J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

A low speed single barrel pellet injector and its
application to complementary study on ablatant in LHD
J. S. Mishra1) , R. Sakamoto2) , G. Motojima2) , H. Yamada2)
1)

Graduate University for Advanced Studies, SOKENDAI, Toki 509-5292, Japan
2)
National Institute for Fusion Science, Toki 509-5292, Japan(Received: 20 November 2009 / Accepted: 29 March 2010)

A simpliﬁed calculation considering the three dimensional helical magnetic ﬁeld and B × ∇B drift
was carried out to study the eﬀect of injection location on plasmoid drift. From the calculations diﬀerent
injection possibilities are considered around the torus for the experiment.
A low speed single barrel pellet injector has been designed for alternative injection in LHD. A pellet
is being injected by the combined operation of mechanical punch and pneumatic propellant system. The
shape of the pellet is cylindrical of 3 mmϕ × 3 mmℓ. Pellet injection speed ranges between 100 ∼ 600
m/s but maximum injection speed during operation is constrained to survivability through the curved
guide tube. Diﬀerent curved guide tubes are designed for eﬃcient launching of pellet to particularlocations of LHD.
Keywords: LHD, Pellet Injector, Solenoid Punch, Plasmoid, E × B drift, Guide tube

1. Introduction

Where Te , ne , rp , vp are the plasma temperature anddensity, pellet radius and speed respectively.
LHD has major radius, averaged minor radius,
magnetic ﬁeld and plasma volume 3.9 m, 0.6 m, 2.9 T
and 29 m3 respectively. LHD has a set of l/m=2/10
continuous superconducting helical coils which generates the magnetic conﬁnement ﬁeld. Since the helicalmodulation is superimposed to the toroidicity, additional High Field Side (HFS) appears under the helical coil and varies from one location to other, hence
the HFS side is not always inboard to the vacuum
vessel as in the case of tokamak. Therefore, one cannot simplify the magnetic ﬁeld strength distribution
like B ∝ 1/R as in axis symmetric tokamak ﬁeld. A
complementary study, focusing on diﬀerences of themagnetic ﬁeld conﬁguration between the helical device and tokamak, is one of the approaches to understand the eﬀect of pellet plasmoid drift on the fuelingeﬃciency in LHD. Previous study on LHD considering∇B drift for the coil side injection (CSI) is still premature due to the lack of local density measurements [11].
The objective of present study is to know the eﬀect ofmagnetic conﬁguration on pellet injection locations byconsidering the ∇B induced drift and subsequent density redistribution process for diﬀerent pellet injection
locations in LHD.
LHD is equipped with a multi barrel high speed(≈1200 m/s) pneumatic pipe gun type pellet injector[12]. Bended guide tubes are needed for injection locations other than outboard side of the torus. Deterioration of pellet mass inside it limits the applicationof high speed pellet injector [13], hence a low speed
pellet injector has been designed. In following sections ﬁnding of a suitable location for pellet injection
considering the ∇B eﬀect and development of a low

Pellet injection using isotopes of Hydrogen has
been proved to be a promising method for magnetically conﬁned plasma devices such as tokamak andstellarators/helical devices [1–3]. Improved plasma
properties obtained for a pellet injected NBI plasma
in Heliotron E, however for a highly peaking density
proﬁle plasma becomes unstable [4]. But in Large Helical Device (LHD) a high density (up-to 1021 ), stablewith low diﬀusion operational regime, so called Internal Diﬀusion Barrier (IDB) has been discovered usingpellet injection [5]. It has been experimentally observe that fueling eﬃciency is more for high ﬁeld side
injection compare to the low ﬁeld side injection of thetorus [6]. This is due to the drift of the ablatant towards the Low ﬁeld side(LFS) of the torus [7, 8]. Thisdrift reported is supposed to be due to the E ×B force
on the ablating ionized cloud by its self-consistent electric ﬁeld, produced by the B × ∇B drift of ions andelectrons inside it [9]. Technological diﬃculty restricts
higher pellet injection speed for core fueling. It canbe seen from equation (1) that, there is a poor dependence of normalized penetration depth λ/a [10] on
injection speed in comparison to plasma temperature.
Therefore it will be a major issue to inject a pellet
into plasma core from LFS for the next generationhigh temperature and large volume plasma devices
like ITER. Therefore considering the above discussed
kind of drift, it is viable to inject a pellet from HFSof the device for better fueling eﬃciency and it is alsoplanned for ITER.
λ= CTe−5/9 n−1/9
rp5/9 vp1/3
e
a

