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

Outline

• Introduction

• LTS and HTS magnets for neutron scattering

• Requirements and specifications

• Design and analysis

• Test results

• Summary

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

HTS-110 products

HTS-110

product

Solenoid magnet

Coils and current leads

Custom magnet

Vector magnet

Split-pair magnet

Applications

Neutron Scattering & Beam-line Magnets

Materials Analysis NMR & MRI

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

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

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/

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

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)

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

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).

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

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

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

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

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

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

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

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.

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.

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.