1. Philippe Baudrenghien, Grégoire Hagmann, BE-RF, 22.06.2016
SPS LLRF upgrade
Philippe Baudrenghien, Grégoire Hagmann, BE-RF,

2. History of SPS LLRF
The original LLRF was designed in the first half 1970s and used NIM standard, with no remote control facilities. One either uses front-panel knobs or functions (designed by the Power Converter group). There was no built-in logging or diagnostics
An upgrade was done in the mid-1980s when the SPS became a part-time LEP injector, requiring some ppm facilities. The LEP platform (G64 crate with CPU) was deployed in the SPS. The RF group developed custom-designed Real-Time Cards (RTC) driven by timing pulses, to make settings cycle dependent (switching from LEP injector to proton fixed-target or p-pbar operation)
The natural ppm solution was to use one Beam Control system per beam (pFT, p-pbar, ee-). Each beam would have dedicated key components (Master Oscillator), re-use some sub-system (beam-based phase loop for example), with analog switches (timing driven) commuting the signals -> bulky hardware and no real ppm implementation
Following the LEP stop, the G64 architecture was found obsolete and dismantled in the SPS
Since the LHC start-up, new SPS LLRF hardware has used the VME-based LHC platform, which was selected around 2003 for the LHC RF.

3. Scope of the upgrade
The original plan was to redesign the Cavity Controllers (dating from late 1990s when the SPS was upgraded as LHC injector) for the additional two cavities
It was later decided to replace the Cavity Controllers of the existing four cavities as well
In 2014, slip stacking was considered for LHC ions. That called for modification to the Beam Control system. New requirements are
Possibility to drive sets of cavities at different frequencies
Beam phase loop measuring selected bunches and acting on the frequency of the corresponding cavities
This upgrade of the Beam Control justifies a full renovation of the LLRF system
Starting with functional specifications including all beams
Implement real ppm operation (no more analog switches)
Include logging, observations
It was also decided to replace all cavity cables Tunnel-Surface.

4. 200 MHz Cavity Controller (1/2)
The design is similar to the 800 MHz Cavity Controllers (operational 2016)
R&D well advanced (see G. Hagmann, LIU-SPS coord. meeting, 24th Sept 2015)
The complexity comes from the operation with Fixed-Frequency Acceleration (FFA) that requires Frequency Shift Keying – FSK – modulation of RF frequency. R&D ongoing on the multi-rate processing (Doct. student J. Galindo)
Collaboration on the parameters optimization (T. Mastoridis)

5. 200MHz Cavity controller (2/2)
Fig2: TWC200 Cavity Controller

6. Collaboration on SPS LLRF upgrade
Collaboration with Pr. T. Mastoridis, CalPoly, San Luis Obispo, CA
The objective is to help the designers by answering the following questions
How much is the beam affected by the LLRF technical choices?
What is the effect of the High Level imperfections?
What is the importance of LLRF imperfections on the overall performance?
What is the impact of possible misalignments between MHz RF systems caused by uncompensated transient beam loading?
Bunch phase modulation along the batch. Measured (blue = Cavity Phase) vs. model (red)

7. Fig2: TWC200 Cavity Controller
Planning & manpower
9.3 FTE in the RF-FB section
+ x FTE in RF-CS
Fig2: TWC200 Cavity Controller

8. Beam Control (tentative)
The most demanding beam is LHC ions (FFA). A possible evolution, compatible with all beams, is an upgrade of the existing ions system. It includes
A Synchro loop (or radial loop) that is responsible for keeping the beam centered in vacuum chamber
(Several) phase loop(s) that damp(s) synchrotron oscillations.
We keep one Master DDS that generates 2n Frev. Its frequency is adjusted from a synchro loop (or radial loop) . The Master DDS is common to both beams, it only adjusts its frequency to keep a hypothetic beam centred. It takes care of the slow (adiabatic changes) such as acceleration
The Slave DDS is upgraded to a Triple (or more outputs) Slave DDS
One output, Favg= 4620 Frev0 , is used by synchro loop
The other outputs can be used in slip stacking, potentially FSK modulated
The DDS implementation allows for fast (non-adiabatic) phase or frequency jumps (bunch rotation, bucket merging at the end of slip stacking,…).

9. Planning & manpower 5 FTE 2016-2020 in the RF-FB section
+ y FTE in RF-CS

10. Conclusion: status
800 MHz Cavity Controller operational on both cavities
R&D on the 200 MHz Cavity Controller (compatibility with FFA)
Block Diagram on the Beam Control
Study of Beam Phase measurement
Simulations to pilot the design of Cavity Controller

11. Conclusion: manpower
14.3 FTE in the RF-FB section, that is 2.86 FTE/yr
Granted resources:
Greg Hagmann 65%
Gerd Kotzian 30%
Philippe Baudrenghien 20%
Daniele Castello da Silva 100% (new staff, Aug 1st 2016)
Lorenz Schmid 100% (LIU-ions fellow, Aug 1st 2016)
Plus
Collaboration with T. Mastoridis (CalPoly, San Luis Obispo)
J. Galindo (Doct.)
Required 1 FTE/yr in the RF-CS section
Hopeful resources:
1 flexibility post LIU-ions in the RF group, assigned to the RF-CS section for LIU-ions software. Hopefully working on the SPS LLRF FESA classes (A. Butterworth)
Cannot do without that post in RF-CS section

