The MD was done at 450GeV using beam 2 only. An MD focussing on injection of bunches with nominal emittance was done in parallel on beam 1.

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1 CERN-ATS-Note MD MKI UFOs at Injection Tobias BAER, Mike BARNES, Wolfgang BARTMANN, Chiara BRACCO, Etienne CARLIER, Christophe CHANAVAT, Lene Norderhaug DROSDAL, Noel GARREL, Brennan GODDARD, Verena KAIN, Volker MERTENS, Jan UYTHOVEN, Jorg WENNINGER, Markus ZERLAUTH Keywords: UFO, dust particle, macro particle, MKI, injection kicker magnet Summary During the MD, the production mechanism of UFOs at the injection kicker magnets (MKIs) was studied. This was done by pulsing the MKIs on a gap in the circulating beam, which led to an increased number of UFOs. In total 43 UFO type beam loss patterns at the MKIs were observed during the MD. The MD showed that pulsing the MKIs directly induces UFO type beam loss patterns. From the temporal characteristics of the loss profile, estimations about the dynamics of the UFOs are made. 1. Introduction An important limitation for the performance of the Large Hadron Collider are so called UFOs ( Unidentified Falling Objects ). UFOs were first observed in July 2010 and caused numerous protection beam dumps since then. They are presumably micrometer sized dust particles that lead to fast beam losses with a duration of about 10 turns when they interact with the beam. Exceptionally many UFOs occur around the injection kicker magnets MKI (cf. Fig. 1). The Aim of the MD was to study the production mechanism of the UFOs at the MKIs. For this, the MKIs were pulsed without injection of beam on a gap in the partly filled machine (1236 circulating bunches). Tests were done with all four MKIs pulsing at the same time and pulsing of individual MKIs only. The MD was done at 450GeV using beam 2 only. An MD focussing on injection of bunches with nominal emittance was done in parallel on beam Machine Protection verification In order to pulse a single MKI individually, the fine delay of the other MKIs was increased beyond the delay of the kicker dump switch. Because this implicates significant changes of the behaviour of the equipment, while operating with high intensity beam (1236 nominal bunches), the MD was classified as MPS class C. Thus, a procedure to guarantee

2 machine protection was put in place for this MD [1]. A key aspect to ensure machine protection was a test with pilot bunches distributed around the machine. This test showed the feasibility of the procedure for individual MKI pulsing and that the individual MKI pulses in a gap of the partly filled machine do not affect the circulating beam. Figure 1: Layout of the MKI for beam 2. Mobile BLMs (red) have been installed in addition to the existing BLMs (blue). 3. Observations and Results During the MD, the MKIs were pulsed in total 21 times (cf. Table 1). 14 times all MKIs were pulsed together, 3 times only MKI.D was pulsed and 4 times only MKI.A was pulsed. In total 43 UFO type loss pattern at the MKIs were identified during the MD (cf. Table 2). Figure 2 gives an overview of the kicker pulses and the observed beam losses. Figure 2: Overview of the MKI pulses and the beam losses at the MKIs with 1236 bunches circulating beam. Most of the loss spikes are UFO type losses

