Bound-free pair production from nuclear collisions and the steady-state quench limit of the main dipole magnets of the CERN Large Hadron Collider

During its Run 2 (2015–2018), the Large Hadron Collider (LHC) operated at almost twice higher energy, and provided Pb-Pb collisions with an order of magnitude higher luminosity, than in the previous Run 1. In consequence, the power of the secondary beams emitted from the interaction points by the bound-free pair production (BFPP) process increased by a factor $\sim 20$, while the propensity of the bending magnets to quench increased with the higher magnetic field. This beam power is about 35 times greater than that contained in the luminosity debris from hadronic interactions and is focused on specific locations that fall naturally inside superconducting magnets. The risk of quenching these magnets has long been recognized as severe and there are operational limitations due to the dynamic heat load that must be evacuated by the cryogenic system. High-luminosity operation was nevertheless possible thanks to orbit bumps that were introduced in the dispersion suppressors around the ATLAS and CMS experiments to prevent quenches by displacing and spreading out these beam losses. Further, in 2015, the BFPP beams were manipulated to induce a controlled quench, thus providing the first direct measurement of the steady-state quench level of an LHC dipole magnet. The same experiment demonstrated the need for new collimators that are being installed around the ALICE experiment to intercept the secondary beams in the future. This paper discusses the experience with BFPP at luminosities very close to the future High Luminosity LHC (HL-LHC) target, gives results on the risk reduction by orbit bumps and presents a detailed analysis of the controlled quench experiment.

Figure 1

Example of main ${{}^{208}\mathrm{Pb}}^{82+}$ (blue, $10-12\sigma$) and BFPP1 ${{}^{208}\mathrm{Pb}}^{81+}$ beam (red, $1-2\sigma$) envelopes, and aperture (grey) in the horizontal plane, Beam 1 direction right of IP5 (at $s=0$). Beam-line elements are indicated schematically as rectangles. Dipoles in light blue, quadrupoles in dark blue (focusing) and red (defocusing). While the main beam travels through the center of the beam-line elements in the dispersion suppressor (starting at about 250 m), the BFPP1 beam separates and impacts in the aperture of the second superconducting dipole magnet of cell 11.

Figure 2

Full view of IR5 and adjusted DS. The incoming beam envelopes, before the collision at IP5, are not shown. The grey shaded area represents the aperture, the coloured rectangles on the top the beam-line elements. The effect of a $-3\mathrm{mm}$ orbit bump around Q11 (at $s=\pm 440\mathrm{m}$) on the main (${{}^{208}\mathrm{Pb}}^{82+}$) and BFPP (${{}^{208}\mathrm{Pb}}^{81+}$) $1\sigma$ beam envelopes is shown. Note that the origin of all beams lies at IP5 (center of the plot) such that Beam 2 travels to the left and Beam 1 to the right.

Figure 3

Zoom into Fig. 2 at the impact location of the BFPP beam right of IP5. Purple trajectory calculated without orbit bump, red with a bump amplitude of $-3\mathrm{mm}$ at Q11 (blue rectangle).

Figure 4

2015 (black) and 2018 (purple) MAD-X BFPP trajectories (straight lines) and BLM loss signals (corresponding colors) in cell 11 left and right of IP5 for operational bump amplitudes in 2015 (on the left for $-3\mathrm{mm}$ and on the right for $-2.6\mathrm{mm}$ bumps). The rectangles on the bottom correspond to the beam-line elements, the shaded area on the top visualizes the reconstructed real aperture.

Figure 5

Evolution of BLM signals around the BFPP impact point to the left (left figure, Beam 2) and right (right figure, Beam 1) of IP5, while stepwise inverting the orbit bump from $-3\mathrm{mm}$ to $+0.5\mathrm{mm}$ on the left of IP5 (grey shadowed period) and increasing luminosity (red shadowed period) during the quench test.

Figure 6

Standard (22 mm, grey shaded area) and real (green shaded area) horizontal aperture from measurement, superimposed with the BFPP beam trajectory with two sigma beam envelope around the impact point left and right of IP5 shown for orbit bump amplitudes between $+0.5\mathrm{mm}$ (blue) and $-5\mathrm{mm}$ (green) calculated for the 2015 operational optics configuration.

Figure 7

Simulated BFPP beam distribution as lost in $s$-direction on the beam screen, assuming the measured aperture along the impact location to the left of IP5 and the 2015 operational optics configuration with different BFPP bump amplitudes. The corresponding BFPP trajectories with a beam envelop of two sigma are shown in the same colors. The rectangles on the bottom indicate the positions of the dipole MB.B11L5 and connection cryostat LEFL.11L5.

Figure 8

BLM signal comparison between experimental data from the quench test (blue) and FLUKA data assuming two different loss locations (red and green). The particle distributions for the different loss locations assumed in the simulations are shown in histograms of the corresponding color.

Figure 9

BLM signal comparison between experimental data from the quench test (blue) and FLUKA data assuming both a real (orange) and an ideal (dark green) aperture model. Note that the blue and dark green lines are identical to the ones in Fig. 8.

Figure 10

Peak power density in the MB.B11L5 coils during the quench test estimated by FLUKA simulations assuming both an ideal and a real aperture model.

Figure 11

Main ($12\sigma$) and BFPP1 ($1\sigma$) radial beam envelopes and aperture (grey) in Beam 1 direction right of IP2 (at $s=0$). Beam-line elements are indicated schematically as rectangles. Dipoles in light blue, quadrupoles in dark blue (focusing) and red (defocusing). The BFPP1 beam impacts in the aperture of the second superconducting dipole magnet of cell 10. An orbit bump around MQML.10R2 of $-3\mathrm{mm}$ can move the impact out of cell 10, such that the BFPP1 beam can be absorbed in a collimator (indicated as black vertical lines at $s=430\mathrm{m}$) placed in the empty connection cryostat in cell 11.