1
L-I-H Transitions Facilitated by Supersonic Molecular Beam Injection
and Pellet Fuelling and Sawtooth Crashes on the HL-2A Tokamak
C.H. Liu, Y. Huang, Y. Liu, L.H. Yao, G.L. Zhu, Z.B. Shi, W. Chen, D.L. Yu, X.Y. Han, X.Q.
Ji, Y. Zhou, C.Y. Cheng, B.B. Feng, W.L. Zhong, H.B. Xu, J.Y. Cao, L. Nie, K. Yao, Z. Feng,
L.W. Yan, X.T. Ding, J.Q. Dong, X.R. Duan, and HL-2A team
Southwestern Institute of Physics, P O Box 432, Chengdu 610041, China
E-mail contact of the main author: [email protected]
Abstract. Details of transitions among low mode (L-mode), intermediate phase (I-phase) and high mode
(H-mode) facilitated by supersonic molecular beam injection (SMBI) and pellet injection (PI) as well as
sawtooth crashes have been studied on HL-2A tokamak with not more than 1MW of neutral beam injection (NBI)
heating. Clear I-phase oscillation occurrence is induced by PI and SMBI after several milliseconds. They are
helpful for the formation of edge transport barrier (ETB), which is very important for L-H transition with heating
power close to threshold heating power. L-I-H transitions induced by sawtooth crashes have the similar
phenomena with PI and SMBI. A sawtooth crash expels particles and energy from plasma core to edge region,
resulted in edge temperature gradient and pressure gradient increase. The statistic results illustrate that I-phase
oscillation duration time decreases with the net heating power normalized by toroidal field (Ploss/BT), and
oscillation frequency increases with line averaged density (ne) increase. In the diagram of Ploss/BT vs. ne, the
density is about 2.2×1019
m-3
when lowest Ploss/BT is 0.4 MW/T. The power threshold of L-I transition and I-H
transition have no much difference. Plasma density could be close to Greenwald density limit in H-mode
discharge.
1. Introduction
H-mode operation is extremely important and has been chosen as the standard operation
scenario for ITER to meet its objectives, and the study of transition dynamics and power
threshold scaling is very important to assess auxiliary heating requirements for ITER
operation. The intermediate phase (I-phase) is an intermediate confinement regime between
that of the L- and H-modes, and it is characterized by several kHz oscillations observed on all
Dα view chords. The L-H transitions induced by a pellet injection and a sawtooth crash have
been studied on DIII-D [1] and ASDEX [2], respectively, with the heating power less than the
prediction of power threshold scaling law. The edge fuelling, such as a pellet injection and a
sawtooth crash expelling particles to plasma edge, has very important effect on edge transport
barrier (ETB) formation and sustainment. A predator-prey model [3], in which time-dependent
zonal flows (ZF) and equilibrium flow shear are two competing predators while drift wave
turbulence is the prey, has been proposed and supported by the experimental results on DIII-D
[4]. The periodic turbulence suppression is observed in a narrow layer at and just inside the
separatrix. The intermediate phase (I-phase) of the L-H transition is recognized with
quasi-periodic oscillations [5,6] and turbulent instability bursts [7]. The role of Zonal flows to
L-I-H transitions has been studied on EAST [8] and ASDEX-U [9]. A new model, which is
based on the criterion that L-H transition occurs when turbulence and shear Alfvén waves
EX/P7-08
2
compete in the vicinity of the last closed flux surface (LCFS), could be used to predict the
scaling of the L- H transition power (PL-H) with plasma veriables [10].
The first H-mode discharge on the HL-2A tokamak
has been carried out with neutral beam injection
(NBI) and Electron Cyclotron Resonance Heating
(ECRH) methods in 2009 [11]. When NBI heating
power is in the range of 0.75 MW to 1 MW,
H-mode access has also been carried out without
ECRH heating. Besides spontaneous slow L-H
transitions, L-I-H transitions are facilitated by
pulsed intense edge fuelling such as pellet injection
(PI) and supersonic molecular beam injection
(SMBI) and a series of sawtooth crashes with
lower NBI power.
