Operational performance of crystal collimation with 6.37 $Z$ TeV Pb ion beams at the LHC

The concept of crystal collimation relies on the use of bent crystals to coherently deflect positively charged particles with suitable impact conditions by trapping them in the potential well generated by adjacent crystalline planes. The resulting deflection is much higher than what can be achieved by multiple scattering on amorphous materials. For this reason, this technique has been explored in the past decades for applications to particle accelerators. In particular, a full test stand was installed in the betatron collimation insertion of the Large Hadron Collider (LHC) to explore applications to hadron beam collimation. This setup was extensively studied in Run 2 (2015–2018), as a way to improve the cleaning performance of the machine in particular with Pb ion beams, in view of the more challenging parameters envisaged for the High Luminosity upgrade (HL-LHC). This paper reports the results of measurements performed with Pb ion beams, demonstrating the capability of crystal collimation to improve the cleaning performance at the LHC. These results supported the integration of this advanced technology in the baseline upgrade program for HL-LHC.

Figure 1

Illustration of the standard collimation layout [14].

Figure 2

Illustration of planar channeling in a bent crystal seen from the top (left) and transverse potential experienced by the proton as a function of horizontal displacement (right).

Figure 3

Illustration of the crystal collimation layout [14].

Figure 4

Projection of the Beam 1 betatron collimation system on the vertical plane [36]. TCPs, TCSGs, TCLAs, and the vertical crystal are shown as light blue, dark blue, green, and orange bars, respectively. The beam envelope and the trajectory of the deflected halo based on the machine optics are shown in red and gray, respectively. The TCSG that intercepts the channeled halo on this plane is labeled as “absorber.”

Figure 5

Schematic representation of the angular scan (left frame) and linear scan (right frame) procedures.

Figure 6

Normalized BLM signal measured at the crystal location during an angular scan (horizontal crystal, Beam 2) with proton beams at 6.5 TeV, compared with sixtrack simulations for different bending angles. The raw BLM signal measured at the absorber location is also shown.

Figure 7

Normalized BLM signal measured at the absorber location during a linear scan (horizontal crystal, Beam 1) with proton beams at 450 GeV, compared with sixtrack simulation results. The error function fit on data is also shown. The origin of the horizontal axis is set to the measured/simulated position of the beam envelope defined by the crystal.

Figure 8

Schematic representation of the collimator hierarchy used in crystal collimation tests with Pb ion beams. Primary (TCPs), secondary (TCSGs), and absorber (TCLAs) collimators are depicted in operational (left side) and MD (right side) settings. The red arrow represents the beam axis.

Figure 9

Loss pattern in the whole LHC ring (normalized to the rate of lost particles) measured during a loss map for the horizontal plane of Beam 1 with 20 bunches of 6.37 $Z$ TeV Pb ions, using standard collimation (top) and crystal collimation with MD settings (bottom). The beam intensity in number of charges as a function of time is shown in the boxed plots (note that apparent increases are due to fluctuations in the intensity reading).

Figure 10

Loss pattern in IR7 (normalized to the rate of lost particles) measured during a loss map for the horizontal plane of Beam 1 with 20 bunches of 6.37 $Z$ TeV Pb ion beams using standard collimation (top) and crystal collimation with MD settings (bottom). The layout of machine elements is schematically depicted in the box plot at the top (collimators in black, dipoles in blue, and quadrupoles in red).

Figure 11

Local leakage ratio between standard collimation and crystal collimation using MD and operational settings (see Table 4), measured in loss maps with 20 bunches of 6.37 $Z$ TeV Pb ion beams using MD and operational settings. The corresponding error bars are also shown.

Figure 12

Normalized BLM signal recorded at TCTs with standard and crystal collimation (operational settings) during loss maps for the horizontal plane of Beam 1 with 20 bunches of 6.37 $Z$ TeV Pb ion beams. Note that the signal recorded for TCTPH.04L8 could not be reliably separated from the background, thus only the corresponding error bar is shown.

Figure 13

Example of an angular scan around the reference optimal orientation of the crystal collimator on the horizontal plane of Beam 1 after its insertion with 648 circulating bunches of Pb ions. The reference of $1613\mathrm{\mu }\mathrm{rad}$ from the previous insertion was adjusted by $7\mathrm{\mu }\mathrm{rad}$.

Figure 14

Loss pattern observed while generating sustained losses on all four planes at the same time, with 20 circulating Pb ion bunches and using crystal collimation with MD settings. The full ring (top) and the IR7 region (bottom) are shown.