Cavity Tuners. Outline. Tuner overview Concepts and examples. Focus: Fast piezo tuners for ERLs. Advanced piezo tuning.

Transcription

1 Cavity Tuners Oliver Kugeler Outline Tuner overview Concepts and examples Focus: Fast piezo tuners for ERLs Advanced piezo tuning ERL workshop 2009, Cornell

2 Objectives for tuners Tune cavity resonance to operating frequency after cool-down De-tune cavity on purpose to bypass operation Find resonance after RF-field loss Compensate slow frequency drift Compensate Lorentz force detuning (in pulsed machines) Compensate microphonics (in CW-machines) Design issues long lifetime tuner resolution compact low hysteresis / backlash limit range to avoid plastic deformation of the cavity limit cross-talk to neighboring cavities limit cryo-heatload provide serviceability 2

3 Tuning concepts Tuner Type Comments Quasi-static, pre tuning only Very slow Niobium plunger Three stub tuner Pneumatic bellows Penetration into RF space Also coupling changed Few moving parts 3

4 Slow tuning niobium plunger used at Spiral-2 High RRR 3 cm niobium plunger into the cavity rf space 1100 Hz/mm tuning sensitivity; 90 khz tuning window 11% additional rf loss at 6.5 MV/m; mostly on SS flange and bellows 4

5 Pneumatic Tuners used at ATLAS flexible bellow moved by 0...1atm ghe tuning range ~ 100 khz pro: low parts count 5

6 Tuning concepts Tuner Type Comments Quasi-static, pre tuning only Very slow Niobium plunger Three stub tuner Pneumatic bellows Penetration into RF space Also coupling changed Few moving parts < 1 Hz, compensation of Helium pressure fluctuations Slow Warm motor + lever + tuning plate Large size, good serviceability Cold motor driven lever Small size 6

7 ELBE tuner Dual spindle lever system Motor outside the vacuum Good serviceability 7

8 Slow Tuner at Triumf External motor drive Brushless servomotor Twin opposing angular contact bearing block Internal preloaded precision ball nut Double preloaded linear ball bearing Precision 2mm lead ball screw Tuner actuator shaft Precision servo-motor and ball screw on top of cryomodule Actuator extends (through bellows) to a lever mechanism to the tuning plate Relatively fast response time, up to 30 Hz Tuner sensitivity 0.04 Hz/step; corresponds to 5nm/step Tuner accurately tracks induced helium pressure fluctuations (lower right) Pressure (Torr) 900 Pressure Tuner Position 0 3 Time (min) 6 8

9 Tuning concepts Tuner Type Comments Quasi-static, pre tuning only Very slow Niobium plunger Three stub tuner Pneumatic bellows Penetration into RF space Also coupling changed Few moving parts < 1 Hz, compensation of Helium pressure fluctuations Slow Warm motor + lever + tuning plate Large size, good serviceability Cold motor driven lever Small size Microphonics compensation, LF compensation Fast None / overcoupling Variable reactance Mechanical with piezo Simple, high RF power needs Low cost, limited applicability Development required 9

10 Variable reactance (VCX) tuner at Argonne Based on a set of 10 parallel 77 K PIN diodes Coupled directly to the cavity fields through an inductive loop mounted on a cavity coupling port Diodes are switched on and off; switching the cavity between two frequency states in order to adjust cavity phase Reliable, inexpensive Only developed for f<150 MHz; limited switching power; a fast mechanical tuner is desired for future ATLAS upgrades Brian Rusnak et al 10

11 Fast electrical tuning - VCX tuner at Argonne Brian Rusnak et al 11

12 Demanding requirements for ERL machines Use high-energy, high-q 0 cavities High amplitude and phase stability ( A/A = , φ=0.02 ) Minimizing of microphonics even more important than in other linacs due to energy recovery process Tuning critical because of desire to increase external Q Best solution so far: Cold motor tuner with piezo 12

13 Coaxial ball screw tuner and slide jack tuner (KEK) 13

14 Renascence tuner (JLAB) 14

15 Tuner Planned for MSU Re-accelerator Niobium push-pull tuning plate with convolutions and cuts Based on a warm linear stepper motor plus piezo electric stack Force applied through to a tuning rod to a tuning plate on the bottom of the cavity ~20 khz tuning range (+/- 25 mm) using stepper motor 300 Hz full range with piezo 15

