1. 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
Chamonix 03 / Presentation 7.5 / J. Wenninger

2. 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.
Chamonix 03 / Presentation 7.5 / J. Wenninger

3. Main parameters to stabilize
Orbit ~ mm
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.
Chamonix 03 / Presentation 7.5 / J. Wenninger

4. 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 !
Chamonix 03 / Presentation 7.5 / J. Wenninger

5. 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 ….
Chamonix 03 / Presentation 7.5 / J. Wenninger

6. 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 ?
Chamonix 03 / Presentation 7.5 / J. Wenninger

7. 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.
Chamonix 03 / Presentation 7.5 / J. Wenninger

8. 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.
Chamonix 03 / Presentation 7.5 / J. Wenninger

9. 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.
Chamonix 03 / Presentation 7.5 / J. Wenninger

10. 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…
Chamonix 03 / Presentation 7.5 / J. Wenninger

11. 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 !
Chamonix 03 / Presentation 7.5 / J. Wenninger

12. 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.
Chamonix 03 / Presentation 7.5 / J. Wenninger

13. Operation without RT FB
Reference magnets / multipole factory :
Track multipoles (n ≥ 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 !
Chamonix 03 / Presentation 7.5 / J. Wenninger

14. Tolerances without RT FB
dQ dQ’
 ± ~ ± 15 units
 ± ~ ± 1-2 units
 ± ~ ± 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 ?
Chamonix 03 / Presentation 7.5 / J. Wenninger

15. 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 !!
Chamonix 03 / Presentation 7.5 / J. Wenninger

16. 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 ?
Chamonix 03 / Presentation 7.5 / J. Wenninger

17. 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 !
Chamonix 03 / Presentation 7.5 / J. Wenninger