Power deposition studies for standard and crystal-assisted heavy ion collimation in the CERN Large Hadron Collider

The LHC heavy-ion program with <math display="inline"><mrow><msup><mrow><mmultiscripts><mrow><mi>Pb</mi></mrow><mprescripts/><none/><mrow><mn>208</mn></mrow></mmultiscripts></mrow><mrow><mn>82</mn><mo>+</mo></mrow></msup></mrow></math> beams will benefit from a significant increase of the beam intensity when entering its high-luminosity era in Run 3 (2023). The stored energy is expected to surpass 20 MJ per beam. The LHC is equipped with a betatron collimation system, which intercepts the transverse beam halo and protects sensitive equipment such as superconducting magnets against beam losses. However, nuclear fragmentation and electromagnetic dissociation of <math display="inline"><mrow><msup><mrow><mmultiscripts><mrow><mi>Pb</mi></mrow><mprescripts/><none/><mrow><mn>208</mn></mrow></mmultiscripts></mrow><mrow><mn>82</mn><mo>+</mo></mrow></msup></mrow></math> ions in collimators generates a flux of secondary fragments, which are lost in downstream dispersion suppressor and arc cells. These secondary ions may pose a performance limitation in upcoming runs since they can induce magnet quenches. In order to mitigate this risk, an alternative collimation technique, relying on bent crystals as primary collimators, will be used in forthcoming heavy-ion runs. In this paper, we study the power deposition in superconducting magnets by means of tracking and fluka shower simulations, comparing the standard collimation system against the crystal-based one. In order to quantify the predictive ability of the simulation model, we present absolute benchmarks against beam loss monitor measurements from the 2018 <math display="inline"><mrow><msup><mrow><mmultiscripts><mrow><mi>Pb</mi></mrow><mprescripts/><none/><mrow><mn>208</mn></mrow></mmultiscripts></mrow><mrow><mn>82</mn><mo>+</mo></mrow></msup></mrow></math> run at <math display="inline"><mrow><mn>6.37</mn><mtext> </mtext><mtext> </mtext><mi>Z</mi><mi>TeV</mi></mrow></math>. The benchmarks cover several hundred meters of beamline, from the primary collimators to the first arc cells. Based on these studies, we provide a detailed analysis of ion fragmentation and leakage to the cold magnets and quantify the expected quench margin in future <math display="inline"><msup><mrow><mmultiscripts><mrow><mi>Pb</mi></mrow><mprescripts/><none/><mrow><mn>208</mn></mrow></mmultiscripts></mrow><mrow><mn>82</mn><mo>+</mo></mrow></msup></math> runs.