16 Saraf tuner developed by ACCEL Soreq 16

17 Fast tuners Saclay I tuner lever system piezo spindle flexure tank Modified piezo holder frame: Higher wall thickness 17

18 Fast tuners Saclay II tuner eccentric spindle piezo flexure tank 18

19 Fast tuners - Blade tuner Latest version tested at HoBiCaT use for booster/gun where RE gradient is vital 2 piezos on opposite sites compensate vertical oscillations 1st DESY prototype (Kaiser, Peters) ILC version (Pagani) modified version (Peters, Pagani) 19

20 Saclay tuners amplitude (Hz) 400 Freq Shift (Hz) Saclay I amplitude (Hz) amplitude (Hz) 0 Piezo step response, meas. by Tom Powers phase ( ) phase ( ) Time (s) 80 Excitation amplitude 22.5 Hz 19 Hz Maximum cavity response 340 Hz 150 Hz phase ( ) Saclay II Cavity Frequency Shift (Hz) Piezo Drive Voltage (V) Cavity Frequency Shift (Hz) 80 Piezo Drive Voltage (V) frequency (Hz) 0 frequency (Hz) Time (s) Saclay I Saclay II Freq Shift (Hz) PZT Drive (V) PZT Drive (V)

21 amplitude (Hz) Transfer functions µs BW Saclay I Group delay: -60 Saclay II Double resonance µs phase ( ) amplitude (Hz) BW d ϕ amplitude (Hz) phase ( ) 5 τ -80 = amplitude (Hz) phase ( ) d ω frequency (Hz) frequency (Hz) cavity is blind to higher frequency microphonics Saclay I Saclay II try to increase lowest resonance Group delay at low frequencies 361 µs (290 µs for starting position) 150 µs Lowest resonance at 40 Hz single double structure make tuner stiffer, increase wall thickness phase ( ) 21

22 Stiffening the cavity difference in transfer function after correction almost no difference 22

23 Fast piezo tuner comparison Design Saclay I Saclay II Blade Tuner Motor tuning range 750 khz 500 khz 550 khz Motor hysteresis satisfying backlash problems at low amplitudes Piezo tuning range 840 Hz 1420 Hz 1400 Hz Group delay 360 µss 150 µss 650 µs Stiffness lower higher Lowest Lowest resonance 40 Hz 40 Hz 35 Hz 23

24 Microphonics compensation with Saclay I tuner detuning (Hz) time (s) Microphonics measurements done at HoBiCaT Is this the limit? What is the piezo resolution? ΣFFT w/o feedback with feedback detuning (Hz) with feedback and feed-forward compensation # counts frequency (Hz)) σ f = 2.52 Hz σ f = 0.89 Hz σ f = 0.36 Hz detuning (Hz)) 24

25 Piezo hysteresis stroke vs. voltage K Strain 2 destructive voltage limit increased stroke reduction = 1 : 9 4 K 3 remanent polarisation coercitive voltage butterfly negative nascent = cycle polarisation, curve 0voltage, stroke remanent positive = stroke 0polarisation polarisation remaining stroke = Voltage coercitive voltage > operating voltage bipolar operation possible double stroke 25

26 Piezo hysteresis for blade tuner 3000 both piezos frequenc cy change (Hz) maximum coercitive voltage maximum frequency remanence piezo 1 & 2 1µm cavity strain piezo voltage (V) used semi-bipolar voltage supply 26

27 Piezo relaxation frequen ncy change ( Hz) piezo relaxation piezo - first cycle piezo - second cycle viscoelastic discrepancy stable behavior second ramp piezo voltage (V) nascent curve 27

28 Hysteresis modeling Assume that piezo behavior is fully deterministic Bouc Wen model: X ( t) = µ 2 u( t) + t Y ( V t s ) du( s) Stroke Voltage (Hooke s law) Strain history Fast tuning algorithm needs to incorporate previous history of piezo voltages. How do we gain access to the history function Y(V(s,t))? 28

