A Passively Shielded HTS Magnet for Polarized Neutron ......A Passively Shielded HTS Magnet for...
Transcript of A Passively Shielded HTS Magnet for Polarized Neutron ......A Passively Shielded HTS Magnet for...
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A Passively Shielded HTS Magnet for Polarized Neutron Scattering
Taotao. Huanga, A. Feoktystov b, D. Pooke a and V. Chamritski a a HTS-110 , New Zealand
bJülich Centre for Neutron Science (JCNS) at Heinz Maier-Leibnitz Zentrum (MLZ), Germany
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Outline
• Introduction
• LTS and HTS magnets for neutron scattering
• Requirements and specifications
• Design and analysis
• Test results
• Summary
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What is HTS-110
HTS-110 is a New Zealand company specialising in the design and manufacture of High Temperature superconducting (HTS) magnets.
Established in April 2004 building on 20 years of HTS R&D in government research labs.
Owned by Scott Technology, a listed New Zealand company.
HTS-110 New Zealand
Wellington
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HTS-110 products
HTS-110
product
Solenoid magnet
Coils and current leads
Custom magnet
Vector magnet
Split-pair magnet
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Applications
Neutron Scattering & Beam-line Magnets
Materials Analysis NMR & MRI
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HTS magnet production in HTS-110
Wire test
Coil winding
HTS current leads
Coil impregnation
Coil LN2 Test
Coil pack assembly
Magnet assembly and integration
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Present LTS magnets for neutron scattering
LTS Magnet technology
Wire: NbTi or Nb3Sn
Split pair geometry
Horizontal or vertical field configuration
Symmetric or asymmetric mode (for polarised neutrons)
Compatible with VTI
Active shielding to reduce magnetic fringe fields
Coil support with Aluminium rings, or “wedge” pillars
Cooling: LHe, Recondensing and Cryogen-free
NbTi Wire in channel Nb3Sn wire
Vertical configuration Horizontal configuration
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HTS vs LTS
Advantages
High Tc ( Top>10 K, HTS indispensable)
Ultra-high field (B > 25 T @ 4.2K, HTS indispensable )
Setbacks
In-field anisotropy
Still expensive
1G BSCCO tape 2G YBCO tape
Plot from: https://nationalmaglab.org/images/magnet_development/asc/plots/
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HTS offers benefits to magnets
What High Tc means for magnet designer
Simple cryogenics Very stable, hardly quench Stiff suspension Low power cooling
What HTS technology offers to neutron scattering sample environments Cryogen free Compact low fringe field Fast ramping Fast cooldown Mobile Any field orientation RT bore compatible with commercial sample
cryostats RT aperture with no material in neutron
beams to cause scattering background Symmetric split-pair possible for polarization
analysis
What HTS Magnets means for Users
Easy to use Flexible Combined Saving time, saving space
and saving money
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Requirements for polarized neutron scattering
One of the key issues is to maintain the polarization of a neutron beam on its way through the large magnetic fringe fields
No zero-field nodes along neutron beam path
Very low fringe fields at the positions of polarizer and analyzer
Adiabatic passage across the entire beam cross-section. The adiabaticity parameter given below needs be greater than 20:
𝛼 =𝜔𝐿𝜔𝐵
= 4.63 × 10−4 ×𝜆𝐵
𝑑𝜃Β 𝑑𝑟 > 20
To guarantee a depolarization of less than 1% for neutron of 5 Å, the following equation must be satisfied along the neutron beam path:
𝑑𝜃𝐵 𝑑𝑟
𝐵(𝑟)≤ 2000𝑟𝑎𝑑/𝑇 ⋅ 𝑚
Setup for polarization analysis on a triple axis instrument by Moon et al.(1969)
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Specifications
Description Value Peak Central Field 3 T
Sample volume 25 mm DSV
Vertical sample access Ø80 mm
Horizontal optical access 4 X Ø60 mm
Horizontal opening angle 32°
Fringe field at 0.5 m distance in the parallel mode
10 Gauss at 3 T
Fringe field at 1 m distance in the perpendicular mode
< 1 Gauss
Zero-field nodes Outside the magnet
Field homogeneity in 25 mm DSV 4.2%
Field homogeneity in 15 mm DSV 1.5%
Maximum current 215 A
Ramping time to full field < 6 min
Cool-down time
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HTS magnet design – Concept and Constraints
‘Classic’ HTS-design with split-pair coil-packs and shaped iron poles and yoke. The yoke also functions as a vacuum cryostat.
Two-stage cryocooler with heat extraction from leads minimises coil-temperature rise at high operating currents
Even at high fields well above iron saturation a ferromagnetic yoke can:
• increase peak achievable field magnitude.
• efficiently reduce stray fields.
• minimise perpendicular field effects on coil Ic.
