EX/4-4Ra
Investigations of impurity seeding and radiation control for long-pulse and
high-density H-mode plasmas in JT-60U
N. Asakura, T. Nakano, N. Oyama, T. Sakamoto, T. Suzuki, S. Konoshima, H. Kawashima, H.
Takenaga, G. Matsunaga, K. Itami, H. Kubo
Japan Atomic Energy Agency, Naka, Ibaraki-ken 311-0193, Japan
E-mail: [email protected]
ABSTRACT: Reduction of heat loading appropriate for the plasma facing components (PFCs) such as the
divertor is crucial for a fusion reactor. Power handling by large radiation power loss has been studied in the
ELMy H-mode plasmas on JT-60U with Ar (Case-1) and combination of Ar and Ne (Case-2) gas seeding. For
Case-1, high main radiation loss caused change in ELM characteristics to Type-III, and reduction in ELM energy
loss fraction (WELM/Wdia) to 0.15%: acceptable fraction for ITER. Both transient and steady-state heat loadings
were reduced. Relatively good energy confinement (HH98y2 = 0.87-0.75) with the large frad (Prad/Pabs > 0.8) and
divertor plasma detachment was sustained continuously for 13 s. For Case-2, where Ar seeding to the main
plasma of the ELMy H-mode plasma with ITB was decreased and Ne was seeded to the divertor, the radiation
power was increased intensively in the divertor by Ne seeding. Radiation regions (main edge and divertor) can
be controlled by different radiators. Better energy confinement (HH98y2 = 0.95-0.8) with the large frad was
sustained continuously for 12 s. Large outgas flux and particle recycling in the divertor are produced by larger
increase rate of the surface temperature (13 °C/s near the strike-point) and by transient heat loading with high
energy plasma exhausted at Type-I ELM.
1. Introduction
Reduction of heat loading appropriate for the plasma facing component (PFC),
particularly the divertor, is crucial for a fusion reactor. Impurity gas seeding is considered as a
primary technique to decrease the heat loading to the divertor, and large radiation fractions in
the SOL and divertor (Prad/Pout larger than 0.5 for ITER [1] and 0.7 for DEMO [2]) are
required. Experiments of the impurity gas seeding such as nitrogen (N2), neon (Ne) and argon
(Ar) in the improved confinement plasmas have been carried out in many tokamaks [3-8]. In
JT-60U, power handling by large radiation loss has been investigated in the ELMy H-mode
and reversed shear plasmas with neon (Ne) and argon (Ar) seeding [9 -12]. Good confinement
(H98(y,2) ≥ 0.85) was maintained up to high density ( ne /nGW~0.8-0.9, nGW is the Greenwald
density) and large radiation fraction (Prad/Pabs = 0.7-0.9), while control of the large radiation in
the good energy confinement plasma was not established in steady-state operation. At the
same time, shielding of impurity ions in the SOL and divertor becomes efficient generally
with increasing recycling flux. Therefore, €feedback control of the puff rate as well as
determination of the seeding gas and its puff location are crucial for sustaining the large
radiation in the good energy confinement plasma under the wall saturating condition in the
long discharges, i.e. outgas from the first wall was dominated in the global particle balance
when the surface temperature of the PFC was increasing [13].
Long pulse (τd = 30 - 35 s) ELMy H-mode discharges with different impurity seeding (Ar
and Ne) were carried out in order to sustain the large radiation fraction in the good
confinement plasmas. Control of the large radiation loses in the main and divertor plasmas is
shown in Section 2. Characteristics of the ELMy H-mode and the edge plasma are described
in Section 3. Particle balance and outgas under the wall saturated condition are discussed in
Section 4. Summary and conclusion are given in Section 5.
EX/4-4Ra
2. Impurity seeding in long pulse H-mode plasma
Impurity seeding of Ar, Ne and their combination was
investigated in the long pulse H-mode discharges (τd =
30-35s) with relatively high heating power (PNBI =12-17
MW). Radiation rate coefficient of Ar ions is relatively less
sensitive to the electron temperature range between 10 and a
few 100 eV, compared to that of Ne ions, which has a peak in
20 – 50 eV [14]. Thus, Ar seeding would expect to increase
radiation loss both in the main and divertor plasmas and have
an advantage to control the radiation loss under the condition
of variable edge temperature. First, Ar gas was injected into
the main plasma edge of the standard ELMy H-mode
discharges (Case 1) as shown in Figure 1, where Ip = 1.2 MA,
Bt = 2.3 T, PNB = 12 MW, Rp = 3.4 m, amid = 0.92 m, safety
factor at 0.95 of the minor radius (q95), elongation (κ) and
triangularity (δ) are 3.53, 1.40 and 0.31, respectively.
