LHC Commissioning Phases

LHC Commissioning Phases Circulating pilot and RF capture presented by G. Arduini for the LHCCWG members Many thanks to the EICs & V. Kain, R. Bailey, P. Baudrenghien, L. Bottura, O. Brüning, A. Butterworth, S. Fartoukh, R. Jones, E. Shaposhnikova, J. Uythoven, J. Wenninger, F. Zimmermann….

LHC Commissioning Phases: Phase 2 – Circulating pilot and RF capture • Phase A.2: Circulating pilot and RF capture • Entry and exit conditions • Objectives • Preconditions and tools • Equipment state/readiness (exit from machine checkout) • Controls, software and tools • Machine setup • Beams, cycles and modes • Basic settings • Commissioning procedure • Breakdown • Details of some steps • Possible problems • Outstanding issues • Summary

LHC Stage A: Commissioning phases Phases for full commissioning Stage A (pilot physics run) Phases for proposed 2007 engineering run

Circulating pilot and RF capture – exit and entry conditions • Entry conditions: • First turn completed • RF OFF • Exit conditions: • Beam circulating for few hundred turns and RF captured • Ready for set-up of the instrumentation for circulating beam and more detailed optics measurements (following 2 phases)

Circulating pilot and RF capture – basic objectives • Establish closed orbit • Commissioning of additional instrumentation: • BPM intensity acquisition • Preliminary orbit, tune, coupling and chromaticity adjustments • Obtain circulating beam (few hundred turns at least) • SPS-LHC energy matching • Commissioning of RF capture

Circulating pilot and RF capture – basic objectives • Tolerances we aim at for this phase: • Closed orbit corrected down to ~1-2 mm r.m.s. • Maximum acceptable radial offset: 0.5 mm • Tune: 64.28/59.38 (tune for commissioning) within few 0.01 • Octant-to-octant MB field offset to few 10-4 • Chromaticity to 10-20 units (to minimize decoherence of oscillations – 1000 turn dynamic aperture requires |Q’|<80) • Coupling to few 0.01 • SPS-LHC energy matching to few 10-4 O. Bruning, Chamonix 2003 S. Fartoukh, M. Hayes, LCC#31

Circulating pilot and RF capture – Objectives Assume that priority 1 objectives of phase A.1 have been achieved

Circulating pilot and RF capture – Objectives

Circulating pilot and RF capture – Equipment state and readiness (entry) Assume that entry conditions for phase A.1 have been met

Circulating pilot and RF capture – Controls, software and tools • Some additional key elements for the Circulating beam and RF capture phase: • Fast Analog Signals available in CCC (OASIS) and Pt. 4 (scopes) • Tools to measure the beam phase shift per turn w.r.t. the reference RF frequency signal • Multi-turn trajectory acquisition • YASP for closed orbit measurement and correction (including closure) • LSA applications, in particular: • Knob for B-field correction (affecting all magnetic elements) by octant and for the overall machine. • Knob for Bdl trim with orbit correctors for the two beams • LOCO available for orbit response analysis • Online MADX model available • Online aperture model available

Circulating pilot and RF capture – Beams, cycles, modes,… • Beam type: pilot Ib: 5 109 p en: 0.3 – 3.5 mm.mrad (small en in case of problems) eL: 0.25 – 0.8 (we can probably go to 2.5) eV.s Dp/p: 0.15 – 0.6 10-3 (r.m.s.) • LHC mode: “inject and dump” (set 10-1000 turns) a) One ring, single injection on demand b) One ring, repeated injection c) Two rings, interleaved repeated injection • Available operational cycles, modes and states (as for injection commissioning) • Cycle: Nominal cycle and wait (> 20 mins after injection level achieved) • Occasional re-cycle (in particular during the energy matching) • Machine protection as for injection commissioning • Beam dump and LHC mode “Inject and Dump” allows screens to be in. Here screens should be OUT and we should have an alarm or interlock when they are IN.