(1)

author’s e-mail: mishra@lhd.nifs.ac.jp106©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

J.S. Mishra et al., A Low Speed Single Barrel Pellet Injector and Its Application to Complementary Study on Ablatant in LHD(a)grad B and PoloidalAngle [ ]

360

˚ 300

Fig. 1 Rotational transform along the major radius direction for Rax = 3.6 m.

240
180
120
60
0

60

120

180

240

300

Toroidal Angle [ ˚ ]

(b)

360

Poloidal position [ ]speed injector is being presented.

˚

2. ∇B and plasmoid drift calculation

A pellet injected into the plasma heated mainly
by electron heat ﬂux from the plasma. After the pellet ablation cloud is established, ablated material isionized and forms a high-density and low temperatureplasmoid. A polarization ﬁeld arises due to the driftof ions and electrons in inhomogeneous magnetic ﬁeld.
This electric ﬁeld interacting with the magnetic ﬁeld
pushes the ablating materials down the magnetic ﬁeld
gradient and hence aﬀects the fueling eﬃciency. Many
processes like Alfven wave generation and overlappingof ﬂux tubes after the plasmoid expands half a toroidal
turn along the ﬁeld lines tend to compensate the polarization ﬁeld and hence the drift of the plasmoid [9].
Thus it is important to study the ∇B drift on plasmoid for a non-axis symmetric magnetic ﬁeld device
like LHD.
LHD has helical ripples and rotational transformdue to external helical coils. Fig.1 indicates the variation of rotational transform along the major radius in
the case with magnetic axis position Rax = 3.6 m. The
rate of change of magnetic ﬁeld on edge region along
the toroidal direction is very high and this is less forinner region of plasma. From the ﬁeld line tracing
along the toroid, direction of ∇B and poloidal position for a point on the ﬁeld line that begins from thepellet ablation location was identiﬁed. Fig.2(a) indicates the ﬁeld line calculation at ρ=0.9 for a plasmoid
starting from the ablation point along the directionof pellet injection from the LFS outboard (On midplane) of the Torus. For ρ=0.9 ∇B makes 9 revolutions poloidally for one toroidal turn and also thepoloidal position variation rate is high. Therefore anelectric ﬁeld which drives the density redistribution
may not be formed suﬃciently inside the plasmoid.
Vector plot shown in Fig.2(b) also indicates that themagnitude and direction of ∇B is not constant along
the magnetic ﬁeld line (Blue arrows for the case of360

grad B

300
Poloidalposition240

0˚Outboard

180
120
60
0

60

120

180

240

300

360

Toroidal angle [ ˚ ]Fig. 2 (a) Variation of ∇B direction (red star) andpoloidal position(Angle) of ﬁeld line (continuosline) (b) Vector plot of ∇B (Blue arrows) along
toroidal line projected on to poloidal plane (for
LHD). Green arrow shows the ∇B direction in thecase of tokamak

