wall were reasonably fitted into exponential func-
ions, and the decay lengths of heat and particle fluxes were
ormed to be λq, ELM
= 25 ± 4 mm and λ�, ELM
= 31 ± 1 mm, respec-
ively. Absolute values of heat and particle fluxes were also given
s q ‖ , ELM
= 0.05 MWm
−2 and �‖ , ELM
= 3 × 10 21 m
−2 s −1 at the wall.
he deduced decay length of heat flux is comparable to those of
UG device ( ∼2–30 mm) [3] . One has to keep in mind that the
hange of r s not only can affect the plasma density and temper-
ture at the separatrix, but also in the near and far-SOL, which can
nfluence filament dynamics [16] .
.2. Velocity of ELM filament
Radial velocities of ELM filament were measured from the time
elay between two radially separated electric probes, PEPs and FRP.
n FRP head is held at a fixed position toroidally close to PEP and
.7 cm radially in front of the PEP. In Fig. 3 (a), signals of ion sat-
ration currents from both FRP and PEP show a good correlation
f the ELM structures on the ion saturation current measurement.
pproximately 200 ELM filament peaks, which can be clearly iden-
ified and corresponded with D α , were selected and these data
ere averaged over 4 ms centered on the D α peak as shown in
ig. 3 (b). There is a time delay of ∼ 0.5 ms between first large fil-
ment peaks of both probes. From the time delay and difference
f distance, radial ELM filament velocity can be deduced as 80–
00 ms −1 .
Fig. 5 (a) and (b) show the trend that the radial velocity of
lament increases with the amplitude of ion saturation current
1262 M.-K. Bae et al. / Nuclear Materials and Energy 12 (2017) 1259–1264
Fig. 3. (a) ELM filament peaks in FRP and PLP measurements were selected by matching with D α peaks. (b) Selected peaks were sliced for 4 ms based on the highest of
peak and averaged for velocity calculation.
Fig. 4. Profile of ELM peak particle flux ( �‖ = I is /e A e f f = J is /e ) and heat flux at the far-SOL measured by PEP during the variation of separatrix in 7 cm. The decay lengths
of heat and particle flux are determined by the least square fitting (red), which are given as about 25 mm and 31 mm, respectively. (shot #13117). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
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density ( J is = I is / A e f f ) and heat flux during the change of separatrix
position. This trend indicates that v r _ ELM
depends on the J is and on
the relative separatrix position ( �r s ), since J is decreases exponen-
tially with �r s as observed in Figs. 2 (b) and 4 . It means v r _ ELM
is strongly correlated with the particle flux of filaments and sep-
aratrix position, while it is weakly correlated with the heat flux.
In addition, radial propagation of ELM filament can be derived
s v r _ ELM
∼ λ/ τ‖ ∼ C s λ/ L ‖ ( C s =
√
T e + T i / M i , L ‖ = πRq ), where τ ‖ nd L ‖ are time scale for transport and parallel connection length,
is the safety factor, C s is ion sound speed [2] , estimated assum-
ng T e = T i = 10 eV which is measured by PEP. The radial velocity
f the ELM filament ( v r _ ELM
) is calculated as 43–73 ms −1 , from the
esults of moving separatrix experiment (#13117) based upon the
alculated decay length of particle flux.
M.-K. Bae et al. / Nuclear Materials and Energy 12 (2017) 1259–1264 1263
Fig. 5. The radial velocity of filaments as a function of (a) ion saturation current density, (b) parallel heat flux, and (c) ELM energy loss ratio ( �W ELM / W tot ). Peak values of
J is and q ‖ are averaged over 10 measurements. In case of (c), 8 discharges with different plasma current were used.
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In order to investigate the correlation with v r _ ELM
and
W ELM
/ W tot , the radial velocity has been obtained from 8 dis-
harges, which contains about 200 ELMs in each discharge, with
ifferent plasma current ( I P = 400–750 kA), at the same separa-
rix position. The average ELM energy loss is determined from
he drop of total stored energy ( W tot ) during ELMs. Fig. 5 (c)
hows v r _ ELM
as a function of the fraction of energy loss due
o ELMs ( �W ELM
/ W tot = 0 . 015 − 0 . 03 ). The range of average fila-
ent radial velocity is 80–100 ms −1 , while separatrix is fixed at
s = 2.23 m. There seems to be no clear correlation with v r _ ELM
and
W ELM
/ W tot . The reason could be that the only largest filament in
ach ELM period has been used for analysis of the radial velocity,
here could lead the uncertainty of v r _ ELM
because each ELM fila-
ent has different ener gy and size. Therefore, to obtain the valid
orrelation between v r _ ELM
and �W ELM
/ W tot , we have to include all
f filaments for analysis during ELM.