Circulating pilot and RF capture – Machine set-up • Machine set-up as for first turn commissioning, in particular: • Sextupole spool pieces ON and at nominal settings (from magnetic measurements) • Lattice sextupoles ON and at nominal setting (to correct for natural chromaticity) • Skew quads ON and at nominal settings (from magnetic measurements) • Expected machine settings with this configuration

Circulating pilot and RF capture – Breakdown ST=Shift Team

Circulating pilot and RF capture – Breakdown

Circulating pilot and RF capture – details • Instrumentation and corrector checks (unless completed in phase A.1) • BPM intensity measurement is surely important at this stage (if not earlier) to determine potential bottle-necks (in particular soft ones) and to select the data to be retained for the turn-by-turn trajectory data averaging • Systematic BPM sanity check (polarity, control chain, plane, ring and other cabling errors) ~2 shifts (R. Steinhagen, F. Zimmermann) is necessary for the following steps. • This could also provide BPM and corrector calibration and data for off-line linear optics check that could then be made available early in the commissioning in case of problems.

Circulating pilot and RF capture – details • Establish closed orbit: • Measure integer tune from first turn difference for two different injection settings and correct if necessary • Close the trajectory on itself with 2 correctors from difference of 2 consecutive turns (at least a few BPMs after the injection point must provide data in the second turn)  YASP • Closed orbit measurement (average of turn-by-turn data over the number of turns available). The BPM system can cope with an increase in r.m.s. bunch length st from 0.4 to 1.3 ns.  • 140 turns for nominal momentum spread. About 280 turns by reducing the momentum spread at extraction from the SPS by using pilot with 0.25 eV.s and reducing the RF voltage to ~2MV at SPS extraction. • We should then aim to correct the orbit with a small number of correctors to avoid biasing the momentum offset determination among sectors later (A.2.4)

Circulating pilot and RF capture – details • Measurements with few turns: • If it has not been determined from first turn data the integer tune can be measured from the difference of two orbits (one of them with a distortion from a kick) and harmonic analysis. • Fractional part can be measured by exciting injection oscillations and comparing turn-by-turn trajectories or turn-by-turn phase advance measurements obtained combining the turn-by-turn data from pairs of BPM separated by 900 in phase advance. The precision of this measurement is expected to be DQ=0.01 • Adjust the coupling empirically by minimizing the CTA • Any oscillation will die-out rapidly because of the de-coherence due to chromaticity: • e-folding time = 4 turns for QH’=-179 for sdE/E0=3.06×10-4 • 43 turns for Q’H=-17, Q’V=17. Can be further increased by reducing the momentum spread at extraction from SPS (88 turns for sdE/E0=1.5×10-4). • Adjust the chromaticity empirically by maximizing the decoherence time • If the lifetime would not be sufficient systematic linear optics checks might be required

Circulating pilot and RF capture – details • Offsets between the different sectors: • Once the beam can circulate for a few hundreds turns the orbit can be measured and corrected possibly down to 1 mm r.m.s. • In these conditions it should be possible to correct the relative octant-to-octant MB field offset to few 10-4 • The B-field trim should be “propagated” to the multipoles (at least quadrupoles) to keep the correct MQ-MB tracking • MQ-MQ tracking might be difficult to correct at this stage unless the coupling has been minimized J.Wenninger LHCCWG #17

Circulating pilot and RF capture – details • SPS-LHC energy matching • Before the energy matching can start the beam must survive for at least ¼ of a synch. period (~45 turns at nominal voltage – 8MV, about ~35 turns at 16 MV). • Starting point beam centred in the SPS at extraction and in the LHC for both rings (RF OFF). Pilot beam with small momentum spread. • Assumed common frequency for B1 and B2 in SPS-LHC (fSPSLHC). Assume same magnetic field at extraction from the SPS for B1 and B2 • Measure frevLHCi (i=1,2) by observing bunch slip w.r.t. SPS-LHC reference frequency fSPSLHC/hLHC (either looking at bunch on longitudinal pickup or at phase detector) vs. time • Trim fSPSLHC, the LHC integrated field for B1 and B2 (via the CO correctors only), and the magnetic field at extraction in the SPS  • There will be a radial offset at extraction in the SPS if CLHC¹27/7 CSPS (this seems to be the case) • There will be a radial offset in the LHC after capture only if the two “rings” have a different circumference (~0.1 mm for ~1.5 mm difference)

