Between electron cloud concerns, ultra-low beam emittance requirements and a handful of wigglers, you might say that the damping rings in the International Linear Collider are high maintenance. In the ILC, the 6-kilometre round damping rings will transform loose bunches of electrons and positrons into tight, disciplined beams before their final acceleration toward the interaction point at the center of the machine. It sounds easy enough, but fine-tuning the beams to be less than the thickness of a human hair is an extremely complex and challenging task.
In order to optimise the performance of the damping rings and at the same time minimise the cost, test facilities, such as the Accelerator Test Facility at KEK, are necessary to study important physics and technology issues. At Cornell University, a proposal is underway to convert the Cornell Electron Storage Ring (CESR), which is scheduled to stop operating for high energy physics in March 2008, into a new damping ring test facility, called CesrTF, for the ILC. This past summer, Cornell physicist Mark Palmer enlisted the skills of three undergraduate students to lend a hand with CesrTF R&D.
Understanding the electron-cloud effect is a very important issue for the positron ring in the ILC. While the ATF at KEK operates with an electron beam, CesrTF will have the ability to flexibly run with either positrons or electrons, allowing scientists to complement damping ring work that is being conducted at other laboratories. CESR is also the only operating wiggler-dominated ring. A wiggler magnet consists of a series of dipole magnets that bend the beam back and forth, causing it to emit synchrotron radiation. In fact, ninety percent of the synchrotron radiation in the ring occurs in the high field superconducting wigglers. Because of their high magnetic fields, the wigglers are very susceptible to the dreaded electron cloud growth. For this reason, the wigglers in the positron ring require a large amount of R&D. And because the proposed ILC wigglers are modelled after the CESR design, converting Cornell's storage ring into a damping ring test facility would provide valuable insight into the electron cloud effects.
"By placing the CESR wigglers at zero dispersion locations in the ring, simulations indicate that CESR can be operated with emittances approaching those required for the ILC damping rings," Palmer said. "This offers an excellent opportunity to study the emittance diluting effects of the electron cloud and other beam dynamics effects, as well as gain further experience with achieving ultra-low emittance beams."
With the larger project goals in mind, Palmer assigned each summer student to three different areas of CesrTF R&D. As part of the National Science Foundation's Research Experiences for Undergraduates program, Jim Shanks, a senior at Michigan State University, conducted a number of lattice design studies to analyse the configuration of the CESR facility and determine how much the design would have to change in order to convert it into a test area for the ILC. At the same time, he also examined what might be accomplished at CESR before making any physical changes to the ring layout -- meaning not move any wigglers to zero dispersion locations and also leave the CLEO detector and solenoid magnet in place. "We tried to design a lattice where we would have to make minimal changes in order to get the low emittance that we need for the ILC," he said. Shanks found that by using only six of the 12 wigglers in the CESR ring and creating a zero dispersion region around them, he could create a lattice with an emittance a little over three times larger than the target for CesrTF operations. "If we turn off six of the twelve wigglers, we can achieve the low emittance," he said. Eventually the remaining six wigglers will be moved to their new locations, and the CLEO detector will be removed. Shank's lattice design will be used as part of a transition plan for conducting initial experiments before CESR has been modified to the final CesrTF configuration.
Supported by an NSF grant, Michael Ehrlichman, a senior at the University of Minnesota, analysed intrabeam scattering (IBS) in CesrTF. When many particles are densely packed into bunches, scattering of particles within a bunch can be another culprit for emittance growth in a damping ring. The high bunch density requirements make it necessary to examine intrabeam scattering in the ILC. Physicists expect this effect to be readily observable in CesrTF. Using a number of different algorithms, Ehrlichman evaluated emittance growth due to IBS as a function of the bunch charge. He then explored how the IBS impact could be controlled by adjusting the CesrTF operating parameters. Ehrlichman's results will help guide experiments to study IBS in CesrTF and to disentangle the impacts of IBS from other beam dynamics effects that are of interest to ILC physicists. With plans to summarise his research at Cornell in his senior thesis this year, Ehrlichman went back to school this fall with a great sense of satisfaction. "It was an amazing summer," he said. "Based upon my results, I was able to have an impact on the design of the CESR test facility."
In some ways, it seems that the damping rings are like mothers -- they are happiest when things are clean, rather than scattered everywhere, and prefer quiet. Another REU student, Joseph Burrell, a junior at Wayne State University, spent his summer characterising the first prototypes of new readout electronics for the CESR beam position monitor (BPM) system. In CESR roughly 100 BPMs are used to monitor the trajectory of the beam around the ring. Burrell had the assignment of verifying the noise and linearity of the readout electronics and identifying any remaining problems in the hardware before the new generation of electronics will go into full production. "We want to know the position of the beam down to the precision of a micron, and noise can have an impact on these measurements," Burrell said. "Noise can occur in a system through such things as computer chips interacting with each other. We need to test all of these things to see what causes the noise." After conducting many tests, Burrell found that the beam position monitor cards may be interacting with each other to create some noise in the system. "They will have to do more tests to determine what exactly is causing the noise," he said. Final updates will continue to be implemented to the design.
While work will continue on all three projects for the CESR test facility, Ehrlichman, Burrell and Shanks made significant contributions to the R&D work. With all three planning to continue on to graduate school for either physics or engineering, they may continue to contribute to ILC research for some time to come. "I had to learn so much over the summer, and it was definitely a confidence booster to know that I am capable of doing all those things," Shanks said. "It is good to know for the future."
Read the Students' Final Reports:
-- Elizabeth Clements
|© International Linear Collider|