Ramping & Snapback

Ramping & Snapback Andy Butterworth AB/RF Chamonix XIV 17 January 2005

Outline • Procedure for ramping the LHC • Initial conditions for ramp commissioning • Outline of ramp commissioning • Moving to the nominal cycle • Getting through snapback • Ramping further • Measurements and commissioning of accelerator equipment • Conclusions

will vary Baseline energy ramp Parabolic-Exponential-Linear-Parabolic (PELP) c/o Paul Collier Parabolic round-out I  t2 Linear I  t PELPs are also used for any synchronized setting – e.g. applying a trim Parabolic I  t2 Exponential I  et

Baseline energy ramp PELP • minimize voltage discontinuities • start with dI/dt = 0 • ramp starts slowly • 9 GeV in the first minute • minimize dynamic errors • slow snapback: ~70 seconds • respect ramp rate of the power converters • round-off at high energy • persistent currents, ramp-induced eddy currents

Ramp implementation: functions & timing • Ramp is driven by current, voltage and frequency as functions of time, pre-loaded to the power converters and RF • as an array of points (delta-time, delta-reference) • 1 ms granularity • arbitrary time spacing • requires linear interpolation of supplied points • for all powering circuits - 1700 power converters • mains, quads, triplets, insertions, spool pieces, orbit correctors... • Functions must take into account static & dynamic magnet errors • geometric, beam screen, eddy current in ramp • persistent current decay, snapback

Functions & timing (contd.) • The control system for the LHC power converters has dedicated controller embedded in every converter • function generation (current versus time) • current regulation • state monitoring and control (on, off, reset etc.) • The same function generator module is also used in the RF systems • voltage & phase, frequency, radial position, power coupler position etc.

Functions & timing (contd.) • Execution of functions is triggered by a “start ramp” timing event • the operator decides when to ramp and requests the timing system to send the event • The ramp stops naturally when the functions come to an end • not possible to stop while the functions are executing • a stop “during the ramp” means generating functions which stop at the desired time

Real time corrections • The function generator controller also has a real-time input • accepts corrections up to 50 Hz • in combination with time dependent functions • as an offset: Ref = F(t) +dFrt • or as a fractional gain: Ref = F(t)(1 + Grt) • Real-time corrections are eventually foreseen... • operator-controlled real-time knobs • beam-based feedback • orbit – local and global • tune, chromaticity - will not be in place for commissioning • feed-forward from online reference magnets • open to question…

Other systems RF • Low-level loops control the beam • phase loop locks RF to beam to avoid emittance blowup • must eventually use synchro loop when we want to collide • both rings locked to the same frequency program • avoids rephasing before physics • but radial loop used during ramp commissioning since exact beam energy is not well known • adjusts RF frequency to centre beam at pickup in IR4 • measure frequency offset and feed correction forward into functions Beam Dump • loaded with the reference energy ramp • supplies strengths for septum, MKD, MKB as function of beam energy • tracks the energy using a hardware ‘energy meter’

Pre-requisites & initial conditions • Circulating beams in both rings at 450 GeV with well adjusted beam parameters • Relaxed tolerances: • low intensities • more tolerance on beam parameter variation: tune, Q' etc. • commissioning tunes • QH and QV widely separated, more coupling allowed • no crossing angle bumps or spectrometer compensation  more aperture

Pre-requisites (contd) • Beam instrumentation • continuous PLL tune measurement available (no feedback) • with tune history • must be possible with pilot bunches • feed-forward to correct the tune – closing the loop will come later • Q’ measurement with RF frequency modulation • online (head-tail) Q’ measurement highly desirable • orbit acquisition through the ramp • and eventually feedback around the beam dump and collimation regions • RMS • predictions of persistent current effects & snapback after a fixed time on injection plateau

Pre-requisites (contd) • Machine Protection • beam dump commissioned at 450 GeV • initial commissioning of beam loss monitors at 450 GeV • beam interlocks verified system by system • initial collimation settings: as for injection • TDI out, collimators at coarse settings (~ 7/8.5) • Controls • function generation and management (trims, incorporation) • sequencer, ramp timing commissioned • RF • beam control: phase, synchro & radial loops operational • Transverse feedback OFF • only needed at this stage for injection damping

Phases of ramp commissioning Start 450 GeV on “degauss” cycle Switch to nominal cycle [ring1, ring2] Single beam through snapback [ring1, ring2] pilot Ramp – single beam [ring1, ring2] pilot++ Single beam to physics energy [ring1, ring2] Two beams to physics energy moderate intensity (3x1010 ppb) single bunch at 7 TeV End

