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Resonance Control
in SCRF Cavities

Resonance Control in SCRF Cavities

SCRF cavities are designed with thin walls to maximize heat transfer to liquid He bath
The thin walls lack stiffness making the cavities susceptible to mechanical oscillations
Longitudinal oscillations can change the resonance frequency of the cavity
Oscillations can be excited
Deterministically (Lorentz Force)
Non-Deterministically (Microphonics)
Lorentz Force Detuning

Electromagnetic fields in the cavity cause the cavity wall to distort
Distortion changes the resonance frequency of the cavity
Effect proportional to the square of the field strength

CW-mode operation

Peak power increases with the fourth power of accelerating gradient:
Example: for Eacc=35MV/m up to 100-150% extra RF power:

-Over-sized Klystron

- More difficult control of RF (LLRF)

Lorentz Force Detuning

and RF Power Requirements
(for FlatTop~800us)

Microphonics
Noise sources (.e.g. pumps, cryo-system; seismic waves, etc. ) excite cavity vibrations
Tuners
Slow Tuner
Compensates for
Static Detuning Forces

Fast Tuner
Compensates for
Lorentz Force Detuning & Microphonics

Stepper motor changes length of the cavity to bring it to the desired resonance frequency

Techniques:
a) Piezoelectric actuators

b) Magnitostrictive actuators

c) Electromagnetic (superconductive)

Fast Tuner Technologies

PIEZOELECTRIC ACTUATORS

Commercially available from multiple sources
Typically used at room temperature (stroke~30mm for 40mm long Piezostack at RT)
Work at cryogenic temperatures with reduced stroke (6-10% of RT stroke ~ 4-5mm at 4K)
Deliver high forces ~5000N for 10*10mm2 cross-section
Actuator of main choice at many labs for detuning compensation studies

Double Piezo (DESY)

Single Piezo (FNAL)

DETUNING COMPENSATION

Resonant Excitation Method

Advantage: small piezo stroke amplified by mechanical resonance

Excitation electrical pulse on the piezoactuator

(control of in-advance timing and amplitude)

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for more details, please visit
http://ilc-dms.fnal.gov/Workgroups/Cryom...Solyak.ppt

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