I. Linscott

Operation and performance of the PEP-II prototype longitudinal damping system at the ALS

A modular programmable longitudinal feedback system has been developed as a component of the PEP-II R+D program. This system is based on a family of VME and VXI packaged signal processing functions which implement a general purpose digital feedback controller for accelerators with bunch spacings of 2 ns. A complete PEP-II prototype system has been configured and installed for use at the LBL Advanced Light Source. The system configuration used for tests at the ALS is described and results are presented showing the action of the feedback system. Open and closed loop results showing the detection and calculation of feedback signals from bunch motion are presented and the system is shown to damp coupled-bunch instabilities in the ALS. Use of the system for accelerator diagnostics is illustrated via measurement of grow-damp transients which quantify growth rates without feedback, damping rates with feedback, and identify unstable modes.

Beam diagnostics based on time-domain bunch-by-bunch data

A bunch-by-bunch longitudinal feedback system has been used to control coupled-bunch longitudinal motion and study the behavior of the beam at ALS, SPEAR, PEP-II, and DAΦNE. 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 coupled-bunch 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.

Software architecture of the longitudinal feedback system for PEP-II, ALS and DA/spl Phi/NE

We describe the software architecture of the Longitudinal Feedback System being built for the PEP-II B-Factory at SLAG, the ALS light source at LBL and the DA/spl Phi/NE phi factory at Frascati. This VME/VXI based system utilizes commercially available embedded CPU controller boards running the VxWorks real time operating system. The operator interface for PEP-II and ALS is based on the EPICS control system package. Embedded processors are used to load, monitor and diagnose various components of the system. The feedback function is implemented using digital filtering techniques on a DSP farm residing in the VME crates. The operator interface is written to allow the loading of applications, e.g., accelerator diagnostic functions, system hardware integrity functions, etc., without intervening controller reboots.

Feedback control of coupled-bunch instabilities [particle accelerators]

The next generation of synchrotron light sources and particle accelerators will require active feedback systems to control multi-bunch instabilities. Stabilizing hundreds or thousands of potentially unstable modes in these accelerator designs presents many technical challenges. Feedback systems to stabilize coupled-bunch instabilities may be understood in the frequency domain (mode-based feedback) or in the time domain (bunch-by-bunch feedback). In both approaches an external amplifier system is used to create damping fields that prevent coupled-bunch oscillations from growing without bound. The system requirements for transverse (betatron) and longitudinal (synchrotron) feedback are presented, and possible implementation options developed. Feedback system designs based on digital signal-processing techniques are described. Experimental results are shown from a synchrotron oscillation damper in the SSRL/SLAC storage ring SPEAR that uses digital signal-processing techniques.<<ETX>>

A longitudinal multi-bunch feedback system using parallel digital signal processors

A programmable longitudinal feedback system based on four AT&T 1610 digital signal processors has been developed as a component of the PEP‐II R&D program. This longitudinal quick prototype is a proof of concept for the PEP‐II system and implements full‐speed bunch‐by‐bunch signal processing for storage rings with bunch spacings of 4 ns. The design incorporates a phase‐detector‐based front end that digitizes the oscillation phases of bunches at the 250 MHz crossing rate, four programmable signal processors that compute correction signals, and a 250‐MHz hold buffer/kicker driver stage that applies correction signals back on the beam. The design implements a general‐purpose, table‐driven downsampler that allows the system to be operated at several accelerator facilities. The hardware architecture of the signal processing is described, and the software algorithms used in the feedback signal computation are discussed. The system configuration used for tests at the LBL Advanced Light Source is presented.

Operation and performance of a longitudinal feedback system using digital signal processing

A programmable longitudinal feedback system using a parallel array of AT&T 1610 digital signal processors has been developed as a component of the PEP‐II R&D program. This system has been installed at the Advanced Light Source (LBL) and implements full speed bunch by bunch signal processing for storage rings with bunch spacing of 4 ns. Open and closed loop results showing the action of the feedback system are presented, and the system is shown to damp coupled‐bunch instabilities in the ALS. A unified PC‐based software environment for the feedback system operation is also described.

Analysis of DSP-based longitudinal feedback system: trials at SPEAR and ALS

Recently a single-channel prototype of the proposed PEP-II longitudinal feedback system was successfully demonstrated at SPEAR and ALS on single-bunch beams. The phase oscillations are detected via a wide-band pick up. The feedback signal is then computed using a digital signal processor (DSP) and applied to the beam by phase modulating the RF. We analyze results in the frequency- and the time-domain and show how the closed-loop transfer functions can be obtained rigorously by proper modeling of the various components of this hybrid continuous/digital system. The technique of downsampling was used in the experiments to reduce the number of computations and allowed the use of the same digital hardware on both machines.<<ETX>>

Feedback control and beam diagnostic algorithms for a multiprocessor DSP system

The multibunch longitudinal feedback system developed for use by PEP-II, ALS, and DA{Phi}NE uses a parallel array of digital signal processors (DSPs) to calculate the feedback signals from measurements of beam motion. The system is designed with general-purpose programmable elements which allow many feedback operating modes as well as system diagnostics, calibrations, and accelerator measurements. The overall signal processing architecture of the system is illustrated. The real-time DSP algorithms and off-line postprocessing tools are presented. The problems in managing 320k samples of data collected in one beam transient measurement are discussed and our solutions are presented. Example software structures are presented showing the beam feedback process, techniques for modal analysis of beam motion (used to quantify growth and damping rates of instabilities), and diagnostic functions (such as timing adjustment of beam pick-up and kicker components). These operating techniques are illustrated with example results obtained from the system installed at the Advanced Light Source at LBL. {copyright} {ital 1997 American Institute of Physics.}

Multibunch feedback—Strategy, technology, and implementation options

The proposed next generation accelerator and synchrotron light facilities will require active feedback systems to control multi‐bunch instabilities. These feedback systems must operate in machines with thousands of circulating bunches and with short (2–4 ns) interbunch intervals. The functional requirements for transferse (betatron) and longitudinal (synchrotron) feedback systems are presented. Several possible implementation options are discussed and system requirements developed. Results are presented from a digital signal processing based synchrotron oscillation damper operating at the SSRL/SLAC SPEAR storage ring.

Measurement of multi‐bunch transfer functions using time‐domain data and Fourier analysis

Multi‐bunch transfer functions are principal ingredients in understanding both the behavior of high‐current storage rings as well as control of their instabilities. The measurement of transfer functions on a bunch‐by‐bunch basis is particularly important in the design of the active feedback systems. Traditional methods of network analysis that work well in the single bunch case become difficult to implement for many bunches. We have developed a method for obtaining empirical estimates of the multi‐bunch longitudinal transfer functions from the time‐domain measurements of the bunches’ phase oscillations. This method involves recording the response of the bunch of interest to a white‐noise excitation. The transfer function can then be computed as the ratio of the fast Fourier transforms (FFTs) of the response and excitation sequences, averaged over several excitations. The calculation is performed off‐line on bunch‐phase data and is well‐suited to the multi‐bunch case. A description of this method and an an...