Circulating pilot and RF capture – details • SPS-LHC energy matching (continued) • Switch the RF ON. It might be worth starting at maximum voltage (16 MV). • Adjust the RF phases (by using the phase pick-up signal) • Set-up the phase loop (gain, offsets, etc.) • Check residual momentum mismatch by observing the phase pick-up signal (sinusoidal oscillation with amplitude proportional to the momentum mismatch) • Iterate • Reduce the voltage to 8 MV (nominal injection value) or lower

Circulating pilot and RF capture – Possible problems • Very low lifetime (few tens of turns) • Causes: Thin obstacle (a “phantom” valve or other objects with thickness in the mm range, pressure bumps), poor optics control • Diagnostic tools: check performance with probe pilot beam (with small transverse and longitudinal emittances) BLMs, mobile BLMs and display, BPM intensity mode, radiation survey piquet, systematic optics measurements from orbit response analysis. • Remedies: obstacle removal, optics correction, leak fixes • Issues: activation of components at problem location – cool-down times • Large coupling • Causes: magnet model uncertainty, wrong MQS settings, polarity/cabling errors, calibration errors. • Diagnostic tools: cross-plane orbit response with local bumps at the MQS • Remedies: Correction from measurements • Issues: analysis tools • Large chromaticity • Causes: magnet model uncertainty, wrong MCS/MS settings, polarity/cabling errors, calibration errors. • Diagnostic tools: phase advance along the ring vs. momentum offset • Remedies: Correction from measurements • Issues: analysis tools

Circulating pilot and RF capture – Possible problems • Large path-length difference (>5 mm) between beam 1 and beam 2 leading to large offsets after capture • Causes: Differences in Bdl among apertures larger than expected • Diagnostic tools: measure the revolution frequency of the 2 beams with RF off • Remedies: Need to have different SPS extraction magnetic fields for B1 and B2 (2 different cycles) for the same RF frequency

Circulating pilot and RF capture – Issues • Software for BPMs in intensity mode… • This is a very useful tool in the SPS at start-up even after 30 years of operation…. • Needed to identify soft bottlenecks and lifetime problems • Some time is required for the commissioning, but well spent • Recycling • How often is recycling required (e.g. if energy mismatch detected and MBs adjusted)? • Cycling strategy to be fully defined for each circuit. • We are trying to have a strategy for the commissioning with beam. Do we have it for the “cold check-out”? • From the “What if” exercise we can learn something for the “cold check-out strategy”  at least to avoid trivial problems (calibration curves, wrong settings, …). • Need to create a list of possible errors and corresponding methods to trace them

Summary • Phase A.2: from first turn to basic machine set-up to obtain circulating beam (few hundred turns) for RF capture and to open the way to the next phase focussed on Beam Instrumentation set-up • Also in this phase aim for simplicity (where possible): • For some of the steps interleaved injection might be required • Stay with single pilot bunch • Systematic measurement limited to the sanity check of the correctors/BPM required to iron-out the orbit and to adjust the LHC Bdl • Need to commission the BPM intensity measurement to identify localized losses that might lead to low lifetime • Some “what-if” scenarios elaborated • We need to create a list of possible errors (cabling, polarity, etc.) with their effect and possible means to detect them

References • Web documentation LHC Commissioning procedures, LHC Commissioning pages Documentation and Procedures: Phase A2 [EICs+V. Kain] • LHCCWG minutes and relevant presentations: Circulating Beam and RF Capture [G. Arduini, A. Butterworth] Circumference Difference Between the 2 LHC Rings [G. Arduini] Beam Instrumentation - BPM, BLM, BCT, Transverse Diagnostics [R. Jones] Commissioning Procedures [V. Kain] Response Matrix Measurements and Analysis [J. Wenninger] Tracking error measurement and correction [J. Wenninger] Summary of Parameter Tolerances [F. Zimmermann] What to Do If We Cannot Get in Tolerance[F. Zimmermann] • Others: The minimum machine and the first 1000 turns or so [O. Brüning] Commissioning tunes to bootstrap the LHC, LCC#31 (23/10/02) [S. Fartoukh, M. Hayes] First Turn, LHC Project Note 308 [A. Verdier]

America’s Marine Corps never makes detailed studies in advance. Leaving important things to the last minute reduces the risk of wasting time on things that may ultimately prove not important at all. The Economist, Jan 4th, 2007 F. Zimmermann LHCCWG#19

LHC Commissioning Phases