Measurements of heavy ion beam losses from collimation

The collimation efficiency for ${}^{208}\mathrm{Pb}^{82+}$ ion beams in the LHC is predicted to be lower than requirements. Nuclear fragmentation and electromagnetic dissociation in the primary collimators create fragments with a wide range of $Z/A$ ratios, which are not intercepted by the secondary collimators but lost where the dispersion has grown sufficiently large. In this article we present measurements and simulations of loss patterns generated by a prototype LHC collimator in the CERN SPS. Measurements were performed at two different energies and angles of the collimator. We also compare with proton loss maps and find a qualitative difference between ${}^{208}\mathrm{Pb}^{82+}$ ions and protons, with the maximum loss rate observed at different places in the ring. This behavior was predicted by simulations and provides a valuable benchmark of our understanding of ion beam losses caused by collimation.

Figure 1

The cross sections for creating heavy isotopes (mass number $A>140$ and charge $Z>60$) by a $106.4\mathrm{GeV}/\mathrm{nuleon}$ ${}^{208}\mathrm{Pb}^{82+}$ beam impacting on a carbon target. The cross sections were simulated by FLUKA.

Figure 2

A collimator jaw of the same type as installed in the SPS. The figure is taken from 31.

Figure 3

The installation of the prototype LHC collimator in the SPS tunnel. The figure is taken from 31.

Figure 4

The $\beta$ functions of the SPS downstream of the collimator and the locally generated dispersion $d_x$ from the collimator. We show also the magnetic elements ($\mathrm{MB}=\mathrm{\text{main bend}}$, $\mathrm{QF}=\mathrm{\text{focusing quadrupole}}$, $\mathrm{QD}=\mathrm{\text{defocusing quadrupole}}$). The BLMs and the collimator are indicated by vertical lines.

Figure 5

Dispersive orbits of fragmented ions produced in one of the collimator jaws, shown together with the aperture. The collimator is located at $s=5222\mathrm{m}$ at the left edge. The vertical dashed lines indicate the locations of the four BLMs closest downstream. A large fraction of the total losses occur at the aperture limitation at $s=5277\mathrm{m}$.

Figure 6

Vertical betatron orbits coming out of the collimator, starting at $y=0$, for different starting values of $y^{\prime }$ (vertical momentum normalized by longitudinal momentum). The collimator is located at $s=5222\mathrm{m}$ at the left edge. The vertical dashed lines indicate the location of the four BLMs closest downstream. Particles with large vertical angles are lost close to BL520.

Figure 7

The installation of a BLM (BL521) in the SPS tunnel, shown together with a close-up of the inner assembly of the chamber.

Figure 8

The ${}^{208}\mathrm{Pb}^{82+}$ momentum per nucleon during three cycles of the SPS in the case of acceleration and a circulating beam at constant energy (CBCE). In the acceleration cycle, the dumping of the beam during the collimation measurements is indicated, as well as the extraction momentum.

Figure 9

Schematic sketch of collimator jaws kept parallel to the beam direction when moved in (left) and with a 2 mrad angle (right).

Figure 10

The 3D geometry as implemented in FLUKA around a BLM in the vertical plane (BL520, top) and in the horizontal plane (BL521, bottom). A photo of the central part of the real geometry around BL521 is shown in Fig. 7.

Figure 11

The geometry as implemented in FLUKA around the monitor BL521 with the simulated energy deposition from a typical loss superimposed. The loss is indicated by the dashed arrow.

Figure 12

Example of simulated ICOSIM (top) and measured (middle) loss map of the whole SPS ring with a $106.4\mathrm{GeV}/\mathrm{nucleon}$ circulating ${}^{208}\mathrm{Pb}^{82+}$ beam. The bottom part shows a close-up of the loss peak in the measurements, with the names of the BLMs with the highest signals indicated. The simulated losses were binned in 5 m intervals. The collimator is located at $s=5222\mathrm{m}$, just upstream of the large loss peak.

Figure 13

Average loss map for $106.4\mathrm{GeV}/\mathrm{nucleon}$ ${}^{208}\mathrm{Pb}^{82+}$ ions (top) and 270 GeV protons (bottom) with parallel jaws (blue bars) and jaws tilted by 2 mrad (red bars). The measurements are compared with simulations using both methods of simulating the particle-matter interaction in the collimator, as described in the text. All results were normalized to ${10}^{10}$ lost particles. The standard deviation between different measurements is indicated.

Figure 14

The simulated distribution of $\delta$ calculated according to Eq. (3), using the FLUKA full method, of all high-energy nuclei except ${}^{208}\mathrm{Pb}^{82+}$ coming out of the collimator (top) and of the ions lost within a 15 m interval upstream of each BLM (bottom) at $106.4\mathrm{GeV}/\mathrm{nucleon}$ with parallel jaws. The heights of the bars show the number of nucleons belonging to ions having $\delta$ within a certain interval.

Figure 15

The $A$ distribution of ion fragments lost within 15 m upstream of each BLM at $106.4\mathrm{GeV}/\mathrm{nucleon}$ and parallel collimator jaws as simulated with the FLUKA full method. The heights of the bars show the number of nucleons from ions having a certain $A$.

Figure 16

The simulated distribution from the FLUKA full method of $y^{\prime }$ of all ion fragments coming out of the collimator (top) and of the ions lost within a 15 m interval upstream of each BLM (bottom). The heights of the bars show the number of nucleons belonging to ions within a certain interval.

Figure 17

Measured and simulated ${}^{208}\mathrm{Pb}^{82+}$ loss maps with parallel jaws for a $5.9\mathrm{GeV}/\mathrm{nucleon}$ beam injected on the collimator (green bars) and circulating $106.4\mathrm{GeV}/\mathrm{nucleon}$ (blue bars).

Figure 18

The simulated $A$ distribution of ion fragments from the FLUKA full method lost within 15 m upstream of each BLM at $5.9\mathrm{GeV}/\mathrm{nucleon}$. The heights of the bars show the number of nucleons from ions having a certain $A$.

Figure 19

The simulated distribution of $\delta$ calculated according to Eq. (3) from the FLUKA full method of all ion fragments except ${}^{208}\mathrm{Pb}^{82+}$ coming out of the collimator at $5.9\mathrm{GeV}/\mathrm{nucleon}$. The heights of the bars show the number of nucleons belonging to ions having $\delta$ within a certain interval.

Figure 20

The simulated distribution of $\delta$ (top) and $y^{\prime }$ (bottom) of all protons coming out of the collimator.