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

Angoletta, Maria Elena

Chohan, V

Ludwig, M

Marqversen, O

Pedersen, F

CERN Document Server

THE NEW DIGITAL-RECEIVER-BASED SYSTEM FORANTIPROTON BEAM DIAGNOSTICSM. E. Angoletta, V. Chohan, M. Ludwig, O. Marqversen, F. PedersenCERN, Geneva, Switzerland.AbstractAn innovative system to measure antiproton beamintensity, momentum spread and mean momentum inCERN’s Antiproton Decelerator (AD) is described. Thissystem 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). Anultra-low-noise, wide-band AC beam transformer (0.2MHz – 30 MHz) is used to measure AC beam currentmodulation. For bunched beams, the intensity is obtainedby measuring the amplitude of the fundamental andsecond RF Fourier components. On the magnetic plateausthe beam is debunched for stochastic or electron coolingand longitudinal beam properties (intensity, momentumspread and mean momentum) are measured by FFT-basedspectral analysis of Schottky signals. The system thusprovides real time information characterising the machineperformance; it has been used for troubleshooting and tofine-tune the AD, thus achieving further improvedperformances. This system has been operating since May2000 and typical results are presented.1 INTRODUCTIONAntiprotons from a target, bombarded with an intensebeam from the 26 GeV Proton-Synchrotron (PS), are fedto the AD [1] where they are decelerated in several rampsto a momentum p of about 100 MeV/c. Figure 1 showsthe basic AD cycle; bunched-beam deceleration ramps areinterspersed with plateaus, where the beam is cooled atfixed p and revolution frequency fREV.01234Time, t/sMomentum, p[GeV/c]13(10) 38(33) 86(58)62(52) 110(60)RF ON:RF h = 6 1 1 133.5720.30.119 sec(20 sec)pbar injectionBunch rotationStochastic coolingStochasticcooling13 sec(15 sec)ElectroncoolingActualdurationDesigndurationElectron coolingRebunchingFast extraction23 sec(6 sec) 16 sec(1 sec)Figure 1: The momentum p during the basic AD cycle.At the end of the AD cycle the beam is fast ejected tothe experimental areas. The intensity of the beam variesfrom some 5x107 particles at 3.5 GeV/c to some 2.5x107at 100 MeV/c. Traditional DC beam transformers do notwork at these low intensities, hence an ultra-low-noisepick-up is used [2]. The new DRX-based, AD beam-diagnostic system acquires and processes the datagenerated by this pick-up. The system measures beam,momentum spread and mean momentum throughout theAD cycle. It therefore enables a real-time evaluation ofthe cooling performance during all plateaus.2 THE SYSTEMThe system is shown in Figure 2. Input from thelongitudinal Pick-Up (LPU) (a) feeds 2 variable-gain,remotely-controlled amplifiers (b) [2]. After a 16 MHzlow-pass filter stage (c), the signal is digitised by a Pentek6441 fast ADC (d). The data are digitally down-converted and processed by Pentek’s 6510 DRX (e),which is set up before each measurement. Finally theReal-Time Task (RTT), running in the Device StubController (DSC) (f), collects, post-processes and storesthe data for later display. The RTT acquires from the ADRF system the information needed to optimise the choiceamong various user settings; it is synchronised to machineevents by a timing reception and generation module(TG8) (g). Users access the system, to evaluate data andto set various parameters, through the local Ethernet viaseveral workstations (WKS), by means of an applicationprogram. The system has been providing beam intensity,momentum spread, revolution frequency and debunchedbeam power spectral density PSD since May 2000.2.1 HardwareThe LPU head amplifier’s response bandwidth isbetween 0.2 and 30 MHz; its lowest noise level is 1.5pA/Hz½ in the 1-3 MHz range [2]. The second stageamplifier has a total gain range of approximately 45 dB,between –10 dB and +35 dB, divided into 3 dB steps. TheADC is the Pentek 6441 module, a dual channel, 12-bit,VMEbus board. We improved its actual S/N ratio by 12dB, via our own analogue input front-end, bypassing