2. Experimental set-up
HL-2A is a divertor tokamak with major radius of 1.65 m and minor radius of 0.40 m [12].
The fuelling systems consist of gas puffing (GP), PI [13] and SMBI [14], and the latter two
share the same port in the low field side (LFS), but they outlet at different positions from the
high field side(HFS). Injection direction of PI and SMBI are both on the middle plane of
vacuum chamber. In the paper, the issues of PI from HFS and SMBI from LFS are studied as
shown in figure 1.
The PI could produce 1 ~ 40 hydrogen/deuterium pellets in one injection cycle at repetition
frequency of 1 ~ 30 Hz, with pellet diameter of 1.3 mm and length of 1.3 ~ 1.7 mm. The
pellet velocity can also be varied from 150 to 1000 m/s. A supersonic molecular beam is
generated by a solenoid-driven pulsed valve with a cylindrical diameter of 0.2 mm, the valve
of the LFS-SMBI with high back pressure (0.2 ~ 8.0 MPa) in the gas tank. The pulse duration
can be changed from 0.3 ~ 50 ms and the pulse number can be varied from 1 to 1000. The
co-NBI system with four ion sources could inject about 1 MW power into the HL-2A plasma.
The injection angle is about 58◦ in tangential injection.
Diagnostic methods used in the reported experiments include three soft x-ray arrays [15],
which have 48 channels totally with a time resolution of 500 µs and spatial resolution of 2.5
cm installed on three positions (up, and middle, and down) in the main chamber; microwave
reflectometry [16], which is used to measure electron density profiles at the pedestal region.
The experiment conditions are plasma current Ip =140 ~ 200 kA, toroidal field Bt = -1.2 ~
-1.4 T, line averaged density ne = 1 ~ 3.5×1019
m-3
.The minimum NBI power for spontaneous
L-H transition is about 0.75 MW, which could be affected by wall conditions, plasma
displacement control, fuelling method and so on.
3. L-I-H transitions induced by supersonic molecular beam injection
Figure 2 illustrates an example of L-I-H transitions induced by SMBI from magnetic LFS in
shot 19302. Figure 3 shows the density profiles change during an L–I–H transition resulting
from SMBI. During I-phase oscillation after the first SMBI, ne and carbon radiation (CII)
SMBI, PI
from LFS PI from
HFS
FIG. 1. Schematic of PI and SMBI from
different directions.
3
decrease, but election temperature and WE increase. During the I-phase, the particle
confinement could not maintain at stable level. The initial response of the edge pressure to the
pellet introduction is adiabatic: the density increases and the temperature drops resulting in no
change in the pressure. Then density decreases slightly while the edge is re-heated. The entry
into I-phase is delayed for ∼ 8 ms until the
edge pressure gradient is sufficient to
achieve an I-phase transition. The plasma
remains in the I- phase during the time ( ∼
14 ms) necessary for re-heating to further
increase in the edge pressure sufficiently to
cause an H-mode transition. Plasma
horizontal shift moves outwards. The
phenomena illustrate that the plasma
energy confinement improves step by step
during I-phase oscillations from 455 ms to
480 ms. Then four large ELMs burst
during H-mode, which induce CII
radiation and D radiation increase sharply.
Plasma ne increases up to 3.3×1019
m-3
,
and plasma stored energy increases up to
37.5 kJ until the following SMB pulse
injection. L-I-H transitions occur again
induced by the following SMBI at 495 ms.
The phenomena could be illustrated by the
scaling diagram of power threshold (Pth)
versus line averaged density in high
density
branch,
where Pth
increases
with
density
increase
[15].
During
the
I-phase induced by the 2nd
SMBI, though
D and CII radiation increases, and
electron temperature decreases, thus
plasma stored energy still maintains in the
level of H-mode plasma.
