G. Stover

Programmable DSP-based multi-bunch feedback—operational experience from six installations

A longitudinal instability control system, originally developed for the PEP-II, DAΦNE and ALS machines has in the last two years been commissioned for use at the PLS and BESSY-II light sources. All of the installations are running identical hardware and use a common software distribution package. This common structure is beneficial in sharing expertise among the labs, and allows rapid commissioning of each new installation based on well-understood diagnostic and operational techniques. While the installations share the common instability control system, there are significant differences in machine dynamics between the various colliders and light sources. These differences require careful specification of the feedback algorithm and system configuration at each installation to achieve good instability control and useful operational margins. This paper highlights some of the operational experience at each installation, using measurements from each facility to illustrate the challenges unique to each machine....

Observation, control, and modal analysis of longitudinal coupled-bunch instabilities in the ALS via a digital feedback system

The operation of a longitudinal multibunch damping system using digital signal processing (DSP) techniques is shown via measurements from the Lawrence Berkeley Laboratory (LBL) Advanced Light Source (ALS). The feedback system (developed for use by PEP-II, ALS, and DAɸNE) uses a parallel array of signal processors to implement a bunch-by-bunch feedback system for sampling rates up to 500 MHz. The programmable DSP system allows feedback control as well as accelerator diagnostics. A diagnostic technique is illustrated which uses the DSP system to excite and then damp the beam. The resulting 12-ms time domain transient is Fourier analyzed to provide the simultaneous measurement of growth rates and damping rates of all unstable coupled-bunch beam 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.

Harmonic cavities and longitudinal beam stability in electron storage rings

Harmonic cavities have been used in storage rings to increase beam lifetime and Landau damping by lengthening the bunch. The need for lifetime increase is particularly great in the present generation of low to medium energy synchrotron light sources where the small transverse beam sizes lead to relatively short lifetimes from large-angle intrabeam (Touschek) scattering. We review the beam dynamics of harmonic radiofrequency (RF) systems and discuss effects on longitudinal beam stability.

Design and implementation of IIR algorithms for control of longitudinal coupled-bunch instabilities

The recent installation of third-harmonic RF cavities at the Advanced Light Source has raised instability growth rates, and also caused tune shifts (coherent and incoherent) of more than an octave over the required range of beam currents and energies. The larger growth rates and tune shifts have rendered control by the original bandpass FIR feedback algorithms unreliable. In this paper we describe an implementation of an IIR feedback algorithm offering more flexible response tailoring. A cascade of up to 6 second-order IIR sections (12 poles and 12 zeros) was implemented in the DSPs of the longitudinal feedback system. Filter design has been formulated as an optimization problem and solved using constrained optimization methods. These IIR filters provided 2.4 times the control bandwidth as compared to the original FIR designs. Here we demonstrate the performance of the designed filters using transient diagnostic measurements from ALS and DAΦNE.

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.}

Control of multibunch longitudinal instabilities and beam diagnostics using a DSP-based feedback system

A bunch-by-bunch longitudinal feedback system has been designed and built to control coupled-bunch instabilities in the PEP-II machine. A prototype system has been installed at the Advanced Light Source at LBNL. Programmable DSPs allow longitudinal feedback processing in conjunction with data acquisition or instrumentation algorithms. Here we describe techniques developed for different beam and system diagnostics, such as measurements of the modal growth and damping rates and measurements of the bunch-by-bunch currents. Results from the Advanced Light Source are presented to illustrate these techniques.

Status of the top-off upgrade of the ALS

The advanced light source is currently being upgraded for top-off operation. This major facility upgrade will provide an improvement in brightness from soft X-ray undulators of about one order of magnitude and keep the ALS competitive with the newest intermediate energy light sources. Major components of the upgrade include making the booster synchrotron capable of full energy operation, radiation safety studies, improvements to interlocks and collimation systems, diagnostics upgrades as well as emittance improvements in the main storage ring. Most hardware necessary as part of the upgrade has been installed and commissioned. The radiation safety studies are making good progress and have passed a first outside peer review successfully.

LUX - A Design Study for a Linac/laser-based Ultrafast X-ray Source

We describe the design concepts for a potential future source of femtosecond x-ray pulses based on synchrotron radiation production in a recirculating electron linac. Using harmonic cascade free-electron lasers (FELs) and spontaneous emission in short-period, narrow-gap insertion devices, a broad range of photon energies are available with tunability from EUV to hard x-ray regimes. Photon pulse durations are controllable and range from 10 fs to 200 fs, with fluxes 107-1012 photons per pulse. Full spatial and temporal coherence is obtained for EUV and soft X-rays. A fiber laser master oscillator and stabilized timing distribution scheme are proposed to synchronize accelerator rf systems and multiple lasers throughout the facility, allowing timing synchronization between sample excitation and X-ray probe of approximately 20-50 fs.

Enhanced functionality and performance of the longitudinal damping system at the ALS using a new VXI based processing system

The ALS longitudinal feedback electronics, designed and developed at the Stanford Linear Accelerator center (SLAC), has been operating very reliably at the ALS for over four years. Recently a new VXI based implementation of the entire RF and timing electronics has been built, tested, and will soon replace an earlier prototype installation. The components designed are contained in three VXI packages along with a VXI controlled system oscillator chassis. The new system contains a number of functional enhancements that most notably include: enhanced timing control, internal temperature monitoring of all VXI modules, simulated beam signal generation, automatic locking of the phase detector servo loop, and the monitoring and alarm of critical RF signal levels. The details of the design and implementation of the VXI modules along with performance characteristics are presented below.