Review of the instabilities observed during the 2012 run and actions taken

Despite the excellent performance of the LHC in 2012, with a record peak luminosity at 4 TeV corresponding to 77% of the 7 TeV design luminosity of 10 cms, the intensity ramp-up was perturbed by several types of instabilities. All the observations (during both physics cycles and dedicated MDs) are critically reviewed, comparing them to past predictions and new findings. The lessons learned and actions taken are then discussed in detail. INTRODUCTION AND MAIN LIMITATION AT THE END OF THE RUN The LHC luminosity has been considerably increased in 2011 and 2012 and the peak luminosity record was ~ 7.7 10 cms, corresponding to 77% of the 7 TeV design luminosity of 10 cms, but at 4 TeV, i.e. 4 / 7 = 57% of the design energy. This great achievement led to the discovery of a Higgs-like boson, announced on July 4, 2012, and to a total integrated luminosity of ~ 23 fb for the two high-luminosity experiments, ATLAS and CMS. Furthermore, only half of the design number of bunches has been used (1374), using a beam with 50 ns bunch spacing instead of the nominal 25 ns bunch spacing. However, and this is the reason of the success, the bunch brightness was up to ~ 2.4 times larger than nominal, as bunches with ~ 1.6 10 p/b within ~ 2.2 μm (transv. r.m.s. norm. emittance) were successfully put in collision, instead of the nominal bunches of 1.15 10 p/b within 3.75 μm. Finally, tight collimators settings (collimator apertures close to nominal ones at 7 TeV) have been used in 2012, leading to larger impedances and more critical instabilities (a factor ~ 2.3 was computed for the transverse plane compared to 2011 [1]). Despite the excellent performance of the LHC in 2012, 3 types of instabilities perturbed the intensity rampup, which are discussed below. In collision: “Snowflakes” These instabilities happened always in the horizontal plane only, for both beams and could happen after several hours of stable beam (see an example in Fig. 1). It concerned initially only the IP8 private bunches, i.e. the bunches colliding only at the Interaction Point 8. This was rapidly identified and these instabilities disappeared once the filling scheme was modified (reducing first the number of private bunches and then removing them all). The interpretation of this mechanism is that it happens on selected bunches with insufficient tune spread (and thus Landau damping) as they have no Head-On collisions in IP1/2/5 and have a transverse offset in IP8 to level luminosity. Figure 1: Example of “snowflakes” instability: (a) bunch intensity vs. number of the bunch (25 ns slot) and (b) horizontal frequency spectrum vs. time. Courtesy of X. Buffat. During the collapsing process (putting the beams into collision) A second type of instabilities happened at the end of the collision process when ending with residual separations of ~ 2.1 σ in IP1 and ~ 1.2 σ in IP5 (values estimated from luminosities at the moment of the dump). These instabilities happened also in the horizontal plane and an example is given in Fig. 2. Figure 2: Example of instability during the collision process (putting the beams into collision): the luminosities vs. time are represented for the different experiments as well as the horizontal oscillations amplitude for beam 1. Courtesy of G. Arduini. During or at the end of the squeeze process: EOSI (End Of Squeeze Instability) A third type of instability happened during or at the end of the squeeze, called EOSI (End Of Squeeze Instability), also in the horizontal plane. A characteristic picture is shown in Fig. 3, where 3 lines spaced by the (small-amplitude) synchrotron tune can be observed. Figure 3: Example of instability observed at the end of the squeeze: horizontal frequency spectrum vs. time after the end of the squeeze. Courtesy of X. Buffat. The first and second types of instabilities did not appear anymore after the change of the Landau octupoles polarity (moving from negative to positive detuning with amplitude) on August 7 (fill #2926) and the increase of the chromaticities and the transverse damper (ADT) gain. Unfortunately these 3 parameters have been modified more or less at the same time and it is therefore not possible to identify the main beneficial effect. The third type of instabilities could not be cured during the entire run despite the increase of the octupoles current close to its maximum value (550 A), the increase of chromaticities from ~ 2 units to ~ 15-20 units and the increase of the ADT gain to its maximum value (50 turns damping time). Note however that this instability appeared then mostly in the vertical plane of beam 1. Furthermore, it became then very reproducible at the end of the squeeze (with even more synchrotron sidebands), when β* is already at 60 cm, and after ~ 16 min from the start of the squeeze process. Examples of this instability are shown in Fig. 4. Figure 4: Examples of the EOSI instability at the end of the run, which could not be cured: (a) Courtesy of N. Mounet and (b) and (c) courtesy of T. Pieloni. LCMS