Orbit control for machine operation and protection

1. Orbit control for machine operation and protection J. Wenninger AB/OP • Orbit control requirements • Feedback performance & limitations • Feedback architecture • Summary & Outlook Main persons involved in orbit FB (past & present) : L. Jensen, R. Jones AB/BDI J. Andersson, S. Chtcherbakov, K. Kostro, T. Wijnands AB/CO M. Lamont, R. Steinhagen, J. Wenninger AB/OP Q. King AB/PO Chamonix 03 / Presentation 5.5 / J. Wenninger

2. Stabilization requirements I • Collimation (see also R. Assmann): • Cleaning section : < 0.3 s  70 mm • TCDQ absorber in IR6 : <  0.5 s  200 mm @ 7 TEV …for nominal performance in physics and for b* = 0.5 m !! Collimation inefficiency versus position error Stabilization to  200 mm is sufficient : • On day 1 + some e. • @ injection and during the ramp. • For larger b* in physics. Chamonix 03 / Presentation 5.5 / J. Wenninger

3. Stabilization requirements II • Beam dumping system (see also B. Goddard) : • CO stabilized to  1 mm (peak) @ kicker & septa in IR6 – H plane only !  in the shadow of the collimation requirements / TCDQ. • Injection : • CO stabilized to 0.2 mm rms at the TDI. • Machine protection : • Stabilize CO around the WHOLE ring to ensure that the aperture limits are always in the collimation section. Very important for the triplets. • Machine performance & operation : • Minimize beam excursions with respect to reference CO to help control feed-downs from multipoles (injection & snapback). • Stabilize the orbit during the squeeze. • Minimize beam movement at the IRs in physics. • Make life (much) easier for operation ! Chamonix 03 / Presentation 5.5 / J. Wenninger

4. Ocean waves (S. Redaelli) 1 mm 1 nm IP4 OPAL cavern Ground motion @ LEP • Observed orbit drifts :~ 200-500 mm rms over a few hours ~ 20-50 mmrms over ~ minute(s) • LEP/LHC tunnel is a quiet place. Ground motion spectrum ~ f-3 @ b = 100 m • orbit rms    ground movement • Uncorrelated motion :   35 • Waves (E. Keil): • f < 5 Hz   1 • f > 5 Hz1 <  < 100 • CO movements at f > 0.1 Hz are • in or below the few mm range ! @ b 100-150 m Chamonix 03 / Presentation 5.5 / J. Wenninger

5. Magnet girders @ LEP Vertical low-b quadrupoles @ LEP moved vertically ~ 100 mm during the machine cycle : • Orbit drifts of 2-5 mm rms • dominant effect on LEP orbit • Not entirely reproducible • Related to temperature • Lot’s of problems in the ramp • due to the absence of a • real-time feedback. • = kick due to low-b movement @ one IP 1 mrad  40 mm rms @ b=100 m We must watch out for : • Triplet movements • Vibrations (cryo…) Chamonix 03 / Presentation 5.5 / J. Wenninger

6. Orbit movements during Snapback and decay • Random b1 errors (~ 0.75 units)  1 mm rms in the horizontal plane (with a large spread). • Random a1 errors (~ 2.6 units)  3-4 mm rms in the vertical plane. • Feed-down from b2 errors  0.2 mm rms in both planes ! Chamonix 03 / Presentation 5.5 / J. Wenninger

7. Ramp, squeeze, collisions • Ramp : • “Experience” shows that drifts of few mm rms have to be expected. • Magnetic centre of the warm quads expected to move by ~ 100 mm. (should be Ok !) • Squeeze : • Large drifts – up to 20 mm rms (IR1 & IR5 b* : 18 m 0.5 m) • Effects are very sensitive to the input conditions : orbit offset, b-fct and strength change in IR quads. Collisions : • Ground motion … • (Parasitic) beam-beam kicks. @ LEP the inability to control the orbit in real-time during ramp & squeeze probably cost us ~ 5% overall efficiency ! and was responsible for > 30% of the lost ramps. Chamonix 03 / Presentation 5.5 / J. Wenninger

8. Orbit drifts & requirements in short • Most drifts occur / build up on time scales of few seconds to minutes.  need a good feedback gain at and above ~ 0.1 Hz. • The squeeze could be the most delicate phase for the orbit FB. • The most critical requirement apply during collisions where slow ground motion is hopefully the main ‘enemy’… • During the initial operation, requirements are not as stringent – 200 mm rms tolerance is probably OK. • Most perturbations produce ~ REPRODUCIBLE drifts (except ground motion)  80% (?) or more of those effects can be anticipated and feed-forward.  reduces load on FB. Chamonix 03 / Presentation 5.5 / J. Wenninger

9. Power converters & magnets • Cold orbit correctors : • Circuit time constants t 10 to 200 s (arc correctors ~ 200 s). • For small signals the PC is limited to frequencies of ~ 1 Hz. Warm orbit correctors : • Circuit time constants t ~ 1 s. • PC could run well beyond 10 Hz ! • Too few of them in the cleaning section to build a closed correction !  would need warm (or super-power cold) correctors in the cold section of the machine ! • Cannot profit from their speed – we could consider slowing them down to remove this source of fast orbit movements ! Controls : • All PCs accept real-time input @ up to 100 Hz. • Each PC can only be controlled by a SINGLE feedback loop ! Chamonix 03 / Presentation 5.5 / J. Wenninger

