FAST FERRITE PHASE SHIFTER FOR THE SNS LINAC

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FAST FERRITE PHASE SHIFTER FOR THE SNS LINAC

• Yoon Kang
• JUNE 19, 2000

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PHASE SHIFTER SPECIFIACTION FOR SNS LINAC

• Frequency 805 MHz ? 1 MHz
• Insertion Loss 0.1 dB max.
• Input VSWR 1.2 1 max.
• Phase shift 0 60 degrees
• Response lt 100 ?sec.
• Peak power 500 kW
• Duty cycle 7

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PHASE SHIFTER TESTING

• 1st Phase
• Demonstrate proof of principle and feasibility of
  the phase shifter
• Show phase shifting range and slew rate
• April - May
• 2nd Phase
• Perform detailed engineering design for high
  power capability
• Build audio amplifier-type power supply
• Perform high power testing
• June - August

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PHASE SHIFTER DESIGN CRITERIA

• High power capability
• Cooling is needed to protect the YIG ferrite
• Impedance matching is needed at the input and the
  output
• Fast response
• Low inductance needed in biasing magnet
• Large phase shifting range
• Impedance matching around the YIG is important
• Low insertion loss
• Operate below spin-wave resonance in YIG ferrite
  with low 4?Ms
• In waveguide, place ferrite slabs where waves are
  circularly polarized with orthogonal magnetic
  bias field

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CRITICAL PARTS FOR PROTOTYPING

• Low RF loss YIG material
• Delivery is 12 weeks
• Use available YIG bars four 0.5 thick with
  higher 4?Ms 250 gauss
• Low RF loss dielectric material (er 14)
• Delivery is 12 weeks
• Use low quality alumina ceramic slabs 0.5 thick
  (er 9.8)
• Magnet yoke material
• Use high permeability ferrite by gluing many
  small I-cores together to form C shaped magnets
• Broadband high power amplifier
• Use available high power pulse generator 1kV
  3A maximum
• Waveguide structure
• Use 2 reduced height waveguide with clamps

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MUST USE ORTHOGONAL BIASED FERRITE

• Parallel biased ferrites have extreme RF power
  loss at f gt 100 MHz.
• Orthogonal biased ferrites can be used at much
  higher frequencies with lower RF loss.
• Orthogonal biased ferrite
• Need to operate with circularly polarized RF
  magnetic field orthogonal to the bias magnetic
  field
• Generally exhibits non-reciprocal RF property due
  to non-reciprocal tensor permeability

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CIRCULARLY POLARIZED MAGNETIC FIELDS OF TE01 MODE
IN WAVEGUIDE

• Magnetic fields look like two opposite circularly
  polarized waves at certain locations inside the
  waveguide
• External bias magnetic fields can be applied
  orthogonal to the plane of RF magnetic field
• The phase shifter is non-reciprocal

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COMPLEX PERMEABILITY OF THE YIG FERRITE4?Ms
175 Gauss, ?Hk 4 Oe, ?n ?o/?
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LOSS-TANGENT OF YIG MATERIAL4?Ms 175 Gauss,
?Hk 4 Oe
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PHASE SHIFTING RESPONSE CONSIDERATION

• Magnetic field bias winding with an inductance Ls
  1 ?H and a resistance Rs 0.1 ? will have 3
  dB frequency, f 16 kHz
• Magnet flux penetration through Al waveguide
  wall One skin depth is 0.026 at f (0.0845/?)2
  16 kHz
• RF skin depth is only 3 ?m in Al

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CROSS SECTION OF WAVEGUIDE PHASE SHIFTER
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YIG OPERATING POINT SELECTION

• Use ? ? varies w.r.t. external magnetic bias
  Ho and at below resonance ? lt 1, but ?- ? 1
• Higher ?n requires higher magnetic field bias
• At resonance, ?n 1 gt Ho 23 kA/m operate
  below spin-wave resonance, ?n lt 1
• Avoid ?n 1 ? lt 0 and higher magnetic loss