But care must be taken in design to balance and counteract significant on-axis and off-axis mechanical forces.
Schematic of the HTS magnet: cryocooler(1), HTS coil pack (2) and cryostat (3).
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Design and Analysis: Coil support
Repulsive forces make Aluminium rings or wedge pillars redundant
Stiff axial and radial support to allow magnets to be oriented in any directions
Big RT aperture possible
Temperature profile of the radial support
Magnetic force on the HTS coils
Axial supports
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Design and Analysis: Magnetic Fields
The maximum magnetic field on HTS coils is 5.8 T because of a big gap between two coils.
The homogeneity over 15 mm DSV is 1.5% and the homogeneity over 25 mm DSV is 4.2% which are limited by the arrangements of the coils.
Field profile over 15 mm DSV
Field profile over 25 mm DSV
3D model of this magnet
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Design and Analysis: Zero-field nodes
Zero-field nodes are moved further from the HTS coils with iron shielding
Zero-field nodes are outside the magnet cryostat, so guiding fields can be used to avoid neutron depolarization.
Vector plot along the neutron passage
Zero-field node Zero-field node Zero-field node
Positions of zero-field node vs magnetic fields
Position of zero-field node without iron shielding
Position of zero-field node with iron shielding
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Design and Analysis: Adiabatic passage
Contour of |dθB/dr|/B(r) at full-field along the neutron beam path
To guarantee a depolarization of less than 1% for neutron of 5 Å, the following equation must be satisfied along the neutron beam path:
𝑑𝜃𝐵 𝑑𝑟
𝐵(𝑟)≤ 2000𝑟𝑎𝑑/𝑇 ⋅ 𝑚
Contour of |dθB/dr|/B(r) in the radial direction Contour of |dθB/dr|/B(r) in the axial direction
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Design and Analysis: Fringe fields
By exploiting the use of iron in the magnetic circuits, the maximum magnetic field increases by 13% and the fringe fields reduce greatly.
Fringe fields with passive shielding B0=3.0 T Fringe fields with passive shielding B0=2.6 T
10 Gauss
1 Gauss 1 Gauss
10 Gauss
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Test Results: Summary
Description Test value Peak Central Field 3.0 T
Temperature at the 1st stage 30 K
Temperature at the 2nd stage 7 K
Temperature at the HTS coil pack A 12 K
Temperature at the HTS coil pack B 12 K
Fringe field at 0.5 m distance in the parallel mode
9 Gauss
Fringe field at 1 m distance in the perpendicular mode
0.8 Gauss
Zero-field nodes Outside the magnet
Field homogeneity in 25 mm DSV 4.1%
Field homogeneity in 15 mm DSV 1.5%
Maximum current 210 A
Ramping rate to full field 5 min
Cool-down time 18 hours
Mass - magnet and cryocooler 344 kg
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Results: Magnetic fields
The magnet was ramped up to 3.0 T at the centre with an operating current of 210 A.
From the test results, the homogeneity of the magnetic fields over 15 mm DSV is 1.46% and the homogeneity of the magnetic fields over 25 mm DSV is 4.1%
The test results agree well with the calculation results.
Distribution of the magnetic fields at 210 A in the X-axis direction
Distribution of the magnetic fields at 210 A in the Y-axis direction
Distribution of the magnetic fields at 210 A in the Z-axis direction
Magnetization curve
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Results: Zero-field nodes
Positions of zero-field nodes as a function of central field
All the zero-field nodes are outside the magnet. .
Please note the positions of zero-field nodes are sensitive to the polarization of earth magnetic fields.
The positions of zero-field nodes would be further away from the magnet cryostat by changing the polarization of this magnet.
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Results: Zero-field nodes
Fringe fields in the axial direction Fringe fields in the radial direction
From the results the fringe field at 0.5 m from the centre of this magnet is less than 10 Gauss in the Y-axis direction and the fringe field at 1 m from the centre of this magnet is less than 1 Gauss in the X-axis direction.
5 Gauss line is at 0.6m from the centre of this magnet in the Y-axis direction. 5 Gauss line is at 0.4m from the centre of this magnet in the X-axis direction.
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Summary
Recently a passively shielded 3T HTS magnet for polarized neutron scattering experiments was successfully designed, constructed and tested by HTS-110.
This split-pair magnet provides a maximum horizontal magnetic field of 3 tesla while the fringe field is less than 1 mT at 0.5 m from the magnetic centre in the magnet axial direction and the fringe field is less than 0.1 mT at 1 m from the magnetic centre in the magnet radial direction. Furthermore the zero-field nodes are located outside the magnet cryostat easing the control of neutron polarization at entry to the magnet.
This magnet exemplifies that the combined requirements of magnetic fields, sample access, optical access and neutron depolarization for polarized neutron experiments can be satisfied by passively shielded , symmetric split-pair HTS magnets.