3 UFOs between MKI pulses Especially in the beginning of the MD, several UFO type loss pattern were observed between the kicker pulses (8 UFOs within 20 minutes after the last injection). Towards the end of the MD, the MKIs were pulsed more frequently, but particularly in the hour after the last MKI pulse only 4 UFO type loss pattern were observed. Figure 3 illustrates the number of UFO type loss pattern between the kicker pulses and the number of kicker pulses for certain time intervals. Figure 3: The number of UFO type loss pattern between the kicker pulses and the number of kicker pulses for different time intervals during the MD. Especially after the last injection, many UFO type loss patterns were observed. UFOs directly after MKI pulses 17 of the identified UFO type loss patterns where observed within the second of the pulsing of the MKIs. Table 1 summarizes the MKI pulses and the detected UFO type loss patterns. A UFO type loss pattern was always observed when all four magnets were pulsed or only MKI.D was pulsed. In the four cases where MKI.A was pulsed, no UFO type loss was observed. The kicker pulses are generally accompanied by a small loss signal with a typical duration of 40 80µs (cf. Fig. 4 and Fig. 5), which is recorded by the BLM injection capture buffer (512 40µs). The amplitude of this loss signal is given in Table 1. In all but the highlighted case, the loss signal in the moment of the kicker pulse is smaller than the peak loss of the UFO type loss pattern and cannot explain it 1. It is concluded that in these cases an UFO is directly following the MKI pulses. In two cases the UFO is even recorded by the BLM injection capture buffer (cf. Fig. 4 and Fig. 5). 1 For the UFO type loss pattern after the injection at :30: (highlighted in Table 1), the signal in RS01 and RS02 corresponds to the signal while pulsing the MKIs. The losses in the higher running sums cannot be explained from the beam losses recorded in the injection capture buffer. Especially the loss in RS05 exceeds with 1.44E-04 Gy/s the usual noise level (cf. Appendix A.1). Possibly, a UFO with lower magnitude than the losses while pulsing the MKIs occurred after the injection capture buffer ended

4 Assuming that a macro particle is released in the moment of the kicker pulse and assuming a constant acceleration a of the particle until it interacts with the beam, for these cases, the acceleration can be calculated from the time difference t between MKI pulse and UFO occurrence by a = 2 s t2, with s = 19mm being the half-aperture of the MKI. The resulting acceleration is 658 m for the case depicted in Fig. 4 and 2055 m s 2 s2 for the case depicted in Fig. 5. The resulting particle velocity v p in the moment of the interaction with the beam would be 5.0 m and 8.8 m respectively. In the limit that the macro particle size is much small than the s s beam size, and assuming a constant particle velocity while interacting with the beam 2, the particle velocity can also be calculated from the temporal width σ t of the loss profile by v p = σ b, with σ σ b being the beam size [3]. With a normalized emittance of ε n = 2.5μm rad t and a beta function of β = 43m it is obtained that v p = 2.96 m, respectively v s p = 2.17 m. s The inconsistency of the calculated particle velocities implicates that the simple model of the macro particle dynamics is not sufficient and that the motion is more complex (cf. e.g. [2]). An increased statistics is needed to draw more specific conclusions about the macro particle dynamics. Figure 4: (left) Spatial beam loss pattern of the UFO directly after the injection at 01:29: The loss pattern downstream of the TDI is due to uncaptured beam lost on the TDI. (right) The temporal distribution of the loss. From Gaussian fit: Amplitude: 0.018Gy/s, temporal width: 160µs, delay to kicker pulse: 7.6ms. 2 This is assumed to be a major simplification. The macro particle dynamics is expected to be much more complex due to ionization processes [2]

