1. Overview of the LLRF Activities at SLAC
R. Akre*, Z. Geng, B. Hong, D. Brown, S. Condamoor, K. Kim, R. Larsen, J. Olsen, Vojtech Pacak, R. Ragle, D. Van Winkle, C. Xu
SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, U.S.A.
October 1st, 2013
Overview of the LLRF Activities at SLAC—LLRF 2013
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2. SLAC Facilities Overview

3. Outline PAD/PAC based LLRF System at SLAC
System Architecture
Phase and Amplitude Detector (PAD) and Controller (PAC)
Projects Adopting the Pizza Box based LLRF System
MicroTCA Development for LLRF
Highlights of the Technology
Results from a Prototype

4. LCLS LCLS required upgrades of LLRF system:
PAC--Fast phase and amplitude control => FPGA + DAC + I/Q Modulator + Solid-state Amplifiers
PAD--Precise phase and amplitude measurement => Down Converter + I/Q Sampling + Digital Demodulation
Fast data acquisition and pulse-to-pulse control at 120 Hz => EPICS + Fast Private Network

5. Architecture of the LLRF System for LCLS
Measurement and control are done locally for each RF station. A new 1KW Solid-State Amp drives Klystron.
Data process and feedback algorithm are performed in the central VME controller
Ethernet was used for communications – good for 120 Hz operation, but almost at the limits

6. Phase and Amplitude Detector and Controller --PAD and PAC
4 Chan - 130MSPS, 16 bit ADCs LTC2208 In PAD
First PAC built in 2006
Phase and Amplitude measurement result
One of the DownMix Channels

7. Projects with PAD/PAC Based LLRF System
LCLS—LINAC Coherent Light Source
13 Fast Control RF stations (Injector, L1S, L1X, etc.)
Phase Amplitude Control for LLRF Reference and Laser Drive System
ASTA—Accelerator Structure Test Area for photocathode QE and beam emittance study
Reference System for LLRF and Laser.
Feedback control for Gun RF signals with one Klystron Station
XTCAV—X-Band Transvers Deflector for Femtosecond Electron/X-ray Pulse Length Measurements
The X-Band Frequency Generator and the Fast Feedback System
A New Modulator Klystron Support Unit (MKSUII) replaces the >25 year legacy unit
XTA—X-Band Test Area for Compact Photo-injector with X-Band Structures.
Customized X-Band Frequency Generator is implemented in the existing NLCTA Test Hall. PADs/PACs are used for several RF stations.

8. Motivations Limits of PAD/PAC based LLRF System
A feedback control loop has to follow the chain of PAD-VME-PAC connected with Ethernet, the real-time performance is limited. It is not possible to do intra-pulse control (pulse width ~ 3 µs)
Computation power of the Coldfire MCU used in PAD/PAC chassis is quite limited. One more Channel Access client connected to the EPICS software in the Coldfire MCU can significantly degrade its real-time performance
One PAD chassis (2U or 3U) only contains 4 ADC channels. Channel density is low to efficiently use the rack space
Custom designed chassis is difficult to maintain
New requirements to LLRF System
Capability for intra-pulse feedback
More ADC channels
Fast waveform acquisition at 120 Hz
More complicated data processing

9. LLRF Frequency Reference
Consists of 14 Chassis located in a temperature controlled enclosure
Generate S-band Ref/LO, X-band Ref/LO, ADC clock and Gun laser clock signals with required stabilities

10. Recent Phase Noise Measurements of the Frequency Reference System
476MHz Master Oscillator and the 60W Amplifier upgrades improved the MDL.
With the LO and Clock, a phase measurement resolution of degree RMS within 1.2 MHz bandwidth can be achieved.
476MHz : 22fSrms 10Hz to 10MHz
2856MHz : 21fSrms 10Hz to 10MHz
476MHz = 22fSrms
2856MHz = 21fSrms
2830.5MHz = 21fSrms
25.5MHz = 165fSrms
119MHz = 51fSrms
102MHz = 65fSrms
2830.5MHz : 21fSrms 10Hz to 1MHz
119MHz : 51fSrms 10Hz to 10MHz
Overview of the LLRF Activities at SLAC—LLRF 2013
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11. Architecture of MicroTCA based LLRF System
RF Support Chassis is for down conversion, up conversion and klystron high voltage conditioning
AMC board contains ADC, DAC and FPGA for RF detection and actuation
CPU and EVR locate in the same crate as the ADC board
Interconnections are via PCI Express

