1. LLRF FOR THE SPS 800 MHZ CAVITIES P. Baudrenghien, G. Hagmann 4/4/2012LIU meeting 1

2. Motivation for upgrade The present system has only amplitude/phase control with low bandwidth (measurement in the center of the beam batch). It cannot control transient beam loading The new system will include 1-T feedback, feedforward, longitudinal damper (dipole and quadrupole – if needed), longitudinal blow-up and built-in observation and post-mortem The design is much inspired by the LHC 400 MHz LLRF. It profits from synergy with the ongoing 352.2 MHz LLRF design for Linac4 Before designing, we will develop a detailed model (including cavity response, transmitter nonlinearities,…) to predict the influence of technical specifications on beam stability. A similar exercise was done for the LHC by the LARP collaboration (SLAC). We wish to involve this collaboration in the SPS upgrade. 4/4/2012LIU meeting 2

3. Retrospective The first design to control transient beam loading (and improve the threshold for longitudinal coupled-bunch instability) in a TWC cavity (SPS 200 MHz) was made by D. Boussard in 1985 with G. Lambert on charge of the electronics [1]. They named their design One-Turn Delay Feedback. Their system used the signals from combiners of all antennas of each cavity (with delays corresponding to the particle time of flight), summing on all four cavities and acting back on the drive of two cavities. We had two 4-sections and two 5-sections cavities. The drive was sent to one cavity of each type with delay/phase balancing. The bandwidth was limited to < 1 MHz. 4/4/2012LIU meeting 3

4. When preparing the SPS as injector into the LHC, improved impedance reduction was needed and I inherited the project of upgrading the design, with the following requirements One system per cavity Increased bandwidth Addition of a feed-forward The LLRF was designed with more than 10 MHz BW… but first tests with the TXs in 2000 showed that the power limitations had not been properly included in the design [2]. The non-linear phase characteristic of the TX, out of band, and the non-linearity, actually localized in the Class A solid state drivers much reduced the usable BW. Others have done similar errors: The PEP II impedance control was effectively limited by the non-linearity of the klystron drivers. 4/4/2012LIU meeting 4

5. For the LHC we have attempted to integrate the LLRF design in its environment from the start Three questions are essential: How much is the beam affected by the LLRF technical choices? Imperfections result in poor transient beam loading compensation, longitudinal stability issues and RF noise driven emittance blow-up What is the effect of the High Level imperfections? The non-linearity and frequency response of the power chain must be considered from the start What is the importance of imperfections in the LLRF on the overall performances Typical imperfections are misalignments (slightly offset phase of an RF feedback for example) or noise figure of the various components. An answer can only come from a detailed model of the RF chain. That was done for the LHC in a LARP collaboration with SLAC. Key players had designed and operated the much similar PEPII cavity controllers (J. Fox et al.) 4/4/2012LIU meeting 5

6. 4/4/2012 6 LIU meeting This resulted in identifying the few key elements in the LLRF that were critical to limit RF noise, and the sensitivity of beam stability to misalignment in the LLRF parameters [3] For example, it was found that a 5 degrees offset in the RF feedback phase severely distorts the flat response of the closed loop feedback, resulting in a four-fold increase in growth rate of the most unstable coupled- bunch mode driven by the LHC cavity impedance at the fundamental (opposite) [4]

7. A detailed model for the 800 MHz We wish to orient the LARP collaboration on the development of a detailed model of the SPS 200-800 MHz RF It would include a detailed description of the hardware: Non-linearity (mainly High Level part), noise (both low and high level), misalignments Our SLAC colleagues are interested This would serve as a guidance when making technical choices in the LLRF design We need backing for the LARP meeting. 4/4/2012LIU meeting 7

8. Basic functionalities The key features of the SPS 800 MHz Cavity Controller are A strong RF feedback to reduce impedance at the fundamental (transient beam loading and stability issue). Given the loop delay in the SPS installation (TX on the surface), this can only be a One-Turn Delay feedback A feedforward, generating a TX drive from a beam current measurement (PU) further reducing the beam induced voltage A local loop around the TX to make the outer feedback insensitive to the TX gain and phase changes, and to locally reduce the noise produced by the high Level (called klystron polar loop in the LHC and Linac4 designs) A channel to modulate the cavity field in phase and amplitude, as actuator for a longitudinal feedback (dipole and quadrupole mode respectively) A similar channel to implement longitudinal blow-up at 800 MHz (either via phase or amplitude excitation) A facility to remotely measure open-loop and closed-loop response for remote setting-up Built-in observation memories for diagnostics and post-mortem Unique to the SPS 800 MHz design No need for tuning (TWC cavity) Requirement to keep phase alignment with the voltage measured in the sum of the four 200 MHz cavities. This system is considered as the master. The SPS RF is not democratic! 4/4/2012LIU meeting 8

9. Block diagram 4/4/2012LIU meeting 9

10. Travelling wave cavity The TX can influence the accelerating field through the main coupler only, while the beam excites the field in each cell, thereby generating a travelling wave that propagates through the structure The Beam loading impedance is therefore different from the Generator Induced impedance and at some frequencies beam loading compensation is impossible [5] 4/4/2012LIU meeting 10 Left: Forward transfer impedance Zrf as a function of deviation from the centre frequency. Right: Beam transfer impedance Zb as a function of deviation from centre frequency (Top = real part, bottom = imaginary part). Above plots correspond to the 4 sections 200 MHz cavities-

