Luminosity studies in a traveling waist regime in the Compact Linear Collider

The final focus system of the Compact Linear Collider (CLIC) features a crab cavity just before the final quadrupole doublet to ensure head-on collisions at the interaction point. A decrease in the expected luminosity with respect to the case without crossing angle has been recently observed in simulations for the CLIC baseline configuration. This effect is explained from the aberrations induced due to a $z$-dependent off-center horizontal orbit in the final focus sextupoles produced by the crab cavity. Three options to recover from this loss are explored. The effect of the longitudinal $E\mathrm{\text{−}}z$ correlation of the incoming beam from the linac is taken into account and its effect on the expected luminosity is evaluated.

Figure 1

Layout of the elements in the last part of the CLIC BDS. The crab cavity is located just before the final doublet. After being deflected by the crab cavity the beam will pass horizontally off center in the two last sextupoles, SF1 and SD0.

Figure 2

Vertical beam size of the colliding slice at different longitudinal locations before and after the IP for an initial distribution with no $E\mathrm{\text{−}}z$ correlation from the linac.

Figure 3

Luminosity scan for different values of traveling waist ($\frac{\partial w_y}{\partial z}$) and waist shift ($w_y$). The luminosity is normalized with respect to the case without crossing angle and no $E\mathrm{\text{−}}z$ correlation in the incoming beam from the linac. The different options studied are depicted with the same notation as in Table . The maximum luminosity region corresponds to positive traveling waist parameters and is centered around a waist shift of $\sim 25\mu \mathrm{m}$ which is consistent with results obtained for ILC [12].

Figure 4

Sketch (not to scale) of turn around (TA), main linac, and beam delivery system (BDS) for present (red) and proposed (blue) crossing schemes. In order to revert the sign of the crossing angle the main linac and BDS have to be rigidly moved from the TA. This can be achieved, for example, by redesigning the basic cell of the TA to decrease the total bending angle.

Figure 5

Vertical beam size of the colliding slice at different longitudinal locations before and after the IP for an initial distribution with real sinusoidal-like $E\mathrm{\text{−}}z$ correlation from the linac.