Digital Low Level RF

The demand on high stability and precision on the RF voltage for modern accelerators, as well as better diagnostics, maintenance and flexibility is driving the community to develop Digital Low Level RF systems (DLLRF) for both linear accelerators and synchrotrons. The state of the art in digital technologies applied to DLLRF systems is reviewed; different designs developed or in development at various laboratories are surveyed

The New Digital-Receiver-Based System for Antiproton Beam Diagnostics

An innovative system to measure antiproton beam intensity, momentum spread and mean momentum in CERN's Antiproton Decelerator (AD) is described. This system is based on a state-of-the-art Digital Receiver (DRX) board, consisting of 8 Digital Down-Converter (DDC) chips and one Digital Signal Processor (DSP). An ultra-low-noise, wide-band AC beam transformer (0.2 MHz - 30 MHz) is used to measure AC beam current modulation. For bunched beams, the intensity is obtained by measuring the amplitude of the fundamental and second RF Fourier components. On the magnetic plateaus the beam is debunched for stochastic or electron cooling and longitudinal beam properties (intensity, momentum spread and mean momentum) are measured by FFT-based spectral analysis of Schottky signals. The system thus provides real time information characterising the machine performance; it has been used for troubleshooting and to fine-tune the AD, thus achieving further improved performances. This system has been operating since May 2000 and typical results are presented

Digital Signal Processing: State-of-the-Art at CERN and Recommendations

Dramatic hardware performance improvements over the last decades have paved the way to the ascent of digital techniques for processing signals, with a concurrent and parallel interest in Digital Signal Processing (DSPing) and in the use of Digital Signal Processors (DSPs). Recent discussions within PS showed that there are needs for DSP-qualified manpower in new projects that cannot be fully satisfied internally. In order to determine how PS can best profit from the growing importance and efficiency of DSP technologies, with an effort compatible with the available divisional resources, a DSP working group was created. Its mandate is to advise PS management on the best way to proceed in the DSPs and DSPing domains. In particular, the issues targeted are wide-ranging, from evaluating the state-of-the-art at CERN to hardware standardisation and required training. This report gives the findings of the working group and presents its closing recommendations

Real-Time Tune Measurements on the CERN Antiproton Decelerator

A novel system for real-time tune measurement during deceleration of a low-intensity particle beam is presented. The CERN Antiproton Decelerator decelerates low intensity (2x107) antiproton beams from 3.5 GeV/c to 100 MeV/c. Because of the eddy-currents in the magnets, a tune-measurement during a pause in the deceleration would not be representative. One must thus be able to measure the tune in real time during the deceleration. The low intensity of the antiproton beam prevents the use of standard Schottky techniques, and swept Beam Transfer Function (BTF) measurements are too slow. A system was therefore developed which uses an M-shaped power spectrum, exciting the beam in a band around the expected frequency of a betatron side-band. Excitation at the betatron frequency, where beam response is highest, is thus minimized and measurements of BTF, and therefore the tune, can be made with much reduced emittance blow-u

Proposal for a Cavity Phase Observation System in the PS Machine

In multi-cavity synchrotrons it is essential to be able to measure the phase difference between RF cavities. Errors in relative phase can have a particularly deleterious effect on the beam during RF gymnastics. Currently, two methods are available to measure the relative phase in the CERN Proton Synchrotron (PS), but neither attains the desired resolution nor covers the full arsenal of cavities. This note describes a system that will measure the relative phase between cavities with high resolution. The system makes use of the digital hardware deployed in the LEIR beam control and of the corresponding DSP and FPGA signal processing. The focus is on beams controlled by the Multi Harmonic Source (MHS) clock. The system described here is also a step towards the deployment of a new generation of digital beam control systems for the PS Complex machines, within the framework of the LHC injector consolidation and following the successful commissioning of the LEIR digital beam control system. Some expected benefits are briefly summarised at the end of this documen