Fig.1 Ar gas injected into
main plasma of the standard
ELMy H-mode. In addition,
Ne gas is injected into the
divertor of the ELMy
H-mode with ITB.
Second, in order to improve the energy confinement better
than the standard H-mode plasma, the impurity seeding into
H-mode plasma discharges with internal transport barrier (ITB) was investigated: Ip = 1.05
MA and Bt= 2.0 T were lower, and PNB = 14 - 16 MW was larger than those for Case 1, and Rp
= 3.4 m, amid= 0.88 m, q95 = 3.53, κ = 1.43, δ= 0.33 were comparable. Here, Ne seeding would
expect to increase the radiation loss in the divertor plasma, and it was injected mostly into the
divertor in addition to the Ar seeding (Case 2).
2.1 Ar seeding in H-mode plasma plasmas
First, Ar gas seeding was investigated in the
long pulse discharges (τd = 30s) as shown in
Figure 2. After a large Ar puff (1.5 Pam3s-1, i.e.
~8.4x1019 Ar s-1) at t = 6 - 6.5s, radiation from
Ar+14 ions in the main plasma starts increasing,
then saturated during a constant Ar puff rate of
0.25 Pam3s-1. Radiation loss, in particular, at the
main plasma (Pradmain) was increased from 1.2 to
3.8-4.4 MW, while the radiation loss in the
divertor (Praddiv) is increased from 3.5 to 5.0-5.7
MW. Recycling particle flux and ne are
gradually increased during the constant Ar puff
until t = 15 s, i.e. ne = 3.5 - 3.8x1019 m-3, where
Greenwald density fraction corresponds to 0.75 –
€
0.84. Active control of Ar gas puff rate is
necessary to sustain the large radiation level and
€
high density.
Feedback of the Ar puff rate is
applied for t = 15 – 25 s, using integrating
bolometer signals viewing the main plasma edge:
a constant level of the radiation loss is maintained
under the condition of increasing the outgas from
the first divertor as shown in Figure 2 (d).
Fig.2 (a) plasma current, NBI power, (b)
electron density, Ar puff rate, where
feedback control is used in 15-25s, and
Ar+14 intensity, (c) radiation power and
(d) deuterium recycling fluxes in main
and divertor, (e) H-factor and total
radiation fraction.
EX/4-4Ra
Large total radiation fraction, fradtot = (Pradmain +
Praddiv)/Pabs = 0.82-0.91, is sustained continuously,
with relatively good energy confinement of H89L =
1.4 - 1.5 (HH98y2 = 0.77-0.84) is maintained, for the
first time, for 13 s (t = 8 - 21 s) until a large
breakdown of NBI (ΔPNBI = -4 MW at t = 21.5 s).
ELM characteristics changed from Type-I to
Type-III with increasing Pradmain, where Pradmain
includes radiation loss in core, edge and SOL
regions. Type-III ELM operation would be
preferable to reduce the transient heat load to the
divertor and the first wall. Figure 3 (a) and (b)
show waveforms of Dα signal viewing the outer
divertor in the transition from Type-I to Type-III,
and sustainment of Type-III ELMs, respectively.
Fig.3 Waveforms of (a) transition
from Type-I to Type-III ELMs, (b)
Type-III ELM.
ELM frequency (fELM), energy loss fraction normalized by the pedestal energy (WELM/Wped),
H-factor (HH98y2) and fradtot are shown in Figure 4 as a function of Pradmain. With increasing
Pradmain, fELM of Type-I ELM is decreased to 25 - 40 Hz at Pradmain = 3 - 3.5 MW, and Type-III
ELM accompanying with fast fELM and small amplitude appears between Type-I ELMs. At the
same time, WELM/Wped is reduced from 3.5-4.5% to below the evaluation level (1%). These
values correspond to the energy loss fraction normalized by the plasma stored energy
(WELM/Wdia) from 0.7-1.1% to 0.15% (2 - 3 times smaller than that acceptable for ITER).
Fig.4 (a) frequency and energy loss
fraction of ELM, (b) electron density,
temperature and pressure at the edge
pedestal, (c) total radiation fraction
and enhancement factor of the energy
confinement, as a function of the
main plasma radiation, Pradmain.