Radial velocity of the filaments has been also calculated by time
f flight (TOF) method from the time delay between the start of
agnetic fluctuation signal and PEP signal. The mean value of ra-
ial velocity by TOF ( v r _ T OF ) is about 200 ms −1 , which is about 2
imes larger than the v r _ ELM
calculated by the above method. The
eason for this difference seems to be due to the time delay be-
ween the magnetic fluctuation, indicating the time of the forma-
ion of filaments within the pedestal, and the time of departure
f filaments from the LCFS, which would usually be happened in-
ide of core plasma with respect to separatrix. Hence, v r _ ELM
mea-
ured at far-SOL would relatively be expected to be smaller than
r _ T OF .
t.3. Energy loss to the wall by ELM filament
Total ELM energy deposition to the wall can be derived as
wal l _ ELM
= q ⊥ S out �t ELM
, where S out is approximated by the total
rea of midplane outer wall (13.3 m
2 ) and q ⊥ �t ELM
( q ⊥ = q ‖ sinθ ,
≈ 5 °) is calculated by integrating the heat flux as a function of
ime measured by PEP. The same method as used to determine the
adial velocity is utilized to find the ELM peaks in heat flux data.
he average energy of eight discharges, deposited on the first wall
y the filament ( W wal l _ ELM
) during the flat-top phase, is 1 ± 0.3 kJ,
hich is significantly lower than those of divertor targets. Since,
LM energy ( �W ELM
) ejected from the core plasma is generally 5–
0 kJ in KSTAR, the ratio of energy deposited on the wall to the
LM energy loss ( W wal l _ ELM
/ �W ELM
) is about 15%. Then ratio of the
LM deposited energy on the wall to the total stored energy of
lasma, W wal l _ ELM
/ W tot is about 0.5%. If we simply extrapolate this
atio to the case of ITER, with 100 MJ plasma energy, deposited
nergy by ELM on outer midplane wall can be about 2.5 MW/m
2
assuming outer midplane wall area is ∼200 m
2 , ELM duration is
ms), which does not exceed thermal limitation of Be, which indi-
ectly validates the results of Kocan’s: neither melting nor signifi-
ant evaporation of Be [17] .
. Conclusion
To investigate the ELM filamentary phenomena in the far SOL
f KSTAR during type-I ELMs, poloidal electric probes (PEPs) were
sed and measured the far-SOL plasma parameters such as elec-
ron temperature, plasma density, heat and particle flux, and radial
1264 M.-K. Bae et al. / Nuclear Materials and Energy 12 (2017) 1259–1264
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velocity of ELM filaments to the wall by the configuration of two
triple probes and one Mach probe. During the slow change of sepa-
ratrix position by 7 cm, the radial decay length of heat and particle
fluxes are deduced as λq, ELM
= 25 ± 4 mm and λ�, ELM
= 31 ± 1 mm,
respectively. The mean radial velocity of ELM filaments for eight
discharges are measured as 80–100 ms −1 by the time of flight
method with two radially separated electric probes, PEPs and FRP,
which a fixed in far-SOL region. Radial velocity of the filaments
have been also calculated as ∼200 ms −1 by TOF method, i.e., using
the time delay between start of magnetic fluctuation signal and
PEP signal, which seems to indicate the faster formation of fila-
ments inside the pedestal far from the separatrix than the depar-
ture of the filaments from the separatrix.
These radial velocities of ELM in KSTAR are smaller than the
other tokamaks, e.g. MAST [1] , AUG [5] and JET [18] , because this
result is based on the measurements in the far-SOL region. There-
fore, energy loss of the ELM filament will be larger and it can have
a relatively smaller value than the other tokamaks. Moreover, typ-
ical ELM and plasma energy in KSTAR is smaller than the other
tokamaks.
Radial velocity is strongly dependent upon the particle flux
of filament and separatrix position, while it is weakly dependent
upon the heat flux. Besides, v r _ ELM
seems to be independent of the
ratio of ELM energy loss to the total stored energy ( �W ELM
/ W tot ).
From the heat flux measurement, energy deposition to the wall
due to convective transport of ELM filament is estimated as 0.5%
of the total stored energy, which could be utilized to energy loss
transport model for the application to ITER [17,19] .
cknowledgement
This research was supported by National R&D Program
hrough the National Research Foundation of Korea (NRF)
unded by the Ministry of Science, ICT & Future Planning
2015M1A7A1A01002784 ), and was partly supported by KSTAR
roject and National Research Council of Science and Technology
NST) under the international collaboration & research in Asian
ountries ( PG-1314 ).
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