10. 10 Hz sampling of the LHC beam cycle in the SPS averaged over 2 hours Start of ramp Orbit acquisition Per ring and plane : 500 orbit measurements ~ @ every quadrupole. The real-time orbit acquisition will run at 10 Hz. For a good FB performance : sampling frequency ≥ 20 x (fastest perturbation to stabilize)  FB limited ~ 0.5 Hz ! • SPS tests in 2002 on 4 BPMsequipped with LHC readout: • Transmission delays over standard SPS network are OK for 10Hz CO. • Very good electronics performance. • CO resolution < 20 mm for nominal intensity. Extr. flat top Chamonix 03 / Presentation 5.5 / J. Wenninger

11. Gain = 10 @ 0.1 Hz Gain = 1 @ 1 Hz Feedback performance Feedback gain (not ultimate performance !) • Delay of 1 period (100 ms). • Limitations due to the correction strategy not included ! • 2 period delay (200 ms) may be more conservative for initial operation…  reduced gain. To improve the performance towards higher frequencies  orbit sampling of 20 Hz or more ! Chamonix 03 / Presentation 5.5 / J. Wenninger

12. Complications, complications… • Ramp : Energy tracking. • Squeeze : • Orbit response matrix must be updated to track optics changes. • Reference orbit must be updated (crossing scheme…). • LHC energy stabilization at injection with horizontal orbit correctors : • The same correctors are also used by orbit FB.  FB also responsible for energy ?  Energy trims not via real-time inputs since very slow changes ! • Ring 1 – Ring 2 coupling in IRs 1,2,5 & 8 : • Handle rings individually or in common ? • Individual ring handling will NOT work well for the squeeze. • Trims : Must allow some form of manual corrections (bumps, Xing angles …). • Post-mortem diagnostics Chamonix 03 / Presentation 5.5 / J. Wenninger

13. Feedback Strategies • Global correction / feedback: • By definition such a FB affects the orbit in (at least) one entire ring. Local correction / feedback : • Uses a subset of monitors and correctors. • Provides a LOCAL correction, i.e. does not affect the orbit outside its ‘working’ range. Requires a buffer region to enforce the closure. Collimation IR This is NOT really what we want (for protection…) ! Chamonix 03 / Presentation 5.5 / J. Wenninger

14. Local Corr. # 1 Local Corr. # 2 Global Corr. Local Corr. # n Input Orbit Corrected Orbit Predicted Orbit Predicted Orbit … Marrying local & global FB loops • The classical approach (Light sources) : frequency de-coupling • Very fastlocal loops (> 50 Hz), sampling rate ~ kHz. • One slowglobal loop (0.1 Hz). • Does not work (well) @ LHC due to the ‘slow’ sampling and large perturbations during snapback and squeeze. • A single global loop with chained corrections : • Can apply both global & local corrections – complete info available ! • Very flexible & easy to (re)configure. • Avoids correction weighting – tricky to tune. • Total correction = S corrections Chamonix 03 / Presentation 5.5 / J. Wenninger

15. FB Centralized feedback architecture • Global correction as “workhorse” – good to satisfy most requirements • entire CO information available. • can be made rather insensitive to bad monitors. • can be easily configured and adapted. • numerical problems are more complex. • large amount of network connections to front-ends. • Local corrections • ensure tight constraints in local sections… • (very) sensitive to faulty monitors. • Data transfer • first tests  OK ! • lightweight ‘protocols’ please ! Chamonix 03 / Presentation 5.5 / J. Wenninger

16. Ground motion correction in collision • Simple global correction : • “Conservative” correction strategy – insensitive to isolated faulty BPMs. • Decouple rings (i.e. common beam pipe elements not used). Residual orbit shifts after ~ few hours of coast / 1 beam Primary Coll. IP1 s =10 mm s = 17 mm Note the very large residual drift @ IP1 despite a 100 x smaller b  correction strategy ! Chamonix 03 / Presentation 5.5 / J. Wenninger

17. FB FB FB FB FB FB FB FB Entirely local feedback architecture • reduced # of network connections. • numerical processing simpler. • less flexibility. • not ideal for global corrections. • coupling/X-talk between loops is an issue. • problem with boundary areas to ensure closures. Example of an aggressive solution… the Swiss Light Source… Chamonix 03 / Presentation 5.5 / J. Wenninger

18.  A = SLS : global correction with local loops ! One can cast the solution of the orbit problem in the form of a matrix multiplication (q = kicks, y = input orbit) LHC matrix Each local FB loop receives a piece of the matrix to perform a global orbit correction (+ needs to talk to its neighbor !). All non-zero elements are very close to the diagonal Equivalent to a MICADO correction using ALL AVAILABLE orbit correctors of the machine – every “bad” monitor kills you ! Chamonix 03 / Presentation 5.5 / J. Wenninger

19. BPM reliability in critical areas • Cleaning Section : • Stabilization to the required accuracy with a local correction can only be achieved throughout the cleaning sections if the BPMs are reliable at the level of  50 mmor better. • To detect systematic errors at the level of 100 mm or less is not simple ! • Those BPMs are installed in a very difficult area (radiation). Triplets – inner IR region : • The directional couplers in the common beam tube have a tough job to separate the beams. • This is a critical region with b* = 0.5 m – aperture ! • Experience will show how much we can trust them. • Fortunately we start with 75 ns bunch spacing  OK ! Chamonix 03 / Presentation 5.5 / J. Wenninger

20. Summary & outlook • Stabilization requirements for protection & collimation • Tough @ 7 TeV + squeezed – but no show-stoppers. • The squeeze is likely to be the most delicate phase. • Architecture & correction strategies • More systematic simulations & tests required to : • choose implementation – local / global… • check ring decoupling and strategies. • Fast orbit movements or failures cannot be avoided by any orbit feedback  interlocks on beam movement / beam position. • SPS tests in 2004 • Test of a closed local orbit FB based on 6 BPMs equipped with standard LHC electronics  good test bed & milestone.  end 2003 Chamonix 03 / Presentation 5.5 / J. Wenninger