12. Thank you for your attention
Questions, comments?

13. Additional slides
Presentation at LIU meeting

14. 200 MHz Cavity Controller (1/2)
Compatible with frequencies from MHz (ions FT) to MHz
Compatible with the full range of synchrotron frequencies
Reduction of impedance at fundamental and transient beam loading. A factor 2 improvement is expected, compared to the late nineties implementation. The corresponding gain/bandwidth requirements are being studied
Alignment of MHz phase. The compensation of the beam loading in the 200 MHz will be improved, resulting in an expected +- 5 deg MHz) maximum misalignment of the two RF along the batch. Gain/bandwidth requirements being studied
Batch per batch longitudinal blow-up. Transition time 225 ns, equal to the p-LHC batch spacing. Given the 640 ns cavity filling time (4-sections) the required RF power must be studied
Longitudinal damper, dipole (and quadrupole modes) usable over the full frequency and fs range (p LHC, p FT, ions) and with FM modulated RF (fixed frequency acceleration)
Settings driven by functions (ppm) so that they can be varied during the cycle

15. 200 MHz Cavity Controller (2/2)
Operation in amplitude modulation mode (LHC p beams, ions FT, ions LHC) to save on CW RF power. Modulation at up to 4 frev (ions FT)
Individual Master RF for each cavity, or at least two groups (for slip stacking of LHC ions)
Fast voltage modulation (<1 microsecond response time): voltage reduction at injection – LHC p and CNGS beams, non-adiabatic voltage jump at flat top for bunch rotation – LHC p, ions, AWAKE. For the voltage reduction at injection, the fall-time must be below 225 ns (batch spacing)
Fast phase modulation (<1 microsecond response time): jump on the unstable phase for bunch rotation (proton FT beam, AWAKE), batch per batch longitudinal blow-up at injection. For the later, the transition-time must be below 225 ns (batch spacing)
Fast frequency change (<1 microsecond response time): non-adiabatic locking of cavity on a common reference frequency at the end of the slip stacking process (LHC ions)
Compatibility with Fixed Frequency Acceleration: FM modulation during 1 turn (ions LHC, ions FT).

16. 800 MHz Cavity Controller
Compatible with frequencies from MHz (4 x MHz = first zero of 4-sections cavity) to MHz (4 x MHz )
Compatible with the full range of synchrotron frequencies
Reduction of impedance at fundamental and transient beam loading compensation
Alignment of MHz phase, resulting in an expected +- 5 deg MHz) maximum misalignment of the two RF along the batch. Required gain/bandwidth being studied
Longitudinal blow-up, batch per batch, with transition time below 225 ns (RF power plant allowing)
Extended range of frequencies i.e. usable for ions for longitudinal blow-up without Frequency Modulation. Goal: equalization of batches before start ramp
Fast voltage modulation (<1 microsecond response time): voltage reduction at injection – LHC p beams, non-adiabatic voltage jump at flat top for bunch rotation – Awake beam
Fast phase modulation (<1 microsecond response time): jump on the unstable phase for bunch rotation – Awake and p-LHC beams
Settings driven by functions (PPM) so that they can be varied during the cycle.

17. Beam Control: Master RF
At present, we have only one Master RF per beam, but we have several different VCO/DDS, depending on the required frequency range and needed RF manipulations (fixed frequency acceleration for ions)
The new LLRF requirements imply
One Master RF per cavity (or at least per group of cavities) if we wish to operate them at different frequencies (slip stacking). At the end of the slip stacking process, the RF frequencies must return to the same common value in a fraction of a synchrotron period. This fast and precise reaction is incompatible with a Phase-Locked Loop implementation (as used in the LHC asymmetric collisions p-Pb for adiabatic cogging of the p and Pb rings), but could be implemented by a DDS (similar to the FM modulation in SPS ions fixed frequency acceleration)
Improvement on RF noise (ions beams). This was traced to the Master DDS module that is being upgraded.

18. Beam Control: Phase Loop
The phase loop is operational for all beams. It damps common-mode synchrotron oscillations and reduces the effect of RF noise on the first synchrotron sidebands, thereby improving beam lifetime
The requirements are
Compatible with bunch spacing multiples of 5 ns
Compatible with bunch intensity from 2E7 (ions FT after debunching- recapture) to 4E11 p per bunch
Compatible with 1 to 80 bunches per batch
Individual bunch-by-bunch measurement
“Averaging” over the selected bunches
One phase loop per group of cavities (slip stacking) with possibility to select the RF voltage measurement as sum of a number of cavities
Linear phase discriminator with 360 degrees range (ions, awake).
Given the very wide range of bunch intensities (4 decades), it may be appropriate to have parallel systems.

19. Beam Control: Radial Loop
The radial loop is needed to cross transition (proton Fixed Target, ions LHC,…). It will keep the beam centred, even in presence of inaccuracy in the frequency program calculation.
The requirements are
Compatible with bunch spacing multiples of 5 ns
Compatible with bunch intensity from 2E7 to 3E11 p per bunch
Compatible with 1 to 80 bunches per batch
Individual bunch-by-bunch measurement. Synergy possible with Transverse Damper front-end
“Averaging” over the concerned bunches.
Given the very wide range of bunch intensities (4 decades), it may be appropriate to have parallel systems
An upgrade of the Radial Loop is not required to implement ions slip-stacking, but is motivated by the obsolescence of the equipment (original SPS equipment dating from the 1970s).