1. Beam Steering Tools ‘Slow’ Steering Orbit feedback Status & plans
Jörg Wenninger
for the steering & feedback team: R. Steinhagen, G.Sivatsky, JW
Acknowledgements to the usual BI friends, L. Jensen & R. Jones.
‘Slow’ Steering
Application overview
Threading
Calibration & K-modulation
Tools
Orbit feedback
Status & plans 1 1

2. High Level (LSA) Steering Application
Developed with LEP/SPS experience in mind.
- Keep the good stuff (algorithms, tricks...).
- Remove limitations, clean structure, from C to JAVA, exit COCU.
- ‚Loose‘ coupling to LSA and CO-standards – can run „LSA-free“.
Generic core + machine specific add-ons.
- Generic is never satisfactory !!
- Able to handle N rings/lines with beams propagating clock-/anticlockwise,
coupled by M common regions.
In the process of invading (being sucked into) all CERN machines.
LEIR in 2005.
SPS and all its transfer lines in 2006 (tests since 2003).
LHC and PS in 2008.
New machines profit from previous ‚debugging‘.
Interventions on code more delicate – consequences on all machines.
Has to handle
11 different ‘BPM’
acq systems. 2

3. Functionality overview
General tools:
- Control of element status and calibration.
- References, reference catalog, autosave.
- Interpolation to any element, time evolution plots.
- Internal post-mortem.
- Usable as fixed display.
- etc
Trim tools:
- Trim via LSA.
- Trim ,scanning‘ (for example bumps).
- Internal trim history (in addition to LSA).
Automation:
Slow feedback based on any algorithm.
Response measurements.
etc
Algorithms:
- MICADO & SVD in their full glory and flexibility. Enhanced diagnostics wrt past.
- Short range corrections a la LEP, single beam and two beam.
- Bumps of any shape anywhere in the ring, automated or manual.
- Threaders.
- Bare corrections.
- Internal simulation tool.
- etc
Optics tools:
- Dispersion measurement & model.
- Response measurements & online check.
- Betatron oscillation fits.
More stuff in the pipeline … 3

4. LHC Position Data Sources
Injection: up to 50 turns, provided by JAVA data concentrators.
‚FIFO‘ mode :
Commissioning mode.
Asynchronous, no bunch timing.
Only for empty machine & single bunch.
Capture mode :
Programmed bunch selection (injected buckets).
Closed Orbit: from orbit feedback data concentration.
1 Hz down-sampled stream :
Data stream of 1 Hz, down-sampled from raw rate of 10/25 Hz.
1 second average stream :
Data stream of 1 Hz.
Raw data averaged over 1 second.
10 second average stream :
Data stream of 0.1 Hz.
Raw data averaged over 10 seconds.
All streams are available from the application.
All streams have been ~ tested.
Fune tuning/stress testing to be done... 4

5. Threading
Fundamentally there is not a large difference between threading & closed orbit correction... What makes threading a bit spicy :
Large machine.
- Beam only reaches ~end first arc without ‚intervention‘.
- ~15-20 iterations to get around the ring, depending on what you find...
Must eventually establish a closed orbit.
First beam ever in the LHC.
- Commissioning of BPMs at same time as the machine is explored.
- Commissioning of the steering SW at same time as the machine is explored.
- There can be surprised....
>> Technically threading LHC and LEP is not very different.
(see also demo to come)‏
>> In the worst case, threading must be interleaved with polarity & optics checks etc.. 5

6. Threading Strategy Efficient LEP strategy :
Work on a limited region where the amplitude grows :
shorter : minimizes possible no. of surprises within region (optics etc).
longer : less sensitive to isolated BPM errors.
Apply MICADO with few correctors (1-3) :
Minimize impact of polarity errors & bad BPMs.
Detection of poor BPMs :
Compare predicted and achieved correction.
Use your experience ???!!
Detection of COD errors :
Compare predicted & achieved correction. Use few CODs / step.
Example for a first turn LHC b1
Horizontal
Vertical 6