4. L-I-H transitions induced by pellet injection
FIG. 2. SMBI induced L-I-H transitions from LFS
direction injection. The discharge parameters are
Ip = 200 kA, BT = -1.4 T, PNBI = 1 MW (440 ms –
940 ms), PECRH = 1.4 MW (715ms - 1115ms). The
waveforms from up to down: (a) line averaged
density ne, (b) plasma stored energy WE, (c) CII
radiation intensity, (d) electron temperature
measured by multi-channel ECE, (e) D
radiation in outer divertor, (f) plasma horizontal
shift, (g) SMBI monitor by D array in the main
chamber and (h) Electron density evolution
measured by microwave reflectometry.
450 460 470 480 500 510 520
3
3.5
ne (
m-3
)
450 460 470 480 490 500 510 520
20
30
40
WE (
kJ
)450 460 470 480 490 500 510 520
0.5
1
1.5
2
I-D
a,d
iv (
a.u
.)
450 460 470 480 490 500 510 5200
0.01
0.02
I-C
II (a
.u.)
440 450 460 470 480 490 500 510 520 5300
1
2
Time (ms)
I-D
a (
a.u
.)
0.8
1
ME
CE
(a
.u.)
440 450 460 470 480 490 500 510 520 5300
1
2
Time (ms)
-1
-0.5F
Dh
(a
.u.)
(a)
(b)
(c)
(d)
(e)
(f)
(g)
I-phase H-mode I-phaseL-mode
MECE16 (r/a=0.75)
MECE10 (r/a=0.4)
SMBI monitor
H-mode
# 19302
440 460 480 500 5200
2
4
6
8
10
12x 10
18
# 19302
Time (ms)
n e (m-3
)
(h)
FIG. 3. Evolution of electron
density profiles at different time.
0.34 0.36 0.38 0.4 0.420
0.2
0.4
0.6
0.8
1
r (m)
ne
(1
019 m
-3)
# 19302
t = 445 ms
t = 458 ms
t = 465 ms
t = 480 ms
4
Figure 4 shows the plasma confinement change induced by PI in shot 17934. Plasma
parameters in the shot are: Bt = -1.28 T, PNBI = 0.85 MW (620 ~ 1125 ms). It could be found
that electron density, stored energy and plasma radiation increase obviously after the pellet
injection, and D radiation in divertor increases as well. After about 8 ms delay, limit-cycle
oscillation of 1.5 kHz could be clearly seen on D radiation signal. The oscillations last about
25 ms, followed by a short ELM-free phase. Electron temperature is higher in I-phase than
that in L-mode before pellet injection into r = -33.5 cm region, but it is almost unchanged in r
= -38 cm region. After pellet injection, plasma density
increases, resulting in the power threshold for L-H
transition decrease. The edge transport barrier forms
gradually at the inside region of the LCFS, then
plasma transport decreases and plasma confinement
improves. Plasma operation enters into I-phase. The
phenomena also could be explained by the scaling
diagram of power threshold (Pth) versus line-averaged
density in low
density branch,
where Pth
decreases with
density
increasing [17].
Pellet injection
depth is deeper
than SMBI on
HL-2A, and the
ablation time of a
pellet is longer
than an SMB
with the similar
plasma discharge parameters. Comparing the plasma
density profiles in figure 3 and 5, it is found that a
pellet takes more particles into the plasma than an
SMB, so the pedestal height is higher after a pellet
than that after an SMB.
5. L-I-H transitions induced by sawtooth crashes
The sawtooth oscillations are m/n = 1/1 internal kink mode, such magnetohydrodynamic
(MHD) instability is characterized by quasi-periodic collapses in the temperature and density
in the plasma core region (q = 1 surface). Sawtooth crashes expelling particles and energy
from plasma core to edge region is helpful for the density and pressure pedestal formation
when auxiliary heating power is close to the L-H transition threshold heating power. The
experimental results of multi-time L-I transitions induced by periodic sawtooth crashes on
HL-2A tokamak are shown in figure 6. After sawtooth crashes, L-I transitions occur as
monitored by D radiation in the lower divertor. ‘I’ time zone is I-phase, and ‘II’ time zone is
FIG. 5. Electron density profile
evolution during L-mode, and
I-phase, and H-mode in shot 17934.
