J. Plasma Fusion Res. SERIES, Vol. 5 (2002) 446449

Development of the Guide Tube for Magnetic
High Field Side Pellet Injection
KIZU Kaname, SASAJIMA Tadayuki, MASAKI Kei, HIRATSUKA Hajime, ICHIGE Hisashi,
HONDA Masao and MIYA Naoyuki
Department of Fusion Facilities, Naka Fusion Research Establishment,
Japan Atomic Energy Research Institute, Naka-gun, Ibaraki 31 1-0193, Japan(Received: 11 December 2001 / Accepted:23 April 2002)

Abstract
For attaining a higher fueling efficiency in JT-60U centrifugal pellet injection experiments, guidetubes for a magnetic high-field side (HFS) injection were developed. The guide tube for HFS (top)injection is about l5 m in length, 5 mm in inner diameter and 600 mm in the smallest curvature radius,and has been connected to the upper port of the vacuum vessel. The speed exceeding 220 m/s led topellet destruction in the guide tube, while the injected pellet mass estimated from the increase of plasma
density reached about 80 Vo of that from low-field side (LFS) injection. A simple impact model wasapplied to explain the above result. The perpendicular pellet velocity limit on the wall inside the tube wasestimated to be 20 m/s indicating that less than 100 m/s pellets can pass through the guide tube even witha minimum curvature of 200 mm. Based on this estimation, a new guide tube for HFS (mid) injection
was designed and installed in March, 2001. Pellets with 100 m/s were successfully injected to the plasma
through the HFS(mid) guide tube.

Keywords:
high field side pellet injection, guide tube, JT-60U, deuterium pellet, pellet velocity limit

1. Introduction
In JT-60U, in order to attain higher density andshows that the HFS (top) pellet can be deposited deeper

in the plasma than that of LFS (mid) and 70 7o ofGreenwald density with Hsep of 1.94 was attained.
Therefore, to attain more effective fueling, anotherguide tube for HFS (mid) injection, in which pellets

higher confinement plasmas by successive refueling to
the plasma core, development of a centrifugal pellet
injector started in 1994. The centrifugal pellet injector
which can eject trains of up to 40 cubic (2.1 mm.1l
pellets at a frequency of 1-10 Hz and speeds of
100-1000 m/s was developed and installed in 1998

were injected from the inboard midplane, was installed
and started operation in March 2001.
In this report, the design of guide tubes for HFS(top) and HFS (mid) injection, and operational results

lr,2l.
On ASDEX-U pellet injection experiments from amagnetic high field side (HFS) in 1996 [3] showed
higher fueling efficiency than magnetic low field side(LFS) injection under the high power heating plasma. In
1999, a guide tube for HFS (top) injection was adopted
in JT-60U. The recent report of Takenaga et al. [41are described.

2. Design of guide tubes
The configuration of two types of guide tube, HFS(top) and HFS (mid), and the vacuum vessel is shown in@2OO2 by The Japan Society of Plasma

Science and Nuclear Fusion Research446

Hojabri A. et al., MHD Activity and Disruption in Low-4(a) in Iran Tokamak I (IR-Tl)

Fig. 1. Pellets ejected from centrifugal pellet injector

through the HFS (top) guide tube can also be investigated by the time of flight method.
The guide tube for HFS (mid) is 5 mm in innerdiameter, 7 mm in outer diameter and about 5 m inlength. Pellets are injected from the inboard side of thepass through the line selector which selects the tube for

HFS (top) or F[FS (mid) injection as also shown in Fig.
1. The line selector has a movable funnel connected tothe guide tubes for HFS (top) or HFS (mid) injection
through a gate valve.
The targe:t point of HFS (top) injection is thebottom area oI the inner divertor baffle plate. The tube

