A quest for measuring ion bunch longitudinal profiles with one picosecond accuracy in the SNS linac

The SNS linac utilizes several accelerating structures operating at different frequencies and with different transverse focusing structures. Low-loss beam transport requires careful matching at the transition points for both the transverse and longitudinal plane. Longitudinal beam parameters are measured using four Bunch Shape Monitors (used at many ion accelerator facilities, aka Feschenko devices). These devices, as initially delivered to the SNS, provided an estimated accuracy of about 5 picoseconds, which was sufficient for the initial beam commissioning. New challenges of improving beam transport for higher power operation will require measuring bunch profiles with 1-2 picoseconds accuracy. We have successfully implemented a number of improvements to maximize the performance characteristics of the delivered devices. We will discuss the current status of this instrument, its ultimate theoretical limit of accuracy, and how we measure its accuracy and resolution with real beam conditions.