LHC RF re- commissioning for run 2 with planned operation at 6.5 TeV/c and 0.55 A DC current P. Baudrenghien BE-RF with contributions from J.E. Muller, E. Shaposhnikova and H. Timko Several slides are copied from previous presentations: on the way we intended to commission the LHC (run 1) from a presentation at the LHCCWG June 2007 on the first commissioning from a presentation in Sept 2008, after the original start-up week LBOC July 29 th, 2014

Period 1.1: Single bunch (or few bunches) captured Period 1.2: ramping single bunch(es) to 6.5 TeV Period 2: 25 ns, 450 GeV Period 3: 50 ns, 6.5 TeV Period 4: 5-20 ns, 450 GeV Period 5: 25 ns, 6.5 TeV

What was done between Sept 2008 and Dec 2009 Period 1.1: Single bunch (or few bunches) captured

A1 First Turn Pilot

June 5, 2007LHCCWG meeting5 Goal Inject pilot and centre first turn RF sub-goal : Label buckets (numerology and cogging) Adjust front end gains to see PU signals (APW and BPM) We do not anticipate changes but it must be checked

June 5, 2007LHCCWG meeting6 Strategy (1) Generate the LHC injection kick (RF). Observe kick+beam. (BT). They adjust their delay (or ours?) to kick the beam. Get the beam to make a few turns (OP) Adjust the gain of the RF front end of the Beam Position and Beam Phase module + time alignment and F rev marker (RF). Dedicated RF 8 hours. To do that: Set the Observation memory to trigger on the “Beam In” timing. Observe the PU signal (APW in Beam Phase module and BPM in Beam Position module). Adjust gain/attenuation Align the signals from the 2 inputs (OK for APW and  from BPM. More difficult for cavity sum as the beam induced voltage will be very small with pilot. Coarse adjustment must be done without beam) Adjust the F rev marker (offset in memory addressing) so that marker points to bucket 1 Sector tests No change in hdw -> 4 hours should be OK

June 5, 2007LHCCWG meeting7 Strategy (2) Adjust delay in the F rev sent to the beam dump. Observe our signal at the beam dump location. Compare to bunch position. Adjust. (BT). The abort gap ends just before the passage of bucket 1 (proposed convention) Adjust delay in the 40 MHz/F rev clock received by the experiments (EXP) Should be quick but must be done for MP. Was done during sector test in 2008 Should be quick. Done during sector test 2008

A2 Capture and Circulating Pilot

June 5, 2007LHCCWG meeting9 Goal Capture and centre the closed orbit. Get pilots to collide at the right point (cogging) RF sub-goal: Commission phase loop and synchro loop. Capture Adjust relative positions of the 2 rings for collisions in IPs (cogging) Still relevant

Sept 17, 2008LLRF commissioning10 Adjust Inj Freq (2008) Sept 11, 14:00 Synchro module not working anymore. 17:30 Synchro modules replaced with lab version Test Synchro module Step Response -> OK 21:00 Beam on ring 2, 100 turns Keep Synchro loop On, Phase loop OFF, Radial loop OFF, RF OFF Measure the Beam/RF phase slip turn after turn, using Beam Phase Module with RF OFF Re-adjust inj freq from MHz to MHz Beam Phase module DAC out Filtered PU signal Bunch by bunch phase Phase avg Should be quick. Magnetic centre should not have changed much

Sept 17, 2008LLRF commissioning11 Capture (2008) Sept 12, 00:30 Cavities are now on same ref as low- level. Switch phase loop ON at injection. Transient lasts for 10 rev period as expected. Trajectory on MR very good… Phase loop can be ON before inj as we have a threshold on PU signal MR of one of the very first capture beams. T. Bohl and U. Wehrle Bunch avg phase at inj (phase loop on) Took 3 h from RF ON to captured beam in Should not be much longer in 2015

June 5, 2007LHCCWG meeting12 Adjust Synchro Loop dynamics With both loops ON, measure the synchro loop step response Adjust synchro loop gain and phase advance to fine tune the response As this depends on the RF voltage via synchrotron freq we want to do it at different voltages. But it is not urgent. Optimization of the LLRF loops dynamics was never done. This is now more important if we ever want to manipulate the RF loops for blow-up in physics, as this likely requires the opening of the phase loop with beam.

June 5, 2007LHCCWG meeting13 Check phasing of cavity sum Now try to capture the beam with one cavity at the time Observe synchro loop phase discri output  sync after transient If non-zero, fine-tune the delay in Cavity Sum for that cavity 8 hours RF Must be done as we have replaced Mod1B2

June 5, 2007LHCCWG meeting14 Align the two rings This stage has been advanced compared to the original OP scenario Get 1 circulating pilot in bucket 1 of each ring Measure the collision point. This requires an acquisition in a PU that sees both rings (first BI, then EXP). Adjust one ring with respect to the second to get collision in IP1 (?), using F rev prog 2 B# of the second ring. See diagram on page 12 4 hours OP,BI and RF Done in Should be quick in But must be done early in the start-up

what was done in 3.5 TeV Period 1.2: Ramping. Longitudinal stability. Blow-up

July 29, 2014LBOC meeting16 May 15 th, First attempt to ramp single bunch nominal. UNSTABLE May 17 th, We inject with phase loop open and 45 degrees phase error -> large blow-up during capture (1.4 – 1.6 ns). First successful ramp. STABLE May 28 th, Blow-up in the SPS (1.6 ns). STABLE June 15 th, Blow-up in the LHC ramp. STABLE

