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SUPERCONDUCTING
MAGNET COIL
FOR K-500 SUPER CONDUCTING CYCLOTRON
1.
INTRODUCTION
One
of the major component of this
cyclotron is the super conducting magnet coil which produces very
high magnetic field required for
rotating high energy charge particles. The magnet coil
of the K-500 Superconducting
Cyclotron is immersed in liquid Helium (4.2K) in a specially built stainless
steel Cryostat. This Cryostat consisting of super conducting coil is finally
placed inside iron core magnet producing magnetic field upto 5.5 Tesla.
2.
SUPERCONDUCTING CABLE
The
cable used for coil winding is multifilamentary superconducting wire (1.29 mm
dia) having 500 filaments of 40 micron diameter Nb-Ti in copper matrix which is
embedded in OFHC grade copper channel (2.794mm X 4.978mm) for cryogenic
stability (Fig-1). The main idea behind using the superconducting cable is to
get very high magnetic field (5.5 Tesla) using a very high overall current
density of 5800 Amp/cm2 and there by reducing the overall size of coil / magnet
to many folds as compared to room temperature magnet. 35KM length of the cable
had been used for fabrication of superconducting magnet coil. Cryogenic
Test laboratory had been set up at VECC for characterization of the
superconducting cable.
3.
DESCRIPTION
OF COIL
The
basic structure of the coil consists of layer type helical winding on a
stainless steel bobbin of 1473mm I.D. x 1930mm O.d. x 1168mm height. The bobbin
is afterwards welded shut to become helium can. The coil is spilt into two
halves (upper & lower sides of median plane) and each half is again split
into large (b-coil) & short (a-coil) coils as shown in figure - 2. The inner
wall of the SS bobbin is covered with two layers of 5 mil mylar, followed by a
first layer of 40 mil thick x 13mm wide strips of fibre glass laminate
(NEMA-G-10CR), called picket fence, placed at a gap of 13mm. The spaces between
the pickets are used for the passage of liquid helium circulation for coil. At
each end of the bobbin there are grooved flange spacers also made of G-10 Glass
Epoxy laminate placed on the collar plate at 20 pitch, which provides radial
alternating ducts (13mm wide) for helium feed passage. The narrow grooves on
flange spacers position the picket fence strips by meshing with tabs of the
pickets. The a & b coils are separated by partition insulating spacer 10mm
thick having alternate grooves also spaced at 20 pitch for helium passage &
lead entry / exit. This partition spacer is also made of Glass epoxy laminate
consisting of 6-segments of 600 each. The coil leads enter through the partition
spacer and first turn is made using climb spacer for progressive increase in
height and helical winding is carried out.
After
each layer again alternate G-10 picket fences are mounted (180 Nos per layer)
which provides insulation between layers and passage of liquid helium axially.
On completion of one layer, climb spacers (180 Nos.) of varying heights are
mounted to fill up the gaps between end turn and flange spacers. This restricts
the movement of conductor during operation to avoid quench. The super conductor
of rectangular cross section (2.794mm x 4.978mm) is insulated at two edges by
4-mil thick mylar adhesive tape leaving the board face of the conductor with
liquid helium. Since the coils and conductor experiences radial & axial
forces of high magnitude, the winding is required to be done at high tension at
2,000 PSI±10% and conductors needed to be placed one above the other with very
close tolerance to restrict movement of conductor. Turn to turn insulations had
been checked in-situ after each layer. Two a coils and b coils had been finally
connected in series and brought out through the lead port placed on the upper
collar of the SS bobbin for termination. After completion of a & b coils in
upper & lower halves of median plane, ten layers of mylar sheet was wrapped.
Then aluminium banding was carried out around the
a & b coils at 20,000 PSI tension for restricting the movement of the
conductor & coil while the magnet is energized. Aluminium banding gives more
compressive stress to the coils at 4.2K as compared to SS because of higher
co-efficient of linear contraction. Special grade of Aluminium (5052 - H34)
strip was used having high hardness and tensile strength.
4.
WINDING MACHINE
The
general layout of the winding machine is shown in figure 3. The winding was done
on a vertical winding (Lathe) machine for handling heavy load of around 7 tonne
(dead weight of final coil & SS bobbin assembly). 2440mm dia bed of the
winding machine was driven by motor with reduction gear assembly to get very
high torque. The conductor from the payoff reel passes first through the
straightening rollers, then to Servo Controlled Tensioning block (2,000PSI ±
10%) using Magnetic particle brake. The tensioned conductor passes through steam
& detergent cleaning system, rinsing with clean water, Ultrasonic flaw
detector to check any air void in the solder bond and then drying by hot air.
