J. Plasma Fusion Res. SERIES, Vol.2 (1999) 393-397

Plasma Density Control by Molecular Beam Iniectionin HT-7 Tokamak
WU Yingxiangl, LI Jiangang2, wAN Baonian2, GONC Xianzu2, GU Xuemao2,
LUO Jiarong2, WANG Xiaoming2 and MORITA Shigeru3
tlnstitute of Mechanics, Chinese Academy of Science, Beijing, China
2lnstitute of Plasma Physics, Chinese Academy of Science, Hefei, China
3National Institutefor Fusion Sciences, Toki, Japan(Received: 9 December 1998 / Accepted: 3 September 1999)

Abstract
Laval nozzle, which produces supersonic molecular beam, is proven to be an effective fuelling tool

for magnetic confinement devices. Details of its design are given in the first part of this paper. Resultswith supersonic beam injection in HT-7 tokamak show higher efficiency and deeper penetrationcompared to normal gas puffing. The peaking factor is almost the same with the off-axis pellet injection.

Experiments demonstrate that it is a suitable fuelling method of steady-state operation with high density
for super-conducting tokamak.

Keywords:
HT-7, Laval nozzle, supersonic molecular beam, gas puffing

1. lntroduction

the experiments is to optimize its parameter for theTo fuel tokamak plasma in more efficient way ishigh-density steady-state fueling, deeper penetration andvery important issue for fusion research. Pellet injection

for reactor relevant performance due to its fuelling

reduce the high edge recycling made by strong gas
puffing. The detail of the Laval nozzle design wasdescribed in the first part of the paper. The experimental
results were given in the following part.

efficiency and recycling problem. Oriented supersonicmolecular beam injection may be a good candidate forfuelling in steady-state operation with high density. The

2. Gas Injector Design
As we know, in order to obtain a supersonic,

is proven to be very effective, but it is difficult forsteady-state operation. Normal gas puffing is not propermolecular beam was successfully injected to the HL-lM
tokamak and proven that it is more effective than thenormal gas puffing [1-2]. According to aerodynamic
principle, Laval nozzle is the key device to produce

parallel, uniform molecular beam at the exit section of
an injector, a Laval-shaped nozzle must be used toachieve this goal. The gas in a Laval nozzle will flowfrom subsonic (convergent section) through transonic(throat section) to supersonic (divergent section) shownin Fig. l. Owing to the different flow properties in eachsupersonic molecular beam [3]. HT-7 is a superconducting tokamak emphasized on high performancesteady-state operation. For getting steady-state highdensity plasma, a Laval nozzle was designed andinstalled in the HT-7 tokamak. Its experiments werecarried out in the HT-7 tokamak. The main purpose offlow section, the design method should be different.

@1999 by The Japan Society of Plasma
C o rre spondin g autho r' s e -mail : yxw u @ ime ch. ac. cn

Science and Nuclear Fusion Research393

WuY. et al., Plasma Density Control by Molecular Beam Injection in HT-7 Tokamakcoth(4b) sinh(26)- , l_^_ cosh(2t? ) + cos(26)H, (6, n)=+M<lMsl

M>lcoth(4b) sin(24)cosh(24)+cos(26)'2coth(q,o)

cosh(2[ ) + cos(25) 'Hr(€,q)=#rn
The constant 46, which is chosen to make the throat radius being l, is computed by./4u = coS ec-r

Fig. 1 Flow property in a Laval Nozzle.

\,/ -\ )2r.

Here r is the throat radius. Substituting the expressionsof y, Hp H2into the equation of Y, and with the maximum discharge condition at the throat section, we canobtain the stream function Y, then we can obtain the velocity, pressure, density and temperature in the flow
field by:

2.1 The design of a Laval nozzle transonicsectionAccording to aerodynamic theory, if the flow at thethroat of a nozzle reaches maximum discharge, the flowin divergent section of the nozzle will be supersonic,
and in supersonic area, the downstream flow will have

Vn v=-- V€
PtHt
PvHz'

u=-.
""

q2 =u2 +v2,

no influence on upstream flow. Therefore, we can deal- ly*t y-t ,\rl '. r=y+t_T-to,
P=l;2s'l
2 2'with the flow in subsonic area and transonic area
separately.