12. MicroTCA Crate and FPGA AMC Board
Vadatech MCH UTC002
ADLINK AMC-1000 CPU
Struck SIS8300 AMC (Virtex 5 FPGA ; 4 lane PCI Express; 10 Channels 125 MS/s 16-bit ADC; Two 250 MS/s 16-bit DACs; Twin SFP Card Cages)
Overview of the LLRF Activities at SLAC—LLRF 2013
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13. FPGA Firmware Design
Fast real-time functions for the RF station control:
I/Q demodulation
Intra-pulse phase amplitude control
RF pulse generation
I/Q modulator calibration

14. Control Algorithm Pulse-Pulse Feedback
Implemented in software as a real-time thread
Use vector summation of multiple signal as an input for the feedback loop
Pulse-Pulse Feedback corrects slow drift
Intra-Pulse Feedback
Implemented in FPGA
Correction only works for the same pulse
The feedback algorithm compares RF pulse and Intra-Pulse I/Q set points table and accumulates the error for the given window. The result is applied to feed- forward value for another later window when the beam is accelerated
Intra-Pulse Feedback takes care fast random jitter

15. Software Architecture
Computation Nodes
Software Architecture in CPU
Pulse-to-pulse feedback and real-time data acquisition is done with EPICS software in MicroTCA CPU
Up to 6 AMC ADC boards are controlled by the same CPU
Integration with Timing system (EVR) and provide Beam Synchronous Acquisition (BSA)
Support function for Intra-Pulse Feedback: Loop Phase/Gain Compensation and Loop Phase/Gain Correction

16. Prototype Installed at LCLS Linac (LI28-2)
SSSB
RF Support Chassis
6-slot MTCA Crate
MKSUII (Interlock)

17. Summary
PAD/PAC systems work robustly and will be continuously supported and maintained for LCLS and other projects
MicroTCA system has been proved to be a powerful and compact solution, providing improved control capabilities and performance
Future development for Linac upgrades or new projects could be built/enforced with the experience of MicroTCA.

18. Note and References
Note: Ron Akre passed away on April 2nd, References: [1] P. Emma, “LCLS-II Conceptual Design Review,” SLAC, April 8, 2011, Chapter 6, Accelerator [2] Z. Geng, “LCLS-II Injector LLRF System-MicroTCA Based Design”, SLAC, June, 4, 2012, SLAC AIP Report [3] Z. Geng, “LCLS-II Low Level RF Controls”, FAC Presentation, SLAC, February 27-28, 2013 [4] Z. Geng, “LCLS-II Injector LLRF Final Design Report”, SLAC, January, 23, 2013 [5] R. Akre, “Linac Coherent Light Source (LCLS) Low Level RF System”, SLAC, September, 19, 2006, LCLS LLRF Review [6] C.G. Limborg-Deprey*, C. Adolphsen, et al. “COMMISSIONING OF THE X-BAND TEST AREA AT SLAC”, MOPB029, Proceedings of LINAC2012, Tel-Aviv, Israel [7] E. Jongewaard et al., “RF GUN PHOTOCATHODE RESEARCH AT SLAC”, IPAC2012, New Orleans, Louisiana, USA [8] R. Akre et al., “Commissioning the Linac Coherent Light Source Injector”, Phys. Rev. ST Accel. Beams 11, (2008) [9] Y. Ding et al., “Femtosecond Electron and X-ray Beam Temporal Diagnostics Using an X-band Transverse Deflector at LCLS”, Phys. Rev. ST Accel. Beams 14, (2011) [10] P. Krejcik et al., “Engineering design of the new LCLS X-Band Transverse Deflecting Cavity”, IBIC 2013, Sept. 16th, 2013, Oxford, UK

19. Thanks!