J. Plasma Fusion Res., SERIES, Vol. 3 (2000) 477480

Fueling Pellet Injection in LHD and lts Prospect
to Steady State Operation
SAKAMOTO Ryuichi*, YAMADA Hiroshi, KATO Shinji, TANAKA Kenji, SAKAKIBARA Satoru,
NARIHARA Kazumichi, MORITA Shigeru, SUZUKI Hajime, INAGAKI Shigeru,
OSAKABE Masaki, KANEKO Osamu, KOMORI Akio, SUDO Shigeru, MOTOJIMA Osamu
and LHD Experimental Group

National Institute for Fusion Science, Toki, Gifu 509-5292, Japan(Received: l8 January 2000 / Accepted: 19 May 2000)

Abstract
In the LHD experiment, pellet injection is placed as a fundamental tool since deterioration of fuelingefficiency by gas puffing is concerned. The fueling pellet injector, which is developed for LHD with fiveindependent barrels, has demonstrated high reliability and reproducibility.
Pellet injection has extended an operational region of NBI plasmas in LHD to higher densities
without confinement deterioration. Several important data, such as plasma stored energy (0.88 MJ),
energy confinement time (0.3 s), B Q.4Vo at 1.3 T) and density (l.l x 1020 m-3) were attained by pellet
injection.

Particle transport has been analyzed for the near-term LHD experiments using repetitive pellet
injection. The analysis suggests that there is prospect ofcore fueling by repeating pellet injection without
perturbation at center.

Keywords:
fueling, particle control, hydrogen pellet, ablation

1. lntroduction

efficiency of gas puffing is not good in large-scale,Through the study of inter-machine scalings ofenergy confinement time [,2], confinement incurrentless helical plasma has been proved to becomparable to L-mode in tokamaks while preferable

high-temperature and steady state plasmas since theneutral gas from gas puffing is ionized at the plasma
surface. Especially gas puffing is not efficient in LHD,
because core plasma region is surrounded with thickdependence on electron density is more pronounced thanergodic layer that connects to the divertor.

in tokamaks. The international stellarator scaline 95

the important issue in research of magnetic confinement

Hydrogen ice pellet injection is one of theimportant techniques used for core plasma fueling.'A
fueling pellet injector, which equips independent 5barrels, has been designed and it has been installed onLHD t3l. The main aim of the present study is toinvestigate the validity of the pellet fueling and thefusion. Gas puff fueling has been using widely to build

pellet ablation behavior in LHD.

(ISS95) is described as follows.
?fses = 0.08 az;zr *o'ut p;tin a," B|'t' r}:ff .
Particle control, or fueling and pumping are one ofup and sustain plasma density, however fueling*C o rr e sp ondin g autho r' s e - mail : s akamot o @ LH D. nifs. acjp477@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchSakamoto R. et al., Fueling Pellet Injection in LHD and Its Prospect to Steady State Operation

Fueling Pellet Injector

1"HD2nd Expambn &Measur€menl lstExpansion

Chambor Chamb€r

Cryog€nbChamber

L-1-L
Fig. 1 Schematic drawing of the fueling pellet injector and plasma vacuum vessel of LHD.

2. Experimental Set-up
Figure I shows the schematic drawing of the pellet
injector and plasma vacuum vessel of LHD. The pellet
injector has been connected to the outer port (3-O). Thepellet injector uses a pneumatic pipe-gun type barrel,
therefore high-pressure helium gas is employed forpellet acceleration. In order to prevent helium gas flow
into LHD vacuum vessel, double expansion chamberswith large capacity vacuum pumps and fast shutter
valves were installed. The size and velocity of the pellet
are 3 mmp x 3 mml and I km/s, respectively. The
pellet is injected from outer side equator of LHD. Pellet
mass was measured by microwave cavity mass detector,
and pellet velocity was measured by time of flightmethod. The injected pellet was checked by shadow
graph, which consists of first flash lamp (70 ns) andCCD camera, at an exit of the injector.
Pellet ablation and penetration are measured byfirst photo diode (500 kHz sampling) and CCD camera(each 33 ms exposure) with Hc filter.

1.0
Peltet lniectionl I

5=-o0.8

$y

0.6
0.4
o.2

00.8

oC

0.6

oo.4

lco.2

AI

00.5 1.0 1.5 2.0 2.5
Time [s]Fig. 2 Typical discharge of pellet injected plasma. Pellet
injection is carried out at 0.8, 0.88, 0.96, 1.04 and1.12 s.

3. Pellet Injection into NBI Heated Plasma(#15440, Bt = 2'75 T, R', = 3'6 m, P"", = 4 MW)

Waveform of a typical discharge of pellet injected
plasma (#154y'.0, Bt = 2.'l5 T, R- = 3.6 m, P *", = 4 MW)
are shown in Fig. 2. Pellet injection is carried out at 0.8,

temperature.

0.88, 0.96, 1.04 and l.l2 s. Line averaged electron
density n, increases sharply at the timing of injection
and then increase rate of the plasma stored energy WpFigure 4 shows plot of the maximum stored energy
versus the density when the stored energy achieved tomaximum. Open circles and close circles are data of gas
puffing and pellet injection, respectively. The dashedline indicates the stored energy predicted by 1.5 times

becomes much steeper. Figure 3 shows the dependenceof the plasma stored energy on the line averaged.

the international stellarator scaling 95 (ISS95) for aheating power of 4 MW. In the case of gas puffing, theelectron density. At the timing of injection, the pellet

ablates adiabatically, thereby the density increase
without change of plasma stored energy, and then themaximum density is 0.5 x 1020 m-3 and the confinement
deterioration is observed at densitv above 0.35 x 1020

stored energy increases along with recovery of electron
478

Sakamoto R. et aI., Fueling Pellet Injection in LHD and Its Prospect to Steady State Operation
2.0#15440
5th pellet

800(#15440)

ps n.,i',JiII,
I

1.50.6.l),

ir .lf,'f 'S.