0.32 0.34 0.36 0.38 0.4 0.42 0.440
0.2
0.4
0.6
0.8
1
1.2
1.4
r (m)
ne
(1
019 m
-3)
# 17934
t = 790 ms
t = 798 ms
t = 810 ms
t = 822 ms
t = 835 ms
FIG. 4. Time evolution of main
parameters during L-I-H transitions
induced by pellet injection from HFS.
The waveforms from up to down are
plasma current Ip, line averaged
density ne, plasma radiation power
Prad, plasma stored energy WE,
electron temperature measured by
ECE at radius r = -33.5 cm and -38
cm, D radiation in outer divertor
and pellet injection monitor.
162
165
168
Ip (kA) #17934
L-mode I-phase H-mode
1
1.8
ne(1013
cm-3
)
190
215
240
Prad (kW)
15
25
WE (kJ)
0.025
0.035
Mece12 (a.u.)r= - 33.5cm
0.32
0.38
Mece14 (a.u.)
r= - 38cm
0.2
0.7D -div(a.u.)
790 800 810 820 830
0.02
0.06
I-D (a.u.)
Delay time: 8ms
time (ms)
5
L-mode phase. A sawtooth crash causes line averaged density decrease, because it expels
particles from plasma central region to edge region, which increases particle transport. Plasma
stored energy increases slightly during I-phase interval. H98 factor is a little higher in I-phase
than that in L-mode. Maybe the edge pressure gradient is not steep enough to maintain the
I-phase, plasma confinement transits back from I-phase to L-mode.
Figure 7 displays the soft x-ray signal and their spectrums in the plasma central region and
edge region. From soft x-ray mode number
analysis, before the sawtooth crash, strong
MHD perturbations show the long-live modes (LLMs) [19, 20] characteristics. The LLMs last
longer time than normal sawtooth precurors. The fundamental frequency of the LLMs is about
15 kHz, and mode number structure is m/n = 1/1, while 2nd
and 3rd
harmonic wave could also
be detected in plasma central and edge
region. The LLM is terminated by sawtooth
crashes. The relative soft x-ray perturbation
induced by LLM activity is 15% at ρ =
0.06 (inside the q=1 surface) and the
perturbation induced by a sawtooth crash is
about 40%.
Figure 8 shows the density profiles at the
time 730 ms, 735 ms and 750 ms as 0.85 0.9 0.95 1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
r/a
ne
(1e1
9 m
-3)
# 19772
750 ms
735 ms
730 ms
FIG. 8. The density profiles
at three different time as
the red and pink and green
dot lines shown in figure 6.
730ms: before a sawtooth
crash; 735ms: during
I-phase interval; 750ms:
L-mode after I-phase.
# 19772
700 720 740 760 7800
10
20
30
700 720 740 760 780 8000
0.2
0.4
Time (ms)
700 720 740 760 780 8000
0.1
0.2
Time (ms)
sawtooth
crash
Isx
/Isx
, r/a=0.06
Isx
, r/a=0.06
Isx
, r/a=0.8
Isx,r/a=0.06
Isx,r/a=0.8 m/n=1/1
FIG. 7. Time evolution of soft x-ray signal and
its spectrum with the same time interval of
figure 8. The waveforms (a) and (b) are soft
x-ray in central region (r/a = 0.06) and
sawteeth crash induced soft x-ray perturbations,
respectively; (c) is soft x-ray in edge region (r/a
= 0.8); (d) and (e) are spectrums of soft x-ray
signal of waveform (a) and (c) respectively.
(a)
(b)
(c)
(e)
(d)
# 19772
700 720 740 760 780 800
216
218
220
Ip (
kA
)
700 720 740 760 780 8002.4
2.6
2.8
ne
(1
01
9 m
-3
)
700 720 740 760 780 800
26
28
We (
kJ
)
700 720 740 760 780 8000.9
1
1.1
H9
8
Time (ms)
700 720 740 760 780 800
1
2
sx
(a
.u.)
0.2
0.4
0.6
Da
,div
(a
.u.)
