A conduction-cooled, 680-mm-long warm bore, 3-T Nb3Sn solenoid for a Cerenkov free electron laser

Abstract

A compact, cryocooler cooled Nb₃Sn superconducting
magnet system for a Cerenkov free electron laser has been designed,
fabricated and tested. The magnet is positioned directly behind the
electron gun of the laser system. The solenoidal field compresses and
guides a tube-shaped 100 A, 500 kV electron beam. A two-stage GM
cryocooler, equipped with a first generation ErNi₅
regenerator, cools the epoxy impregnated solenoid down to the operating
temperature of about 7.5 K. This leaves a conservative operational
margin for the MJR type of Nb₃Sn conductor of about 350% (I
_c) at 3T. Current leads with HTS sections between the first
and second stage of the cryocooler effectively reduce the conductive
heat leak. In the warm bore, 60 mm in diameter and 680 mm long, an
operational magnetic field of 2 T has been achieved during laser
operation without any quench. The field homogeneity is better than 99.5%
over 450 mm axial length. Design details and test results of this magnet
system are presented

Full text

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, NO. 2, JUNE 1999
A conduction-cooled, 680-mm-long warm bore, 3-T Nb3Sn solenoid for a
Cerenkov Free Electron Laser.
W.A.J. Wessel, A. den Ouden, H.J.G. Krooshoop, H.H.J. ten Kate, J. Wieland* and P.J.M. van der Slot'
LOW Temperature Division, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
*Quantum Electronics Division, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
'Nederlands Centrum voor Laser Research B.V., P.O. Box 2662,7500 CR Enschede, The Netherlands
Abstract -
ducting magnet system for a Cerenkov Free Electron Laser has
been designed, fabricated and tested. The magnet is positioned
directly behind the electron gun of the laser system. The sole-
noidal field compresses and guides a tube-shaped 100 A, 500 kV
electron beam. A two-stage G.M. cryocooler, equipped with a
first generation ErNi5 regenerator, cools the epoxy impregnated
solenoid down to the operating temperature of about 7.5 K. This
leaves a conservative operational margin for the MJR type of
Nb3Sn conductor of about 350% (I,) at 3T. Current leads with
HTS sections between the first and second stage of the cryo-
cooler effectively reduce the conductive heat leak. In the warm
bore, 60 mm in diameter and 680 mm long, an operational mag-
netic field of 2 T has been achieved during laser operation with-
out any quench. The field homogeneity is better than 99.5%
over 450 mm axial length. Design details and test results of this
magnet system are presented.
A compact, cryocooler cooled Nb3Sn supercon-
I. INTRODUCTION
For the first time a cryocooler cooled superconducting
magnet system is used for electron beam compression and
guidance of a Cerenkov FEL. A cryocooler cooled system
was chosen because it is easy to use, compact and safe. Its
long-term reliability has been shown in a model MRI magnet
with the same cryocoolers [ 11.
The wavelength of this laser is adjustable between 600 pm
and 6 mm. The calculated output power increases with the
wavelength from 100 kW to several MW. For this type of
Cerenkov FEL an annular electron beam is required, see
Fig. 1. The electron beam is enclosed by a wave-guide, lined
with a dielectric. Due to the dielectric liner, electromagnetic
wave modes exist, for which the phase velocity inside the
wave-guide becomes smaller than the speed of light. Laser
action occurs by the coupling of the electron beam with these
wave modes. A maximum coupling is obtained with a thin
walled annular electron beam, located as close as possible to
the wave-guide without loss of electrons in the wave-guide.
A ring-shaped Lanthanum Hexaboride thermo-ionic elec-
tron emitter produces the electron beam. The surface of this
cathode should be large enough to produce a beam current of
100 A. To obtain a beam diameter of 6-10 mm, a compres-
sion factor of 10 is required. This factor depends on the dis-
Manuscript received September 14, 1998.
This work has been supported, in part, by the Netherlands Technology
Foundation STW.
tance between the cathode and the coil, which should be
about 200 mm. After beam compression the electrons will
follow a spiralled path. The pitch depends on the speed of
the electrons and the magnetic field strength. This means that
the magnetic field has to be proportional to the accelerating
voltage. The wavelength is determined by the dielectric in
the wave-guide and the (tuneable) accelerating voltage. A
more detailed description of this laser system in given in [2].
11. COIL DESIGN AND FABRICATION
A. Magnetic field design
The FEL design aims at the production of 0.6-6 mm waves
with an optical power between 100 kW and several MW re-
spectively. This requires a maximum solenoidal magnetic
field strength of 2.5 T. Because the distance between the
electron beam and the wave-guide is only 0.5 mm, the field
homogeneity over the length of the wave-guide (400 mm)
should be better than 99 %. The field shape in the beam
compression region affects strongly the efficiency of the la-
ser. A dedicated computer program is developed to calculate
the most efficient beam path. The desired field shape and the
basic homogeneity could be obtained by adapting the sole-
noid length and the number of turns of the notch coils at
ends of the solenoid.
The final magnet characteristics are summarised
Table I.
the
in
Metallic waveguide \
Thmannular -
electron beam
\
Dielectric liner
Electron gun
Superconducting magnet
beam campmsron
guiding
beam dump
Fig. 1. Schematic layout of the electron gun, electron beam, wave-guide and
superconducting magnet.
1051-8223/99$10.00 0 1999 IEEE