Gas puff (#1 1606)
q-6

0.4

fIt

1.0.lin@3c;
Io.2

o0.9
0.5

0.4

0.6

0.8
0.0
1 .130I tro4m-t]

1.131

s 0.71.132

Time [s]Fig. 3 Stored energy Wo evolution as a function of line
averaged electron density r4-. Dashed line andFig.5 Temporal evolution of Ha light emission fromsolid line shows data of gas puffing and pellet

ablating pellet and line density nl.

injection, respectively. The dashed line indicatesthe stored energy predicted by 1.5 times theinternational stellarator scaling 95 (lSS95) for a

4. Ablation

heating power of 4 MW.

Figure 5 shows the temporal evolution of Ha lightemission from ablating pellet and line density. Ha lightemission is consistent with density increase in detailstructure. CCD image shows that the trajectory of theHa light emission is straight, therefore time trace of theHo light emission shows the spatial penetration depthon the assumption that velocity of the pellet is constantduring the ablation. In this case, the penetration depth is

1.0

3
S0.6(Dabout 0.8 m.

culEI

Neutral gas shielding (NGS) model [4], which issimple and the most widely adopted ablation model, cano.4

U'

be expressed as follows [5],d o.z)"la = C T-"to ,;t" *]r, ,rf; ,00.2

0.4

0.60.8where 2,, n", m, V, is the central electron temperature,
the central electron density, the pellet mass and the pellet velocity, respectively. The scaling suggests that thepenetration depth depends mainly on the electron temperature. Triangle symbols in Figure 6 show the comparison of measured penetration depth with prediction
from the NGS expression. The measured penetration
depth is systematically smaller than NGS model. Regression analysis for ablation database of LHD has been
carried out (Open circles in Fig. 6). The result is as follows,1.0' nax. W ) -I1O2o m-31
ne(aIrp.
Fig. 4 Maximum plasma stored energy as a function ofline averaged electron density. Open circles andclose circles show data of gas puffing and pellet
injection, respectively.

m-3. In the case of pellet injection, there is noconfinement deterioration up to 0.8 x 1020 m-3, and thestored energy attains to 0.875 MJ. The maximum)" / a = O.l23T;o'8s n-0.o7 *oo.ro ,2ro .

density achieved is l.l x l02o m-3. However, the storedenergy is saturated at density above 0.8 x 1020 m-3 forlack of heating power.

The penetration depth has pronounced undesirable
dependence on the temperature in comparison with NGS.
479

Sakamoto R. et al., FrclingPellet Injection in LHD and lts Prospect to Steady State Operation't0
LHD pellet Scaling
NGS Scalingcf)

i'It(',Er(UOro,.sil |lrlrll | trrlrrrrlrl | |rlrltl[l'I Itr lraIt rrr,lhrrtxt{1ltrrrrXililrlrr tt tr tl tl rl:lIIIIItrlrl!trrrlr

rrr
rr lllI ll
rrrrrlilrrrr,lffrrraall,
rtrrtlil

iiarrlilrrf
lt rt tl tl lr tl 13 ll
ll ll ll ll l' lr lr lrc(uaaob.9U'v,o)23

ITime (s)o)

o

t

|

0.5

Fig. 7 Temporal behavior of densities at center p = 0 in asolid line and p = 0.7 in a dashed line.
0.5

1.5

stored energy and energy confinement time. Several
important data, such as plasma stored energy, energy
confinement time, B and density were attained by pelletMeasured Pellet Penetration VaFig. 6 Comparison of measured penetration depth withinjection. The maximum electron density attain to 1.1 x1020 m-3 transiently, the value is twice the maximum
density of gas puff fueling plasma. The maximum
plasma stored energy attained to 0.875 MJ with 3.6 MW

predicted penetration depth.

5. Density Sustainment Simulation byRepetitive Pellet Injection

NBI, 0.8 MW ICRF and pellet injection.
In LHD, pellet ablation process and density
increase is consistent with each other, but measured

Particle transport has been analyzed for the nearterm LHD experiments using repetitive pellet injection.
l-D particle balance is modeled in the three regions,which are plasma, scrape-off layer and vacuum. Figure
7 shows the temporal behavior of densities at the centerand in the intermediate radius (p = 0.7). Temperature
profiles are fixed and assumed to be constant in time.
When pellets with the diameter of 2.6 mm are injected
every 0.2 s with the velocity of I km, the central density
is kept constant at 5 x l0re m-3 without any gas puffing
and is not perturbed by pellet injection. The pellet isablated completely outside of p = 0.6. Although thepenetration depth is smaller than the theory (NGS). The
regression analysis shows that the dependbnce of thepenetration depth on the electron temperature is the 0.85th power. This exponent is 1.5 times larger thanNGS prediction (-5l9).
The particle transport analysis, which is combinedwith pellet ablation code, suggests that there is prospectof core fueling by repeating pellet injection without
perturbation at center.

largest perturbation seen at p = 0.7 teaches 30Vo,

Referencetll U. Stroth e/ a/., Nucl. Fusion, 36, 1063 (1996).

perturbation in the core region inside p = 0.5 can besuppressed to less than 107o.

[2] S. Sudo et a/., Nucl. Fusion, 30, l1 (1990).
t3l H. Yamada et a/., (ISFNT-S).
t4l P.B. Parks e/ al., Nucl. Fusion, 17, 539 (1917).
t5l L.R. Baylor et a/., Nucl. Fusion, 37, 445 (1997).

6. Conclusion
Pellet injection has extended operational region ofLHD to high density plasma with increased plasma

480