Experience with high loaded Q cavity operation at HZB Axel Neumann, TTC CW-SRF meeting U Cornell, 06/11-06/14/2013 Ithaca, NY

Test set up: Horizontal test facility HoBiCaT Testing fully equipped cavities including helium vessel, motor- and piezo tuner, CW modified TTF couplers, magnetic shielding, etc. Temperature range down to 1.5 K, typically 1.8 K with 100 W @ 1.8 K: 16 mbar ±30 mbar rms Coupling variable, installations down to bc=1 possible RF set up: 19 kW IOT, 400 W solid state amplifier driven by PLL or Cornell’s LLRF system Two cavities tested in parallel or sample studies Gun cavity tested with diagnostic beam- line

High QL Measurements done/planned at BESSY/HZB Cavity type RF requirements or achievements Design/operated QL Project Eacc=15-20 MV/m, Q0=1-3.1010 , sf=0.02 deg., sA/A=1.10-4 3.107 Operated: 5.106-2.108 BESSY FEL, SC CW, low beam loading Epeak=12-25 MV/m Q0=2-7.109 sf=0.02 deg., sA/A=1.5.10-4 Operated: 3.106-1.5.107 Lead cathode, all SC gun for low current FELs Eacc=20 MV/m, Q0=1-2.1010 , sf≤0.05 deg. 5.107 BERLinPro Linac, zero beam-loading E0=30 MV/m, Q0≤4.109 3.6.106 BERLinPro Gun prototype, 4mA Eacc=10 MV/m see Cornell’s presentation 1.105 BERLinPro Injector, 100 mA, low QL TESLA Tested in HoBiCaT 1.6 cell Pb/Nb hybrid, J. Sekutowicz Tested in HoBiCaT HZB 1.4 cell Expected 2014 Cornell booster Expected 2014

Mechanical oscillations of the Cavity: CW operation: Field stability determined by Microphonics Helium pressure fluctuations Df/Dp = 50-60 Hz/mbar, Gun:100 Hz/mbar Heat transport dynamics 16 mbar Field amplitude variation: Dynamic Lorentz force, Df/DEacc² = 1Hz/(MV/m)² 2.5 mm Niobium walls Stochastic background noise Deterministic, narrow-band sources: Vacuum pumps Mechanical oscillations of the Cavity: Microphonics G. Bissofi 222 Hz 151 Hz Response of the Cavity-Helium vessel-Tuner system: (FEM simulations, e.g.: Devanz et al. EPAC 2002)

X X Measurements at high loaded Q Low beam-loading CW SRF linacs allow operation at high QL narrow bandwidth (order of 10s Hz) X CW operation: Microphonics, peak events? Field stability at the presence of microphonics High cw gradient: E.g. 20 MV/m  Ponderomotive instabilities by LF detuning High cryogenic dynamic losses, helium bath stability Beam transients during ramping to 100 mA, how to handle? (ERL) Residual beam-loading due to beam losses  non- perfect recovery, time jitter? Combine microphonics compensation with LLRF at high QL for a multi-cell cavity

Power requirements and parameter space Studied within this work Pros: High QL: low forward power for a given field level, reduction of thermal stress for RF transmission line/coupler But: Effective detuning is a convolution of the detuning spectrum with the cavity response (bandwidth) Cavity transfer function itself altered by controller settings (feedback gains) QL 7-cell cavity, 20 MV/m, Ib=0

Detuning spectrum versus bandwidth (TESLA cavity) For two different tuning schemes (Saclay I and INFN Blade) open loop measurements of microphonics vs. QL were performed Both tuners showed to have different transfer functions and thus detuning spectra QL,Saclay: 3.107-4.108 QL,Blade: 7.105-2.107 Blade: Mechanical eigenmode at 300 Hz, vacuum pump freq. Saclay: Excitation of 1st mechanical eigenmode sets in

Detuning characterization of a TESLA cavity, short-term Characterisation: Measurement results Detuning (Hz) sf = 1.56 Hz SFFT Excited Eigenmode He pressure- variations FFT Detuning (Hz) Time (s) HoBiCaT: sf = 1 - 5 Hz (rms)  2-13° phase error (Aim: 10-2 °) „open loop“ „closed loop“ He pressure variations: fmod < 1 Hz Cavity specific: Lines at 30 or 41 Hz 8

Longterm stability: Peak events Microphonics recorded at HoBiCaT with TESLA cavity for 48 hours RMS Values around 1-5 Hz  Determines field stability and thermal loading of RF system (5 kW) Peak values extend out to 17 σ!  Determines RF power installation (15 kW) Peak events occur 10-20 times a day! (This was partly improved by changes to the control settings of the under-press. pumps.) Expected field stability: 0.02 - 0.1° For „comfort“ want to reduce the microphonics Gaussian sub-range 0.8 Hz rms 9