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TABLE I
MAGNET CHARACTERISTICS
350 %. This current margin can be translated into a tem-
perature margin, which should amount about 4 K under these
conditions.
-
cryostat
Warm bore diameter
Warm bore length
60 mrn
680 mrn
Refrigerator
2Dd stage temperature
1” stage temperature
G.M. type, LH RGD580
8 K @ 0.5 W
35 K @ 40 W
Coil type
Superconductor
Innedouter diameter
Coil length
Number of turns
Selfinductance
Protection
Solenoid with notches
Nb3Sn, MJR type, diameter 0.63 mrn
100/140 mrn
610 mm
11209
2.25 H
Full warm bore, stainless steel
7 pairs anti-parallel Schottky diodes
Field range for FEL
Homogeneity
Field constant
Stored energy
0.5-2.5 T
21.32 mT/A
20 kJ
> 99.5% over 450 mm in the warm bore
B. Conductor characteristics
A Nb3Sn superconductor of the MJR type is used for this
coil. The coil is designed to generate a field of 3 T at a tem-
perature of 8 K. At 3 T central field, the maximum field in
the coil is 3.2 T at a current of 140 A. The quench current
(at 3 T, 8 K) of a short sample, reacted together with the
coil, is 500 A. This means that the current margin amounts
C. Coil fabrication
The coil is fabricated according the “wind-and-react’’
technique. As a consequence heat resistant wire insulation in
the form of a glass fibre braid had to be used. Because this
type of insulation is brittle, an extra layer to layer insulation
of two layers glass fibre tape is applied. The terminals for the
joints to the current leads are positioned in the low field re-
gion at the ends of the coil. Before reaction they are mounted
on a copper strip over a half turn around the coil. The coil is
impregnated with the CIBA-GEIGY MY740-HY906-DY062
epoxy combination. Because of the relatively long coil length
the stainless steel coil former is not removed. To prevent
shear cracking of the impregnated coil from the mandrel
during cool-down and excitation of the magnet, the surface
of the coil former is coated with Boron Nitride [2].
The outer surface of the central part of the impregnated
coil is machined down to 0.5 mm distance from the windings
over a length of 450 mm. Four copper shells, with the con-
nections to the cryocooler soldered to them, are glued with
STYCAST 2850FT to the machine-finished coil. Holes in
the copper shells prevent enclosure of air bubbles. To ensure
perfect contact between the shells and the coil during cool-
down and excitation of the magnet, two layers of stainless
steel wire are wound under tension around the copper shells
~31.
current feedlhrough
I I
GM cryocooler, RGD58
d
Fig. 2. Longitudinal and transverse cross-sections of the magnet system

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111. CRYOSTAT DESIGN AND FABRICATION
A. Cryostat design
The stainless steel 3 16 cryostat (also vacuum chamber) is
designed to be de-mountable to provide easy access to the
inner parts. A cross-section of the magnet is shown in Fig. 2,
the total FEL system is shown in Fig. 3.
The 2-stage G.M. cryocooler (LH RGD580), originally
equipped with a Pb regenerator, operates successfully with
an ErNi5 regenerator. It is positioned in the vertical symme-
try axis of the solenoid. In this set-up the field at the motor of
the cryocooler is less than 10 mT and the cooling surface of
the 2nd stage is only 50 mm away from the coil. The connec-
tive cryostat piece between the coil part and cryocooler part
of the cryostat can be lifted towards the cryocooler during
the assembly. Flexible Litze cables connect the 2"d stage
cooling surface to the copper shells on the coil. Cold welded
Indium seals ensure a low thermal resistance.
The whole cryostat is mounted on an axial guidance rail,
which enables both easy mounting of the dielectric wave-
guide and the possibility to find the optimum distance be-
tween the cathode and the coil.
B. Coil support
The copper radiation shield serves also as radial coil sup-
port. Coil and shield are stacked concentrically on to the
warm bore with glass-fibre reinforced plastic spacers. Be-
cause the space between the electron beam and the wave-
guide is only 0.5 mm the alignment tolerance is about 0.1
mm over the whole length. The axial spacers between the
cryostat and the radiation shield are also made of glass-fibre
reinforced plastic. For the axial support four stainless steel
wires of 1 mm diameter connect the centre of the coil with
the end flanges of the radiation shield.
C. Heat balance
The estimated heat loads on the lSt- and 2nd stages are sum-
marised in Table 11. Note that on the 2nd stage HTS current
leads, type CryoSaverTM, effectively reduce the conductive
heat leak. With a total heat load of 0.3 W on the 2nd stage the
final coil temperature was expected to be around 8 K.
TABLE
I1
CALCULATED HEAT LOADS (IN w) ON THE COOLING SYSTEM
Stage Leads Support Radiation Total
1
16
4
10
30
2"d
0.12
0.18
nil.
0.3
D. Electrical connections and coil protection
. The current leads in the magnet system consist out of four
sections: the vacuum feedthrough, the copper wire between
room temperature and the lst stage, the HTS leads between
lst and 2"d stage and a Nb3Sn tape between the 2nd stage and
the joints at the ends of the coil. Note that also the HTS cur-
rent leads are located close to the coil in the low field region,
ranging from 60 mT at the coil surface to 30 mT at the
1"stage.
Seven pairs of Schottky diodes are connected anti-parallel
to the coil for quench protection. The base plates of these
diodes are mounted on the copper shells.
Fig. 3. The Cerenkov FEL with the magnet placed directly before the big vacuum chamber of the electron gun.

