Design of A 25barrel Pellet Injector With Cycle Refrigerator For the HL2A Device XIAO Zhenggui LI Bo

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Design of A 25-barrel Pellet Injector With Cycle
Refrigerator For the HL-2A DeviceXIAO Zhenggui
LI Bo LI Li

• Contents
• 1. Introduction
• 2. Why consider use a refrigerator for
• pellet forming?
• 3. Basic design parameter
• 4. Cryogenic component
• 5. Calculation of heat loss for the cryostat
• 6. Heat sink mass
• 7. Summary

2
Abstract

• The first 25-barrel hydrogen pellet injector
  employed a
• Gifford-McMahon (GM) cycle refrigerator has been
• designed as an advantage plasma fuelling tool
  for
• HL-2A tokamak. The in-situ condensation is used
  for
• making 25 pellets in 25 gun barrels
  simultaneously.
• The advantages of use refrigerator are that
  ease for
• operation, high reliability and flexibility,
  reduction the
• operation cost. The prime designed principle and
  goal,
• the construction and layout, the heat load
  calculation of
• the components, as well as the engineering
  parameters
• choice of the injector were introduced in this
  paper.
• Key words Pellet injector Cycle refrigerator
  
• Heat loss Heat sink

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1. Introduction

• HL-2A the first symmetry divertor tokamak in
  China
• has been built at SWIP in the end of 2002. The
  main
• objectives of HL-2A are to produce more
  adaptable
• divertor configuration for study enhanced plasma
  
• confinement and improved stabilities of plasma
  with
• high discharge parameters. The divertor physics
  and
• engineering problems are also should be
  researched
• on this machine. Since one of a primary
  concern in
• fusion research is particles transportation and
  density
• control of plasma that deeply related to the
  fuelling
• scheme. The conventional fuelling way is edge
  gas
• puffing, however many experiments indicated that
  the

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• the fuelling efficiency of gas puffing
  usually degraded
• with increase of heat power and the density
  profile of
• plasma hard to control. Pellet injection has a
  potential to
• overcome these shortages. Injected pellet can
  penetrate
• rather deep even in high auxiliary heat plasma
  and
• produce more favorable particles deposition
  profile
• deeper inside the plasma column. So the PI has
  not
• only been used on current fusion experiments
  widely,
• but has also been adopted at next largest
  fusion
• device , ITER, an ambitious international
  research
• project to harness the promise of fusion energy,
  for
• plasma fuelling.

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2. Why do we consider use a refrigerator
for pellet forming?

• In HL-1M an EPI has been developed and used
  for
• Plasma Fuelling Experiments(PFE). The basic
  operation
• performances of EPI have been managed.
  However,
• the equipment needs use liquid helium(LHe) as
  cooling
• medium. Some factor, the expensive cost of LHe
  and
• hardness to control the temperature of the
  cryogenic
• elements, limited the equipment operation in
  any time,
• the further engineering tests and the more deep
  study
• of PFE are also limited. Thus, we present a new
  design
• of PI to overcome above shortages, use this
  design the
• injector will operate easily and flexibly,
  thereby the PFE
• will carry out in any time.

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• The design goal of this injector is (1)
  Operational
• flexibility with low cost (2) High reliability
  for operation.
• Based on this goal and in consideration of the
  needs
• of HL-2A fuelling experiments, in this design a
  G-M cycle
• refrigerator is chosen to instead of use of L He
  ,and the
• pneumatic pipe-gun mechanism is adopted to pellet
  
• freeze and accelerate.
• Flexibility in fuelling experiments requires
  that the PI
• has capable of producing pellets with different
  size and
• firing them with any desirable time sequence.
  This goal
• was met by produce the pellet in 25 gun barrels
  with 2 or
• 3 different inner diameter. The injector can be
  easily
• operated and each pellet can be managed and fired
  

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• independently at any desired time by a
  programmable
• logic controller. The low operation cost was
  realized
• by use the refrigerator to freeze the pellet.
• The high reliability of PI operation is also
  necessary.
• The young scientists in SWIP should put them
  more
• vitality on research the performances and
  characteristics
• of fuelling plasma. The operation of the PI
  should be one
• routine business. The use of the in-situ
  condensation in
• pellet injector does not need mechanical motion
  of the
• elements, thus reduces more trouble in low
  temperature
• condition, so the high reliability of injector
  operation is
• guaranteed.
• In this design the largest barrels number of
  25 is adopted up to date.