vacuum vessel at 45" to the plasma midplane. The guidetube was set up between the inner vacuum vessel skinand the first wall tiles. The outlet of the guide tube is 5mm lower than the first wall surface to protect from the(5 mm in inner diameter, 7 mm in outer diameter and-15 m in length) has four curves. The curvature radiusof the first curve is limited to R = 600 mm because of
obstructive devices around the tube. According to thereports [5,6], the inner diameter (D) of the guide tube
should be less than twice of the pellet size (d), D/d < 2.
In JT-60, the inner diameter (5 mm) approximately twotimes the diagonal line of a pellet was adopted.
The guide tube has a double-tubed structure. The
outer secondary tube is pumped out to lower theinfluence on unexpected air leakages. The guide tube
consists of several sections of tubes. Each tube wasplasma exposure. For this modification, LFS (mid)injection was canceled. Based on the estimation from
HFS (top) injection results, the inlet and outlet curvatureof the guide tube are designed to be 200 mm and 100mm, respectively.

3. Operational Results
HFS (top) pellet injection experiments wereperformed from January to November 2000. Pellet
injection speed into the plasma is important for thecentral fueling. Average pellet speed was measured by
two microwave cavities. Figure 2 shows the relationshipbetween the ejected pellet speed calculated by therotational frequency of an acceleration motor and themeasured average pellet speed. For these measurements,
different fuel gases H1 Dz and Ne were used. The

connected by a flange structure. To prevent thedestruction of the pellet, inlet sides of the tubes havetapered structures.

Two microwave cavities were installed for measuring the number and mass of passing pellets. One wasinstalled betwoen the injector and the line selector. The
other was installed in the outlet side of the HFS (top)guide tube (Fig. 1, point A). The average pellet speed(a,measured pellet speeds were about 79 Vo of thecalculated speed and a difference in the pellet gas
species was not clear.

R=1134

IGuide tube

HFS(mid)

Guidb tube

R==100<_f"t et45"
Vacuum Vessel

Line selector

Line selector

Fig. 1 Schematic drawing of guide tubes for JT-60U (a) and the line selector (b).
44'7

Hojabri A. et al., MHD Activity and Disruption in Low-q(a) in Iran Tokamak 1 (IR-TI)

A simple impact model was applied to theexperimental data. The collision of a pellet on the guidetube inner wall was supposed to be like Fig. 4. A pellet
whose speed is V impacts on the guide tube inner wall

Pellet masses injected from the HFS were also
investigated. Figure 3 shows the pellet speed
dependence of the increase of the electron number inplasma (AN.). It was supposed that AN" is nearly equal
to the injected pellet mass because of OH plasma.

which curvature radius is R. When perpendicularvelocity (Vo"*) of the pellet to the wall is larger than alimit value, the pellet will be broken. The relationshipbetween V andVr",p is written as,

Pellets were injected into OH plasma with pellet speed

from 100 m/s to 400 m/s. Here, pellet speeds arecalculated ones. Open circles are the results of HFS(top) injection. The solid line shows the maximum valueof HFS (top) injection. It is clear that pellet masses fromHFS (top) increase as the pellet speed decreases. Thisindicates that the most of ejected pellets are destroyedthrough the guide tube even at 300 m/s. Destruction ofVpe* = l'z 5i1 I(1)cos0=l-(D-d/2)/RQ)

pellets was also observed by the microwave cavity
located at point A. Pellets with 220 m/s successfully
show the pellet signal. On the other hand, in the rangeof 320 to 400 m/s, pellet size reduction and pellet

where 0, D and d mean the pellet impact angle on thewall, inner radius of guide tube and pellet size, respectively. From eqs. (l) and (2), the maximum perpendicular pellet velocity without destruction by the impact isderived. From Fig. 3, we supposed that the maximum

breaking was observed.

pellet speed to pass through the guide tube was 220 m/s.

Jft,300i2504..

E 2ooe."3 r50
3 100500050 100 150 200 250 300 350
Calculated speed (m/s)
Fig. 4 Schematic diagram of simple impact model.

Fig. 2 Relationship between calculated pellet speed andobserved pellet speed.