7. Demo :Threading Test LHC Beam1
Trajectories simulated in MADX with errors…
MADX output is conditioned to take into account aperture, add noise…
Conditioned data is imported into steering application.
Corrections are evaluated and re-exported to MADX.
Iterate, iterate…
‘Conditioned’
trajectory file
Corrector
changes
Aperture cuts,
noise….
MADX
Applied an ‘effective’ aperture of 16mm (H) & 11 mm (V) everywhere.
Beam considered lost if trajectory exceeds those values.
Trajectory file
LTC / June 2005 7

8. Threading Test 13 iterations, fluctuates between ~10-20 Conditions :
BPMs : ± 3 mm errors, flat distribution.
Quads :
misalignment : 0.4 mm rms (gauss).
b2 = ± 200 units random, flat distribution
 > 100% -beat (closed orbit)‏
Dipoles :
b3 = -20 units (systematic)‏
other components : standard error table
Multipole correctors : OFF
Position (mm)
LTC / June 2005 8

9. !!! Energy Mismatch !!!
The precise energy at extraction from the SPS is GeV/c (for a ‘nominal’ setting of 450 GeV/c), i.e. off by -1.8 permill.
For the moment we have not adjusted that value…
Wait and see how it goes in or adjust – to be decided.
If the energy mismatch between SPS and LHC is too large (few permills…) the beam may not make it into the first arc !
>> If the beam is stuck at the entrance of the first arc we may have to make an energy scan of the beam extracted from the SPS (TI2/8 dp/p aperture ~ 0.3%). 9

10. !!! Common Regions !!!
While threading one beam, one has to pay attention to COD in the common regions, since exciting them will also affect the beam that comes later.
The algorithm(s) may pick a common region COD to correct an error that is actually located nearby in the non-common region  not ideal for the other beam !!
There are various strategies to cope with the common regions:
At one extreme…
Do not touch common region CODs before both beams are circulating together.
May not work….
At the other extreme…
Steer each beam without paying attention to the common regions.
When switching from one beam to the other, reset the common region CODs to zero or whatever optimum kick of the other beam.
When both beams are circulating individually:
Compute the bare orbit for each beam (unfold all kicks).
Calculate a combined correction for both beams.
Inject both beams on the newly corrected orbit, it should work… 10

11. First Turn to Closed Orbit
When the beam reaches the end of the first turn, either it will make N (N > 2) turns right away, or one has to work a bit more on the first turn....
When the beam makes ‚multiple‘ turns, a closed orbit can be obtained by
Closing the second turn on the first
- a priori only a fraction of turn 2 needed (< 20%).
Estimating the CO from average of turn 1 to N, followed by CO correction
>> Both options are available
When you start working on the CO, optics errors (tune off, beta-beat) start to strike you and can make corrections diverge:
In case of convergence problems, need to check Q / optics. 11

12. Steering is affected by…
BPM calibration factors (and offsets)‏
- Isolated errors are not critical at the beginning (depends on HOW one steers).
Corrector calibrations and polarities.
Optics model uncertainties.
- Beta beating. CO steering Ok until ~50%, stops working around 100%...
- Tune error : CO response ~ 1/sin(Q)‏
>> divergence if Q approaches integer, or if Q far away from model.
>> The steering application provides automated response measurements to probe calibrations & optics.
Output in format suitable to inject into fit program...
Used extensively all over the SPS + LEIR.
For the LHC results may not be available immediately. The fits can be tricky due to the large number of parameters.... 12

13. Response Measurements : how much?
The response measurement of a corrector involves applying a +kick and then a –kick and record the orbit (or trajectory) for each case. Time requirement :
~ 20 seconds /corrector.
Possibly more if done with trajectory (SPS supercycle length defines time).
Data sample sizes :
BPM calibration only : 2-4 correctors/plane/arc sufficient.
Rather fast (~< hour).
Corrector calibration: ALL ~1060 correctors in the ring
Lengthy... count 2 shifts.
Must be done sooner or later.
Optics model: similar to BPM calibration case 13