\-The flow parameters in transonic area can besolved by the method created by Lin and Jia [4] who
used a stream function to sovem the flow field:v-t ,\']'
P=lIy*,) - ) s-l/"'' '\r-rwG,n)=21r!'l\21\-tl r- | 8, G) rll'-'c6l lr - y.t ,rJ€nl6lH'(4, n)El{,1)l-I

2.2 The design of a Laval nozzle in supersonicsection
In order to obtain the flow parameters and the wallshape of a Laval nozzle in supersonic area, we hady(€, Ddlt,

better use characteristic method. The basic equations togovern the flow-field are:where F/1({, r) and H2((,4) are the inverse of the Lamedq .^. sin(d;'sin(/)tan(P) dr-!aO.hn(t)+" q-=0,Ycos(O +P;

coefficients for the transformation between theorthogonal curvilinear coordinates (€, n, 0) and the/av \orthogonal curvilinear coordinates (x, y,0); 7 is thespecific heat ratio; g(6) is rather complex, we do notdescribe it here. The reader who is interestins in it canHere q is the velocity in the flow-field, 0 is the flow di-refer to reference [4].

rection, and p is the Mach angle defined byl. | =tan(dtl).
\or/'t,1For simplicity, we choose a circular type wall as

Il=sin-'4.
'M

the throat section of the Laval nozzle, so theemployment of double circular coordinates will be

M is the local Mach number in the flow field.
In order to obtain the parallel gas flow at the exit

convenient. The coordinates and the Lame coefficientfor this conformal curvilinear coordinate transformation

section of the Laval nozzle, there must be no reflection

wave from the wall of the nozzle into the inner flow
field of the nozzle. To realize this, when the expansion

are as follow:

394

WuY. et al., Plasma Density Control by Molecular Beam Injection in HT-7 Tokamak,.-r-*t4"-F18.scm
SS bean'--\ilFig. 2 Design of an ideal gas injector in supersonicregion.

\,

162Oru@m@07@01@mm0time(ms)HT-7 Shot 18230 time (ms)Fig. 3 Typical results in supersonic gas puffing at 200ms

for 1Oms. B, =1.8T.

waves reach the wall, they must be absorbed totally by

the wall. By this condition, we can obtain the shape ofthe nozzle step by step with characteristic equations

until the flow parameters at the exit section of thenozzle are parallel and uniform. In this way, theafter switching on the beam. Chord averaged electron
density at l8.5cm increased too. But its rate was lower

supersonic section of the Laval nozzle can be designed

than that at central chord, implying more peaked(see Fig. 2).

electron density profile. I1o emission increased slightly

with the increasing of electron density, which meansthat supersonic beam puffing does not cause serious
recycling comparing with strong gas puffing case.

2.3 The design of a Laval nozzle in subsonicsectionThe design of a Laval nozzle in subsonic section isvery simple, as we can use one-dimensional flow theory
to treat it. The relation between section areas and Mach
numbers is:

However global particle confinement was improved
considerably. Z"n decreased to 2 after supersonic beam
was puffed. The ratio of CIII emission intensity toelectron density implied that supersonic beam puffing
did not bring any impurity problem. Radiation power
and ratio of radiation power to ohmic power increased
with increasing of electron density. Measurement by SX
PHA showed a slight electron temperature variation

Y'l/'I 1 +'y_1 .\:rr_rr|llili.'-'.t
A,M"l"| _____________Ll*---_A,-M, l T-t_-,III t+_M: "l1\2

Iafter supersonic beam was puffed. The stored energy inplasma was increased after supersonic beam injection.
This might be an effect of improved global confinement

By using the Mach number next to the transonic area
and adjusting the Mach number at each section, we canobtain the shape of the nozzle in subsonic section. In
this way, we designed a helium gas injector with I mmin diameter at throat section and 6mm in diameter at andexit section that can produce parallel and uniform gas
beam at about 220omls at exit section of the injector.
For deuterium working gas, the speed at the exit section

and decreased impurity contents.

As comparison with supersonic gas puffing, Fig. 4
showed a typical result for normal gas puffing at l40ms

lasting for 150ms duration. To keep electron density
increasing, gas puffing must be applied for a much
longer duration compared to supersonic gas puffing.
Similar plasma performance was obtained. However,radiation power increased strongly with increasing ofelectron density due to little variation in ^2"6. Increasingrate of electron density at central chord and chord at
18.5cm are nearly same that implied little change inelectron density profile during gas puffing. To compare
the delayed time of electron density after two different
gas puffing and electron density profile, we show theratio of electron densities from central chord to theis about 3.2km/s.

3. Experimental Results
Supersonic beam has been proven to be a very
effective fuelling method in HT-7 tokamak. Typicalresults were shown in Fig. 3. The valve was switched on

at 200ms and lasted for 10ms. Central chord line
averaged electron density raised quickly and kept
increasing from I x l013cm-3 to 4 x l0r3cm-3 in 250ms395

WuY. et al., Plasma Density Control by Molecular Beam Iniection in HT-7 Tokamak

chord at l8.5cm shown in Fig. 5. Clearly, electron
density started to increase in supersonic gas puffing

more quickly than that in normal gas puffing. Also
electron density profile after supersonic gas puffing
became more peaked.