LHD). A comparison for the case of a same aspect ratio tokamak for the same ρ value represented by thegreen arrows indicates that ∇B is always towards inboard side of torus and there is a little variation of
this.
To know the eﬀect of E × B drift on pellet injection location a simple calculation based on the eﬀectof B × ∇B drift on plasmoid was performed for anumber of pellet injection locations of the torus. A
discrete set of points on the direction of pellet injection is presumed. A plasmoid is assumed to be at thesesets of points as soon as the pellet ablation completes.
Expansion of the plasmoid from that point along thetoroidal magnetic ﬁeld direction was taken into consideration. The geometry of LHD indicating HFS,
LFS, horizontally elongated, vertically elongated section and ∇B direction is shown in Fig.3. A comparison between the case for a HFS and a LFS injection
is shown in Fig.4. The LFS location is horizontallyelongated outboard side and the HFS side (Coil side)107

J.S. Mishra et al., A Low Speed Single Barrel Pellet Injector and Its Application to Complementary Study on Ablatant in LHD

(a)

(a)
0.50

1.5

1.00
15.0

1.0

Helical coil

30.0

0.25

60.0

Z(m)

0.5

Z (m)

45.0

High Field

Low Field

Low Field

0.0

0.00
r

−0.5

High Field-0.25

Helical coil

−1.0

-0.50

−1.5
2.5

3.0

3.5

4.0

4.5

5.0

R (m)
1.5

0.0

0.2

0.4

5.5

(b)

0.50

ρ 0.6

0.8

0.6

0.8

1.0

(b)

1.00
15.0

Low Field

30.0

1.0

0.25

45.0

High Field

Helical coil

Z (m)Helical coilHigh Field

0.0

Z(m)

60.0

0.5

0.00

-0.25

−0.5
−1.0

-0.50
0.0

Low Field

−1.5
2.5

3.0

3.5

4.0

4.5

5.0

0.2

5.5

0.4

ρ

1.0

R (m)

Fig. 4 E × B drift direction along the ﬁeld line projected
on to poloidal plane for (a) LFS outboard consideration and (b) HFS inboard consideration. Coloredlines indicates the plasmoid expansion length along
the ﬁeld line direction.

Fig. 3 Contour plot (Black line) of magnetic ﬁeld strength
at a (a) Horizontally elongated cross section and(b) vertically elongated cross section of LHD. Bluearrows indicates the direction of ∇B and coloredellipses are the magnetic ﬂux surfaces. Location of
HFS positions are near the helical coils.

considered(Fig.6(b)) for upper port injection from a
vertically elongated section. For both the cases calculation suggests a signiﬁcant amount of drift eﬀectalong the ﬁeld line towards the center of the torus.
Based on the calculations following injection possibilities are considered for the experiments, (1) Horizontally elongated outer port injection (2) shallow angle
inner port injection and (3) Shallow angle upper port
injection. First one is the LFS while other two possibilities considered as the HFS injection.

is taken at the vertically elongated inboard side of thetorus. The vector plots shown are the projection ofdrift direction along the ﬁeld line on the poloidal planeof the LHD. The black arrow indicates the pellet ablation and plasmoid formation region along the pellet
trajectory. Colored lines indicate the 60 cm expansionof plasmoid along the ﬁeld line up to which plasmoid
expansion can be observed. For the LFS, drift is lessand nearly constant along the ﬁeld lines compare to
HFS and the drift direction is always out of the torus.
For the HFS drift is signiﬁcantly high and in-fact forthis position drift is maximum compare to all otherlocations. Due to space constraint injection of a pellet from the inboard coil side is not possible, hence
a diﬀerent approach by injecting a pellet from a low
ﬁeld side pointing to a location close to helical coil isbeing considered(Fig.5). Fig.6(a) shows the drift direction for an inner port injection pointing to a region
close to helical coil above the mid-plane at z=0.13m(51◦ I port cross section). A similar approach also

3. Pellet Injector
A single barrel in-situ pipe gun type pellet injector
has been developed for alternative injection in LHD.
Due to inadequate ﬂoor space the overall dimension of
the pellet injector is kept minimized. A solenoid operated mechanical punch is used to initialize the pellet
motion. It also helps to minimize the total lengthof the injector by minimizing the requirement for differential pumping system by reducing He propellant
gas. A schematic of the injector is shown in Fig.7. In
following subsections a detail of various parts of the108

