10 The Largest Accelerators and Colliders of Their Time
629
and KEKB smaller. KEKB has solved the conflict with the injection to the light
sources by introducing a pulse-to-pulse switching of the linac.
10.4.11 Crab Crossing
KEKB operated with crab crossing from 2007 through 2010 using superconducting
crab cavities [163] installed one cavity per ring [164]. KEKB had already achieved
a luminosity 1.76 × 10 34 cm −2 s −1 by then with the crossing angle, so the intension
of the crab crossing was not simply the backup. Simulations of beam-beam effect
indicated that a head-on or crab collision could increase the beam-beam parameter
ξ y even higher than 0.15 combining with a horizontal betatron tune close to a half
integer [165]. Thus the hope was to experimentally verify the possibility of such a
high beam-beam parameter, considering super B-factories.
The crab cavities were successfully installed and operated for more than 3 years.
The luminosity was actually increased as shown in Table 10.12. The resulted beambeam parameter was 0.09, which was indeed higher than the value with crossing
angle, but much less than the simulation (0.15). It was not easy to single out the
cause, but there were indications that remaining higher-order terms of the beam
optics at the IP degraded the luminosity and ξ y . One example was the chromatic
x-y coupling terms at the IP [166]. By installing skew sextupoles in the arc as the
tuning knobs, the luminosity was improved by up to 10% [124]. Then a speculation
was that any higher order terms at the IP could degrade the performance. It was not
easy to estimate how many terms were relevant and how to correct them, as there
were almost no direct beam diagnostics at the IP except the luminosity. What was
verified at KEKB was that the crab crossing itself should work for any colliders up
to ξ x, y 0.1, including the Large Hadron Collider (LHC).
10.4.12 SuperKEKB
Although the crab crossing scheme tested at KEKB achieved then-highest luminosity, it also showed the difficulty to realize the very high beam-beam parameter 0.1.
Thus the direction toward a higher luminosity asymmetric collider must change. An
alternative idea, Nano-beam scheme, developed by P. Raimondi [167] saved the next
generation of KEKB. The idea by Raimondi consists of:
• A large crossing angle, in terms of Piwinski angle θ x σ z /σ ∗
x 1, where θ x is the
half horizontal crossing angle.
• A very short β ∗
y ∼ σ ∗
x /θ x σ z .
• Small horizontal/vertical emittances.
629
and KEKB smaller. KEKB has solved the conflict with the injection to the light
sources by introducing a pulse-to-pulse switching of the linac.
10.4.11 Crab Crossing
KEKB operated with crab crossing from 2007 through 2010 using superconducting
crab cavities [163] installed one cavity per ring [164]. KEKB had already achieved
a luminosity 1.76 × 10 34 cm −2 s −1 by then with the crossing angle, so the intension
of the crab crossing was not simply the backup. Simulations of beam-beam effect
indicated that a head-on or crab collision could increase the beam-beam parameter
ξ y even higher than 0.15 combining with a horizontal betatron tune close to a half
integer [165]. Thus the hope was to experimentally verify the possibility of such a
high beam-beam parameter, considering super B-factories.
The crab cavities were successfully installed and operated for more than 3 years.
The luminosity was actually increased as shown in Table 10.12. The resulted beambeam parameter was 0.09, which was indeed higher than the value with crossing
angle, but much less than the simulation (0.15). It was not easy to single out the
cause, but there were indications that remaining higher-order terms of the beam
optics at the IP degraded the luminosity and ξ y . One example was the chromatic
x-y coupling terms at the IP [166]. By installing skew sextupoles in the arc as the
tuning knobs, the luminosity was improved by up to 10% [124]. Then a speculation
was that any higher order terms at the IP could degrade the performance. It was not
easy to estimate how many terms were relevant and how to correct them, as there
were almost no direct beam diagnostics at the IP except the luminosity. What was
verified at KEKB was that the crab crossing itself should work for any colliders up
to ξ x, y 0.1, including the Large Hadron Collider (LHC).
10.4.12 SuperKEKB
Although the crab crossing scheme tested at KEKB achieved then-highest luminosity, it also showed the difficulty to realize the very high beam-beam parameter 0.1.
Thus the direction toward a higher luminosity asymmetric collider must change. An
alternative idea, Nano-beam scheme, developed by P. Raimondi [167] saved the next
generation of KEKB. The idea by Raimondi consists of:
• A large crossing angle, in terms of Piwinski angle θ x σ z /σ ∗
x 1, where θ x is the
half horizontal crossing angle.
• A very short β ∗
y ∼ σ ∗
x /θ x σ z .
• Small horizontal/vertical emittances.