FIG. 6. Time evolution of main parameters in
shot 19772 during sawtooth periodic crashes
induced L-I transitions. In the shot, Bt = -1.35
T, PNBI = 0.76 MW (420 ms ~ 920 ms); PECW
= 1.68MW (865ms ~ 1265ms). The waveforms
from up to down are: (a) plasma current Ip;
(b) line averaged density ne; (c) plasma stored
energy WE; (d) soft X-ray radiation intensity
in plasma central region; (e) Da radiation in
lower divertor; (f) H98 factor.
(a) #
19772
(b)
(c)
(d)
(e)
(f)
I
II
6
denoted in figure 6. After a sawtooth crash, density is higher in I-phase than that before the
sawtooth crash; the soft x-ray signal intensity increases in edge region and it decreases in
central region. A sawtooth crash expels particles and energy from plasma central region to
edge region, plasm density and pressure as well as their gradients increase in the edge region.
Another kind of interesting phenomena is the oscillation frequency during I-phase induced by
successive sawtooth crashes, which changes step by step in shot 19720 as shown in figure 9.
The discharge parameters are Ip = 200 kA, BT = -1.39 T, PNBI = 0.9 MW (410 ms ~ 810 ms),
and ECRH heating power PECRH = 1.45 MW (866 ms ~ 1266 ms, not in the I-phase time zone).
The reversal of sawtooth crash occurs at the r/a = 0.5 and r/a = 0.6 region (q = 1 surface
location), which could be seen in figure 9(a). Soft x-ray signal intensity decreases inside the q
= 1 surface. They increases outside the q = 1 surface. Plasma confinement condition inters
into I-phase from L-mode after the 1st sawtooth crash. The 2
nd sawtooth crash decreases
I-phase oscillation frequency from 2.2 kHz to 1.9 kHz, and the 3rd
sawtooth crash decreases
the frequency to 1.4 kHz further. There are two type-III ELM bursts after the 3rd
sawtooth
crash. I-H transition occurs after the 4th
sawtooth crash, and then plasma inters into
type-III ELMy H-mode. Type-III ELM
bursts induce strong perturbation in soft
x-ray intensity signal. In the whole I-phase
interval, plasma density maintains the same
level and plasma stored energy increases
slightly though I-phase oscillation frequency
decreases step by step.
Figure 10 shows the electron pressure and pressure gradient analyzed by multi-channel soft
x-ray inversion method [20] in shot 19720. Edge pressure and its gradient increases step by
step in L-mode and I-phase and H-mode zone. The large pressure and pressure barriers are
formed at r/a = 0.75 inside of the LCFS.
FIG. 9. Time evolution of L-I-H transitions induced
by several sawtooth crashes and I-phase oscillation
frequency in shot 19720. The sawtooth crash time is
marked by four black dotted lines. The waveforms
from up to down are: (a) soft x-ray signal in
different plasma region (The location of the soft
x-ray from up to down: r/a = 0.40, 0.30, 0.18, 0.5,
0.6, 0.67, 0.74, 0.80 and 0.82); (b) plasma
line-averaged density ne (blue color) and plasma
stored energy WE (red color); (c) D radiation in
lower divertor.
600 620 640 660 680 700 720 740 760
0.1
0.2
0.3
0.4
Time (ms)
I-D
,div
(a
.u.)
0
0.2
0.4
0.6
0.8
1
1.2
I-s
x (
a.u
.)
1.5
2
2.5
3
0.82
0.30
0.80
0.67
0.18
0.50
0.60
0.74
L-
mode
(b)
2.2 kHz 1.9 kHz 1.4 kHz
(c)
I-phase
WE (10 kJ)
ne(10
19 m
-3)
type-III ELMs type-III ELMy
H-mode
0.40(a) # 19720
FIG. 10. Electron pressure and pressure
gradient results analyzed by soft x-ray signal
inversion method in shot 19720. (a) electron
pressure; (b) pressure gradient.