5 Figure 5: (left) Spatial beam loss pattern of the UFO directly after the injection at 23:54: The loss pattern downstream of the TDI is due to uncaptured beam, lost on the TDI. (right) The temporal distribution of the loss. From Gaussian fit: Amplitude: Gy/s, temporal width: 218µs, delay to kicker pulse: 4.3ms. Timestamp (local time) MKI pulsing UFO detected (BLM) UFO Peak loss RS01 [Gy/s] Peak loss at pulse RS01 [Gy/s] :38: A, B, C, D BLMEI.05R8.B2E10_MKI.D5R8.B2 3.95E E :54: A, B, C, D BLMEI.05R8.B2E10_MKI.D5R8.B2 5.25E E :56: A, B, C, D BLMEI.05R8.B2E20_MKI.C5R8.B2 6.34E E :15: D BLMEI.05R8.B2E10_MKI.D5R8.B2 3.17E E :22: D BLMEI.05R8.B2E10_MKI.D5R8.B2 8.60E E :29: D BLMEI.05R8.B2E10_MKI.D5R8.B2 1.27E E :35: A, B, C, D BLMMI.05R8.B2E30_MKI.B5R8.B2 2.12E E :40: A None N/A 0.0 (MKI.D) :48: A None N/A 0.0 (MKI.D) :56: A None N/A 0.0 (MKI.D) :02: A None N/A 0.0 (MKI.D) :07: A, B, C, D BLMMI.05R8.B2E31_MKI.A5R8.B2 7.96E E :18: A, B, C, D BLMEI.05R8.B2E10_MKI.D5R8.B2 7.78E E :29: A, B, C, D BLMEI.05R8.B2E10_MKI.D5R8.B2 1.38E E :29: A, B, C, D BLMEI.05R8.B2E10_MKI.D5R8.B2 2.05E E :30: A, B, C, D BLMEI.05R8.B2E10_MKI.D5R8.B2 1.36E E :32: A, B, C, D BLMMI.05R8.B2E30_MKI.B5R8.B2 2.05E E :35: A, B, C, D BLMMI.05R8.B2E30_MKI.B5R8.B2 7.02E E :35: A, B, C, D BLMMI.05R8.B2E30_MKI.B5R8.B2 1.18E E :37: A, B, C, D BLMMI.05R8.B2E31_MKI.A5R8.B2 1.18E E :38: A, B, C, D BLMEI.05R8.B2E10_MKI.D5R8.B2 1.81E E-03 Table 1: Overview of MKI pulses and detected UFO type loss patterns in the second of the MKI pulse

6 Timestamp (local time) UFO BLM Peak loss RS01 Peak loss RS05 [Gy/s] [Gy/s] :01:31 BLMMI.05R8.B2E31_MKI.A5R8.B2 7.42E E :05:22 BLMEI.05R8.B2E20_MKI.C5R8.B2 1.73E E :05:29 BLMEI.05R8.B2E20_MKI.C5R8.B2 1.36E E :05:30 BLMEI.05R8.B2E20_MKI.C5R8.B2 3.26E E :09:37 BLMMI.05R8.B2E30_MKI.B5R8.B2 3.62E E :15:26 BLMMI.05R8.B2E31_MKI.A5R8.B2 1.65E E :16:38 BLMMI.05R8.B2E30_MKI.B5R8.B2 2.44E E :21:20 BLMMI.05R8.B2E31_MKI.A5R8.B2 2.72E E :38:06 BLMEI.05R8.B2E10_MKI.D5R8.B2 3.95E E :38:17 BLMMI.05R8.B2E30_MKI.B5R8.B2 5.43E E :39:03 BLMEI.05R8.B2E10_MKI.D5R8.B2 8.15E E :39:09 BLMEI.05R8.B2E20_MKI.C5R8.B2 1.54E E :54:49 BLMEI.05R8.B2E10_MKI.D5R8.B2 5.25E E :55:17 BLMEI.05R8.B2E20_MKI.C5R8.B2 5.43E E :56:20 BLMEI.05R8.B2E20_MKI.C5R8.B2 6.34E E :03:13 BLMEI.05R8.B2E10_MKI.D5R8.B2 1.18E E :15:20 BLMEI.05R8.B2E10_MKI.D5R8.B2 3.17E E :21:31 BLMMI.05R8.B2E31_MKI.A5R8.B2 1.27E E :22:10 BLMEI.05R8.B2E10_MKI.D5R8.B2 8.60E E :29:01 BLMEI.05R8.B2E10_MKI.D5R8.B2 1.27E E :35:06 BLMMI.05R8.B2E30_MKI.B5R8.B2 2.12E E :37:21 BLMMI.05R8.B2E30_MKI.B5R8.B2 9.96E E :45:51 BLMMI.05R8.B2E30_MKI.B5R8.B2 8.24E E :48:26 BLMMI.05R8.B2E30_MKI.B5R8.B2 2.90E E :05:05 BLMMI.05R8.B2E31_MKI.A5R8.B2 3.62E E :07:46 BLMMI.05R8.B2E31_MKI.A5R8.B2 7.96E E :18:25 BLMEI.05R8.B2E10_MKI.D5R8.B2 7.78E E :19:16 BLMEI.05R8.B2E10_MKI.D5R8.B2 3.89E E :19:19 BLMEI.05R8.B2E10_MKI.D5R8.B2 3.80E E :19:40 BLMEI.05R8.B2E20_MKI.C5R8.B2 1.27E E :20:50 BLMEI.05R8.B2E10_MKI.D5R8.B2 1.63E E :21:26 BLMEI.05R8.B2E10_MKI.D5R8.B2 2.08E E :29:03 BLMEI.05R8.B2E10_MKI.D5R8.B2 1.38E E :29:49 BLMEI.05R8.B2E10_MKI.D5R8.B2 2.05E E :30:34 BLMEI.05R8.B2E10_MKI.D5R8.B2 1.36E E :30:46 BLMEI.05R8.B2E10_MKI.D5R8.B2 1.29E E :32:07 BLMMI.05R8.B2E30_MKI.B5R8.B2 2.05E E :35:08 BLMMI.05R8.B2E30_MKI.B5R8.B2 7.02E E :35:23 BLMEI.05R8.B2E20_MKI.C5R8.B2 8.15E E :35:54 BLMMI.05R8.B2E30_MKI.B5R8.B2 1.18E E :37:25 BLMMI.05R8.B2E31_MKI.A5R8.B2 1.18E E :38:10 BLMEI.05R8.B2E10_MKI.D5R8.B2 1.81E E :38:36 BLMEI.05R8.B2E10_MKI.D5R8.B2 1.63E E :06:12 BLMMI.05R8.B2E30_MKI.B5R8.B2 4.53E E :17:51 BLMMI.05R8.B2E30_MKI.B5R8.B2 4.53E E :18:47 BLMMI.05R8.B2E30_MKI.B5R8.B2 3.62E E-05 Table 2: Beam losses at the MKIs with UFO type loss patterns, which were identified during the MD. The highlighted events occurred within the second of the pulsing of the MKIs

