1. Status of SPARC synchronization system and possible upgrades
M. Bellaveglia
On behalf of the LNF timing, synchronization and LLRF group

2. SUMMARY SPARC present synchronization system
Some option to enhance the system without the transition to the optical distribution
Future development and requirements
Study of an optical synchronization system with cost estimation
Conclusion

3. SPARC present synchronization system
One optical master oscillator (MIRA)
Feedbacks with BW <<1Hz, ≈5kHz and ≈1MHz to synchronize the subsystems
Shot-to-shot (10Hz) analysis for amplitude and phase calculation

4. Phase detection – standard mixing technique
Sampling
Board
DtRMS ≈ 55 fs
Tested Sampling Boards:
ADLINK 9812: 12-bit, 4-channels, 20 Ms/s
ADLINK 9820: 14-bit, 2-channels, 65 Ms/s
NI PXI 5105: 12-bit, 8-channels, 60 Ms/s
Phase detection resolution ≈40fs

5. PLLs - Laser and RF phase lock
Electro-opto-mechanical PLL: BW limited to some kHz (piezo-crystal maximum frequency)
It locks the laser oscillator repetition rate (79.33MHz=RF/36) to a stable Reference Master Oscillator (RMO) to keep the main frequency constant in time
Full electronic PLL: BW measured in lab ≈1MHz
We have chosen to use this system architecture to bypass the laser Synchrolock and overcome the PLL bandwidth limitation problem
The main oscillator of the system is presently the photo- cathode laser oscillator

6. How the Synchrolock works
Stepper motor,
coarse positioning
Laser oscillator cavity
12.6ns
Galvo motor
low freq. correction
79.33MHz
From RMO
Piezo motor
high freq. correction
Synchrolock board
Motor drivers
Fundamental loop
@79.33MHz
EO transducer
and active filtering
Frequency x9
Harmonic

7. Upgrading the Synchrolock
New harmonic loop phase detection
Actual scheme:
Active frequency x9 multiplier and active filtering after photo detection -> high electronic noise
714MHz phase comparison -> low resolution
Reference lock limited to <300fsRMS rms of relative jitter
New scheme (1k€ to 5k€):
Phase comparison at higher frequency -> higher resolution, locking performance <100fsRMS
Interface with motors to be studied (level of signals, interconnections, changing motor drivers?…)
Simpler upgrade to optical phase detection

8. Comparison with FLAME oscillator PLL
We measured the performance of the FLAME IR oscillator in terms of phase noise at the manufacturer site and after the installation at LNF
High frequency (high resolution) phase comparison directly at 2856MHz
We measured with Agilent SSA E5052A an absolute phase noise of about 130fs
We measured phase noise relative to the RF reference less than 90fs

9. PLLs – Klystron intra-pulse phase lock
PLL on:
Dt<70fsRMS
PLL off:
Dt≈630fsRMS
Phase noise is introduced at the RF power generation level (klystron)
It can be reduced by phase locking the klystron output to the RF reference with an analog loop (like in CW RF in storage rings);
Very short time available to reach steady state (in case of SLED ≈1ms): wideband loop transfer function (≈1 MHz) required.

10. RF gun feeding system upgrade
Gun accelerating gradient increased by increasing the RF pulse level and shortening its duration
Unfortunately the PLL to compress the phase noise introduced by klystron needs 1us to set up correctly
To maintain the Klystron PLL performance the RF pulse has been amplitude modulated:
in the first 3us the RF level is kept as low as possible to make the PLL working
the RF is brought to the maximum level in the last 0.8us

11. PLLs - Slow drifts compesation
Temp.
Phase

12. Measurements on system performance
Phase noise detection resolution: <50fsRMS
Linac RF devices phase noise (standard phase detection): 40÷100 fsRMS
Photo-cathode LAM measured time jitter (resonant monitor): ≈250fsRMS (resolution problem)
e-bunch time jitter
BAM (bunch arrival resonant monitor): ≈250fsRMS
RF deflector centroid jitter (image analysis): ≈150fsRMS

13. Upgrading the resonant monitor
Use of a Mach-Zender 10GHz EO modulator
Already purchased and tested in the synch-lab

14. Electron-photon bunch synchronization
Most strict requirement for laser PWFA with external injection
SPARC and Flame pulses injected in a gas jet (or capillar), requires synchronization at the level of the period of the plasma wave.
Request: Δt<100fsRMS
SEEDING
LASER
PLASMON-X
FLAME parameters
PHOTOINJECTOR
LASER
Wavelength
800 nm
Compressed pulse energy 5 J
Pulse duration (bandwidth)
30 (80)
fs (nm)
Repetition rate 10 Hz
Energy stability
10%
Pointing stability
<2
urad
THOMSON
HHG
DGL
PHOTOINJECTOR
UNDULATOR

15. SPARC-FLAME synchronization
FLAME AREA
FLAME oscillator
In 2856MHz
Out 79.33MHz
Next future solution:
Coaxial cable distribution
Easiest and quickest
Low cost
Possible temperature stabilized cable bundle
100fsRMS laser-to-RF synchronization
Requires PC Synchrolock upgrade
Present phase detection resolution ≈40fs
PC laser
Oscillator
79.33MHz
RF reference
2856MHz
SPARC HALL

16. Enhanced synchronization
FLAME AREA
Optical synchronization
Fiber laser OMO
Major system modification needed
Higher cost
Fiber stabilized links to distribute the signal
Possible optical mixing for laser clients
Sub-100fsRMS laser-to-RF synchronization
FLAME laser
oscillator
PC laser
oscillator
SPARC HALL
OMO
fiber laser
oscillator RF
synthesizer

17. Upgrading to optical synchronization

18. Reference Master Oscillator
Used to long term stabilize OMO frequency avoiding slow drifts
No extreme performances needed
Choice of commercial product (jitter <50fs from 10Hz to 10kHz)
Already tested and installed at SPARC

19. Reference signal generation
Phase noise of an Optical Master Oscillator (OMO) from 1kHz to 10MHz is <5fsRMS
Quotation from MENLO Systems (excluded RMO)

20. Reference signal distribution
Point-to-point distribution with a jitter <10fsRMS
Quotation from MENLO Systems (excluded RMO)
Possibilily to avoid one of the links installing the dedicated rack in the PC laser clean room
Possibility to avoid both the links using special temperature drifts insensitive fibers
Special fibers available from OFS (Optical division of FURUKAWA ELECTRIC CO., LTD.)
Prices are:
without jacket (FA-KF5062) 13.5 k€/km (675€ for 50m)
with jacket (FA-KC4108B) k€/km (3245€ for 50m)
cable bundle ≈200k€/km

21. Optical phase detection for laser clients
Phase detection resolution in the range of attoseconds
Client locks in the worst case with a jitter well below 50fsRMS
We are testing a dummy setup in the synch-lab
Presently we are limitated in the measurement by the low laser power available
Quotation from MENLO

22. Conclusions Coaxial cable distribution:
Can be upgraded to ≈100fsRMS between the electron and the photon beams
PC laser Synchrolock should be upgraded
Resonant monitor should be upgraded
Optical reference distribution (locking sub-systems with <50fsRMS jitter):
It is mandatory in case of LPWA acceleration with external e-bunch injection
It is already developed and installed by MENLO systems in
Total quotation from MENLO systems (SMF28 fibers excluded):
We can choose to use the special fiber from Furukawa instead of the actively stabilized fiber links from MENLO
In this case can yield to a total system cost of ≈245k€ (included fibers for sub-systems interconnection)