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: liuchunhua@swip.ac.cn

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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