J.S. Mishra et al., A Low Speed Single Barrel Pellet Injector and Its Application to Complementary Study on Ablatant in LHD

(a)
0.50

1.00
15.0
30.0

0.25

45.0
60.0

Z(m)

Helical
coil

0.00

-0.25

-0.50
0.0
Helical
coil

1.0

0.2

0.4

ρ 0.6

0.8

0.6

0.8

1.0

(b)

1.00
15.0
30.0

0.8

45.0
60.0

Z(m)

0.6
0.4

Fig. 5 Upper ﬁgure: Shallow angle inner port injection at
z=0.13m (51◦ cross-sectional view) close to helicalcoil; Lower ﬁgure: Shallow angle top port injection
to a HFS (outboard side view). Guide tube withbending at the end towards the HFS and pellet path
are shown by green line and red arrow respectively.

0.2
0.0

0.2

0.4

ρ

1.0

Fig. 6 E × B drift direction along the ﬁeld line projected
on to poloidal plane for for (a) inboard side shallow
angle consideration and (b) upper port shallow angle consideration. Colored lines indicates the plasmoid expansion length along the toroidal direction.

pellet injector is presented.

3.1

0.0

Cryogenic Chamber

For the pellet formation a Giﬀord-McMahon cycle compact cryo-cooler is being used. The GM-cycle
cryo-cooler has the advantage that it is free from replenishment of cooling media such as liquid heliumand can be easily and remotely operate by using thecommon utilities such as cooling water and electricity inside experimental hall. The cooling capacity ofthis cryocoller is 10 W at 8 K on the second stage.
The ﬁrst stage at 40 K temperature is connected toa cylindrical thermal shield (18 cmϕ × 35 cmℓ) madeup of copper. The second stage is connected to theheat sink made up of Copper. A barrel of internal
diameter 3 mm made up of stainless steel is brazed toa Cu disk of 3 mm width, which is connected to theheat sink. All these equipments are placed inside avacuum chamber which maintains a vacuum level ofthe order 10−5 Pa. A turbomolecular pump of capacity 0.06 m3 /s is connected to this chamber. It takes
around 1.25 hour to cool down the heat sink from theroom temperature to 4 K. A heater is being installed
to maintain the temperature at required level. The
injector will be ready for operation after one and halfhour of switching on cooling down. As the heat load

First expansion
chamberTO
LHD

Second expansion
chamber

Cryo cooler

Formation chamber
Solenoid punch

Guide tube
selector

10 cm.

Fig. 7 Schematic of Punch mechanism pellet injector

to heat sink is negligible and rate of cooling is very
high, temperature increase due to a pellet injection
can be recovered quickly and it can be ready for the
next pellet within few seconds.

109

J.S. Mishra et al., A Low Speed Single Barrel Pellet Injector and Its Application to Complementary Study on Ablatant in LHD

Pellet Size and Speed

3.3

Pressure [ Pa ]In LHD cylindrical pellet size of 3 mmϕ × 3mmℓ is regularly injected to the NBI heated plasma
by multi barrel pipe gun injector. Hence the same
size pellet will be used for the above-discussed kindof pellet injection. A pellet contains 1021 hydrogenatoms, hence for a plasma of volume 30 m3 increaseof density 3×1019 is expected. As the pellet will
be launched through the curved guide tubes, pellet
launching speed will be depend on the survivabilityof the pellet through it, which is ≈ 300 m/s but the
injector can be operated at higher speed.

