A powerful and complex superconducting magnetic lens, the last missing part of the SuperKEKB accelerator, was delivered to KEK on February 13th . This is a major milestone in the SuperKEKB/Belle II project.
In the SuperKEKB accelerator, electron and positron beams are accelerated and then collide to produce pairs of B mesons (particles containing beauty quarks) at the center of the Belle II detector. The success of the Belle II experiment depends crucially on the number of B mesons produced per second in these collisions. This production rate, in turn, depends on how well the two beams can be focused to the collision point. The two focusing elements, huge complicated magnetic lenses are some of the most important parts of the accelerator.
In the novel scheme adopted for the SuperKEKB accelerator, the colliding beams will be extremely thin ribbons (the vertical thickness of 60nm corresponds to 1/1000 of width of a human hair!). The two magnetic lenses that are able to focus beams to such a small size in the focal plane, are sophisticated combinations of 55 interleaved electromagnets. Another important feature of the electromagnetic lenses are their temperature, 4 degrees above absolute zero (-273 C). The electric leads of the magnet coils are made of NbTi, a material that only becomes superconducting and loses all its electrical resistance once it is cooled down to very low temperatures, a few degrees above absolute zero.