Fast digital orbit feedback systems at NSLS

Abstract

We are implementing digital orbit feedback systems to replace the analog ones in both the VUV and the X-ray rings. We developed an original VME-based design which is run by a powerful Motorola 2305 CPU and consists entirely of off-the-shelf VME boards. This makes the system inexpensive and easy to configure, and allows for high digitizing rates. The new 5 kHz digital global feedback system is currently operational in the VUV ring, and the X-ray system is in the commissioning phase. Some of the parameters achieved include vertical correction bandwidth of 200 Hz (at DC gain of 100) and typical orbit drift over a fill of <3% of the rms beam size. In this paper we discuss the system architecture, implementation and performance. HISTORY AND MOTIVATIONNSLS is a synchrotron light user facility providing radiation to a large number of the X-ray and VUV ring users. Although both rings generally provide high quality beam, as with any accelerator, they are prone to environmental noise creating a nuisance to some users. Main sources of noise include NSLS and AGS booster operation, floor vibrations due to water pumps and other equipment, and powerline noise at harmonics of 60 Hz. There is also a gradual orbit drift over a fill mainly caused by the mechanical motion of ring components under variable heat load. To combat these, from very early on, the NSLS had a vigorous program to improve orbit stability. In the early 80s, local feedbacks based on analog electronics were put in all of the X-ray ring insertion device beamlines [1]. Later, a global orbit correction principle was pioneered at NSLS [2] which eventually resulted in analog global feedback system installation in both the X-ray and VUV rings [3].While generally providing good stability, present analog systems have some limitations. Most importantly, they use a limited number of BPMs and corrector magnets (trims). This results in good control only at certain locations of each ring. Since the analog design is not easily scalable, it would be an enormous effort to expand the system. In addition, analog architecture is not flexible enough in the choice of system configuration, correction algorithms, filters, etc.With the progress in digital electronics it was realized that a digital design can effectively address all of these issues. A prototype global digital feedback system operating at 550 Hz was built and tested at the X-ray ring, proving a clear advantage of a digital design [4]. It demonstrated a significant reduction in the long term orbit drift compared Work supported by US DOE under contract DE-AC02-98CH10886 y E-mail: boris@bnl.gov to the analog systems in operation, as well as a much better correction of 1.2 Hz harmonics of NSLS booster noise. However, due to its low sampling rate this "old digital system" could not match the bandwidth of the analog systems and it was never put to regular operation. This is why, as the next step in improving orbit stability, we decided to build a much faster digital orbit feedback system. To date both the X-ray and the...