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• The challenge with such a large number pipe
• gun is that the space layout of the 25-barrels
  with
• 25 propellant valves and the temperature rise of
• the heat sink caused by the admitted propellant
  during
• the pellet firing.
• The similar design of 10-barrel pellet
  injector has
• been realized on LHD at NIFS.

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3. Basic design parameter

• The HL-2A is a medium size tokamak device with
  a
• major radius of 1.65m and miner radius of 0.4m
  and a
• symmetrical circular divertor, Ip? 480 kA and
  current flat
• top time of 25s.
• The designed pellet size with nominal
  diameter of
• 1.3mm(15pellet)and 1.4mm(10 pellet) and length
  of
• 1.31.6mm, the contained particles of each
  pellet is in
• the range of 9141019 atom (H) /pellet
  resulting a
• rise of average density by 1.62.51019/m3 in
  HL-2A
• plasma.
• Next figure shows the arrangement of the
  pellet
• injector and connection on HL-2A.

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Fig.1 The layout of multi-barrel pellet injector
on HL-2A
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• In this design the pellets are accelerated by
  high-
• pressure propellant, it bring a large of gas
  diffusion into
• injecting line. A multi-stage vacuum
  differential pumping
• chambers are adopted to evacuate the propellant
  from
• the injecting line and prevent it from diffusion
  into HL-2A
• discharge vessel.
• The pellet can be horizontal injected at
  equatorial
• plane (LFS) with velocity of 8001500 m/s and
  also
• incline injected in the HFS with velocity of
  200400 m/s.

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4. Cryogenic component

• In this design an advanced 4k refrigerator (model
  RDK-415D) was employed. Its refrigeration
  capacity is 1.5W at 4.2 k, 10W at 8 k on the 2nd
  stage and 35W at 50 k, 75w at 70 k on the 1st
  stage .
• The in-situ condensation concept is used for
  pellet forming.

• Fig. 2 The principal of the
• in-situ condensation
• concept

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As the design of the first 25-barrels pellet
injector with refrigerator we must consider how
to arrange the 25-barrel assembly on a common
heat sink. It needs have equal heat
distribution and easily for accessing installing
and replacement. The cooling power of the
refrigerator transmitted from the 2nd stage to
the heat sink?disk?gun barrel and freeze down
the barrel in local zone less than 8k. The heat
sink and the disk are made of OFHC copper tokeep
the temperature uniformity and the disk is brazed
to a thin wall (length of 500mm and thickness
of 0.20.25mm) stainless steel tube. Fig.
3 shows the main construction of 25-barrels
pellet injector cryostat and barrel
assembly.
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Fig.3 Construction of multi-barrel pellet
injector cryostat and barrel
assembly
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• In this PI, the 1st cool head with a
  thermal shield, the
• 2nd cool head with a T-shape heat sink, the
  25-cooling
• disk brazed to 25-gun barrel assembly are all
  contained
• in a cryostat chamber with high vacuum to provide
• thermal insulation for the low temperature
  components.
• The thermal shield absorbed the primary heat
  radiation
• from the chamber wall with room temperature.
  In order
• to guarantee the local zone of barrel cooled
  down less
• than 8k, besides of use the 2nd stage of the
  refrigerator
• as cooling source, the tight connection of the
  disk to the
• heat sink is very importan. The small disk is
  tight
• connected with indium foil by screw through a
• couple of bolt plate and clamp flange as shown
  in
• Fig 3.

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• In addition to this construction, at the both
  side of each
• disk on the gun barrel and gun breech apart from
  the
• disk by 100 mm, two OFHC cooper wire are brazed
• and its another end is connected the shield to
  avoid
• heat penetration directly from the room
  temperature
• breech and barrel to the disk.
• The upstream of each gun breech is connected
  the exit
• of each propellant valve respectively, when open
  the
• valve the high pressure propellant admitted into
  gun
• breech push and accelerated pellet in the
  barrel. The
• downstream of the 25 gun barrels are bent
  firstly and
• formed a cone, then parallel inserted into a
  common
• guide tube, after then the pellets being flying
  into the
• diagnostic chamber and the injection line.