Fig.5 (a) Radiation powers in the inner
and outer divertors, Prad-in and Prad-out,
power to the divertor, Pdiv = Pabs – Pradmain,
and power to the target, Ptarget = Pdiv –
Praddiv, (b) thermocouple signal, (c) ion
saturation current, and (d) electron
temperature near the outer strike-point.
EX/4-4Ra
For the Type-III H-mode, on the other hand, electron pressure at the edge pedestal (peped) is
decreased from 2.5 - 1.8 kPa with increase in Pradmain. It is due to saturation of neped at
1.8-2.0x1019 m-3, while Teped is decreased from 0.8 to 0.6 keV. Thus, the energy confinement of
the core plasma is degraded as shown in Figure 4(c). As a result, it is important to maintain
Pradmain slightly above the threshold (3.4 - 3.5 MW) of the Type-I and Type-III ELM transition
in order to minimize the degradation of the energy confinement with mitigating the transient
ELM peak heat loading.
During the Type-III ELMy H-mode, the steady-state heat fluxes was also largely reduced.
Due to the problem of the infrared TV camera, the target temperature was measured with
thermocouple installed 4 mm below the tile surface. Figure 5 (a) shows the time evolutions of
radiation powers in the inner and outer divertors (Prad-in and Prad-out), total power to the divertor
(Pdiv = Pabs – Pradmain), and calculated total power to the target (Ptarget = Pdiv – Praddiv). Ptarget is
decreased from 5.5 to ~1 MW mostly due to enhancement of both Pradmain and Prad-in. Increase
rate of the target temperature near the outer strike-point (Ttarget-out) becomes small (ΔTtarget-out = 4
°C/s). Ion saturation current (Isdiv) and electron temperature (Tediv) are measured with the time
resolution of 5 ms, where Is corresponds mostly to values between ELMs and Tediv is often
influenced by ELM activity. Since base-level of Tediv at the outer strike-point is decrease to ~5
eV in the Type-III ELMy H-mode, the detachment of the outer divertor plasma is maintained
without outgas from the target plate.
2.2 Ar and Ne gas seeding in H-mode plasma
with an internal transport barrier
Next, the impurity seeding into H-mode
plasma discharges with ITB was investigated in
order to improve the energy confinement better
than the standard H-mode plasma. Figure 6
shows temporal evolutions of the plasma
parameters with combination of Ar and Ne gas
seeding (Case 2). NBI heating power is
controlled using feedback of the NB units to
maintain the plasma stored energy (Wdia) of 2.0
MJ during t = 7 – 30 s. During increase in the Ar
puff into the main plasma (up to 0.85 Pam3s-1, i.e.
4.2x1020 Ar s-1), Ar+14 and Prmain are increased
while they are maintained smaller than those in
Case 1. Thus, Type-I ELM activity was observed
during NBI. From t = 7 s, the small Ne gas puff
is intermittently injected to the divertor (0.05
Pam3s-1, i.e. 2.5x1019 Ne s-1) as shown in Figure
5(c), which causes significant increase in Prdiv.
Good energy confinement plasma (H89P-L
=1.65-1.7) with ITB was sustained at the high
density plasma (line averaged ne = 3.0-3.5x1019
m-3 and central ne(ρ=0.1) = 4.4-5.4x1019 m-3)
until t = 15.5 s. Radiation power is gradually
increased (Pradtot = 8 - 10 MW), and fradtot > 0.7
€ s. On the other hand,
was seen after t = 12.5
H89P-L is decreased gradually from 1.70 (t ~14 s)
Fig.6 (a) plasma current, NBI power, (b)
electron density, Ar puff rate and Ar+14
intensity, (c) Ne puff rate and Ne+9
intensity, (d) radiation power and (e)
deuterium recycling fluxes in main and
divertor, (f) H-factor and total radiation
fraction.
EX/4-4Ra
to 1.53 (t ~25.5 s), in particular, accompanying with
increases in the divertor recycling and Praddiv
although all NBI units of 16 MW are injected. As a
result, using combination of Ar and Ne gas seeding,
good confinement (H89L=1.73-1.5, HH98y2 = 0.95-0.8)
with large radiation fraction (fradtot = 0.7 - 0.9) was
continuously (in quasi steady-state) sustained in
Type-I H-mode during 13 s (t = 12.5 – 25.5 s) until a
large breakdown of NBI.
