CRAB CAVITY FAILURES COMBINEDWITH A LOSS OF THE BEAM-BEAM KICK IN THE HIGH LUMINOSITY LHC ∗

Crab cavities are an essential component of the High Luminosity LHC (HL-LHC) project. In case of a failure they can create large transverse kicks on the beam within tens of microseconds and, therefore, require a fast extraction of the circulating beam. In this paper, the effects of different crab cavity failures in combination with the missing beam-beam kick following the dump of only one HL-LHC beam are presented and consequences for the interlocking strategy of crab cavities are discussed. CRAB CAVITIES IN THE HL-LHC Crab cavities (CCs) are to be employed in the High Luminosity LHC (HL-LHC) to compensate for the loss of luminosity caused by the increased crossing angle [1, 2]. This compensation is achieved by tilting the proton bunches in the crossing plane to restore head-on collisions. However, a tilted bunch also means a wider transverse beam size, which, if not compensated downstream of the interaction point (IP), can lead to increased losses were the beam to move due to another failure. The current baseline is to have two CCs per beam and side for the IPs associated with the ATLAS (IP1) and CMS (IP5) experiments, totaling 16 cavities. Since each of them is to be powered and controlled individually by separate systems, it is assumed that only one CC would fail at a given time. The CC failures considered in this paper are any kind of failure leading to a voltage drop in one cavity or a change of the RF phase of the CC. Typical examples include powering failures and quenches. The high coupling of the cavity implies a short time constant on the order of 400 μs (∼ 4.5 LHC turns) for such a voltage drop, having an exponential decay [3]. As for the phase change, the rate of change is limited by the available power of the RF system with an upper limit at about 28.6 deg per LHC turn (89 μs) [4]. For the sake of simplicity, phase and voltage failures are treated separately, however most failures would have an effect on both. To simulate the worst-case scenarios, the Low Level RF (LLRF) is assumed to be actively driving the phase failure, e.g. due to incorrect user input. Another failure occurring in conjunction with a CC failure could aggravate its severity and such combined failures that are causally linked need to be studied. It is assumed that the instrumentation that the cavities are to be equipped with allows detection of any failure within one μs [1,5], whereafter the beam permits are removed and the beams are dumped within three LHC turns (270μs) [6]. However, the beams are ∗ Work supported by the High Luminosity LHC project † bjorn.hans. filip.lindstrom@cern.ch not dumped at the same moment and this paper focuses on a combination with a loss of the so-called beam-beam kick (BBK). COHERENT BEAM-BEAM KICK The coherent BBK is an electromagnetic interaction between counter-circulating bunches, which results in a transverse kick. In the LHC, the loss of the BBK due to the dump of one beam can move the other beam by up to 1. 35σ [7]. During high intensity beam operation the beam permits of the two beams in the LHC are linked, i.e. if a fault requiring a protection dump of one beam occurs, the second beam will be dumped as well. Nevertheless, the dumping of the second beam can be delayed by up to two turns. As illustrated in Fig. 1, the abort gaps of the two beams overlap only in IPs 1 and 5. Therefore, parts of the remaining beam will experience a loss of the BBK in different IPs for the first two turns and the beams can in practice not be dumped at exactly the same moment. The resulting RMS orbit offsets are summarized in Table 1 in units of beam σ during the first two turns, indicating in which IPs the BBK is lost. The horizontal and vertical orbit excursions of beam 1 and beam 2 for the most critical cases are shown in Fig. 2. The tracking simulations were performed with MAD-X [8] by sequentially removing the long range BBK in the IPs. Figure 1: Schematic of the beams during normal operations (left) and during the dump of beam 1 (right). The abort gaps are only synchronized in IPs 1 and 5. The beam dumping system is located in IR6 and the betatron collimation system in IR7. Different parts of the beams experience a loss of the BBK due to dumping the other beam in different IPs on the first two turns. 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-MOPMF042