7 4. Conclusions and Follow-up The MD showed that pulsing the MKIs directly induces UFO type loss patterns. A post-analysis of injections for normal physics fills revealed several additional cases where a UFO type loss pattern is occurring a few ms after an injection. From the observed loss patterns, it is calculated that the acceleration of macro particle released in the moment of the MKI pulse can exceed the gravitational acceleration by orders of magnitude. A possible explanation could be initially charged dust particles. Following these observations, the triggering of the BLM injection capture buffer was adjusted for nominal operation to increase the detection efficiency of UFOs occurring directly after an injection. The previously observed asymmetry between the four MKI magnets [4] was confirmed by the MD: Whereas there was always observed an UFO type loss pattern after pulsing all four MKIs or MKI.D only, no UFOs were observed directly after pulsing MKI.A. The observations indicate that the UFO rate between the MKI pulses is higher after the last injection with beam and is only secondarily determined by the pulsing of the MKIs. The MD showed that by pulsing the MKIs a sufficient number of UFOs can be produced to study the effect and especially the dynamics of the macro particles. A dedicated UFO MD focusing on comprehensive studies of the production mechanism of UFOs at the MKIs and the MKQA is planned. The use of a dedicated BLM buffer will significantly improve the diagnostics for UFOs which follow directly the kicker pulses. References [1] C. Bracco et al., MD2-2011: Injection studies and MKI UFOs, CERN EDMS Document No , Geneva, June [2] F. Zimmermann, Interaction of macro-particles with the LHC proton beam, IPAC 10, Kyoto, May [3] J. Wenninger, Analysis attempt of dump UFOs, LHC Machine Protection Panel, Geneva, March [4] T. Baer, UFO Update, Mini-Chamonix Workshop, Crozet, July

8 Appendix BLM Noise Appendix A.1: Typical BLM noise of BLMEI.05R8.B2E10_MKI.D5R8.B2 for two minutes stating at :24:00. The spikes are typical noise spikes. Appendix A.2: Typical BLM noise for the mobile monitors 5R8 for two minutes starting at :24:00. The noise is much less than for the permanent monitors 5R8, which can be explained by cabling and acquisition card specific differences