Conceptual design of an off-momentum collimation system in the CERN Super Proton Synchrotron for High-Luminosity Large Hadron Collider proton beams

The CERN accelerator complex is undergoing an upgrade in order to meet the requirements of the High-Luminosity Large Hadron Collider (HL-LHC). One of the key needs is to increase the intensity of beams injected into the LHC from the current $1.15\times {10}^{11}$ to $2.3\times {10}^{11}{\mathrm{p}}^{+}/\mathrm{bunch}$. This requires a beam intensity at injection in the Super Proton Synchrotron (SPS), the last injector before the LHC, as high as $2.6\times {10}^{11}{\mathrm{p}}^{+}/\mathrm{bunch}$, given that a budget for losses of about 10% is included in the design. However, previous experience with high intensity beams suggests that the total amount of losses in the SPS can be even higher, causing an increased activation, faster aging, and consequent early failure of machine equipment, in addition to an increase in the requirements for preinjectors in order to still meet the HL-LHC target in spite of higher losses. In this paper, we propose a collimation system to be used with HL-LHC proton beams in the SPS in order to intercept and safely dispose of beam losses in ad hoc locations and therefore provide the machine with protection against activation and equipment aging. The design is based on a two-stage concept, where the primary stage intercepts particles that otherwise would be lost, and the secondary stage employing an absorber where lost particles are finally disposed of. Numerical simulations prove a cleaning efficiency of the proposed system of at least 80%, substantially reducing the spreading of particle losses around the machine. Furthermore, the performance of the collimation system has a low sensitivity to common machine errors, and the design is cost efficient, with minimal hardware changes.

Figure 1

Horizontal dispersion function for Q20 and Q26 optics. Thanks to the sixfold periodicity of the SPS [1], the horizontal axis includes only half of an arc followed by half of an LSS. The machine layout, comprising dipoles (blue) and quadrupoles (purple), is given in the top panel.

Figure 2

Momentum beam size calculated as $D_x{\sigma }_{\text{dpp}}$ with three values of momentum offset for the Q20 optics: one standard deviation (purple), bucket height (orange) and momentum cut (blue). Mechanical aperture from the machine model is given in black.

Figure 3

Optics functions and location of collimators (black vertical lines).

Figure 4

Histograms of betatron amplitude increase (top) and relative momentum loss (bottom) of protons passing through the primary stage. Please note the logarithmic horizontal scale of both plots.

Figure 5

Orbit bump (purple), beam envelope with (green) and without the orbit bump (orange) at the location of collimators (black rectangles). The machine aperture is given in black and the machine layout is shown in the top panel.

Figure 6

Cut in phase space provided by the two stages of the baseline proposal of the SPS off-momentum collimation system. The expected post-LS2 aperture has been used.

Figure 7

Local cleaning inefficiency for three aperture models: ideal (top), measured (middle), after LS2 (bottom).

Figure 8

Global cleaning efficiency after fixing a flange next to a given defocusing quadrupole. For each data point all the flanges to the left of it are also fixed. Data points are joined to guide the eye.

Figure 9

Local cleaning inefficiency for the measured aperture and with tighter collimation cuts ($4{\sigma }_{\beta }+0.9D_x{\delta }_{\mathrm{p},\text{bh}}$ at the primary stage, $4.5{\sigma }_{\beta }+0.9D_x{\delta }_{\mathrm{p},\text{bh}}$ at the secondary stage).

Figure 10

Local cleaning inefficiency in case of a hierarchy breakage. Aperture model: after LS2.

Figure 11

Global cleaning efficiency as a function of the bump amplitude at the TIDP. Aperture model: after LS2. 17.5 mm is the baseline value of orbit bump amplitude at the TIDP (see Table 5).

Figure 12

Global cleaning efficiency distribution for about 1000 random machines with optics errors.

Figure 13

Distribution of machines attaining a given global cleaning efficiency as a function of $\beta$-beating (top) and D-beating (bottom).

Figure 14

Global cleaning efficiency distribution for about 2000 random machines with combined orbit and optics errors.