During the Ar and Ne seeding, Ptarget was
decreased from 5.2 to 2.8 MW mostly due to
enhancement of Praddiv both at the inner and outer
divertors as shown in Figure 7 (a). Ptarget and ΔTtarget-out
of 13 °C/s are by the factor of three larger than those
for Case 1. Figure 7 (c) and (d) show that ion flux
near the outer strike-point is increased larger than
neutral recycling flux, while the local Tediv is
decreased to 5-10 eV. The outer divertor plasma is
attached during Type-I ELMs for Case 2. Active
control of the Ne seeding would be also necessary to
form and maintain divertor detachment.
3 Radiation fraction and confinement with
impurity seeding
Radiation power and location change depending
on the seeding gas specie and electron temperature
distribution. Radiation power fractions at the main
and divertor are compared in Figure 8 for three
cases, i.e. Case 1, Case 2 and Ne gas seeding to
the main plasma (Case 3). For the Ar seeding
(Case 1), radiation fraction from the main plasma
(fradmain = Pradmain/Pabs) becomes comparable to
radiation fraction from the divertor (fraddiv =
Praddiv/Pabs) for the large radiation plasma (fradtot =
0.7 – 1). On the other hand, fraddiv is strongly
enhanced for the Ne seeding cases (Case 2 and
Case 3) independent of the injection location: for
fradtot = 0.7 – 1, fraddiv = 0.46-0.74 while fradmain =
0.15 - 0.28. Note that Praddiv for Case 2 increase
with small Ne gas puff rate (~0.05 Pam3s-1)
compared to Praddiv with 1-2 Pam3s-1 for Case 3.
Praddiv was enhanced more efficiently for the
combined seeding case.
Fig.7 (a) Radiation powers in the
inner and outer divertors, Prad-in and
Prad-out, power to the divertor, Pdiv =
Pabs – Pradmain, and power to the target,
Ptarget = Pdiv – Praddiv, (b) thermocouple
signal, (c) particle recycling flux, (d)
ion saturation current, and (e)
electron temperature near the outer
strike-point.
Fig.8 Radiation power fractions from
the main and divertor plasmas: squires
and triangles represent Ar and Ne
seeding
to
the
main
plasma,
respectively. Cross-squares correspond
to Type-III H-mode. Circles show
combination seeding of Ar to the main
plasma and Ne to the divertor.
Comparison of the thermal energy confinement
for the three cases is shown in Figure 9.
Enhancement factor for the Ar and Ne seeding
(Case 2) is high (HH98y2 ≥ 0.95) due to the ITB formation up to fradtot ~0.86, compared to HH98y2
= 0.88-0.86 for Case 1 (the standard ELMy H-mode). HH98y2 for Case 2 is also higher than that
EX/4-4Ra
HH98y2 = 0.91 for Case 3 (Ne injection into the
same target plasma). With increasing fradtot, such
high HH98y2 is decreasing to be comparable to those
for Case 1 (HH98y2 = 0.88-0.84) and Case 3 (HH98y2
= 0.85). Here, intense Ne gas puff for the
combination seeding cases, HH98y2 is reduced
transiently and it is recovered with pump-out of
the Ne ions. Greenwald density fraction reaches
up to 0.84, 0.80 and 0.78 for Case 1, 2 and 3 as
shown in Fig. 9 (b).
4. Wall saturation in long pulse discharges
Characteristics of the wall pumping and
outgassing in the long H-mode discharges with
the impurity seeding are investigated. Global
deuterium pumping flux of the PFCs was
evaluated from the particle balance: Γwall = Γgas +
ΓNB – Γdiv, where Γgas and ΓNB are deuterium gas
puff and NB fuelling fluxes, and Γdiv is the
divertor pumping flux. In the long pulse discharge
of JT-60U, Γwall becomes negative mostly due to
an increment of the surface temperature of the
divertor and the first wall [13].
Fig.9 Enhance factor of thermal energy
confinement as a function of (a) total
radiation power fraction, (b) density
fraction normalize by Green world
density: squires, triangles and circles
correspond to those in Fig. 8.
