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Portable Chamber for Spin-Stabilized Devices
Interest in portable spin-stabilized atomic devices arises due to increased demand for reliable frequency standards, gyroscopes for inertial navigation, and magnetometers for geophysics and geolocation application and for biomedical studies. Magnetic field homogeneity is critical for performance of atomic clocks and inertia navigation devices; similarly correct interpretation of NMR spectrum in non-uniform magnetic field is crucial in spin-stabilized atomic magnetometers. We  introduce a Chamber Transfer Function (CTF) which links applied magnetic field, the chamber geometry, signal detection approach and resulting spectral power distribution of the Larmor frequency. CTF of our ferrofluidic chamber is calculated and compared with traditional Helmholtz-coil designs. Current enclosure size is 12x8x8 mm3 and working area is hemisphere of 2 mm diameter. The obtained results demonstrate that designed chip-size enclosure has field homogeneity better 10-6, the field strength on two order magnitude higher and ten times less footprint than corresponding Helmholtz coils based designs. 
 
How it works: Spin-stabilized devices utilize Nuclear Magnetic Resonance (NMR) 

 Phase-coherent oscillation

-Phase-coherent oscillation is achieved by field B1.

-Larmor frequency (𝜔𝐿) is proportional to B0 and rotation of the chamber Ω :

𝜔𝐿=𝛾𝐵0−Ω

-The frequency is detected as a modulation of the probe light intensity.  

 

Larmor frequency is the intrinsic property of corresponding atoms. Frequency detection can be utilizes in different instruments such as:

Atomic Clocks (𝜔𝐿)

Inertial Navigation Systems (Ω)

Magnetic Detector (𝐵0)

Medical MRI (𝛾)

 Accuracy of any instrument is determined by only our ability to control its environment: mostly magnetic field and temperature

Portable Environmental Chamber

Portable Environmental Chamber- Chamber sizes: 12 x 8 x 8 mm3;
 
- NMR working chamber: Hemisphere 2mm diameter;
 
- Magnetic field strength: 40 G;
 
- Field homogeneity: better 10-6;
 
- Operational temperature: -60 to 140 oC (Flash temperature 160oC and boiling temperature >205 oC of the working media);
 
- External electromagnetic field isolation: TBD


 

NMR working chamber

NMR working chamber

Side and top views of the working chamber:

(a) reflective spots for laser beam;

(b) optical fiber sealed in optical ports;

(c) conductor for magnetic excitation of NMR response;

(d) µ-metal layer.

Hemispherical NMR working chamber manufactured at UCI:

  

 

 
Detected by NMR field is ... Average field? Maximum field? Something else?

 
Signal detection schematics

- Probe light excites NMR oscillation,

- Individual oscillations are determined by local magnetic field

- Detected signal has the power spectral distribution

- Described by Weighted Magnetic Field (WMF) of the chamber.

 
 
Weighted Magnetic Field Distribution (WMF): Definition

Definition of weighted magnetic field distribution

- Distribution of magnetic field B(r) in the volume, V, is mapped by function β(B│B0)- Volume fraction of ΔV(=∑ΔVi ) is linked to magnetic field within a given range.Weighted Magnetic Field (WMF) distribution  links volume fraction, 𝑑𝑉, and magnetic field variation 𝑑𝐵 as β(B│B0 )dB=dV
 

  B0 is the referencing field: What is the reference field? How to define this field?

 

Homogenous Magnetic field
Weighted field distribution of homogenous magnetic field

A uniform magnetic field has value  across the chamber

β(B│B0 )=V𝛿(B-B0 );  β(B* )=V𝛿(B*-1)

𝑝(𝜔)=𝜔2/(2𝛾𝜇0𝜔0 ) 𝛿(𝜔𝜔0 −1)=𝜔0/(2𝛾𝜇0 ) 𝜔2𝛿(𝜔1)

        - Valid for a chamber having arbitrary shape

        - It can be used as a reference for small inhomogeneity or small chamber

Helmholtz Coil

Helmholtz coils

Shown are two coils R=20mm separated 20mm

Field homogeneity inside sphere r=1mm is 10-5.


Current Design

Weighted field distribution of Helmholtz coils and CoMMET's designCurrent design uses a single coil radius R=3.5mm and length 3mm on 1.5 mm from the bottom.
 
Field homogeneity is compared with the Helmholtz coils above (two coils R=20mm separated 20mm). Lines represents WMF distribution for two systems - Helmholtz coils and CoMMET's design:
- Brown dashed line represents weighted magnetic field distribution for Helmholtz coils, and
- Black solid lie represents current design.
 
Conclusion:
- Field uniformity in hemisphere r=1mm is better 10-5;
- 35 times higher field strength with same current as in Helmholtz coils;
- No heating issues.