1. Machine Tolerances in Cleaning Insertions
J. Wenninger AB-OP
SPS Beam Operation
Tolerances
Orbit stabilization
Beam optics
Collimation Review / J. Wenninger

2. Collimation Review / J. Wenninger
A world of collimators
Operation without collimators (‘All OUT’) is only possible at the LHC with very low intensity and around injection :
One pilot-ish bunch (5 ×109 p)  no quench expected
One nominal bunch (1011 p)  no damage, but risk of quench
At high energy use of collimators will be mandatory, but coarse settings are acceptable for ‘low’ intensity … and during the initial phases.
This is of course independent of the collimator design issue.
Collimation Review / J. Wenninger

3. Getting started at injection
The available machine aperture at injection is 8.5s with a margin of
 4 mm closed orbit + 20% b-beat + mom. offset + mech. tolerances
Protection devices for injection will be set  7s
 primary collimators will be set to 5.5-6s
Both absolute orbit excursions and b-beat must be under control, or else the collimator settings of 5-6 s must be tightened even more !
Collimation Review / J. Wenninger

4. Constraints on machine changes
The following machines changes (wrt a reference situation) lead to a 50% degradation of the nominal betatron cleaning efficiency :
8% b-beating
0.6 s orbit shift
50 mrad angle change
Collimation inefficiency
versus position error
Tolerances are cleary tigher if there is a combined change of b-beat, orbit … as is often the case !
 may have to take a factor 2 off from those numbers !
Note : the simulations were made for an older ‘version’ (2002) of the cleaning system !
Collimation Review / J. Wenninger

5. Orbit tolerances for the LHC
With time demands for orbit stabilization have poped up everywhere around the LHC. A more or less exhaustive list :
Cleaning section IR3/ <  0.3 s mm
TCDQ absorber in IR <  0.5 s mm
Q-meter and transverse damper in IR  200 mm
Injection points IR2/IR mm
Injection protection devices IR2/IR8 <  0.5 s  500 mm (?)
Stabilization for collisions
TOTEM experiment IR mm (!!!)
Protection – global orbit ~ 500 mm rms
e-cloud(*) – global orbit <1000 mm rms ?
7 TeV
Note the expected BPM systematic errors :
intensity (pilot  nominal bunch) 100 mm
bunch length changes (injection – flat top) 100 mm ?
Presently
‘studied’
at the SPS…
(*) : not formally expressed – but to be expected from SPS experience…
Collimation Review / J. Wenninger

6. Collimation Review / J. Wenninger
Expected orbit drifts
Phase Total drift / rms Time scale Comment
Injection 2 mm min Decay
Start ramp 2 mm sec Snapback
Ramp few mm min SPS/LEP experience
Squeeze mm few min Depends on orbit quality in insertions
Collisions few mm hours LEP orbit
Some drifts (ramp, squeeze) are probably sufficiently reproducible to use feed-forward from one fill to the next for the bulk part.
Most drifts become critical on time scales > 1-10 seconds.
Collimation Review / J. Wenninger

7. Ground motion at LEP  If the LHC moves like LEP we are safe.
f  0.1 Hz : no problem expected…
Average ±1s
LEP rms orbit drifts in 1998 for 390 fills, normalized to b=1 m
f > 0.1 Hz :
Amplitudes  O(few mm)
 should be OK
 If the LHC moves like LEP we are safe.
Note :
The large time constants of the orbit corrector power converter ( s) and the available voltage limit useful orbit corrections to f  1 Hz (at 7 TeV) !
Collimation Review / J. Wenninger

8. Orbit feedback overview
A global real-time orbit stabilization + local ‘refinements’ is considered to satisfy all the demands :
Sampling rate 5 – 25 Hz (design is 10 Hz) for corrections at up to Hz.
Upper limit is 50 Hz due to power converter controls.
Data transmission from  70 front-end computers (1000 readings/plane) to central feedback over switched Gigabit Ethernet (LHC technical network).
Central processing on Linux systems (multi-processor) with (almost) hard real-time capabilities. A total processing delay < 40 ms is feasible.
Fan-out of corrections to PC front-end systems (500 correctors/plane).
If there are performance problems  local loops in cleaning insertions !
Collimation Review / J. Wenninger

9. Collimation Review / J. Wenninger
b-beating
With only ~ 8% b-beating change tolerated, a good correction of the optics is required at all stages
Ramp (decay & snapback seem OK).
Squeeze
Dynamic squeeze in collisions for LHCb.
Fancy knobies (arrrgh !!). …
There are lot’s of distributed sources of b-beat
Spool-piece corrector alignement.
Orbit in sextupoles.
Quadrupole calibrations (nominal accuracy ±2 x 10-4  5% b-beat change during squeeze, Ok but near the limit…).
 work ahead !
Collimation Review / J. Wenninger

10. b-beating measurement
Careful optics adjustments will have to be made :
K-modulation : available (as far as I know !) in cleaning IRs
- control of hysteresis effects ? At 7 TeV the quadrupoles will not be far from saturation.
- good measurements require continuous / PLL Q measurements. Not expected to be available before some months after startup.
Kicks / AC dipoles combined with multi-turn BPM data :
- beware of oscillation amplitude limits !
This issue clearly deserves a closer look….
Collimation Review / J. Wenninger

11. On the road to a nominal LHC…
Beam cleaning offers new challenges for retired LEP operation cowboys :
lot’s of tuning ahead – at least as far as the tight tolerance allow it !
The relatively tight orbit control in the cleaning sections is manageable.
Tight optics control is much more tricky. Deserves further studies and … lot’s of work on the beam !
Collimation Review / J. Wenninger