Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 /241 PRELIMINARY FINDINGS FROM INSTABILITY MEASUREMENTS DURING THE 75ns AND 50ns BUNCH SPACING OPERATION Elias Métral (for the collective effects team with ABP, OP, BI, RF etc.). Elias Métral (for the collective effects team with ABP, OP, BI, RF etc.). Many thanks to Benoit, Nicolas, Riccardo and Tatiana for the data analysis u Introduction u Reminder on some predictions as concerns ecloud-induced coherent instabilities u 1 st measurements in the LHC with 50 ns => 02/11/10 and 20/11/10 u 1 st measurements in the LHC with 75 ns => 18/11/10 u Conclusions

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 /242 INTRODUCTION u Reminder with a single-bunch: An instability (from the machine impedance) is predicted and observed with octupoles OFF above 1-2 TeV/c (depending on beam parameters) => Famous “Christmas tree”, reproduced with HEADTAIL (Benoit Salvant): head-tail instability m = -1 + subsequent loss (revealing all the other modes). No problem during the last months when the octupoles were ON (K3 = -6 at top) u With many bunches (not discussing beam-beam):  Transverse coupled-bunch instability expected. Prediction of a rise-time of ~ 50 ms for the 25 ns operation (0 chromaticity, equidistant bunches etc.) and should be damped by the feedback. Chromaticity can help  With bunch trains and chromaticities, it is being looked at (N. Mounet, extension of HEADTAIL code)  Ecloud recently observed => It represents a “generalized impedance” and the beam can also become “coherently” unstable

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 /243 PREDICTIONS FOR ECLOUD-INDUCED COHERENT INSTABILITIES (1/5) u See for instance G. Arduini et al., Present understanding of electron cloud effects in the Large Hadron Collider, PAC2003:  Observation in the SPS with LHC beams (train of 72 b, 25 ns): Horizontal coupled-bunch instability  Wave-length of ~ a 72 b train  Rise-time ~ 1 ms (50 turns), ~ independent of Nb  Transverse damper can damp it Vertical single-bunch instability => TMCI-like  Rise-time ~ 2 ms (100 turns) for Nb = 1.15E11 p/b and heavily depends on Nb  High chromaticity needed (up to 20 units in Qy’ quoted. Since several years xiy ~ 0.4, i.e. Qy’ ~ 10 units) => Explained by the 2 vertical stripes in dipoles (depending on Nb)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 /244 PREDICTIONS FOR ECLOUD-INDUCED COHERENT INSTABILITIES (2/5) From Elena Benedetto’s PHD thesis

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 /245 PREDICTIONS FOR ECLOUD-INDUCED COHERENT INSTABILITIES (3/5)  Extrapolation to the LHC using scaling laws: Horizontal CBI rise-time  In the SPS  In the LHC => This number is comparable to the LHC feedback damping time Assuming the same e- density

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 /246 PREDICTIONS FOR ECLOUD-INDUCED COHERENT INSTABILITIES (4/5) Vertical SBI intensity threshold or as in saturation Chamber cross section Above the threshold, instability rise-time of the order of (or smaller) than T s are foreseen for low chromaticity (TMCI- like)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 /247 PREDICTIONS FOR ECLOUD-INDUCED COHERENT INSTABILITIES (5/5)  In the SPS  In the LHC => 1E11 p/b is quoted in the paper… To be checked ~ 3E11 p/b from HEADTAIL simulations (cf. FZ’s talk)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 450 GeV/c on 02/11/2010 (1/4) u Beams dumped at ~ 00:26:12: B1 was already circulating with bunches (~ 1E11 p/b) and we injected the (12+) 36 bunches of B2 => Coherent instability in H of B2 (see on the BBQ). Then it was re- observed in V essentially… Rise-time of ~ 1 s

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 450 GeV/c on 02/11/2010 (2/4) u B1, (starting at slot 1501), ~ 1E11 p/b u Qprime ~ 10 (H&V, both planes) u Instability in V seen on plates of the Transverse feedback Head of batch Middle Tail “Optical fit” => ~ 4000 turns ~ 0.4 s

