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Photon Sciences Directorate at Brookhaven National Laboratory

NSLS-II's Beam Position Monitoring System

Beam Position Monitor

Beam position monitoring system

One of the many built-in technologies that will make NSLS-II one of the most advanced synchrotrons in the world is the system that will monitor the position of the electron beam as it circles the main ring.

To deliver light of the highest quality, the beam must be ultra-stable: It cannot stray more than 200 nanometers in the vertical direction. This level of stability is unprecedented and, accordingly, requires a highly advanced beam position monitoring (BPM) system.

The mechanical requirements for the system's diagnostics and instrumentation are extremely challenging. Research and development of the BPM began in August 2009 and made significant progress in fiscal year 2010. The system will consist, in part, of more than 300 individual monitors – 60 in the injection system and approximately 250 placed around the main storage ring. The monitors will each have two circular electrodes, known as “button pick-ups,” that will detect position-dependent electrical impulses from the beam. This information is then converted to digital data and distributed to computers in the NSLS-II control room to be analyzed and displayed. It takes only one 10,000th of a second for a complete set of beam-orbit data to be collected, which allows even the tiniest deviation of the beam orbit to be corrected. It is in this way that the extreme orbit stability of NSLS-II is maintained.

As they design the system, the NSLS-II diagnostics team is very concerned how thermal effects will impact the long-term performance of the BPM. Powerful microwaves generated in the beam vacuum chamber can substantially heat certain BPM components, such as the button pick-ups, causing them to expand and the monitor to malfunction. The team must come up with solutions to these issues. For example, they decided to make the buttons from molybdenum, a metal with high electrical and thermal conductivity.

The first test of the system took place in June at the Advanced Light Source in Berkeley, California, and was successful. The test also showed that the system as it is currently designed will hold up well over time.