Machine Tolerances in Cleaning Insertions J. Wenninger AB-OP SPS Beam Operation Tolerances Orbit stabilization Beam optics 30.06.2004 Collimation Review / J. Wenninger

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. 30.06.2004 Collimation Review / J. Wenninger

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 ! 30.06.2004 Collimation Review / J. Wenninger

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 ! 30.06.2004 Collimation Review / J. Wenninger

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 IR3/7 <  0.3 s 70 mm TCDQ absorber in IR6 <  0.5 s 200 mm Q-meter and transverse damper in IR4  200 mm Injection points IR2/IR8 200 mm Injection protection devices IR2/IR8 <  0.5 s  500 mm (?) Stabilization for collisions TOTEM experiment IR5 20 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… 30.06.2004 Collimation Review / J. Wenninger

Collimation Review / J. Wenninger Expected orbit drifts Phase Total drift / rms Time scale Comment Injection 2 mm 15 min Decay Start ramp 2 mm 20 sec Snapback Ramp few mm 20 min SPS/LEP experience Squeeze 2-20 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. 30.06.2004 Collimation Review / J. Wenninger

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 (10-200 s) and the available voltage limit useful orbit corrections to f  1 Hz (at 7 TeV) ! 30.06.2004 Collimation Review / J. Wenninger

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 0.2-1 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 ! 30.06.2004 Collimation Review / J. Wenninger

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 ! 30.06.2004 Collimation Review / J. Wenninger

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…. 30.06.2004 Collimation Review / J. Wenninger

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 ! 30.06.2004 Collimation Review / J. Wenninger