1. CW Measurements of TESLA and Gun Cavities with the Cornell LLRF System at HoBiCaT
Axel Neumann
SRF Science and Technology
Helmholtz-Zentrum Berlin
for a collaboration by Cornell University and HZB:
S. Belomestnykh*, J. Dobbins, R. Kaplan, M. Liepe, C. Strohman
W. Anders, R. Goergen, J. Knobloch, O. Kugeler (*now BNL)

2. Lunar Landing Research Facility, Hampton, Virginia,
Another perspective of LLRF:
NASA
Lunar Landing Research Facility, Hampton, Virginia,
NASA Langley Research Center

3. Motivation: Field control for SRF cavities in Energy Recovery Linacs
Visit:
HZB ERL: BERLinPro
SRF Photo-injector
Main linac cavity
(with JLAB style HOM
dampers)
High beam current,
low beam power
Riemann et al. IPAC 2011
SRF booster cavity
(Cornell type)
High beam power
and high beam
current
Parameters
Beam energy
50 MeV
Max. current
100 mA
Nominal bunch charge
77pc
Max. rep. rate
1.3GHz
Normalized emittance
< 1mm mrad

4. Overview and challenges for LLRF:
Low beam-loading (zero?) allows high QL operation
CW operation: Microphonics, peak events?
High cw gradient: 20 MV/m  Ponderomotive instabilities
High cryogenic dynamic losses, helium bath stability
Beam transients during ramping to 100 mA, how to handle? Low current diagnostic mode….
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
TESLA cavity as testbed
for cw driven ERL main linac cavity
II.
Synchronization to cathode laser
CW operation at strong microphonics
Strong response to LHe pressure variations
First version without tuning scheme
Field ramp at strong Lorentz-force detuning
Frequency deviates from design frequency
1.6-cell SC gun cavity as
first step towards ERL photo- injector in collaboration with:
JLAB, DESY, A.Soltan Institute, MBI,
BNL for beam dynamic studies

5. Collaboration HZB with Cornell University at HoBiCaT
Goals of the measurements
Determine the ultimatively achievable field stability:
What are the maximum loop gain settings?
What is the optimum cavity bandwidth for ERL main linacs (with respect to stability and forward power)?
Determine microphonics detuning level, peak events and spectrum
How to ramp cavity field at small bandwidth in the presence of Lorentz force detuning
Combine detuning and LLRF field control for a most reliable and stable system
Detailed description of the LLRF hardware and performance at Cornell
see: Cornell Lab talk by Vivian Ka Mun Ho on Monday

6. Testbed: The Cornell LLRF system at HoBiCaT
Reference oscillator:
1.3 GHz, below 36 fs
integral time jitter of
10 Hz – 5 MHz sideband
spectrum
Gun cavity
TESLA cavity
TESLA cavity, courtesy DESY
17 kW CPI IOT
6xADCs (16 bit): Pfor,Pref,Ptrans, Beam signal,…
Slow DAC/ADC interface
2xDACs
2nd generation system
Saclay I tuner
with high voltage piezos
Here piezo in PI loop only

7. 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
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

8. 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:
QL,Blade:
Blade: Mechanical eigenmode
at 300 Hz, vacuum pump freq.
Saclay: Excitation of 1st mechanical eigenmode sets in

9. Detuning and transfer functions
Measured detuning spectrum
Mechanical
resonances
Turbo pump
lowest mechanical eigenmodes at and 36 Hz
group delay between ms
piezo tuning range above 1 kHz

10. Time domain results: Proportional gain scans
Residual error evolves with 1/(1+KP)
Upper limit by noise of RF reference system
At low gain strong coupling of detuning and amplitude via Lorentz force detuning
Lorentz force effect

11. Parameter studies: Optimum gain and cavity bandwidth
log(sf)
Best results: ° ° °
QL=5.107, f1/2=13 Hz
QL=2.108 , f1/2=3.25 Hz
9 cell TESLA cavity
Eacc= 10 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
Areas with sf>0.1 were blanked out

12. Limitations for combining LLRF and piezo tuner control?
Amplitude and
phase spectrum
vs. proportional
gain KP
Detuning
information
Amplified
phase noise?
sf=6 Hz
sf=9.3 Hz
sf=9.6 Hz
Spectrum of calculated
tuning angle by Pt and Pf
Simulation
Noise dominated

13. Best results achieved / Summary for main linacQL
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
But cavity was limited by field emission to 10 MV/m. We plan to repeat test at 20 MV/m with a good and clean cavity.
Piezo loop only operated at low frequency PI mode, still plan to implement adaptive feed forward (see DOI /PhysRevSTAB )
The low noise reference source broke down when tests were started, performed test with a R&S standard frequency synthesizer  results limited by noise
Klystron ripple loop was not implemented, the IOT’s power supply has intrinsically low noise, but very non-linear behavior of IOT at power levels below 1 kW
Next: Gun cavity