11. 800 MHz TX-cavity chain Cavity parameters Cavity response first zeros at +- 3.145 MHz TX response: - 3 dB at +- 1 MHz We aim at +- 6 MHz for the feedback. 4/4/2012LIU meeting 11 Centre freq.800.888 MHz Phase advance per cell  /2 Group velocity v g /c+0.035 Cell length93.5 mm Total length L (37 cells)3.460 m Series impedance R 2 0.647 M  /m

12. OTFB response [6] The One-Turn feedback response has peaks at n f rev This response is easily synthesized if the sampling clock is a multiple of the revolution frequency The sign inversions due to the cavity response occur at fixed frequencies (independent of f rev ). These 180 degrees phase inversions must be compensated in the RF feedback to avoid Close-Loop instability. This is done by inserting a filtering that mimics the cavity zeros, the synthesized cavity We use a sampling clock locked to the revolution frequency and must therefore shift the synthesized cavity response during the acceleration ramp (demodulation-filtering-modulation). 4/4/2012LIU meeting 12

13. Planning The cavities have been equipped with one antenna in every two cells The summing network is being designed (help from D. Valuch). We will have a prototype before end p run 2012 In the meantime the electronics is being designed with simplified functionalities (no feed-forward). It will be tested with beam in second half 2012, using center probe only (and fixed frequency?) The design will be modified according to these first results and completed during LS1 4/4/2012LIU meeting 13

14. Current 800MHz System (NIM) LL Cavity 1LL Cavity 2 LL Common 4/4/2012LIU meeting 14

15. New 800MHz System (VME) LL Cavity 1LL Cavity 2 4/4/2012LIU meeting 15

16. New 800MHz System (VME) RFFGCMMSwitch&Limit Cavity Loops 200MHZ Quadrupler Clock Distributor Linux FrontEnd CTRV VME bus (A24D16) RF LowLevel Backplane 4/4/2012LIU meeting 16

17. VME Cards Switch & Limit Clock Distributor RF design & FPGA (Controls, Acquisitions,…) on the same board 4/4/2012LIU meeting 17

18. Needs for one 800MHz Cavity : Module NameStatus Linux Front-end (CPU)Installed CTRV (timing)Installed CMM (Crate Management)Installed WBS (Wide Band Switch)LHC Spares available, need new series production (only needed for ions operation) RFFG (Function generator)PCB under design, V1 mid-June 2012 Switch&LimitProto V2 under test Clock DistributorProto V2 under test 200MHz QuadruplerProto in production, V1 mid-May 2012 Cavity Loops (RF Feedback)PCB Under design, V1 August 2012 Veto SumUnder specification VME Cards 4/4/2012LIU meeting 18

19. Digital filtering & IQ demodulation TWC 800 MHz pLHC Frequencies : PLL : K=2, M=31, N=8 Frf 200 = 199.943MHz -> 200.395MHz ∆Frf 200 = 452KHz ∆Frf 800 ≈ 1.8MHz LO = M/N * Frf 200 ≈ 775 MHz Fs = Frf 200 / K ≈ 100 MHz F if = Fs/4 ≈ 25 MHz Fs 4/4/2012LIU meeting 19

20. 4/4/2012LIU meeting 20

21. TWC 200 MHz Phase Σ 4/4/2012LIU meeting 21

22. Comb filter (filter with beam synchronous clock) Gain Frev @ injection Freq Gain Frev @ extraction Freq 4/4/2012LIU meeting 22

23. Cavity filter Cavity impedance is complex [sin(x)/x], sign changes @ the zeros => Need absolute filter response (The cavity response does not change during the acceleration ramp!) Freq Fcav 800.888MHz ~3.2MHz Frf flat top ~801.6MHz ∆f=+0.71MHz Frf flat botom ~799.8MHz ∆f=-1.12MHz  ~12MHz (first two zeros) feedback bandwidth is requested  Digital filter clock is derived from the RF (Beam sync clock) 4/4/2012LIU meeting 23

24. Cavity filter (in baseband) As the Digital Cavity is beam synchronous clocked, its absolute transfer function changes with the RF frequency. In order to compensate this effect, prior and after the filter, the feedback signal is modulated (digitally) with the beat frequency (∆f=Frf – Fcav) ∆f F F Before Filtering F ∆f After Filtering F After Filtering without down/up modulation 4/4/2012LIU meeting 24

25. Cavity filter (implementation) 4/4/2012LIU meeting 25

26. References [1] D. Boussard et al., Controls of Strong Beam Loading, IEEE Transaction on Nuclear Sciences., 1985 [2] P. Baudrenghien et al., Control of strong beam loading. Results with beam, Chamonix 2001 [3] T. Mastoridis et al., RF system models for the CERN Large Hadron Collider with application to longitudinal dynamics, Phys. Rev. Sp. Topics. AB, 13, 102801, 2010 [4] P. Baudrenghien et al., The LHC RF System. Is it working well enough ? Chamonix 2011 [5] D. Boussard, Travelling-Wave structures, Joint US-Cern-Japan Intl School, Tsukuba, 1996 [6] P. Baudrenghien, CAS RF 2000 and 2011 4/4/2012LIU meeting 26