Mechanical Properties of Spoke Cavities

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Mechanical Properties of Spoke Cavities

• Zachary Conway
• Physics
• August 30, 2005

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Collaborators

• ANL
• Ken Shepard, Mike Kelly, Joel Fuerst, Gary
  Zinkann, Sergey Sharamentov, Mark Kedzie
• JLAB
• Jean Delayen
• AES
• John Rathke, Tom Schultheiss
• Energen
• Chad Joshi, Chiu-Ying Tai

b0.5 Triple Spoke Resonator
Energen Magnetostrictive Fast Tuner Actuator
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Outline

• Cavity mechanical properties.
• Mechanical Model
• ANL spoke cavity measurements
• Fast mechanical tuners.

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Spoke Cavity Mechanical Properties

• SC cavities have a small intrinsic bandwidth.
• The loaded bandwidth is application dependent
• cw low beam current operation gt microphonic
  induced noise. Low and Medium b RIA cavities
  have loaded bandwidths ranging from 50 to 150Hz
  and must accommodate microphonic frequency
  variations in the 10s of Hz.
• pulsed operation gt Lorentz Detuning. FNAL 8 GeV
  upgrade low energy cavities have loaded
  bandwidths of 800Hz and Lorentz detuning of
  600Hz.

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Controlling amplitude phase errors in spoke
cavities

• There are currently only two options available
• Overcoupling gt RF power increases as 1/QL
• Fast Mechanical Tuners gt Mechanically Tune the
  cavity

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Cavity Electromagnetic and Mechanical Properties.

• The spoke cavities are a complete electromagnetic
  and mechanical system.
• The electromagnetic system can be described in
  terms of the RF eigenmodes.
• The mechanical system can be described in terms
  of the mechanical eigenmodes.

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Cavity Electromagnetic and Mechanical Properties.

• Changes in the mechanical structure of the cavity
  alter the electromagnetic structure and vice
  versa.

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Lorentz Detuning

• The radiation pressure due the the RF fields
  alters the resonators mechanical structure.
• The modified mechanical structure alters the
  electromagnetic properties.

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Lorentz Transfer Function

• To go from mechanical work on the resonator to RF
  frequency variations involves the mechanical
  eigenmodes.
• Expressing this in terms of the RF and mechanical
  modes gives

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Lorentz Transfer Function
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Lorentz Transfer Function Measurement
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b 0.4 Double Spoke Lorentz Transfer Function
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b 0.5 Triple Spoke Lorentz Transfer Function
KL-6.5 Hz/(MV/m)2
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0.5 TSR Frequency Response to Pulsed RF Fields
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Response of b0.5 TSR to an RF Pulse
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To be done

• I will do more when more RF power becomes
  available.

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Microphonics and CW Operation

• Microphonics refers to all cavity frequency
  deviations resulting from external forces.
• Mechanical Vibrations
• LHe bath pressure

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b 0.4 Double Spoke Microphonics

• Results of first cold test of a production model
  double spoke cavity with an integral stainless
  steel housing holding the liquid helium bath.
  (M.P. Kelly et al, PAC 2003)
• Df/DP -76 Hz/torr
• srms 5.3Hz

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b 0.5 Triple Spoke Cavity

• b0.5 triple spoke cavity designed to minimize
  Df/DP.
• Designed to balance the electric and magnetic
  field contributions to frequency shifts due to
  uniform external pressure.

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b 0.5 Triple Spoke Cavity
measured Df/DP(predicted) -12.4(-8.7) Hz/torr
Df/DP -6.3(-4.7) Hz/torr
Df/DP -2.5(-0.3) Hz/torr
Df/DP -0.5(5.4) Hz/torr
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b 0.5 Triple Spoke Microphonics

• Cavity tested in a realistic accelerator
  environment.
• Df/DP -12.4 Hz/torr
• srms 1.04Hz (0.42 DSR srms 5.3Hz)

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Spoke Cavity Fast Mechanical Tuning
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Piezoelectric Actuated Fast Tuner

• Response time lt1ms.
• Layered piezo-ceramic material electrically
  connected in parallel operating at 20K with a
  resolution of 2nm purchased from Physik
  Instrumente.
• Not designed for high frequency operation.

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b 0.4 Double Spoke Piezo Transfer Function
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b 0.5 Triple Spoke Piezo Transfer Function
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Magnetostrictive Actuated Fast Tuner

• Magnetostrictive actuator designed and built by
  Energen, Inc and slated to be tested at ANL.
• Response time lt6ms.
• Magnetostrictive rod coaxial with an external
  solenoid operating at 4K.

4.6
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b 0.5 TSR Magnetostrictive Tuner Transfer
Function
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Future Work

• Measure microphonic behavior of b 0.5 TSR now
  that the support ribs have been modified.
• Pulse a spoke cavity and measure the frequency
  ringing induced so see if it agrees with the
  predicted response.
• Use fast mechanical tuners to phase lock an
  intermediate beta cavity without over coupling.