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5. The calculation of heat loss of cryostat

• As the first design of cryostat for pellet
  producer with
• refrigerator the calculation of heat load of
  cryostat is a
• very important step. The refrigerator has large
  cooling
• capacity of 1.5W at 4.2 k, 10W at 8 k on the 2nd
  stage,
• and 35W at 50 k, 75w at 70 k on the 1st stage.
• The minimum temperature of 3 k can be
  reached in
• condition of without any heat load on the 2nd
  stage.
• Since the pellet size and the sum of the fuelling
  mass is
• very small ( 5mg), so the released heat of the
  sample
• is only a little if consider only the pellet
  producing.
• However, as a cryostat, its all heat loss
  included the
• cryogenic components ,the supports and connecting

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• wire are un-negligible, especially in the
  case of use the multi-barrel.
• The total heat loss of the cryostat
  consists of (1) the solid heat conduction Q1 of
  25-gun pipe to the cool disk, and (2) of all
  measuring wires and the heat power of the heat
  anchor Q2 , (3) radiation heat of the shield Q3
  to the heat sink. These heat losses can be
  calculated by following formula.
• Q1? F bar (T2-T1)/L
  (1)
• ?- Thermal conductivity of the gun barrel
  material
• F bar - The cross section area of the gun
  barrel
• L- The distance between the connection point
  of gun
• barrel on shield and the cooling disk.
• T2,T1-The temperature of the shield and the
  heat sink

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• Q2?F3(T3-T1) /L3w1w2
  (2)
• ? - Thermal conductivity of the measuring wire
• F3, L3 -Cross section area and length of the
• measuring wire respectively
• T3,T1,-The temperature of the measuring wire
  on
• warm end (300k) and on freeze disk
  (8k)
• w1 - The power produced by working current
• through measuring wire
• w2- The power of electrical heat anchors

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•   Q3E?? ? F1(T24-T14 )
  (3)
• Where E
  (4)
• e1,e2-The radiance of the heat sink outer surface
  and the
• inner surface of the radiation shield
  respectively.
• F1,F2-The area of the heat sink outer surface and
  the
• inner surface of the radiation shield
  respectively.
• ?- The Stefan constant of 5.6710-8 (W/m 2
  K 4)
• T2,T1-The temperature of the radiation shield and
  the
• heat sink
• After calculation, the each item of heat loss is
  obtained
• Q10.32 w, Q20.2 w, Q30.015 w
• The total heat loss Qt is about 0.6 w.

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• The heat shield (77k) plays an important
  role in
• reducing the heat loss.
• The total mass of 25-hydrogen pellet is
  about 5mg,
• in the all process, included cooled down to
  triple point
• and solidification, it released sum heat of 30
  joule. This
• process usually completed in 3 minutes, so the
  required
• freeze power is less than 0.2 watt.
• According to above calculation, to make 25
  pellets
• simultaneously the sum heat losses less than 1.0
  w,
• which only a small part of the total
  refrigerating capacity
• of the refrigerator.
• The freeze capacity of RDK-415D refrigerator
  to
• produce 25 pellets is more than enough.

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6. The heat sink mass

• In the original, the heat sink was designed
  a T-shape
• (see Fig.2) and its mass only about 1.2 kg, it
  benefits to
• quickly cool down and easily install the low
  temperature
• components. If only consider for pellet
  forming, this is a
• reasonable choice.
• However, as a pneumatic 25-barrel pellet
  injector, the
• temperature rise of the heat sink caused by
  admitted
• propellant during continual pellet firing should
  be treated
• carefully. In according to the experience of
  the EPI
• operated on HL-1M, each pellet shot brings heat
  of
• 0.4 cal, increase in temperature up to 1.5K
  for the
• original heat sink.

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• This temperature rise is
• intolerable obviously. A
• new heat sink with large
• mass of 18 kg has been
• replaced it as shown in
• Fig.4
• The temperature rise for
• the new heat sink is0.1K
• for each pellet shot and
• the sum in temperature
• rise for 25 pellet firing is
• 2.5K(from8.0K to 10.5K).

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7. Summary

• A 25-barrel pellet injector employed a G-M
  refrigerator
• and in-situ condensation has been designed for
  the mew
• fuelling tool for HL-2A. The advantages of using
  G-M
• refrigerator are that of reduction the operation
  cost and
• has large cooling capacity, ease for operation,
  high
• reliability and high flexibility.
• By careful calculation of the heat losses and
  reasonable
• construction for each cryogenic component, it is
  indicted
• that the cool capacity of RDK-415D refrigerator
  to
• freeze 25 hydrogen pellets simultaneously is more
  than
• enough, the heat shield played an important role
  in
• reduce the heat loss of cryogenic component.
• This design is reasonable and reliable.