Vacuum system and guide tube

V2

dP2
= C12 (P1 − P2 ) − S2 P2 + L2
dt

6
He entry rate
into final stage

4

st

1 Stage2nd

2 Stage

0

1

2

0

rate of the stage i, respectively. Fig.8 shows the characteristic curve for the diﬀerential pumping system
based on the above calculations. The vacuum level ofthe ﬁrst expansion chamber(1st stage) increases to 5
Pa just after the pellet injection as there is a directinﬂow of propellant gas into this chamber, but due
to diﬀerential pumping the vacuum level of 2nd stage
increases approximately to 10−4 Pa and the increasein pressure in this chamber can be recovered withinshort interval of time.
The ﬁnal expansion chamber is equipped with aguide tube selector and three guide tubes to launch apellet from diﬀerent locations. The guide tube selector can be controlled remotely during experiment . A
schematic of the pellet injector with guide tubes used
for the diﬀerent pellet injection locations are shown in
Fig.9. The total length of the guide tube for low ﬁeld
side is 2.2 m with two bending. The inner port guidetube is 20 m length and the upper port guide tube isof 14 m length with ﬁve bending in each.
Pellet erosion occurs by the centrifugal force, due
to the curvature of the guiding tube. The tensile
strength σ t of a pellet in terms of centrifugal force
is given by [14]Two stage diﬀerential pumping system has been
installed to restrict the inﬂow of propellant gas into
the plasma chamber. The volume of 1st and 2nd expansion chamber are 0.015 and 0.02 m3 respectively.
Both the expansion chambers are connected to turbodrag pumps of pumping speed 0.25 m3 /s and 0.5 m3 /s
respectively. Exapansion chambers-1 and 2 are maintained a vacuum level of 10−5 and 10−6 Pa respectively. Considering maximum of 1 Pam3 propellant
gas required for single pellet injection, the characteristic curve for diﬀerential pumping system can be calculated from the following equations.
dP1= Iacc + C12 (P2 − P1 ) − S1 P1 + L1 (2)dt

8Fig. 8 Vacuum characteristic curve (Theoretically calculated value; experimental data is not available) for
two stage diﬀerential pumping.

Pellet Launching

V1

10Time [s]A pellet needs high pressure or a little warm up
to break away from the freezing zone. For the case ofhigh pressure, the pellet is being accelerated to higher
speed. The pellets with higher speed cannot be survive through the curved guide tube, and again due tospace constraint the injector dimension is minimized.
Therefore to balance these situations a solenoid punch
is added to the breech side of the injector which initializes the motion of the pellet from the freezing zone.
After that a small burst of He propellant gas can be
add to accelerate it to higher speed. The propellant
gas can be easily removed by the two stage diﬀerential
pumping system. Using this technique the velocity ofpellet can be controlled precisely and with additionof more gas the pellet can be reach up-to high speed.
The punch head, operated by a solenoid used in thissystem is originally procured from the Oak Ridge National Laboratory (ORNL)

3.4

2
10
1
10
0
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
10
-7
10
-1

16
He-Entry Rate 10 [ He/s]

3.2

σt =

mvp 2
AR

(4)

Where m is mass of the pellet, vp is velocity of Pellet,
A is area of the pellet in contact with guide tube walland R is bending radius of the guide tube. Taking σ t= 0.1 MPa [15] at 10 K temperature for a Hydrogenpellet and A = 10 % of total surface area, the√relation
between vp and R can be written as v p = 400 R m/s.
For R = 0.8 m a pellet up to a speed of 350 m/s
can be delivered intact. An intact ratio of 65% and100% at vp ≤ 350 m/s and 300 m/s respectively has
been obtained using the guide tube of R = 0.8 m [13].
Hence all the guide tubes used are of bending radius
0.8 m.