14. Design: J. Sekutowicz* TTF-III FPC: QL= 1.109-6.106
Cavity fabrication at Thomas Jefferson Lab (P. Kneisel, Proc. PAC 2011)
Fundamental power
coupler port
Stiffening ring
Status Anfang des Jahres:
Large grain cavity backwall for
cathode
Cavity half-
cells
Pick-up Port
Passive stiffening
System: “Spider“
Design: J. Sekutowicz*
„Helium vessel endplate“
TTF-III FPC: QL=
including 3-stub tuner
Cavity in helium vessel
Pb cathode on back
plane
*Sekutowicz et al., Proc. PAC 2009

15. Mechanical design: Countermeasures to increase field stability
Beam quality dominated by field stability
Field stability in SC cavity: Avoid detuning (deformation) of the cavity
Combined FEM mechanical and
Electro-magnetic field simulations
 low deformation (detuning) design
Backwall stiffening
Detuning in Hz/mbar F
Lorentz-force detuning: Df/Epeak² = 1.33 Hz/(MV/m)²
114
Neumann et al., Proc. Linac 2010

16. Synchronizing Laser and LLRF without tuner
Both, GDR LLRF and Laser
PLL synchronized to reference
follow slow frequency drifts of
gun cavity.
Tested laser PLLs bandwidth
by lock-in measurement F
Laser+PLL
1/N
Loop filter
Gain
Use modified piezo loop to
add slow PLL loop for drift stabilization of cavity
and to ramp field
First beam
21st of April 2011
on YAG viewscreen

17. QL Epeak sf sF sA/A Dfpeak (Hz) 6.6.106 20 MV/m 7.0 Hz 0.017 deg
Field stability
Microphonics spectrum
Mechanical
eigenmode QL
Epeak sf sF
sA/A
Dfpeak (Hz)
20 MV/m
7.0 Hz
0.017 deg
25.6 Hz
12 MV/m
5.0 Hz
0.02 deg
20 Hz
Using Cornell LLRF system+ slow PLL loop

18. Thank you for your attention
Summary for gun cavity
Adapted Cornell’s LLRF system within a few days to be operated with gun cavity without tuner
System routinely ramps cavity to peak fields on cathode of 22 MV/m and even more
Piezo tuning algorithm used in lowpass PI configuration to follow cavity drift by reference
Laser PLL showed no sign of loosing lock so far
Within the framework of the performed beam measurements the set up is stable
Jacek Sekutowicz (DESY) and Peter Kneisel (JLab) are working on a new cavity with tuning scheme adopting the Saclay I tuner for the 1.6 cell design
Thank you for your attention

19. Backup transparencies

20. Possible reasons for instability
IOT response time and stability:
Example: QL=2.108, 1.5 Hz detuning
 Eacc drops to 9.07 MV/m:
(10 MV/m)²-(9.07 MV/m)²
Lorentz force detuning: ~17.6 Hz
LLRF needs to compensate
by 30 times RF power
At high loop gains the requested
power even increases on short
time scales
Beam of IOT becomes unstable
leading to an RF trip
The cure:
Combine detuning control and LLRF
to minimize „high power“ events
Operation at high QL possible,
thus saves RF power installations!

21. Extension of the HoBiCaT Cavity Test Facility
Laser
hut
Laser beamline C o n t r l ro m
HoBiCaT cryostat
Diagnostic
e- beamline
RF amplifier

22. Gun with Diagnostic beamline at HZB
SC Solenoid
Stripline BPM,
ICT
THz diagnostics SQ
Faraday Cups
SC Cavity
Dipole magnet
Viewscreens
HoBiCaT
cryostat
Mass Spectrometer
First time operated fully SC photoinjector ensemble (SC Cathode, Cavity, Solenoid)
Source/upgrade for CW low current machines (POLFEL, XFEL, FLASH)

23. EM design: Highest fields at cathode region
SC RF Gun0
Pick-up probe
Excited p-TM010 mode
Original design by
Jacek Sekutowicz*
Lead cathode,
Tc= 7.2 K
Input coupler
EM design: Highest fields at cathode region
SC lead cathode on half- cell backwall: QEPb~10.QENb
Study beam dynamics at short pulses, ERL parameter range
Frequency p-mode
1300 MHz
Epeak/Eacc
1.86
Hpeak/Eacc
4.4 mT/(MV/m)
Geometry factor
212.2 W
R/Q (linac, b=1)
190 W
*Sekutowicz et al., Proc. PAC 2009

24. Ibeam=50 nA at Ecath=20 MV/m
Kamps et al., Proc. IPAC 2011
Neumann et al., Proc. IPAC 2011
Beam energy
First beam
21st of April 2011
on YAG viewscreen
Beam current (nA)
Laser phase (deg)
3-4 ps pulse length
during extraction
Bunch charge: 5-6 pC
Ibeam=50 nA at Ecath=20 MV/m
at 8 kHz Laser rep. Rate, l=258 nm best QE=1.10-4

25. Reconstruction of the transfer function
Fit by par. systems of 2nd order
Include response of higher
modes at lower frequencies
20 modes needed for the range shown
Complex system complicates model-based design (e.g. Kalman filter)
Transfer function implemented as table
Tuning relevant range