Time-frequency analysis by Wavelets ^ t/s s. w Morlet Df (Hz) Variation up to Df =10 Hz on a ~100 ms time scale Spectrum of He-pressure variations of stochastic nature Adaptive, „learning“ (dynamic) compensation mandatory Need for classic feedback control 10

Detuning compensation: Characterize the tunercavity action TESLA Cavity 1.6 cell gun Cavity Mechanical resonances Turbo pump Mechanical resonance Helium Activity?

Model based controller: Fit of the transfer function TESLA Cavity Fit: Parallel acting 2nd order systems Evaluate response of higher modes at lower frequencies >20 modes needed for fit Systems complexity complicates use of model based feedbacks (e.g. Kalman filter) Transfer function as look-up table Kalman approach tested within a Master thesis (P. Lauinger), test in prep. Relevant for tuning 12

A tested scheme: Least-mean-square based adaptive feedforward Dl (nm) t (s) External mechanical oscillations FFT t (s) DU (V) Compensating signal t (s) Df (Hz) Detuning of the cavity IFFT ∙H-1 Calculation of optimal FIR filter parameters For white noise excitation The FIR filter would be H-1piezoDf 13

Compensation results Single-resonance control: SFFT QL=6.4.107 Feedback only sf = 0.89 Hz Feedback and Feed- forward sf = 0.36 Hz Open loop sf = 2.52 Hz SFFT QL=6.4.107 Multi-resonance control: Piezo resolution seems to limit control of neighboring modes

LLRF studies with U Cornell: Limits of QL log(sf) Best results: 5.107 0.008° 1.108 0.0093° 2.108 0.0236° QL=5.107, f1/2=13 Hz QL=2.108 , f1/2=3.25 Hz 9 cell TESLA cavity Eacc= 10-12 MV/m Tbath= 1.8 K PI piezo loop 8/9-p filter optimized QL=1.108, f1/2=6.5 Hz LF detuning  IOT beam instable Cavity field trip Areas with sf>0.1 were blanked out

LLRF studies QL sf (Hz) sf (deg) sA/A Pf (kW) 5.107 9.5 0.008 1.10-4 1.106 1.108 7.9 0.009 2.10-4 0.595 2.108 4.2 0.024 3.10-4 0.324

Gun cavity LLRF and microphonics studies Insufficient cooling of cathode More about Gun Cavity in talk by A. Burrill @25 MV/m, quench would occur after few minutes A lot of power dissipated in LHe bath  effect on microphonics?

Results with the SC Gun Cavity QL: 1.4.107, due to ponderomotive instability changed to 6.6.106 At 25 MV/m the cavity losses increase due to bad thermal contact of cathode plug and back wall via indium seal Microphonics increase by factor of three, mechanical resonance excited Strong line at 35 Hz appears: Eigenmode of the Helium bath?

On-going studies: Thesis P. Lauinger Further studies with TESLA cavity planned as well as microphonics compensation using Kalman filter approach Microphonics (Hz) Qcrit (W/cm²) Pheater (Dfmax) (W) Heater power (W) Cavity driven by LLRF at Epeak=15 MV/m Piezo compensation in PI loop mode with low-pass filtering Additional power dissipated in LHe bath by heater within liquid Microphonics recorded while heater is powered TLHe (K) T. Peterson, TESLA-Report 1994-18

Summary Microphonics as main error source for field stability extensively characterized for TESLA and 1.6 cell Gun Cavities at various QL Microphonics compensation demonstrated with TESLA cavity at high QL, an order of magnitude feasible Needs to be implemented within operating LLRF system LLRF studies showed a stable operation at up to QL=2.108, still needs to be demonstrated for fields larger than Eacc>12 MV/m Experiments to correlated microphonics and helium heat transport dynamics were started, more results hopefully this summer New microphonics compensation schemes will be tested soon For both cavity types studied a field stability of at least sf≤0.02 deg and sA/A≤1.10-4 was demonstrated Thanks to, people involved: S. Belomestnykh*, J. Dobbins, R. Kaplan, M. Liepe, C. Strohman (Cornell, *now BNL) for the LLRF system J. Sekutowicz (DESY), P. Kneisel (JLab) for the 1.6 cell Gun Cavity W. Anders, A. Burrill, R. Goergen, J. Knobloch, O. Kugeler, P. Lauinger + HoBiCaT personell (HZB)