Beam Diagnostics Based on Time - Domain

A bunch-by-bunch longitudinal feedback system has been used to control coupledbunch longitudinal motion and study the behavior of the beam at ALS, SPEAR, PEP-II, and DA Φ NE. Each of these machines presents unique challenges to feedback control of unstable motion and data analysis. Here we present techniques developed to adapt this feedback system to operating conditions at these accelerators. A diverse array of techniques has been developed to extract information on different aspects of beam behavior from the time-domain data captured by the feedback system. These include measurements of growth and damping rates of coupledbunch modes, bunch-by-bunch current monitoring, measurements of bunch-by-bunch synchronous phases and longitudinal tunes, and beam noise spectra. A technique is presented which uses the longitudinal feedback system to measure transverse growth and damping rates. Techniques are illustrated with data acquired at all of the four above-mentioned machines. INTRODUCTION A bunch-by-bunch feedback system has been developed by a multi-laboratory collaboration for control of coupled-bunch longitudinal motion at ALS, PEP-II, and DA Φ NE. The architecture of the system has been described in detail in earlier publications (1), (2), (3). DSP-based design allows synchronized real-time data acquisition in conjunction with feedback processing. Table 1 summarizes the parameters of different machines on which the feedback system has been used. The feedback system is configured in each case to maintain a constant ratio between the bunch sampling frequency and the synchrotron frequency. Downsampling matches the feedback processing rate to the longitudinal oscillation frequency and results in a significant reduction in the computational load on the DSP array * Work supported by DOE contract No. DE-AC03-76SF00515 as compared to the non-downsampling approach. A table-driven programmable downsampler module allows operation on the machines with widely different numbers of bunches and downsampling factors. DIAGNOSTIC TECHNIQUES A large number of diagnostic techniques based on the time-domain transient and steady-state data have been developed. Transient data is used for measurements of growth and damping rates and injection transients. From steady-state data one can extract information on the system noise floor and a set of bunch-by-bunch parameters such as currents, synchronous phases and synchrotron frequencies. The different accelerators listed in Table 1 vary significantly in the growth times of the unstable modes. For example, at SPEAR, growth time is comparable to the number of samples stored by the DSP. Techniques have been developed to facilitate growth and damping rate measurement in such cases. For weakly unstable modes positive feedback is used to speed up the growth. In cases when the damping rates of naturally stable modes are to be determined, the external excitation method is used (4). Records of steady-state bunch motion provide a wealth of information about the beam and the performance of the feedback and rf systems. By capturing bunch motion while in negative feedback mode, one can quantify the residual noise level due to quantization, as well as determine frequencies and amplitudes of driven motion. Such measurements of driven motion were used during the PEP-II HER commissioning to characterize the performance of the rf system (5). From steady-state records one can also extract information about bunch currents and synchronous phases. To measure bunch currents, we detect the level of low-frequency TABLE 1. Machine Parameters ALS DA NE PEP-II SPEAR Number of bunches 328 12