14. Calibrations : when? The more parameters, the heavier the fit...
>> Ideally perform systematic BPM/corrector calibration when the optics model is corrected (by other means).
If there are problems with CO correction/first turn and there is a suspicion on the optics, limited number of measurements with first turn.
Determine tune value.
Identify major error sources.
>> Successful for TI2, TI8 & CNGS – but much reduced ‘scale’.
Else
Insert into commissioning as needed/when suitable.
Corrector calibration:
At the latest just before orbit FB commissioning 14

15. Trajectory difference The phase advance (tune)
TI2 Example : online diagnostics possible !
One can detect optics errors ONLINE with a few clicks by comparing the model response with a measurement. And correct the machine…
Below an example for TI2 :
Model
Trajectory difference
(response to a kick)
The phase advance (tune)
is wrong (by ~1%) 15

16. Calibration Factors
Calibration factors can be edited ‘on-the-fly’ (very useful for SIGN errors) while steering to avoid loosing a BPM.
>> BI does not have to intervene right away !!
Default calibration factors may be saved (file) and loaded at program startup. DB storage may be implemented if required…..
Editable :
Position, calibration & offset
‘OP Enable’ : manual enable/disable 16

17. BPM offsets & K-modulation
The response measurements do not determine possible BPM offsets.
In case we have suspicions on BPM offsets, we have the option the perform K- modulation on some magnets.
BPM offset determination by K-modulation. One has to :
Modulate (individual quadrupole) strength:
OK for insertions (ind. powering), not for arcs.
Strength changes at the 0.1% level, sinusoid.
Bump beam back and forth in the quadrupole (steering application).
Measure orbit oscillation.
Track with BBQ system.
>> ‚Bumping‘ will be automated in the steering application.
>> Need SW for the analysis :
Correlation of orbit data in quadrupole with BBQ data.
Fit of offset. 17

18. Intensity Mode
When the BPM system is switched to intensity mode, the intensity data of beam X is returned as position data of beam Y. A sort of mess...
- Need special mapping of the data of one beam to the other.
- For the moment intensity would appear as position of the other beam.
The BPM trigger threshold is around 1-2x109 p:
With pilot intensity the usefulness of the intensity mode is rather limited...
With pilot : not trigger  gone below 1-2x109 p...
To be useful the intensity mode requires more beam to be injected !
See R. Jones’ presentation ! 18

19. Energy Offset Measurement & Correction
Relative energy can be easily determined arc by arc for each beam.
Dynamically generated ‘internal knobs’ are available to correct individual arcs with orbit correctors. Perturbations to rest of the machine is minimized.
Example : 0.1% dp/p arc 23
Position change
Corrector settings 19

20. Local CO Corrections
The LEP ‘short length’ correction algorithm (Limberg & Herr) is available. It was used extensively at LEP for local corrections in insertions.
Applicable only for single beam/outside the common regions !!
Before correction
Difference
After correction 20

21. Local 2-beam CO Corrections
The LEP ‘short length’ correction algorithm was extended to provide a simultaneous correction of both beams in the insertions including the common regions. Solves both boundary conditions.
Before correction
Difference
After correction
Beam 1
Beam 1 21

22. Detailed bump shape plot
3C and 4C bumps may be build at any location in a ring or a line.
The correctors may be defined automatically or manually (long bumps, tricky local optics conditions…)‏
Once it is calculated a bump can be scanned back and forth with simple clicks.
Automated bump scans will become available:
It is be possible to implement “simple” automatic scans to constant amplitude in mm or in sigma based on a user selection of locations.
>> Note that such scans may drive the beam into the aperture…
Detailed bump shape plot
(SPS example)‏ 22

23. A feedback module is incorporated in the application for :
Slow Feedback
A feedback module is incorporated in the application for :
‚Slow‘ orbit feedback : ~0.2 Hz sampling & LHC.
Not applicable when PCs are executing a ramp/table.
Only for injection plateau, top energy.
Energy correction feedback with CODs (injection).
- Can be implemented on short timescale.
- Wait to gather some experience before implementing.
First turn correction feedback with transfer line (TI2/TI8).
- Can be implemented on short timescale, see above.
- TIx lines & SPS extraction are very stable. Not needed on a shot by shot basis (tbc).
!!! Slow feedback will not work during ramp & squeeze where you need it most !!! 23

