SUPERCONDUCTING HELMHOLZ COIL
A. Shimizu, K. Enoki and J. Seki
Research Center for Nuclear Physics, Osaka University
Abstract
A superconducting Helmholz coil and a superconducting shielding tube have been built and preliminarily tested. This program,is aimed to study the technical problem for windings of cable,
current lead, cryostat, cooling etc.
1. Superconducting Helmholz CoilThe Helmholz coil is of 14 em bore and of 1.6 x 2.5 mm 2
multifilament NbTi alloy superconductor embedded in a copper
matrix. Designed field strength at Helmholz coil center is 50 kGat 750 A excitation. The specifications of superconducting coil
are listed in table 1 and the coil assembly is shown in fig. 1.
Table 1.

Specification of superconducting Helmholz coil

Bore diameter

140 mmNumber of turns

748/coil

Muximum current

750 A

Current density

187.5 A/mm 2 at 750 A

Coil gap

45 mm as Helmholz coiladjustable between 40 mrn and 60

Field strength at center
Stored energyn;L:t

50 kG at 750 A
74 kJ at 750 A

The cable was wound up tightly to an FRP bobbin with radial
grooves for cooling channel. After winding, the coil was hardened
The space factor of thewith epoxy resin in a dry furnace.
conductor of the coil finished is approximately 0.88. Two coils
are assembled with a stainless steel cylinder and tightened up byend plates. The gap between the coils are adjusted by stainless
These structures also have cooling channels. Theste~l spacers.
weight of coil assembly is about 75 kg.
Vapor cooled current leads are made of OFHC copper tubes
covered with a stainless steel sheath. The vapor flows insidethese tubes. The current density of the lead is 18 A/mm 2 at 750A.
The cryostat, .66 em in diameter and 100 em in height, ismade of stainless steel vessel, copper radiation shields and LHe
vessel made of SUS304L. It has 8.5 em diameter warm bore coaxialwith the Helmholz coil. The outside and the bottom of the LHe
vessel are shielded by LN 2 cooled copper plates whereas the bore
is by superinsulator from the warm bore of the cryostat. Threelayers of polystyrene is placed on the shield plates to avoid
thermal convection above the vessel. The estimated heat load atLHe temperature is about 4 W with no current.
The magnet was assembled with the gap between coils adjusted
to be 5 em and installed to the cryostat. A series of coils wereconnected to a power supply with the external current leads of
200 mm 2 • The magnet was cooled down with LN 2 of 60 liters. The
resistance of the magnet decreased from 4.4· r1 at room temperature
to 0.8 S1 at 80°K. Then the LN 2 was removed and LHe was introduced-317-

from a dewar through a transfer tube. In about two hours, thecoils became a superconducting state and the LHe starts to stay.
The boiled-off gas was drawn out through current leads and a top
flange of the cryostat. With 40 liters of LHe, the magnet wasfully immersed and another 15 liters was appended.
First the magnet was excited with an excitation rate of100A/5min. Up to 400 A excitation, which corresponds 26 kG at
the center, the axial and radial field distributions on and offaxes were measured at every 100 A by Hall probes calibrated witha conventional magnet. In this run, the current reached to 550 A
and then decreased with a rate of 100A/3min. Refilling with LHe,excitation up to 650 A was made. The field strength at thecenter of the coil was measured at every 100 A by a search coilwith a mirror integrator. Examples of the measured fields areshown in fig. 2. These results are consistent with the calculation. The measured boil-off rate of LHe is about 3.5 ~/H with nocurrent and 8 ~/H with 650 A excitation, respectively·. Thesefigures have a space to be improved to account a gas enthalpy.

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1. Superconducting
Helmholz coil assembly.

Fig~010Fig. 2. Field distribution
measured by Hall probe.

2. Superconducting Shielding Tube
A superconducting tube which shields the external magnetic
field was made. The inner and outer diameter of the tube is
26 mm and 46 mm; respectively·. The tube, made from 10 mm widthand 0.14 mm thick V3 Ga tape sandwiched by soldered OFHC Cu (20~m thick) sheets, was impregn*ted with wood's metal.
The test has indicated that:(1) The shielding tube has woiked well at the external field up
to 9 kG.
(2) Over 9 kG the flux jump has been observed and themagnetic flux at 9 kG has been trapped in the superconducting
tube.
(3) The trapped flux h4s not disappeared unless the temP7rature of the tube has risel).up to the super-to-normal transit1on temperature.

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