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DESIGN CONSIDERATION FOR INSIDE OF THE WAVEGUIDE

• Impedance matching is crucial inside the
  waveguide for wider phase shifting range, low
  insertion loss, and high power handling
  capability
• Dielectric loading between YIG bars
• Helps impedance matching for RF
• Enhances phase shifting capability d?/dl
• Reduces the magnetic path length through the
  external biasing magnet for lower inductance

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POWER SUPPLY CONSIDERATION

• Using a YIG ferrite with saturation magnetization
  4?Ms 175 gauss with gyromagnetic ratio ? 2.8
  MHz/Oe ( 35 kHz/A/m) needs bias magnetic field
  Ho 18 kA/m for a range of ferrite
  permeability ?r 0.8 0.7 below resonance
  operation
• Spin-wave resonance at 23 kA/m for 805 MHz
• Effective magnetic path length of the magnetic
  circuit is 0.15 m required current is up to
  3 kA in the YIG if external magnet has similar
  permeability to that of YIG.
• External biasing field current may be lower with
  higher permeability external magnet yoke.

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BIAS FIELD MAGNET CONSIDERATION

• Scalar permeabilities of the materials used in
  the phase shifter
• Magnet ferrite (operating frequency up to 1MHz)
  ?r 2000
• YIG ?r 40
• Aluminum ?r 1
• Inductance - effective permeability of magnetic
  circuit with 2 reduced height waveguide
• with l1 0.12 m, l2 0.11 m, l3 0.0005 m,
  ?r,e 71
• Single turn inductance of the magnet winding Ls
  0.95 ?H/turn
• Voltage requirement
• For 300A _at_ 10uSec 30V

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2 HEIGHT WAVEGUIDE LOADED WITH YIG AND ALUMINA
ALONG WITH TWO COAX-to-WG TRANSITIONS AND
MATCHING POSTS
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PHASE SHIFTER MEASUREMENT SETUP
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WAVEGUIDE WITH FERRITE AND DIELECTRIC
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BIAS FIELD FERRITE MAGNET (upper)
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MEASUREMENT SETUP
1
Oscope
2
3
4
Pulse Trans
Pulse Power Supply
Magnetic Bias
Phase Shifter
Power Splitter
Phase Detector
Signal Source
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MEASURED INSERTION LOSS AND RETURN LOSS Ib 0A
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INSERTION LOSS PHASE SHIFT vs. BIAS CURRENT
(forward propagation)
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INSERTION LOSS PHASE SHIFT vs. BIAS CURRENT
(reverse propagation)
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RETURN LOSS vs. BIAS CURRENT
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PHASE SHIFTER RESPONSE

• 1. Phase detector output
• 2. Pulse transformer output voltage
• 3. Pulse transformer input voltage
• 4. Phase shifter bias current (1V100A)

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PHASE SHIFTER RESPONSE

• 1. Phase detector output
• 2. Pulse transformer output voltage
• 3. Pulse transformer input voltage
• 4. Phase shifter bias current (1V100A)

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CROSS SECTION OF WAVEGUIDE PHASE SHIFTERwith air
gap in the yoke
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PHASE SHIFTER RESPONSEwith air gap in the magnet
yoke (current limited)

• 1. Phase detector output
• 2. Pulse transformer output voltage
• 3. Pulse transformer input voltage
• 4. Phase shifter bias current (1V100A)

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SUMMARY

• Phase shifter functions as expected
• 2 reduced height waveguide is 9.75 wide
• 6section achieves phase shift gt 40 deg _at_ 805 MHz
• Power supply capacity 30V _at_ 300A for lt 50 uSec
  response
• New power supply design is needed for faster
  response
• Large bandwidth current source power supply is
  needed
• Phase shift vs. bias current is not linear need
  to optimize the external magnet shape for better
  performance