High power microwave beam-splitter Tatiana Yugay1,2, Thierry Dubroca2, Eden Steven2, Stephen Hill2,3 1. Simmons College 2. National High Magnetic Field Laboratory 3. Florida State University Funding: NSF-MRI CHE-1229170, NSF DMR-1157490, State of Florida

Introduction

Component Characterization Methods

Dynamic nuclear polarization is the process of irradiating a sample with microwaves to increase its nuclear resonance lines’ intensity. A quasi-optical setup is used to guide microwaves from a 395 GHz gyrotron source to the sample. Transmission losses of the quasi-optical components were evaluated. Additionally, a beam-splitter was designed and fabricated to simultaneously run two dynamic nuclear polarization experiments in parallel.

Beam-splitter Fabrication Methods

Left: an airbrush was used to spray solution of polymer and silver particles onto various substrates such as polyethylene (middle) and quartz (right).

Beam-splitter Characterization Results

Conclusion

Out of the six beam-splitters created by two different methods (spray-coating and evaporation) on three different substrates, only the beam-splitter created by evaporating a thin layer of silver onto a 1 mm thick quartz was able to sustain microwave beam powers up to 50 watt. There are therefore four requirements to making a successful high power microwave beam-splitter:

Component Characterization Results

• 3D horn:

compared transmission with and without the horn

• Cu horn: compared transmission with and without horn

• Shutter: compared transmission with open and without shutter

• Back-to-back horn: compared transmission at entrance and exit

• Mirrors: Measured reflection

• Grid: Measured transmission from 0° to 90° rotation.

Left: beam-splitter, made with silver sprayed onto film, melted at 3 watts of microwave power from a 395 GHz gyrotron. Middle: 150 μm thick quartz with evaporated silver damaged by a 20 watt microwave beam. Right: no observable damages were made to a 1 mm thick quartz with evaporated silver, up to the maximum source power of 50 watt.

Optical Component Transmission

3D Horn 33%

Cu Horn 38%

Open Shutter 99%

Back-to-back Horn 92%

Sample Low Power High Power Polyethylene film ✔ ✔

Quartz ✔ ✔

Polyethylene + spray-coated silver ✔ ✗

Polyethylene + deposited silver ✔ ✗

150 μm quartz + spray-coated silver ✔ ✗

150 μm quartz + deposited silver ✔ ✗

1 mm quartz + deposited silver (20 nm) ✔ ✔

395 GHz gyrotron

600 MHz NMR

magnet

References: 1. Overhauser A., Phys. Rev. 92, 2 (1953); 2. Griffin R. et al., PCCP 12, 5737 (2010); 3. Ung B. et al., Optics Express. 20, 5 (2012).

COPPER HORN 3D HORN PYROMETER BEAM

BACK-TO-BACK HORN GRID

SHUTTER

FLAT MIRROR

CURVED MIRROR

BEAM SPLITTER

POLARIZER #1

GYROTRON

EXPERIMENT 1

EXPE

RIM

ENT

2

• Gyrotron: 395

GHz beam source • Polarizer #1:

filters out beam of wrong polarization

• Beam-splitter: splits beam in two

• Curved Mirror: converges and propagates beam

• Flat Mirror: changes beam direction

• Shutter: on/off beam switch

• Back-to-back Horn: Gaussian beam filter

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140 160

Mea

sure

d Tr

ansm

issi

on (m

W)

Distance (mm)

0

50

100

150

200

250

0 20 40 60 80 100

Mea

sure

d Tr

ansm

issi

on (m

W)

Angle Rotated (Degrees)

MeasuredMalus Law

Where beam diameter = 3D horn diameter

100% transmission

Transmission plot of microwave power as a function of distance between source and 3D horn (blue dots). Linear regression model (solid black).

Transmission plot of microwave power as a function of rotation angle of a polarization grid (blue dots). Malus Law model overlayed (solid orange).

Left: a deposition chamber was used to deposit silver particles onto quartz. Middle: high homogeneity silver deposition on quartz substrate. Mounted beam-splitter in quasi-optical bench with airflow cooling (bottom right).

1. A substrate transparent to microwaves, yet thick (i.e.

strong) enough to mechanically handle thermal stress.

2. A metal layer of high thickness homogeneity, ensuring the beams’ shape remains unchanged.

3. Silver layer of high conductivity, ensuring minimal heat absorption (minimizes thermal stress).

4. A cooling source, to reduce thermal stress on the substrate caused by microwave heating of the metal layer.