Feedbacks & Stabilization Getting them going J. Wenninger AB/OP Machine parameters to stabilize. How far can one get without FBs ? Thanks to A. Burns & R. Jones 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Chamonix 03 / Presentation 7.5 / J. Wenninger Not discussed here The following (really) fast feedback systems : Feedback loops within the RF system. Transverse damper system : This system is required at an early stage to damp injection oscillations and instabilities. Instrumentation details : BPMs, Q-meters, PLLs, AC-dipoles, reference magnets… Please refer to the various presentations of the instrumentation and magnet groups. 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Main parameters to stabilize Orbit ~ 50 - 250 mm Energy @ injection 10-4 Tunes < 0.003 Chromaticity ~ 1-2 units Coupling < 0.005 Luminosity in physics  Mike’s presentation(s). Growing complexity Injection and snapback (ramp)… affected by large dynamic effects ! are the first steps we have to master ! Concentrate the discussion on those phases for the nominal cycle. 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Anticipated variations during decay & snapback Energy : few 10-4 b1 decay, Earth tides… Orbit : ~ 1-4 mm rms. Tunes : not so tough but … snapback Chromaticity : ~ 80 units ! Coupling : feed-down… snapback Q’ is the really tough one ! 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Chamonix 03 / Presentation 7.5 / J. Wenninger Feedbacks… Open loop feedback : The reference magnet / multipole factory is our main OL system. Such systems are very sensitive to model errors. Closed loop feedback : Preferred choice ! Feed-forward : Measurements at time t  anticipate changes at t+ Dt (injection/filling  snapback, one fill to the next, …) Relies on the machine reproducibility  magnet cycling …. 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Setting up for injection… Magnets on pre-injection plateau Multipole factory & reference magnets : evaluate corrections for b2, b3…. incorporated into the PC settings. With experience : include ‘empirical’ corrections based on beam measurements. Magnets to injection settings The persistent curr. decay changes orbit, energy, Q, Q’… during the next ~ 15 minutes. Changes snapback at the start of the ramp. Get some beam – perform rough corrections… If we have them, beam based FBs take full control with beam !  need beam all the time to track the parameters. When do we need the beam-based feedbacks ? 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Chamonix 03 / Presentation 7.5 / J. Wenninger Orbit feedback Measurement and correction are well established, details to be sorted out / finalized  yesterday’s talk ! At injection one can survive with a simple LEP-like measure-correct orbit ‘auto-pilot’ feedback : - relaxed tolerances - drifts are slow enough The orbit FB should be implemented at an early stage with moderate gain, in any case before we start ramping. 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Energy feedback on injected beam First beam(s) : match energy scale of SPS & LHC ! Regular operation : Follow energy changes by adjusting hor. orbit correctors in the LHC. ! Sort out interferences with orbit FB – both use the same elements. Measurements : Comparison trajectory of injected beam – closed orbit. Phase of injected beam… This feedback is not very critical with first beams – mainly required for larger intensity to keep the amount of un-captured beam at an acceptable level. 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Q, Q’ & coupling : single kick measurements Good for Q, Q’ (head-tail monitor) and coupling. Detection : Dedicated high resolution monitors (mandatory for head-tail). Orbit system (multi-turn). data readout : problem for the orbit FB due to front-end load ? Main side effect : emittance blow up  limit on kick amplitude and (rate) number ! Collimation : damage to collimators, coll. efficiency, BLM dump triggers  kick amplitude limits : s/4 to s/2 “default” up to ~ 1 s for low intensity (1 nominal bunch ?) Simple “measure and correct” system, once every few seconds ! Not suited for real-time feedback. 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Q kicking We have to accept some e blow-up Emittance increase due to single kicks (A. Burns) : 0.4 mm kicks ~ s/2 resolution 20 mm 50 turns damping time  reasonable Q’.. dQ < 10-3 Snapback Injection Will not work as well at higher energy because s decreases… We have to accept some e blow-up if we kick all the way up to 7 TeV… 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Q, Q’ & coupling : AC dipole measurements Good for Q and (best for ?) coupling. Detection : Orbit system (multi-turn).  data readout : problem for the orbit FB due to front-end load ? ‘No’ emittance blow up if Q is ~ known  excitation outside Q spectrum. Collimation : same amplitude limitations than for single kicks. Could be operated continuously (!) for a real-time correction. Comments on single kick/AC dipole : They are potential collimator-busters ! Careful amplitude interlocking and energy tracking is mandatory ! Difficult to operate at high energy  very small amplitude ! 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Q, Q’ & coupling : resonant BPM Excellent for Q measurements. Q’ obtained from RF frequency (slow) or phase modulation (fast). Detection of ~ mm excitations : Requires a 40 MHz structure on the beam  not suited for single bunches, TOTEM beams !  test ramps must be performed with 12 cons. bunches or so ! Not limited by e blow-up or collimation. Input signal for the Q PLL system  real-time Q FB. Delicate device  not available immediately. Q’ measurement remains delicate even with Q PLL. 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Operation without RT FB Reference magnets / multipole factory : Track multipoles (n ≥ 2) @ few Hz during injection, snapback… Single kick Q,Q’… measurements : Adjust / fine tune beam parameters. Improve predictions of ref. Magnet / M-factory. During filling : Track parameter changes. Limit number of kicks or use sufficient number of ‘test’ shots. At end of filling, before launching the ramp : Collect the integrated changes observed during injection. Anticipate snapback effects by feed-forward to the PC functions.  should take care of the worst dynamic effects.  need a very strict book-keeping of trims ! 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Tolerances without RT FB dQ dQ’  ± 0.01 ~ ± 15 units  ± 0.001 ~ ± 1-2 units  ± 0.003 ~ ± 6 units Assume 20% error on the predictions/ corrections from ref. magnets & M-factory : Beam measurements during filling : Feed-forward for snapback (prediction) : Measure & correct every few seconds in snapback… With experience we might achieve for the snapback : Q ~ OK 0 < Q’ < 10 or better – unlikely to reach ultimate tolerances. How far can we go like that ? With what kind of beams ? 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

ASARP !! Remarks on operation without FB : All effects come together – makes life a bit more difficult ! We must rely on a good understanding of the dynamic effect and on a reproducible machine – with a machine that just started up !! The emittances will surely be large(r) ! There will be interferences with machine protection system that is also in the ‘commissioning’ phase. Turn-around time is long, over 1 hour even in the best case.  learning by trial and error costs a lot of time… We need a good control system (applications !). … To minimize frustrations and keep a reasonable efficiency (LEPI best was ~ 55 %), we should aim to have operational feedbacks… ASARP !! 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

When should we get FBs into operation ? Orbit : We should start using it at an early stage, before we ramp. Energy : Required at latest when RF capture losses become too high. Q,Q’ : We could survive without FB … but life will be difficult ! It is worth investigating this point in detail (LHC-OP WG ?). We should aim for an operational Q PLL system ASARP ! This will make life  easier ! TOTEM beam , single bunches : Must rely on AC-dipole and beam kicking + feed-forward. Beam-kicking incompatible with small TOTEM e ? 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger

Chamonix 03 / Presentation 7.5 / J. Wenninger This is not LEP ! LEP was commissioned 1989 and … The tune feedback was operational in 1997, A simple orbit feedback in 1995 (physics only). …for operation of the LHC we cannot wait such a long time ! 06.03.2003 Chamonix 03 / Presentation 7.5 / J. Wenninger