em field simulations for - indico.slac.stanford.edu

EM FIELD SIMULATIONS FOR

Chiara Salemi, MITDM Radio + ABRACADABRA First Collaboration Meeting24 Jan 2020

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OUTLINE

WHY— the need for field simulations

WHAT— an assortment of calculations

HOW— EM simulation software

2

WHY

the need for field simulations

3

UNDERSTANDING OUR SENSITIVITY

4

DAQ

Axion Effective Current Pickup

SQUID Magnetometer

UNDERSTANDING OUR SENSITIVITY

5

DAQ

Axion Effective Current Pickup

SQUID Magnetometer

SQUID response

UNDERSTANDING OUR SENSITIVITY

6

DAQ

Axion Effective Current Pickup

SQUID Magnetometer

Axion interactions in magnetic field

UNDERSTANDING OUR SENSITIVITY

7

DAQ

Axion Effective Current Pickup

SQUID Magnetometer

Coupling between axion current and pickup circuit

UNDERSTANDING OUR SENSITIVITY

8

DAQ

Axion Effective Current Pickup

SQUID Magnetometer

Impedance in pickup circuit

WHAT

an assortment of calculations

9

WHAT CAN WE MEASURE?

C. Salemi 10

axion effective current in toroid

induced real current in pickup

norm

(cur

rent

)

WHAT CAN WE MEASURE?

§ Coupling between axion effective current and pickup

§ Self inductance of pickup

§ Parasitic impedance in wiring

§ …

C. Salemi 11

axion effective current in toroid

induced real current in pickup

norm

(cur

rent

)

BASICS OF HOW TO CALCULATE

• Mutual inductance (current ratio)

• Enforce current through magnet volume (proportional to B field)

• Measure current in pickup

• Self inductance

• Enforce current through pickup

• Use

12

L = 2

ZWm

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WIRE LOOP(ABRA RUN 1)

13

• Current ratio ~ 0.11

• Self inductance ~ 100 nH

• Calculable analytically

• Confirms COMSOL calculation

B z

pickup wire

superconducting shield

magnet toroid

CYLINDER(ABRA RUN 2)

14

• Coupling ~7x higher than with wire loop

• Current ratio ~ 0.74

• Reduces inductance in pickup circuit by ~5x

• Self inductance ~ 20 nH

B z

pickup cylinder

SHEATH(FUTURE)

15

• Optimal coupling between axion effective current and pickup (current ratio = 1)

• 10x better than wire loop!

• Difficult engineering

• Construction

• Access

B z

pickup sheath

HOW

EM simulation software

16

HOW DO FINITE ELEMENT SOLVERS WORK?

COMSOL MULTIPHYSICS

• Commercial finite element modeling software

• $$$, but UNC has licenses

• Helpful support team

• Can link together various physics modules (EM, mechanical, CAD imports, …)

• Include vibration?

• Automatic meshing

• Allows precise field modeling for complex geometries, but…

• Not good at large aspect ratios/large scale differences: wire windings are hard

• Complex 3D systems are hard/slow

18

axion effective current in ABRACADABRA-10cm toroid

THERE ARE MANY OTHER OPTIONS

• ANSYS HFSS• Designed for high frequency antenna design

• Only EM

• Kassiopeia• Designed for particle tracking in EM fields for Project 8 experiment

• Doesn’t really do AC

• Boundary element method (BEM)

• Opera• Primary applications are magnets, motors, etc.

• EM and electromechanical in 2D and 3D

• ANSYS Maxwell, FastHenry, …

19

(that I don’t know as much about)

probably not

maybe!

SUMMARY

20

To determine our sensitivity we

must fully understand the effective circuit from axions to

DAQ

DAQ

SUMMARY

21

To determine our sensitivity we

must fully understand the effective circuit from axions to

DAQ

This can be achieved with finite element

modeling of the system’s EM fields

SUMMARY

22

To determine our sensitivity we

must fully understand the effective circuit from axions to

DAQ

This can be achieved with finite element

modeling of the system’s EM fields

We have modeled and tested the ABRA-10cm setups and

demonstrated the usefulness of this

method

SUMMARY

23

To determine our sensitivity we

must fully understand the effective circuit from axions to

DAQ

This can be achieved with finite element

modeling of the system’s EM fields

We have modeled and tested the ABRA-10cm setups and

demonstrated the usefulness of this

method

Simulations will continue to be necessary to optimize the sensitivity of

future generations

BACKUP

24

ERROR AND CHECKS

25

• Is this even right?

• Compare to analytic calculations where possible (e.g. twisted pair, simple wire pickup loop, …)

• How precise is COMSOL anyway?

• Vary parameters to see effects (e.g. conductivity, geometry, boundary conditions, …)

Wire Diameter (mm)

COMSOL inductance,

surface current (nH)

Analytical inductance,

surface current (nH)

COMSOL inductance,

volume current (nH)

Analytical inductance,

volume current (nH)

0.5 112.5 112.8 118.8 119.1

1 95.0 95.3 101.3 101.6

2 77.5 77.8 83.8 84.1

Example: Simple pickup loop geometry self inductance

PARASITIC IMPEDANCE

§ Must particularly consider the parasitic inductance in the system

§ Twisted pair wire between pickup and SQUID input a major contributor

C. Salemi 26

superconducting shielding

NbTi wires

PARASITIC IMPEDANCE

• For a realistic configuration, simulations give 0.48 µH/m and 29 pF/m

• Wire diameter: 0.075 mm

• Insulation thickness: 0.08 mm

• Shield inner radius: 0.5 mm

• Conductivity: varied [107,1025]

27