24. Middleware, JAVA & LSA have conquered the world !
…. a small group in remote corners of BI & OP is still resisting…
They think that we need to control the beam Faster than what is provided by
‘world standards’!
All the world ?? 24

25. LHC Orbit Feedback
The requirements on the stability of the LHC orbit imply real-time control in critical phases of the ramp and the squeeze which is not provided by LSA.
See Ralph’s presentation on collimation…
At some stage manual playing will be ‘forbidden’, or may lead to immediate quench !
Given the number of requirements for beam stability the feedback will perform a GLOBAL stabilization (and not multiple local FBs).
The feedback stabilizes the orbit around a PREDEFINED REFERENCE. It will not ‚find‘ the golden orbit.
Although the absolute LHC look more relaxed than for third generation light sources, LHC has unique difficulties:
Size
Optics changes in the presence of high intensity beams
Xing angles & separation bumps
Intensity variations 25

26. Orbit Feedback Architecture
Central collection of BPM data from in ~ 70 front-ends.
Correction via RT input of the power converters (~ 50 gateways).‏
Data transfer over Ethernet/UDP protocol – no CMW !!
>> Same architecture used for Q & Q’ FBs
Database &OP
Operating frequency Hz
CMW
feedback unit
BPM-Frontend
Ethernet UDP/IP
Ethernet UDP/IP
PC-Gateway
Service Unit
Interface to DB & OP
Data publication
Matrix preparation…
BPM-Frontend
PC-Gateway
...
...
BPM-Frontend
PC-Gateway
BPM-Frontend
PC-Gateway
Orbit Feedback
Controller
BPM-Frontend
PC-Gateway
Surface
Tunnel
18 BPMs/crate
16 CODs/gateway
...
... 26

27. Full LHC Beam-Based FB Control Scheme
Qref,C-ref Σ
ΔQ,ΔC-
Tune/Coupling
Controller
(Skew-) Quadrupole settings
Qavg
Q'ref
ΔQmod
Chromaticity
Reconstr. Q' Σ
Chromaticity
Controller
Sextupole Settings
f0+Δf, Δp/p
A∙sin(2πfe)‏
reference signal
Phase
Detector
Low-pass
Filter φ
PLL-Control Law
e.g. PID Δf
NCO
orbit ref.
δ, Δp/p, Δf Σ
Orbit Feedback
Controller
Δp/p RF
modulation
A∙sin(2πfe)‏
beam response
R(fe)∙sin(2πfe+φ)‏ Δf
(12x/10x)‏
16x2
32x
1075x2
530x2 x2
2x2
LHC
beam response
BBQ
BPMs
CODs RF
2 (+2) x 2
mini- AC
dipole/
damper
Orbit/Energy Feedback
Tune/Coupling PLL
Chromaticity Tracker/Feedback
Tune/Coupling Feedback
LHC FBs: 2158 input devices, output devices → total: ~3300 devices!

28. Orbit Feedback Status
Data concentration of BPM data in FB controller operational.
- Note that only a fraction of the BPM FECs is operational.
FB controller in advanced state.
Data exchange to PC gateways tested.
- Systematic tests to be done.
Publication of concentrated data to the rest of the CERN World almost ready.
User interface for FB control to be done. Only expert control so far.
Interface to LSA (optics) in progress.
- Waiting for LSA refactoring to finish...
Advanced features (squeeze, Xing angles ...) in preparation...
>> We expect to be ready to operate the orbit FB by June
for injection & ramp, maybe more... 28

29. Orbit Feedback Plans
>> Aim is to switch on as soon as possible...
Requirements :
BPM calibrations checked.
COD calibrations and mappings checked.
Reasonable optics (beta-beat < 50%).
Profit from ‘Space-time’ decoupling for commissioning :
Control in ‘space’ (steering algorithms ) must be working.
Control in ‘time’ (FB gains …) relaxed in the beginning.
A stable orbit is one problem less to worry about ! 29

30. Conclusions
The steering application for the LHC is well advanced and tested.
More powerful than the LEP application of October
Too powerful for a safe LHC??
Orbit FB is expected to be ready for the first beam.
The invincible gauls are prepared for the commissioning battle ! 30