Antiproton beam parameters measurement by a new digital-receiver-based system

The Antiproton Decelerator (AD) provides the users with very low intensity beams, in the 107 particles range, hence prompting the development of an innovative measuring system, which was completed in early 2000. This system measures antiproton beam intensity for bunched and debunched beams, together with momentum spread and mean momentum for debunched beams. It uses a state-of-the-art Digital Receiver board, which processes data obtained from two ultra-low-noise, wide-band AC beam transformers. These have a combined bandwidth in the range 0.02 MHz - 30 MHz and are used to measure AC beam current modulation. For bunched beams, the intensity is obtained by measuring the amplitude of the fundamental and second RF Fourier components. On the magnetic plateaus the beam is debunched for stochastic or electron cooling and longitudinal beam properties (intensity, momentum spread and mean momentum) are measured by FFT-based spectral analysis of Schottky signals. The system provides real-time information characterising the machine performance; it has been used for troubleshooting and to fine-tune the AD, thus allowing further improved performance. This system has been operating since May 2000 and providing beam intensity data to the users on a routine basis since late 2000. A dedicated software package was expressly developed to take care of the control, data acquisition and processing phases. It consists of three main codes, namely a GUI, a Real Time Task and a Low Level Code. This report gives an overview of both the hardware and software developed

Beam Measurement Systems for the CERN Antiproton Decelerator (AD)

The new, low-energy antiproton physics facility at CERN has been successfully commissioned and has been delivering decelerated antiprotons at 100 MeV/c since July 2000. The AD consists of one ring where the 3.5 GeV/c antiprotons produced from a production target are injected, rf manipulated, stochastically cooled, decelerated (with further stages involving additional stochastic and electron cooling and rf manipulation) and extracted at 100 MeV/c. While proton test beams of sufficient intensity could be used for certain procedures in AD commissioning, this was not possible for setting-up and routine operation. Hence, special diagnostics systems had to be developed to obtain the beam and accelerator characteristics using the weak antiproton beams of a few 10E7 particles at all momenta from 3.5 GeV/c down to 100 MeV/c. These include systems for position measurement, intensity, beam size measurements using transverse aperture limiters and scintillators and Schottky-based tools. This paper gives an overall view of these systems and their usage

Digital Signal Processing in Beam Instrumentation: Latest Trends and Typical Applications

The last decade has seen major improvements in digital hardware, algortithms and software, which have trickled down to the Beam Instrumentation (BI) area. An advantageous transition is taking place towards systems with an ever-stronger digital presence. Digital systems are assembled by means of a rather small number of basic building blocks, with improved speed, precision, signal-to-noise ratio, dynamic range, flexibility, and accompanied by a range of powerful and user-friendly development tools. The paper reviews current digital BI trends, including using Digital Signal Processors, Field Programmable Gate Arrays, Digital Receivers and General Purpose Processors as well as some useful processing algorithms. Selected digital applications are illustrated on control/feedback and beam diagnostics

Low-level software for the pentek 6510 digital receiver board applied to the new AD beam measurement system

The new beam measurement system for the CERN Antiproton Decelerator heavily relies on a Pentek 6510 Digital Receiver (DRX) board. The new system goal is to extract beam parameters from pickup signals. Its digital implementation allows for higher precision, easier management of the hardware as well as modification and improvement with no hardware change. In this scheme, this innovative VME DRX board is responsible for parallel data acquisition, independent digital down conversion and processing of up to 4 digitised inputs. The in-house- developed low-level code (LLC), running on the board, takes care of several tasks, such as interfacing with the Real Time Task (RTT), data processing and board managing. The RTT runs on a PowerPC VME board and controls the DRX board as a master. The LLC is a state machine developed in C and Assembler, which services several interrupts and performs the FFT of complex input data. The DRX low-level system developed is highly modular and easily adaptable to other processing scenarios. In fact it implements a multi-window spectrum analyser with additional customised data processing capabilities. After a thorough description of the DRX board, a detailed account of the accompanying software is given, together with an overview of the development environment used