Move to nominal cycle “Degauss” cycle used during initial commissioning at 450 GeV • degauss “blip” eliminates persistent current decay on the injection plateau • but hysteresis & snapback mean we cannot ramp  must switch to nominal cycle to continue • need to transfer all trims made on the degauss cycle to the nominal cycle • transfer of 450 GeV corrections & incorporation into the ramp functions

nominal cycle degauss cycle Initial Commissioning – Nominal Cycle • Wait 15-20 minutes on injection plateau before injecting: • reduced persistent currents  “nominal” snapback – bigger but reproducible • limited further decay • but will still need to re-commission 450 GeV • Look at decay with beam: • energy offset vs. time • tune, chromaticity, orbit drifts • reproducibility after cycling etc. • Establish standard operational procedure at 450 GeV • standard checks: momentum, tune, orbit, chromaticity, coupling, dispersion • then launch the ramp as quickly as possible... • try to avoid large scale readjustment of beam parameters every cycle • Full recycle to top energy after every attempt • may initially set up for cycling to a lower energy to save time

Decay & snapback – the problem Injection ~ 1200 s Snapback ~ 70 s decay snap-back c/o L. Bottura • Drift in multipole components due to decay of persistent currents and consequent snapback at start of ramp • The challenge will be anticipating the depth of the snapback and attempting to deal with associated swing of beam parameters

Managing snapback • Procedure: • establish length of time on injection plateau • reference model or measurements to establish depth of snapback • predict required corrector functions and incorporate into the machine settings • load functions to hardware • launch ramp (timing event) • track key beam parameters through snapback: tune, orbit, Q‘ • for feed-forward to the next cycle

e.g. Chromaticity  talk: L. Bottura slow Q’ measurements and b3 corrections during injection Extract sextupole change in dipoles b3 Infer persistent current change I since b3 and I are correlated Just before ramping… Extract total b3 correction Invoke fit for snapback prediction Convert to currents for b3 spool pieces Incorporate into ramp functions & download Timing event Functions invoked at ramp start RT corrections still possible

RMS First attempts at ramping need model predictions good enough to get low intensities through full snapback • If online RMS measurements were available, feed-forward of these to the next cycle would help refine the prediction of the snapback

Beyond Snapback • Things should calm down once the snapback is over • dynamic effects considerably reduced after first 100 GeV • eddy currents small & reproducible L. Bottura • correctors optimised using feed-forward • Measurements made ‘on-the-fly’ during the ramp are used to modify the corrector functions for subsequent cycles

Stopping with beam in the ramp • Must be programmed before starting the ramp • with appropriate round-off behaviour of the functions • need to handle decay after the stop • Restart with beam is possible in theory, but problematic • requires a new set of PELP functions to be loaded • including corrections for handling the associated snapback Used for commissioning of beam dump, beam loss monitors, beam measurements, optics checks, physics...

Machine Protection Start Low intensity, single bunch, low energy... same as at 450 GeV • BLMs: acquisition – no dump, check losses against thresholds • collimators & TDCQ coarse settings Switch to nominal cycle Single beam through snapback Critical machine protection systems must be in place • minimum subset of BLMs connected to beam interlock system • collimators interlocked in place • local orbit stabilisation around beam cleaning insertions and dump region • further commissioning of beam dump & BLMs Ramp – single beam Single beam to physics energy Two beams to physics energy End

System commissioning: Beam dump • Commissioning in ramp (with pilot): • extract at pre-defined energies (small steps - to be defined) • energy tracking (MKD, MSD, MKB) • measure, correct & check trajectories and settings, dilution kicker sweep • check instrumentation, feedback, reference settings, interlock thresholds, kicker timings/retriggering • interpolate to build reference functions for settings and interlock thresholds • increase intensity gradually at each energy • optimisation of interlocks and settings where necessary

System commissioning: RF • Control B-field correction procedures to obtain dB/B < 10-4 • single bunch pilot, inject/dump • Control acceleration through snapback • single bunch pilot, accelerate • dump at progressively higher energies • Measure: • capture losses (flash loss of out-of-bucket beam at start of ramp) • continuous measurements of frequency response of loops during ramp • bunch length (emittance growth) - injection mismatch, RF noise • beam losses • Feed-forward of measured frequency offset • for eventual switch to synchro loop operation • In preparation for physics beams: • Commissioning of programmed longitudinal emittance blowup via RF noise

Conclusions • The procedure and mechanisms for ramping the LHC are well defined • Strategy for commissioning the nominal cycle • wait for current decay to play out before injecting • aim for reproducibility of snapback • iterative process with feed-forward of corrections • relaxed tolerances during commissioning but the target is likely to be moving • RMS • Predictions essential for getting through snapback • Machine protection • starts to become critical when ramping further than the end of snapback • even at low intensities

Ramping & Snapback