(3)

Where Iacc , Pi , Vi , Si , Ci and Li denote propellant gas inﬂow rate, pressure of the stage i, volumeof the stage i, pumping capacity of the stage i, conductance between the stages i and i + 1 which areconnected by guide tube and leak and/or degassing

110

J.S. Mishra et al., A Low Speed Single Barrel Pellet Injector and Its Application to Complementary Study on Ablatant in LHD

4.1m

pellet injection compare to LFS injection when thepellet plasmoid approaches a region close to helicalcoil. Based on the calculation three diﬀerent injection
locations are considered for experiment. Curved guidetubes of bending radius 0.8 m are installed for pellet
launching. Pellet mass erosion occurs at high speed
while passing through bended guide tubes. Hence apellet injector of low injection speed using mechanical punch and small amount of propellant gas was
designed. Pellet injection and comparison of E × B
drift eﬀect with the theoretical models will be carriedout to investigate the ablation and mass deposition
process for optimization of the pellet injection location.

R
0.
8

3.9m
4.8m

.8
R0

4.1m

R0
.8

1.2m

6.3m

1m

Fig. 9 Schematic of guide tubes connected between pellet
injector and inboard, outboard, upper port of LHD.

3.5

Acknowledgements
The support of technical peoples involved in makingof this injector is greatly acknowledged. This work is
supported by the Ministry of Education, Sports, Culture, Science and Technology, Grant-in-Aid for Scientiﬁc Research (S) 20226018.

Injector diagnostics

The injector is equipped with two light gate system to measure the pellet speed by the time of ﬂight
measurement between two ﬁxed positions. Each lightgate consists of a laser and photodiode. A shadowgraph system consists of a CCD camera and a ﬂash
lamp of 180 ns pulse width is installed on the 2nd
chamber to image the pellet during its passage in theﬁeld of view of the camera. From the size and density,pellet mass can be measured, which will be useful for
fuelling eﬃciency calculations. Fast photodiodes andimaging ﬁber will be used for the pellet ablation studies inside the plasma.

3.6

References[1] S.L. Milora, Nucl. Fusion 35, 657 (1995).
[2] B. Pegourie, Plasma Phys. Control. Fusion 49, R87(2007).
[3] R. Sakamoto, Nucl. Fusion 4, 38, (2001)[4] S.Sudo et al, Fusion technology, Vol. 1, 656-660(1992)[5] R. Sakamoto, Nucl. Fusion 49, 085002, (2009)[6] P.T.Lang Phys. Rev.Lett., 79, 1487(1997)[7] H. W. Muller, Phys. Rev. Lett. 83, 2199 (1999)[8] J D Kloe, Phys. Rev.Lett, 82, 2685 (1999)[9] Rozhansky V, Plasma Phys. Control. Fusion 46, 575(2004)[10] L.R.Baylor, Nuclear Fusion 37, 4 (1997)[11] R. Sakamoto, Nucl. Fusion 44, 624 (2004)[12] H. Yamada et al., Fusion Eng. Des. 49-50, 915(2000).
[13] S.K. Combs et al, Fusion Eng. Des. 58-59, 343(2001).
[14] S.Sudo et al, Fusion technology, 14, 1334(1988).
[15] S.K. Combs, Rev. Sci. Instrum. 64, 1679 (1993).

Controls and Data acquisition

Due to inaccessibility of the experimental hall, thepellet injector should be controlled remotely during
plasma operation. A PC based control system is developed using National Instruments Compact Field Point(CFP) controller, input output card and LABVIEW
software. CFP placed near the injector in experimental hall can easily be accessed via ethernet networkfrom LHD control room. All the vacuum valves andpumps are controlled by CFP. Also the pellet formation cycle synchronized with 160 second before plasma
discharge is controlled by CFP, whereas timing signal
for the solenoid punch and fast valves are controlled
by software developed on visual basic platform. The
pressure and temperature data of the injector is alsocollected by CFP analog modules. Diagnostic systemdata’s such as light gate shadowgraph and fast photodiode data is collected by fast ADC.

4. Summary
A simple calculation considering the ∇B drift on
plasmoid in non axis symmetric magnetic ﬁeld of LHD
was performed for diﬀerent pellet injection possibilities. Calculation shows signiﬁcant drift for shallow

111