[xt0201 6+(2.1 mm)3 cubiclVp".p-* = 20 nr/s^ 1<n4o@z3& 100oEo2ooa50oo80L0100200300

Pellet speed (m/s)

4000500

100 200 300 400 500 600Curvature radius (mm)Fig. 3 Ejected pellet speed dependence of increment inFig. 5 Relationship between curvature radius and survival pellet speed derived from adopted model.

electron number.
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Hojabri A. et al., MHD Activity and Disruption in Low-4(a) in Iran Tokamak I (IR-TI)

The pellet speed passing through the HFS (top) guidetube became around 80 Vo of calculated speed. Therefore, the limit velocity which can pass through the curveof 600 mm in ourvature radius supposed to be 2OO rrls,indicating that the V** is from about 20 m/s. A theoretical calculation by Artaud et al. [6] shows that the maximum V** (V*,p-.*) below 15 K is 20 m/s and that V*"-

Pellet (H2, D2 and Ne) speeds were measured bytime of flight technique using two microwave cavities.
The measured pellet speeds were about 79 Vo of.^ gradually becomes smaller with increasing tempera-

density reached about 80 Vo of that from low-field side(LFS) injection. The increase of plasma density afterinjection depends on the pellet velocity. These resultsmean that the pellets wear away through the guide tube.

calculated speed.
The speed exceeding 220 m/s led to pellet destruc-tion in the HFS (top) guide tube, while the injectedpellet mass estimated from the increase of plasma

ture. This means that the Vp".p--u* derived from thesimple impact model agrees with the literature value.

The guide tube for HFS (mid) injection wasdesigned based on the estimation with the above simple

The simple impact model was applied to explain

impact model. Figure 5 shows the calculated result of

the HFS (top) results. Perpendicular pellet velocity limit

survival pellet speed versus curvature radius. The simple
impact model shows that the pellet with the velocity up
to 100 m/s can go through the first curve of R = 200mm. Unfortunately, because the space between thevacuum vessel skin and the first wall was insufficient,
the curvature radius at the outlet was limited to 100 mm
and pellet speed below 80 m/s was available for thison the wall inside the tube was estimated as 20 m/s
indicating that pellets with a speed less than 100 m/s canpass through the guide tube even with a minimum
curvature of 200 mm. Based on this estimation, a new
guide tube for HFS (mid) injection was designed andinstalled in March, 2001. Pellets with 100 m/s in speed
were successfully injected to the plasma.

guide tube. Atter the first R = 200 mm curve, a pellet
shifts its position to the outside of the tube wall being
forced outwar,l by an acceleration up to the end of theguide tube. Therefore, the centrifugal model [7] may be
adoptable in the outlet of HFS (mid) rather than theAcknowledgement
We would like to thank the members of JT-60
Facility Division II for their help. In addition, we aregrateful to Dr. M. Shimizu and Dr. M. Kuriyama forsimple impact model. Considering the centrifugal

their continuous support.

model, a pellet speed of 280 m/s is allowed even withthe minimal ctrrve of R = 100 mm.
Pellet inje'ction experiments from HFS (mid) begun
in March 200 t. The results of the increase of electron
number by HIiS (mid) injection were also indicated inFig. 3. Pellets injection with 100 m/s from HFS (mid)increases the rrumber of electron in plasma much larger
than HFS (top) injection. This demonstrates that thecentrifugal model is applicable to HFS (mid) injection

References
tll K. Kizu et a/., JAERI-Tech200l-022 (2001).
t2l K. Kizt et c/., Fusion Eng. Des. 58, 331 (2001).
[3] P.T. Lang et a/., Phys. Rev. Lett. 79, No. 8, 1487
(1997).

t4l H. Takenaga and JT-60 team, Phys. Plasmas 8,2217 (2001).

t5l K. Biichl and W. Sandmann, IPP-Report, MaxPlanck-Institut ftir Plasmaphysik, IPP 5/83, Aprilresults.

1999.

t6l J.F. Artaud and A. G6raud, Proc. 20th Symposium

4. SummaryFor attaining a higher fueling efficiency, guideon Fusion Technology, Marseille, 971 (1998).

tubes were developed for JT-60U HFS (top) pellet
injection experiments with the centrifugal pellet injector.

t7l S.K. Combs et al., Proc. 17th Symposium onFusion Engineering, San Diego, llOZ (1997).

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