In early March, after more than 23 years of continuous operation, the CESR staff and the CLEO collaboration at Cornell completed their programme of b quark physics. Now the conversion of CESR to operate in the lower-energy region of the c quark, or charm, threshold has begun, with the installation of the first new “wiggler” magnets, which are a key component of the conversion. The US National Science Foundation has approved the proposal for this new programme and awarded a five-year grant to support it. At the same time, the NSF approved the continuation of the CHESS facility, which supports the utilization of synchrotron radiation X-rays produced in CESR.
Conversion of CESR’s e+e– storage ring to operate with sufficient luminosity in the charm-threshold region requires the installation of 18 m of wiggler magnets operating at magnetic fields of 2.1 T. Wiggler magnets have alternating north and south poles which induce rapid radial oscillations of the beams, increasing dramatically the emission of synchrotron radiation in the form of X-rays. Emission of synchrotron radiation “damps” the beams – that is, it decreases the sizes of the beams and increases the luminosity that the collider can provide.
When CESR was operating in the region of the upsilon particle, near 5 GeV per beam, the emission of synchrotron radiation in the collider’s bending magnets was sufficient to achieve small beams and high luminosity. At the lower energies of the c quark threshold region, between 1.5 and 2 GeV per beam, a factor of 20 in the radiation damping rate is lost. The wigglers will make up for this loss and achieve the high luminosities required.
The wigglers for “CESR-c” are superferric magnets, with iron poles excited by superconducting coils. A prototype wiggler, constructed in early 2002, was placed in CESR last August. Beam tests showed that the effects from the wiggler are consistent with estimates based on computer tracking and dynamic aperture analysis. With the extra damping of this one wiggler, as well as two weaker wigglers from the CHESS synchrotron radiation source, the luminosity in CESR in the charm-threshold region approached 2 x 1031 cm-2 s-1, which is already above luminosities achieved at other colliders in this energy range. The final luminosity with all wigglers installed will be around 3 x 1032 cm-2 s-1.
Six wigglers, representing 8 m of the 18 m required, have already been built and will be installed during a machine shutdown from March through June. For cooling, these wigglers will use the cryogenic facilities in place for CESR’s superconducting RF cavities. Space in the ring is being created by removing two dipole bending magnets, in the third of the circumference closest to the central lab, and increasing the field of adjacent magnets.
Other hardware changes are minor, such as using thinner windows in injection lines, improving the regulation of the power supply, and optimizing the superconducting RF field control for higher-field and lower-beam loading. The modifications of the CLEO detector are also modest. The main upgrade is replacing the silicon vertex detector with a small drift chamber.
Because of the lower beam energy, synchrotron-radiation users will no longer be able to run in parallel with high-energy physics operation. Dedicated periods of operation with beam energies above 5 GeV will serve the needs of X-ray users. The higher-energy running also benefits the beam lifetime for high-energy physics operations by keeping the vacuum chamber clean thanks to the action of the higher-energy synchrotron radiation photons.
CESR will operate in the charm-threshold region during the second half of 2003, after a period of commissioning and synchrotron-radiation running. At the same time the remaining wiggler units will be built and tested, ready for installation when convenient, so that full-intensity operation for high-energy physics can follow shortly afterwards.