This activity also required removing about 17 m of the storage ring where it passed through the center of CLEO, then installing bridge supports and finally re-installing the bridging I-beams, holding the accelerator magnets and vacuum system. All of this was accomplished on schedule in three months. The success of the re-installation is underscored by the fact that positron beam was stored with no adjustments to CESR on the very first attempt!
In addition to the CLEO removal, several other accelerator projects were undertaken during the summer of 2016 shutdown. CESR-TA, the test accelerator program, is an NSF-funded program (separate from CHESS) that uses CESR for machine studies and development projects. An important summer project for CESR-TA was continuing the installation of the x-ray optics and detector for the next generation x-ray beam size monitor (NGxBSM). The previous incarnation of this detector required installation in the D-line during CESR test accelerator (CESR-TA) runs two or more times per year. Like its predecessor, the NGxBSM will be able to measure the positron beam size bunch-by-bunch and turn-by-turn for bunches spaced as closely as 4 nsec with a resolution less than 20 mm. The turn-by-turn capability of these detectors permits the determination and removal of any centroid motion of each bunch, producing accurate beam size measurements. As part of CESR-TA, this instrument along with the equivalent electron monitor has been utilized for the study of the growth of electron clouds (ECs) in CESR. ECs are generated from the photoelectron effect when synchrotron radiation from positron bunches strike the vacuum chamber walls. The EC is attracted toward the subsequent positron bunches and then accelerated in their electromagnetic (EM) fields, causing the EC to strike the walls and create secondary electrons. This reaction can cascade down a train of bunches resulting in the growth of the bunches’ centroid motion and beam sizes. The two beam size monitors have only been used in CESR-TA operation as they require the C- and D-line hutches for the installation of the detectors. When initial commissioning is complete, the NGxBSM for the positron beam will be available for use during CHESS operations, providing more accurate beam size diagnostics.
The second major project for CESR-TA is the installation of a new, test quadrupole in CESR and a vacuum chamber, containing a fast EC detector and button electrodes used for transverse electric (TE)-wave measurements. Using DC-biased electrodes, the former of these two instruments detects the growth of ECs as trains of positron bunches pass through the quadrupole and has a 1 nsec time resolution. The TE-wave electrodes excite and detect standing EM modes in the vacuum chamber. As the positron train passes, the growing EC behaves as a plasma, whose dielectric strength depends on the plasma density, which causes the resonant frequency of the EM mode to shift. Measurements of this frequency shift allow the inference of EC density growth along the train of positron bunches. Formed as a part of the study of the damping rings, required for a possible future international linear collider for HEP, the CESR-TA project has measured the growth of ECs in the drift sections, dipole magnets and superconducting wigglers of the storage ring with concentration on methods to mitigate the growth of ECs and their effects. For these accelerator components in CESR the EC will tend to dissipate over 200-300 nsec after the passage of the train of positrons-much shorter than the beam’s circulation time. However, the measurements of EC in quadrupoles indicate that the cloud will be trapped in regions of the magnetic field allowing the EC to persist for many times the 2.5 msec circulation time. One of the effects of this remnant EC from quadrupoles is conjectured to cause the vertical beam size of the lead bunch of the train to be enlarged. The installation of the test quadrupole permits the study of the EC effects as the focal strength of the quadrupole is varied, while maintaining the CESR-TA optical functions by compensating with adjacent quadrupoles. This is the first time that such systematic measurements for quadrupoles have been undertaken in an accelerator.
One of the many experiments planned for the CESR-TA accelerator physics run, scheduled for mid-December 2016, will have a coherent Cherenkov radiator installed in CESR. This effort is part of a sequence of joint Cornell-CERN experiments, using electron bunches in CESR to study possible beam size and position monitors for beam lines for future electron accelerators. The first set of the experiments observed coherent diffraction radiation from a stored electron bunch passing through different height slits in a metal film. The experimental group has successfully observed the coherent radiation from slits ranging in height from 0.5 mm to 4 mm, the smallest apertures for a stored electron beam. While the beam lifetime of 2 minutes, typical for the 0.5 mm slits, would be considered very short for storage rings, it is long enough to obtain accurate measurements of the dependence of the intensity and polarization of coherent diffraction radiation from different slits as the beam position, vertical size and wavelength of the radiation were varied. The experiment, planned for December 2016, would install a 4 mm high, 1 cm-long silicon carbide dielectric Cherenkov radiator to characterize the shockwave, produced by the EM fields of the beam passing through the dielectric at faster than the speed of light in the medium.
Mid-November 2016 saw the completion of the re-installation and cool down of the superconducting wigglers (SCWs) in CESR. Six of the wigglers were removed during the decommissioning of the CLEO experiment. The SCWs are necessary for the operation of low energy beams in CESR. This includes the 2 GeV operation of CESR for the CESR-TA project. In collaboration with an experimenter from Jefferson Laboratory one of the other projects, scheduled for the CESR-TA run, will use counter-rotating electron and positron bunches at 2 GeV for a precision measurement over a 12 hour-period to determine whether there is a variation in the speed of light depending on the direction of the earth’s motion.