As comparison with pellet injection, an off-axispellet injection shot was shown as Fig. 6. The pellet wasinjected to the radius r = l lcm, which was nearly thesame penetration depth with supersonic beam. The pellet

Eowas fired at 772ms. The plasma density at chord l8.5cm
raised immediately and followed the increase of density

Ein the center. The ration of n" (0)/n" (18.5) dropped in
the first few milliseconds that implied the density profile

became flatten. The density profile became peaked
within l0ms and reached its highest value in 30ms,
which was close to the particle confinement time. The
central electron temperature dropped from 400eV to200eV after pellet injection and gradually recovered.
The radiation power increased slowly with the time andshowed a different pattern with the density as the caseof gas puffing. Since there was only three channels for
HCN interferometer, it was very difficult to get accurate
peaking factor measurement. Here, the ratio of n, (O)ln"(18.5) was used as quantitative factor to compare thedensity peaking during normal gas puffing, supersonicbeam and pellet injection. The ratio is 1.5 for the gas
puffing, 2.4 for supersonic beam and2.6 for the off-axis
pellet injection. For on-axis pellet injection, this ratio isabout 3.1 that implied more peaked density profile. For
the off-axis fueling, the pellet didn't show it advantage
to supersonic beam since the peaking factor was almostoo
FoItime (ms)Fig. 4 Typical results for normal gas puffing.

l?

'b
20
ao
D2
@
a0g?

01@2@S046
time (ms)1same. But for the particle-fueling rate for pellet
injection, it showed higher than 87Vo. The particle
fueling rates for supersonic injection and normal gas
puffing were 657o and 3OVo respectively. On the otherhand, the multi-pellet injection for the stable density

o1s2m3@@0time (ms)Fig. 5 Comparison of performance in electron densitywith supersonic and normal gas puffing.

fueling under steady state condition is very difficult atmoment.

To monitor penetration of supersonic beam into
plasma, A photo diode array was installed in the bottom

window of the same port of Laval nozzle to observe lloline emission. Results were shown in Fig. 7. The spatial
resolution of the array was l.7cm. Results showed that a2oE

mount of supersonic beam could penetrate nearly up toplasma center for this special case, while the beam only

penetrated to a depth of l2-l5cm inside of plasma fornormal condition. The penetration depth observed wasooz

more than prediction by Monte-Carlo code, which
predicted penetration of l5cm inside of plasma (r =12cm) in present HT-7 target plasma. This discrepancy
might be caused by neglecting the molecular processes
and steady-state assumption in simulation. The beamt (ms)Fig.6 Off-axis pellet injection shot for compare withsupersonic gas injection. Br = 1.8T.
396

Wu Y. et al., Plasma Density Control by Molecular Beam Injection in HT-7 Tokamak

should penetrate more if ionization and dissociation ofmolecules are considered. In principle, assumption ofsteady state is not true because the cold channel formed

4. Conclusion
To get steady-state high-density plasma, the Laval
nozzle has been installed in the HT-7 superconducting
tokamak and experiments have been carried out. Thehigh-density plasma could easily be controlled by pulsed
high-speed molecular beam that comes from the Lavalnozzle. The speed ofthe hydrogen beam is about 1.52.8km/s that mainly depends on the temperature of theinjection gas and the plasma condition. With penetration
depth up to plasma center, the density peaking factor isalmost the same with the one achieved by off-axis pellet

around beam is beneficial for penetration of next
coming neutral particles. More detail work about thiswill be carried out on HT-7 in near future.

E

E:
R:

injection. The improvements for both energy andEsxicvr

particle confinements were achieved by molecular beam
injection. Fueling efficiency of about 657o demonstrates

Bo

that it is a useful tool for the steady-state tokamako-ooperation.

oh;Referencesx?
do200time (ms)300

tll L.H. Yao et al., Nucl. Fusion 38,736 (1998).

400[2) L.H. Yao et al., lTth IAEA Fusion Energy

HT-7 Shot 21449

Conference, Yokahama, Japan, Oct. 1998, IAEACN-60/FTP/25.
t3l G. Scoles, Atomic and Molecular Beam Methods

Fig.7 H" photo diode array monitoring the penetrationof supersonic beam.

Vol. 1, (New York, Oxford, Oxford Press, 1998)
P.15.

t4l T.J. Lin and Z.X. Jia, Proc. Indian Academy of Sci.
Vol. 4, Part 3, 315 (1983).

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