Longitudinal blow-up… …is essential! Several types of blow-up were proposed during run1 Promising alternative methods were tried at start-up 2012, but after initial problems, were discarded due to lack of time for optimizing Theoretical studies (diffusion models) and simulations (PyHEADTAIL tracking code) have been developed during LS1 to include the effect of controlled RF noise We wish to optimize the process in 2015 We also want to set-up longitudinal profile flattening with RF modulation LBOC meeting17July 29, 2014

Loss of Landau damping and impedance Extrapolating from results in 4 TeV, the min. long. emittance is TeV with 1.15E11 pp bunch (Evian 2014) We want to check it by ramping a few bunches, nominal intensity but different long. emittances, or different intensities and similar emittances. (CERN-ATS-Note MD) That will set the minimum voltage required during physics LBOC meeting18July 29, 2014

Bunch lengthening at 6.5 /eV Emittance growth is caused by IBS and RF noise Damping comes from synchrotron radiation The net effect was predicted to be bunch shortening at 6.5 TeV (20% reduction in length over 6 hours,Evian2014) If so, we may need to apply longitudinal emittance blow-up or bunch flattening, on regular intervals at flat top To be identified asap July 29, 2014LBOC meeting19 Reproduced from J. Tuckmantel, LHC Project Report 819, Synchrotron Radiation Damping in LHC and Longitudinal Bunch Shape, 2005

25 ns scrubbing 450 GeV/c Period 2

Potential problems Capture losses will increase sharply with bunch intensity. The SPS can produce bunches with nominal longitudinal parameters (0.5 eVs, 1.5 ns) up to 1.15E11 p but the bunch length increases quickly above that intensity (1.65 ns with 1.35E11 p). That will create large capture losses. Heating of parasitic resonators (HOMs) must be monitored carefully. The beam spectrum will have lines spaced by 40 MHz (used to be 20 MHz). For narrow-band resonator the power scales as the square of the total beam current -> large increase LBOC meeting21July 29, 2014 Sunglasses?

Potential problems (cont’d) The ACS HOMs have indicated less heating than expected, so far. We have monitoring of both temperature and power. Do we need interlocks? CBI: we do not anticipate stability problems from the cavity impedance at the fundamental. How about the HOMs? LBOC meeting22July 29, 2014 Scrubbing May 19 th, 2011, 1092b, 1.3E14 total

50 ns 6.5 TeV/c Period 3

50 ns intensity ramp-up Monitoring of HOM power Check stability. If we have a few ramps to 4 TeV/c, we can check that the stability is comparable to 2012 (no large modification to the machine impedance) LBOC meeting24July 29, 2014

5-20 ns scrubbing 450 GeV/c Period 4

5-20 ns scrubbing These beams have not been accelerated in the SPS yet. What longitudinal parameters can we expect at injection into the LHC? We anticipate much problems with capture losses. 1.6E11 p per pair! On the hardware side the beam phase loop must be optimized for 5-20ns Diagnostics tools are being developed to monitor the bunch-by-bunch stable phase (energy loss caused by e-cloud) LBOC meeting26July 29, 2014 Sunglasses?

25 ns intensity 6.5 TeV/c Period 5

25 ns intensity ramp-up We will reach record beam current at high energy ( TeV vs A 4 TeV in 2012)-> potential Coupled-Bunch instability (CBI) We want to measure the CBI stability margin by a few test ramps with reduced target bunch length If blow-up does not work for a fill, the high intensity/energy beam will be unstable. That happened in 2012, and the beam was dumped by an interlock on heating. Do we need a specific interlock on bunch length? Do we need dedicated hardware? LBOC meeting28July 29, 2014

25 ns intensity ramp-up (cont’d) What is the optimal voltage in physics? In 2012 we have observed a “saturation” effect in bunch lengthening suggesting limitation of momentum aperture. Lower voltage may be better. To be tested with a few physics fills at different voltages Bunch shaping may have to be applied at regular intervals in physics as the shape will return to Gaussian. Delicate as we must limit the risk of debunching LBOC meeting29July 29, 2014

A word on Controls The RF controls became “robust” after the implementation of basic macro-operations in the sequencer (begin 2010) All RF CPUs (RIO3) are being replaced by MEN A20 running Linux. FESA classes have been re- compiled but remain in FESA 2.10 Very limited upgrades -> We do not anticipate big problems Many features will be re-commissioned during dry runs and sector tests LBOC meeting30July 29, 2014

Conclusions

Except for the replacement of one full cryomodule (four cavities) the RF upgrade has been limited and we do not anticipate hardware problems with the re-commissioning Early in the restart we wish to measure the minimum longitudinal emittance that preserves Landau damping at 6.5 TeV We also want to optimize the longitudinal blow-up in the ramp using single bunch or a few bunches per beam. This manipulation has a big effect on the bunch spectrum and should best be optimized (and understood) before moving to multi-bunch With multi-bunch the challenges are the total beam current (heating and CBI) and the capture losses due to the large SPS bunch length (25 ns and 5-20 ns operation). For the later sunglasses at injection will make life easier. Heating must be monitored closely. Test ramps with reduced target bunch length would identify the CBI limit LBOC meeting32July 29, 2014 Thank you for your attention