(a)
(b)
7
6. I-phase oscillation statistic results
I-phase oscillation is the intermediate phase of L-H transition, but the scaling laws of its
duration, frequency are not clear until now. Figure 11 displays the statistics results of I-phase
duration, and averaged frequency with NBI heating power bellow 1 MW on HL-2A. The
power flow through the separatrix subtracting plasma radiation power Prad is defined as Ploss,
that is Ploss = Paux + Pohm – Prad – dWdia/dt. Where Paux is the auxillary heating power, and Pohm
is the ohmic heating power, and
dWdia/dt is time derivative of the
total plasma stored energy.
Figure 11(a) and 11(b) display
I-phase oscillation duration time
decreasing with Ploss (normalized
by BT) increase, and it has the
same tendency with ne increase.
The results are consistent with
the conclusion in [21] that the
numbers of dithering cycles
occurring at the transition
decrease with the ramp rate γP of
the transition threshold power.
I-phase oscillation could be
detected from low density
branch to high density branch. I-phase
oscillation frequency range is 1 kHz ~ 3 kHz.
And it increases with ne increase, but
decreases with Ploss/Bt increase. The law of
oscillation amplitude is not very clear. If
more data are added future, results could be
revealed. Figure 12 shows the statistics
results of Ploss/BT vs. ne with SMBI and PI
and sawtooth crashes inducing L-I-H
transitions. The density vs. lowest Ploss/BT is
about 2.2×1019
m-3
with lowest Ploss/BT 0.4
MW/T as shown in figure 11(c). The power
threshold of L-I transition and I-H transition
has no clearly different. In H-mode, plasma
density could be close to Greenwald density
limit as shown in figure 12.
7. Summary and discussion
The L-I-H transitions facilitated by supersonic molecular beam injection (from low field
side) , pellet injection (from high field side) and sawtooth crashes have been studied on
HL-2A tokamak with not more than 1 MW neutral beam injection heating power. Though
FIG. 11. The statistics results of I-phase duration, averaged
frequency and oscillation amplitude with NBI heating power
less than 1MW.
0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.750
10
20
30
40
50
60
70
Ploss/Bt (MW/T)
Du
rati
on
(m
s)
1 1.5 2 2.5 3 3.50
20
40
60
ne (1019
m-3
)
Du
rati
on
(m
s)
0.4 0.5 0.6 0.7 0.8 0.9
1
1.5
2
2.5
3
Ploss/Bt (MW/T)F
req
ue
ncy
(k
Hz)
1 1.5 2 2.5 3 3.5
1
1.5
2
2.5
3
ne (1019
m-3
)
Freq
uen
cy (
kH
z)
(a) (b)
(c) (d)
FIG. 12. Statistics results of L-H transition
power threshold. Ploss/Bt vs. ne with Ip = 200 kA.
The dark blue line is the Pth (Pth
=0.042ne200.73
Bt0.74
S0.98
) line; the light blue line is
Greenwald density limit; the plasma is heated by
NBI and ECH in the lowest three points.
8
SMBI penetration depth is shallower and its fuelling efficiency is less than those of PI, SMBI
has the similar function to PI. Because of sawtooth crashes expelling energy and particles to
the edge region, the density gradient and pressure gradient increase at the edge region. Plasma
confinement phase evolution is caused by a series of sawtooth crashes. The statistic results
illustrate that I-phase oscillation duration time decreases with the net heating power
normalized by toroidal field (Ploss/BT) increases, and oscillation frequency increases with line
averaged density (ne) increase. In the diagram of Ploss/BT vs. ne, the density vs. minimum
value of Ploss/BT is about 2.2×1019
m-3
with Ploss/BT ~ 0.4 MW/T. The power threshold of L-I
transition and I-H transition has no clearly different. Plasma density is close to Greenwald
density limit in H-mode. Some shortcomings still exist in the article. For example, (1) the
interplay of turbulence and ZFs is not analyzed; (2) the I-phase oscillations are also found in
the high density branch, which is not consistent with the results on ASDEX-U [7]; (3) though
the diagram of density vs. Ploss/BT is obtained, maybe the error is large because of no enough
reference shots. These all need further research.
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