Quadrupole-free detector optics design for the Compact Linear Collider final focus system at 3 TeV

Aiming to simplify the machine detector interface of the Compact Linear Collider (CLIC), a new detector model has been designed allowing the last quadrupole QD0 of the final focus system (FFS) to be located outside of the experiment with a distance $L^{*}$ from the interaction point of 6 m. In this paper, the beam delivery system (BDS) has been reoptimized, offering a luminosity performance that exceeds the design requirements by 11% for the total luminosity and by 7% in the energy peak. A simulation campaign has been carried out and has proved the feasibility of recovering the luminosity under realistic transverse misalignments of the FFS optics, by means of different orbit and aberration correction techniques, making this long $L^{*}$ design a realistic candidate for the future CLIC BDS.

Figure 1

Vertical cut through the SiD experiment. QD0 is located inside the detector and partially supported by the preinsulator (green block) in the tunnel [4].

Figure 2

Schematic overview of the SiD interaction region layout from the last 12 m of the FFS (upper plot). Simulation of the longitudinal and radial fields (bottom plot). QD0 overlaps with the SiD solenoid field for $L^{*}=3.5\mathrm{m}$.

Figure 3

Vertical cut through the new detector model CLICdet allowing QD0 to be located outside of the experiment. No preinsulator or QD0 shielding are needed as opposed to the short $L^{*}$ design in Fig. 1 [14].

Figure 4

Schematic overview of the new detector (CLICdet) interaction region layout from the last 12 m of the FFS (upper plot). Simulation of the longitudinal and radial fields (bottom plot). No overlapping between QD0 and the new detector field with $L^{*}=6\mathrm{m}$.

Figure 5

Optical functions through the local correction scheme of the FFS for $L^{*}=3.5\mathrm{m}$ (top plot) and $L^{*}=6\mathrm{m}$ (bottom plot), where ${\eta }_x$ is the dispersion function. The lattice for $L^{*}=6\mathrm{m}$ has been lengthened with respect to the increase of $L^{*}$ from the nominal design.

Figure 6

Impact of the dispersion level in the FFS on the horizontal and vertical beam sizes ${\sigma }_{x,y}^{*}$ (top plot), luminosity, and sextupole strength $k_2$ (bottom plot) when ${\eta }_x$ is reduced up to 40%.

Figure 7

Peak luminosity maximization versus ${\beta }_y^{*}$ for $L^{*}=6\mathrm{m}$.

Figure 8

Energy bandwidth comparison between $L^{*}=3.5\mathrm{m}$ and $L^{*}=6\mathrm{m}$. ${\mathcal{L}}_{1\%}$ is normalized to their respective maximum peak luminosity ${\mathcal{L}}_{1\%,0}$.

Figure 9

Luminosity distribution after BBA and one scan of the linear knobs for different values of ${\beta }_y^{*}$. 100 randomly misaligned machines are simulated, and the luminosity is normalized to the design luminosity ${\mathcal{L}}_0=5.9\times {10}^{34}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$.

Figure 10

Tuning performance results for the optimized $L^{*}=6\mathrm{m}$ design with ${\beta }_y^{*}=0.12\mathrm{mm}$. 87% of the machines achieve at least 110% of the design luminosity after 12 iterations of linear knobs, corresponding to approximately 5000 luminosity measurements.

Figure 11

QD0 vertical offset scan as a function of the relative peak luminosity loss for $L^{*}=6\mathrm{m}$ and $L^{*}=3.5\mathrm{m}$ designs.

Figure 12

Normal (left plot) and skew (right plot) sextupole, octupole, and decapole field error tolerances for QD0.

Figure 13

Normal (left plot) and skew (right plot) sextupole, octupole, and decapole field error tolerances for QF1.

Figure 14

Halo distribution after collimation to $15{\sigma }_x$ and $55{\sigma }_y$ at the entrance of the FFS.

Figure 15

Horizontal position of the emitted photons through the FD and CLIC detector, up to the entrance of the first postcollision magnet, for $L^{*}=3.5\mathrm{m}$ (top plot) and $L^{*}=6\mathrm{m}$ (bottom plot).

Figure 16

Vertical position of the emitted photons through the FD and CLIC detector, up to the entrance of the first postcollision magnet, for $L^{*}=3.5\mathrm{m}$ (top plot) and $L^{*}=6\mathrm{m}$ (bottom plot).

Figure 17

Horizontal beam size ${\sigma }_x^{*}$ including only second-order aberrations, simulated before $L_{\mathrm{QF}1\text{−}\mathrm{QD}0}$ optimization, as a function of the FFS length.

Figure 18

Example of $L_{\mathrm{QF}1\text{−}\mathrm{QD}0}$ scan for ${\sigma }_{x,2\mathrm{nd}\text{order}}^{*}$ minimization applied on the $L_{\mathrm{FFS}}=658.9\mathrm{m}$ lattice. The original distance $L_{\mathrm{QF}1\text{−}\mathrm{QD}0}$ after FFS length reduction was 5.8 m.

Figure 19

Parabolic fits of the $L_{\mathrm{QF}1\text{−}\mathrm{QD}0}$ scan for different FFS lengths.

Figure 20

Optimal distance $L_{\mathrm{QF}1\text{−}\mathrm{QD}0}$ for ${\sigma }_{x,2\mathrm{nd}\text{order}}^{*}$ minimization as a function of the FFS length.

Figure 21

Dispersion profile for different FFS lengths with $L^{*}=6\mathrm{m}$.

Figure 22

Average sextupole strength and dispersion at the sextupole locations as a function of the FFS length.

Figure 23

Tuning efficiency comparison after one iteration of BBA and three iterations of linear knobs for $L_{\mathrm{FFS}}=495.7\mathrm{m}$ and $L_{\mathrm{FFS}}=770\mathrm{m}$ with $L^{*}=6\mathrm{m}$.

Figure 24

Tuning efficiency after one iteration of BBA and three iterations of linear knobs for $L_{\mathrm{FFS}}=450\mathrm{m}$ (nominal design) and $L_{\mathrm{FFS}}=770\mathrm{m}$ with $L^{*}=3.5\mathrm{m}$.

Figure 25

Comparison of the average luminosity, over 100 machines simulated, achieved after BBA and iterations of linear knobs applied on the $L^{*}=6\mathrm{m}$ design and the $L^{*}=3.5\mathrm{m}$ design with $L_{\mathrm{FFS}}=770\mathrm{m}$. Five scans of linear knobs were applied, which represents approximately 2000 luminosity measurements.