29 Piezo dynamic hysteresis behavior amplitude frequency amplitude resolved transfer function dynamic piezo response 29

30 High voltage vs. low voltage piezos HV vs. Multilayer piezo: Sparking voltage considerations Paschen s law [F. Paschen, Wied. Ann. 37, 69 (1889)] Vs = Bpd Apd ln( ) ln(1/ γ ) Literature: Helium: V s, min = 156 V (pd) min = 4.0 torr/cm = 0.53 mbar/mm V/V s,min normalized Paschen curve sparks no sparks p*d/(pd) min 1mm piezolayers: sparks at 0.5 mbar 40 mbar 0.1mm distance: sparks at 0.05mbar 4 mbar Can use HV piezos! V 30

31 Outlook Combined stepper motor and piezo tuning is the method of choice, but: Most piezo tuners have been developed for pulsed operation What could be improved in a CW-only tuner? Sacrifice tuning range for stiffness: use shorter piezos Shorter piezos also reduce hysteresis effects Use high voltage piezos for stiffness Use 2 piezos on radially opposing sites in order to access vertical vibrational modes of the cavity Increase cavity wallsize to increase frequency of lowest tuner resonance Improve stability of microphonics compensation algorithms Incorporate piezo hysteresis into compensation algorithm in order to effectively increase piezo resolution Use bipolar power supplies (and increase mechanical pre-stress on piezo) Increase cavity stiffness to increase frequency of lowest resonance 31

32 Summary and acknowledgements Thanks are due to A. Neumann, A. Bosotti, R. Papparella, M. Luong, G. Devanz (measurements) M. Kelly, T. Powers, J. Delayen, S. Simrock, Z. Conway, E. Daly, B. Rusnak and many others I have borrowed transparencies from 32

33 SRF 2009 Conference Invitation SRF 2009 conference held at Helmholtz-Zentrum Berlin (formerly BESSY) Sept 20th 25th srf2009.helmholtz-berlin.de 33

34 Hysteresis effects on detuning compensation 293 K 4 K 34

35 New/prototype Fast Tuners Piezo fast tuner ANL Magnetostrictive tuner ANL/Energen 26 cm Combination fast/slow tuner SARAF/ACCEL No fast mechanical tuners of these types in routine operation with low-, mid-beta SRF linacs 35

36 KEK Slide Jack Tuner 36

37 Microphonics compensation Approaches to Compensate Microphonic Detuning Microphonic detuning is more a cost and implementation challenge than a technical show stopper It comes down to where the effort and resources are placed: Overcoupling: costly, wastes RF, but is effective VCX fast tuning: efficient, needs further development Piezoelectric, Magnetostrictive: need further investigation Cavity stiffening: mechanical engineering design and cost challenge 37

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39 Tuning concepts tuner electrical mechanical purpose slow niobium plunger penetration into rf space lever/motor + tuning plate cold motor driven lever pneumatic bellows pre-tuning compensate Hepressure fluctuations < 1 Hz fast none / overcoupling high rf power needs, use stiffer cavities variable reactance, limited applicability mechanical lever + piezo microphonics compensation Lorentz force compensation limited range, simplicity high range 39

40 Feature list from ERL 2005 (by E. Daly) Coarse Tuning Mechanism Typically cold, must be reliable and maintainable access ports Direct cavity drive reduces stiffness requirements on helium vessel Tuner/HV stiffness > 10x cavity Flexures exhibit reduced backlash Fine Tuning Actuators Piezo operate in compression, warm range 5-10x > cold range, capacitive device, minimize voltage, consider hysteresis MST must operate cold, consider lead thermal design, inductive element, minimize current, consider hysteresis Transmission Location (maintainability) Cold placement requires proper materials, cyclic life testing and access for repair or replacement, electrical feedthroughs Warm placement requires cooldown/tuning compliance, access ports, bellows Testing (minimizes risk associated with reliability and availability) Perform accelerated life tests on critical components Feedback results into design prior to production Develop thorough acceptance tests to verify operation 40

41 Piezo hysteresis for blade tuner 3000 both piezos frequenc cy change (Hz) maximum coercitive voltage maximum frequency remanence piezo 1 & 2 1µm cavity strain piezo voltage (V) used semibipolar voltage supply only 41