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389
Iv. TEST RESULTS
A. Temperatures
Four sensors are used to monitor the temperatures in the
magnet system, see Fig. 2. The temperatures on the lst and
2nd stage during cool down are shown in Fig. 4. The final
temperature of the 1” stage is 35 K, the 2nd stage cools down
to 7.3 K. The temperature in the middle of the coil close to
the connection to the cryocooler is 7.4 K and at the joint at
the end of the coils 7.6 K. This means that the temperature
margin of the windings is 0.4 K larger than expected. Note
that the gradient over the thermal connections is no more
than 0.3 K.
During a field ramp of 3 mT/s to 2 T, the temperature in-
crease in the middle of the coil is 0.2 K and at the joint
0.4 K. After the ramp these temperatures return to their ini-
tial values. Only the temperature on the 1” stage remains 2 K
higher at the operating current.
1
0.998
3
iz
‘cf 0.996
3
1- 3
z
0.992
a ,
0.994
0.99
-
mas ured
-
250 -250 -150
-50
50 150
Axial position (mm)
Fig. 5. Field profile along the central axis in the warm bore.
needs to be improved. Nevertheless laser output has already
been observed for accelerating voltages between 85 and
150 kV at a maximum field of 1.5 T. A more detailed de-
scription of these first experiments is given in [4].
B. Magneticfield
V. CONCLUSIONS
The field profile in the centre of the warm bore is meas-
ured with a Hall probe. Fig. 5 shows the measured and cal-
culated profile. The field varies over the length of the wave-
guide (400mm) less than 0.5 %, which corresponds to a
maximum variation in the electron beam diameter of 50 pm.
C. First laser output
The first experiments with this Cerenkov FEL have been
performed recently at a maximum operational field of 1.75 T.
The optimal position of the wave-guide and the supercon-
ducting magnet have not been established yet. Also the
alignment of the cathode with respect to the wave-guide still
0 1
0
1 1 I I
I
10
20 30
Tim: (hour)
40
50
60
Fig. 4. Cool down behaviour of the 1st and 2nd stage.
For the first time a conduction cooled Nb3Sn supercon-
ducting magnet system for a Cerenkov Free Electron Laser
has been designed, fabricated and tested. With a two-stage
G.M. cryocooler close to the coil, a very compact design has
been achieved. A description has been given of the magnet
system. The magnet system is completed and operates suc-
cessfully for already more then 6 months. In a non-optimised
configuration first laser output has been observed
ACKNOWLEDGMENT
The authors would like to thank Mr. Takashi Yazawa (To-
shiba Co.) for his generous support and expert advice during
the realisation of this magnet system.
REFERENCES
[l] M.T.G. van der Laan, R.B. Tax, H.H.J. ten Kate, L.J.M. van de Klun-
dert, “A 1 T, 0.33 m bore superconducting magnet operating with cryo-
coolers at 12 K,” IEEE Trans. on Mag., vol. 28, no. 1, pp. 633-636,
Januaryl992.
[2] P.J.M. van der Slot, J. Couperus, W.J. Witteman, A.N. Lebedev, E.G.
Krastelev, et al., “A Cherenkov free electron laser with high peak
power,” Nucl. Instr. and Meth, in Phys. Res. A, 358 (1995) 100
[3] K. Koyanagi, M. Urata, Y. Ohtani, T. Kuriyama, K. Yamamoto, S.
Nakayama, T. Yazawa, et al, “A Cryocooler-cooled 10 T Supercon-
ducting Magnet with 100 mm Room Temperature Bore;’ IEEE Trans.
on Mag., vol. 32, no. 4, pp. 2558-2561, July 1996.
[4] J. Wieland, J. Couperus, P.J.M. van der Slot, W.J. Witteman, “First
lasing of a Cerenkov free-electron laser with annular electron beam,”
submitted to Nucl. Instr. and Meth. in Phys. Res. A