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 450 GeV/c on 02/11/2010 (3/4) 24b Batch spacing: 3 micros

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 450 GeV/c on 02/11/2010 (4/4) u The transverse instability seems to move from the tail to the head of the batch, as mentioned in G. Arduini et al., The Electron Cloud Instability of the LHC Beam in the CERN SPS, PAC2003 u Conclusions:  Could really come from the ecloud  The H instability initially seen should be coupled-bunch…  The V one could be the “TMCI-like” instability. Rise-time ~ 0.4 s with Qprime ~ 10. Could be stabilized with Qprime ~ 18 (beneficial effect seen on the transverse emittances), as predicted in Elena Benedetto’s PHD thesis (see FZ’s talk)  Difficult to disentangle from coupled-bunch instability induced by the machine impedance (more precise estimates needed from the simulations point of view. Ongoing)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 /2412 u B2, 9 batches of 12 bunches (50 ns), see pattern on next slides u 3.5 TeV/c, Qprime ~ 4 u Transverse feedback OFF => Beam still stable u Reducing the octupoles => At some point the beam is unstable in H 50 ns beam at 3.5 TeV/c on 20/11/2010 (1/7)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 3.5 TeV/c on 20/11/2010 (2/7)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 3.5 TeV/c on 20/11/2010 (3/7)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 3.5 TeV/c on 20/11/2010 (4/7)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 3.5 TeV/c on 20/11/2010 (5/7) Rise-time of ~ 30 s?

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 3.5 TeV/c on 20/11/2010 (6/7)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 3.5 TeV/c on 20/11/2010 (7/7) u Conclusion => Certainly coming for the machine impedance (as already observed in the past. Analysis still ongoing for the threshold stabilization from the octupoles)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 450 GeV/c on 18/11/2010 (1/5) u Before increase in chromaticity (Qprime ~ 10 in H&V) 11 batches, 2 * 24 b spaced by 225 ns (batch spacing 1.85 micros)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 450 GeV/c on 18/11/2010 (2/5) Low-order coupled-bunch modes observed

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 /2421 Subtracting the constant part (analysis ongoing) 75 ns beam at 450 GeV/c on 18/11/2010 (3/5)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 450 GeV/c on 18/11/2010 (4/5) u Conclusions:  Significant motion on B1 only (both H and V), but nothing on B2  Started to be observed after ~ 3-4 batches  No growing coherent oscillations (but losses observed)  Transverse feedback was ON, no octupoles  Motion seems to be coupled between bunches, low-order mode  Intra-bunch motion also observed (|m| = 1?)  Increase of chromaticity at ~ 15:30 (from ~ 10 to ~ 20 units) seems to have instantly killed this motion and the losses disappeared Origin of this observation not yet known (1 possibility discussed on next slide)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 / ns beam at 450 GeV/c on 18/11/2010 (5/5) The transverse CBI from machine impedance could explain all the features qualitatively (~ same in H and V) for B1 => Analysis ongoing (picture shown here for nominal 25 ns bunch) B2 could be in fact stabilized by the TF if the chromaticities were small (however, they were the same as B1…)

Elias Métral, LHC Beam Commissioning Working Group meeting, 30/11/2010 /2424 CONCLUSIONS u Difficult to conclude at this stage on the origin of all the observed instabilities u More simulations needed to make more precise estimates in the conditions of the experiment u Then, redo some controlled MDs u In the case of transverse coupled-bunch instabilities from (impedance and/or ecloud), they should be damped by the TF, and it is better to have the smallest chromaticity u One should not have TMCI from the machine impedance, therefore there is no reason to increase the chromaticity for that u The only reason to increase the chromaticity could come from the ecloud-induced vertical single-bunch “TMCI-like” instability, which was most probably observed during the 1 st MD with 50 ns beam on 02/11/10 …