Demonstration of advanced iGp functionality at CESR-TA

From the 9^th through the 13^th of March 2009, two iGp-1281F systems were demonstrated at CESR-TA. One iGp system, together with the FBE-500L processor was used to stabilize the longitudinal motion in the ring. Another bunch-by-bunch processor was used to perform vertical feedback and diagnostics.

A diagnostic feature, especially important for CESR-TA studies of electron cloud, is the ability of iGp to perform bunch-by-bunch tune measurement. During the tests we have successfully demonstrated such measurement capability, as illustrated on the right.

The information on the individual bunch characteristics is extracted from the closed-loop bunch spectra, shaped by the bunch dipole response. These measurements are fully parasitic in that the beam is not perturbed in any way from the normal steady-state feedback-damped conditions.

Measurement of individual bunch tunes in CESR-TA was complicated by the fact that the feedback kicker striplines are 3.5 ns long, creating minimum kick response of 7 ns. Such response couples adjacent bunches spaced by 4 ns. Advanced features of the iGp, such as the ability to select an arbitrary subset of bunches for feedback, were critical in enabling bunch-by-bunch tune measurements.

The plot on the right shows measured bunch-by-bunch vertical tunes for a train of 45 positron bunches with 32.25 mA total current.

Another advanced feature, successfully demonstrated at CESR-TA, is the bunch cleaning. The feature allows the user to removed unwanted bunches, while maintaining full stability of the rest of the beam. For each individual bunch one can independently select whether to apply swept sine excitation and/or perform feedback. Typical configuration for bunch cleaning enables the feedback for all bunches that must be retained. Then, excitation is applied to the unwanted bunches, driving them to large oscillation amplitudes and scraping off the charge.