Figure 10 shows time evolution of the particle recycling in the inner and outer divertors
(ΓDα-in and ΓDα-out), neutral pressure below the outer baffle (PD2), particle flux balance and the
integration for Case 1 and Case 2. For these series of impurity seeding experiments,
deuterium gas puff was not injected (Γgas = 0) and external fuel source was NBI: ΓNB =
0.9x1021 Ds-1 (Case 1) and 1-1.2x1021 Ds-1 (Case 2). On the other hand, particle recycling and
PD2 for Case 2 are largely increased by the factors of 5 and 3, respectively, compared to those
Fig.10 (a) (d) recycling particle flux and neutral pressure under the outer baffle, (b) (e)
particle flux balance, (c) (f) integrated flux balance. Left and right show Ar puff into the
standard ELMy H-mode and combination seeding into ELMy H-mode with ITB.
EX/4-4Ra
for Case 1. Thus, Γwall for Case 2 changes from wall pumping to outgas from the plasma
facing components after t = 8.5 s, and the outgas flux is increasing to -3.2x1021 Ds-1 at t = 32 s.
The change in Γwall is observed for Case 1, but Γwall is small by an order of magnitude (<
4x1020 Ds-1). The large outgas flux for Case 2 is caused by large ΔTtarget-out and by transient heat
loading with high energy plasma exhausted at ELM.
5. Summary
Long sustainment of the ELMy H-mode plasmas with large radiation fraction (frad > 0.7)
has been progressed in JT-60U with Ar seeding (Case 1) and combination of Ar and Ne
seeding (Case 2).
For Case 1, the radiation power was increased particularly in the main plasma, which
became comparable to that at the divertor. The high radiation loss in the main plasma caused
change in ELM characteristics from Type-I to Type-III, and large reduction in ELM energy
loss fraction (WELM/Wdia) to 0.15%, which is acceptable for ITER. Both transient and
steady-state heat loadings were reduced. Relatively good energy confinement (HH98y2 =
0.87-0.75) with the large frad (>0.8) and divertor plasma detachment was sustained
continuously for 13 s until a large breakdown of NBI. On the other hand, H-factor (HH98y2 <
0.87) for Type-III ELM scenario was lower than the that for Type-I ELM plasmas, and higher
density (0.9-1xnGW) operation would be required for Type-III ELM operation in ITER.
For Case 2, Ar seeding to the main plasma was reduced and Type-I ELMy H-mode plasma
with ITB was used in order to sustain better energy confinement. The radiation power was
increased intensively in the divertor by Ne seeding. Radiation regions (main edge and
divertor) can be controlled by different radiators. Better energy confinement (HH98y2 =
0.95-0.8) with the large frad (> 0.7) was sustained continuously for 13 s until a large
breakdown of NBI. Large outgas flux and particle recycling in the divertor were produced by
larger ΔTtarget-out = 13 °C/s and by transient heat loading with high energy plasma exhausted at
Type-I ELM. Active control of Ne seeding in the divertor will be necessary for formation and
sustainment of the divertor detachment.
References
[1] M. Shimada, et al., Nucl. Fusion, 47 (2007) S1.
[2] K. Tobita, S. Nishio, M. Sato, et al., Nucl. Fusion 47 (2007) 892.
[3] A.M. Messiaen, J. Ongena, U. Samm, et al., Nucl. Fusion, 34 (1994) 825.
[4] A. Kallenbach, R. Dux, H-S. Bosch, et al., Plasma Phys. Control. Fusion, 38 (1996) 2097.
[5] P. Dumortier, P. Andrew, G. Bounheure, et al. Plasma Phys. Control. Fusion, 44 (2002)
1845.
[6] G. P. Maddison, R.V. Budny, P. Dumortier, et al., Plasma Phys. Control. Fusion, 45 (2003)
1657.
[7] J. Ongena, P. Monier-Garbet, W. Suttrop, et al., Nucl. Fusion, 44 (2004) 124.
[8] J. Rapp, P. Monier-Garbet, G. Matthews, et al., Nucl. Fusion, 44 (2004) 312.
[9] S. Sakurai, H. Kubo, H. Takenaga, et al., J. Nucl. Matters, 290-293 (2001) 1102
[10] Hill K., et al., FEC 2002, Lyon.
[11] H. Kubo, et al., Phys. Plasmas 9 (2002) 2127.
[12] S. Higashijima, N. Asakura, H. Kubo, et al. J. Nucl. Mater. 313-316 (2003) 1123
[13] T. Nakano et al., J. Nucl. Matters, 363-365 (2007) 1315.
[14] D. Post, J. Abdallah, R. E. H. Clark, N. Putvinskaya, Phys. Plasmas 2 (1995) 2328.