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RF shielding and eddy currents in NMR probes.

Mark S Conradi1, Albert P Zens2

  • 1ABQMR, Inc., 2301 Yale Blvd SE, Suite C2, Albuquerque, NM 87106, USA.

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|July 14, 2019
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Summary

Thin metal coatings can effectively shield radio frequency (RF) probe circuits. The key shielding criterion involves the geometric mean of coating thickness and radius, not just the electromagnetic skin depth.

Keywords:
Eddy currentsGradientsInfrared shieldingNMR probesShieldingSkin depthrf heating

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Area of Science:

  • Physics
  • Materials Science
  • Electrical Engineering

Background:

  • Radio frequency (RF) interference can degrade Nuclear Magnetic Resonance (NMR) signal quality.
  • Metal coatings are used in NMR probes for various purposes, including heating and infrared reflectivity.
  • Existing theories often incorrectly assume shielding depends solely on electromagnetic skin depth.

Purpose of the Study:

  • To re-evaluate the criteria for effective RF shielding by thin metal coatings in NMR probe geometries.
  • To provide a more accurate theoretical framework for designing RF shields.
  • To validate the theoretical findings with experimental measurements.

Main Methods:

  • Developed approximate analytical derivations for RF shielding effectiveness.
  • Defined a new shielding criterion based on the geometric mean of coating thickness and relevant radius.
  • Conducted experimental measurements at audio frequencies using relevant coil geometries (saddle and solenoid).
  • Measured the impact of shields on pulsed magnetic field gradients.

Main Results:

  • The classical electromagnetic skin depth is not the sole determinant of shielding effectiveness.
  • A conducting layer can provide effective shielding even if its thickness is less than the skin depth.
  • The geometric mean of coating thickness and radius (r) exceeding the skin depth (δ) is the critical shielding condition.
  • Experimental results at audio frequencies confirmed the theoretical predictions.

Conclusions:

  • Thin metal coatings can be effective RF shields in NMR probes when designed according to the geometric mean criterion.
  • This finding allows for the use of thinner, potentially more cost-effective shielding materials.
  • The study provides a more accurate basis for the design and application of RF shields in sensitive electronic circuits.