After this process the conductor is insulated at edges by 4 mil adhesive mylar
tape and this operation is to be carried out on line automatically. After edge
insulation the dimension of the conductor is checked by electronic dimension
checker. The conductor is now ready for winding on the SS bobbin which is
mounted on the Turn Table of vertical winding machine via pneumatic pressure arm
/ conductor guide. The positioning / laying of conductor was done by pressure
arm. All these equipments are mounted on the tool carriage of the lathe and the
lathe feed is used to move the entire carriage up & down as the
winding advances. The winding system is mostly automatic. At each end of the
layer substantial handwork is involved both in placing last turn and in filling
the transition blocks / climb spacers which carries the conductor to next layer.
All the winding parameters are monitored in a PC and they were logged date wise.
In case of any fault in subsystems, the machines gets off and corresponding
fault is indicated with alarm in the control panel.
5. STATUS
Coil winding
was started on 17th April,2003 after receiving the Cryostat SS bobbin
from M/S.Air Liquide, France and superconducting cable from M/S.Outokumpu
Advanced Superconductor, USA. Superconducting Coils (2-alpha & 2-beta) were
completed by 21st July, 2003. Three joints (splicing) in lower b-coil
& two joints in upper b-coil
had been successfully carried out and tested on line. After completion of
superconducting coils, aluminium banding had been done after changing the
tension device to wind at 20,000PSI. Joining of upper & lower coils (a&b)
had been done by soldering and finally three leads were brought out for
connection with vapour cooled current lead fixed on the Cryostat lead port.
Final coil resistances at room temperature were checked (15.31W
for a-coil
& 31.83 W
for b-coil)
and insulation resistance of coil w.r.t. bobbin was 100 MW.
The whole coil winding, aluminium banding & lead termination took about 6
months time.
COIL
DATA
1)
Conductor material
Nb Ti Super Conducting wire of 500 filaments of
40m
dia in copper matrix (1.29mm dia) which is embedded in copper stabilizer by soft
soldering using Pb & Sn alloy (50:50)
2)
Conductor Cross Section
2.794mm x 4.978mm
3)
Nominal current Density:
5800 Amp/cm2
4)
Design Current
800 Amp.
5)
SS (316L) Bobbin :
ID – 1486mm, OD – 1835mm
Height – 1160mm
Wall thickness – 17.5mm
Collar thickness – 19mm
Weight -2 Tonne
6)
(a) a-Coil
(short coil):
No. of Coils - 2
No. of turns/Coil - 1083
No. of layers/Coils - 36
Total length of conductor – 5.7KM
I.D. of coil –1521mm
O.D.
of coil – 1793mm
Height
of coil – 172mm
Total wt of coil -690 Kg
Inductance
of Coils = 13.8H
(b)
b-Coil
(Large Coil):
No. of coils - 2
No of turns/Coil - 2234
No of layers/Coil - 36
Total length of conductor- 11 .7KM
I.D. of coil – 1521mm
O.D. of coil –1793mm
Height of coil – 333mm
Wt.of coil –1410 kg
Inductance of coils -27.6 H
(c)
Total weight of four coils
4200 Kg
(d)
Total length of Super Conducting Cable used
35 km
7)
Aluminium Banding :
Aluminium strip of material 5052-H34
and cross section of 2.48mm x 5.13mm.
b-coil
:
No. of turns/layer
- 62
No. of layers
- 10
a-coil
:
No. of turns/layer
- 32
No. of layers
- 10
Total wt. of Aluminium Banding for
4-coils
- 320Kg.
8)
Stored Energy of Coils (with iron)
- 22 MJ
1) Conductor type
:
Nb Ti Multifilamentary composite
Superconducting
Wire soldered in
copper
(OFHC) channel.
2) Critical current at
4.2K and 5.5T :
1030 Amp.
3) Filament diameter
:
40 micron
4) No. of filaments
:
500
5) Wire diameter
:
1.29mm
6) Critical current
density (Jc) of
superconductor
:
1813 A/mm2
7) Overall Dimension of
:
4.978mm x 2.794mm
superconductor
8) Overall current
density
:
69.6 A/mm2
9) Copper to S.C. ratio
(Overall) :
20
10) Copper to S.C. ratio of wire
:
1.3
11) Residual Resistivity ratio (RRR) : 150
=R300K
/ R10K
12) Yield Strength
:
117Mpa
13) Twist Pitch
:
<12.7mm
14) Superconducting alloy
:
Nb/46.4% Ti
15) Resistivity
:
10-11ohm-cm
16) Design field
:
5.5 Tesla
17) Design temperature : 4.2K
Superconducting Magnet Coil for K-500 S.C.Cyclotron
Pressure
arm assembly of Coil Winding Machine
Coil
winding in progress