theoriginal input amplifier and low-pass filter. For filtering,we deployed an external 16 MHz-bandwidth anti-aliasinglow-pass filter. The ADC operates at externally-controlled sampling frequencies fS of up to 40 MHz,depending on the beam state and on fREV. The externalADC clock is switchable between a fixed 40 MHz for adebunched beam and Ki·fREV for a bunched beam, whereKI is the largest integer chosen such that Ki·fREV < 40MHz. The DRX is the Pentek 6510 module, a VME board0-7803-7191-7/01/$10.00 ©2001 IEEE. 2371Proceedings of the 2001 Particle Accelerator Conference, Chicago(a)Second stageamplifier(g)(c)(f)AD RFINPUTSTO RTTADC CLOCKGENERATIONKi / 4fREV1823beambunched / debunchedRF harmonic numberLPUDIGITAL I/OCONTROLDAUDIGITALLATCHDDSFixed 10MHz clockWKS WKSVME BUSETHERNETHeadamplifierLOWPASSFILTER(b)Toggle switchTG8DSCDIGITAL I/ODAQ(e)(d)x 4Sampleclock fS16ADC DRXSwitch controlsignalFigure 2: The new DRX-based, AD beam diagnostic system, schematic view. For details on the LPU see [2].carrying 8 Harris HSP50016 Digital Down Converters(DDC) and one TI TMS320C40 Digital Signal Processor(DSP). This board is responsible for parallel dataacquisition, independent digital down conversion andprocessing of up to 4 digitised inputs. These inputs arepre-processed by the DDC chips, by digitally translating(downmix) the input signal to DC, then zooming(decimation) on the frequency window of interest. Digitaldown-mixing, filtering and decimation has theseadvantages over analogue means: 1) strong rejection ofaliased signals due to the digital local oscillator’s greateraccuracy and to the digital mixer’s higher precision, 2)DSP workload greatly reduced and 3) increased flexibilitydue to the DSP and the DDC chips’ re-programmability.Implementing new functions, as a consequence, onlyrequires a software upgrade and no hardware changes.The DSC is a PowerPC VME board running LynxOS.2.2 SoftwareTwo separate codes have been implemented, the RTTand the low-level code (LLC), running on the DSC and onthe DRX, respectively. The RTT sets up the ADC clockand the second stage amplifier. It then instructs the LLChow to set up the DRX board hardware and whatprocessing to use. All these set-up stages depend on beamstate, fREV and user requirements. After instructing theLLC, the RTT generates a VMEbus interrupt, thustriggering the LLC to start the DRX acquisition andprocessing. At the end of the LLC processing stage, theRTT retrieves single output data such as intensity andmomentum spread as well as spectra from the DRX.The LLC performs the measurements from digitisedpickup signals. It also manages the DRX board, sets upthe required DDCs after receiving a VME-bus interrupt(as fast as one every 20 ms), acquires data from eachDDC, processes them and makes results available to theRTT. The LLC also provides the RTT with full diagnosticinformation.The RTT/DRX interface protocol developed is dictatedby stringent DRX memory space constraints, combinedwith users’ needs. For example, WKS users need tofollow beam status and progress in as closely as possibleto real-time. At WKS level, the information is refreshedevery few seconds, but actual data are sampled atintervals as short as 20 ms. The RTT/DRX datainterchange takes place in the DRX global memory, asthis is the only memory visible from the VMEbus.3 SIGNALPROCESSINGThe processing is carried out at the DRX level, first bythe DDCs and then by the LLC. The output is then passedon to the RTT for post-processing and to derive theappropriate physical quantities. The actual processingdepends on the beam state: in particular, the Schottkysignals [3] are processed only with a debunched beam,thus producing PSD plots. With a bunched beam, only theamplitudes at the fundamental (fRF) and second RFFourier components are considered. Debunched beamdata are more affected by noise, whereas bunched beamsignals, which are much larger, are less influenced. Thedata processing stages are detailed in [4] and [5].3.1 Debunched beam.At high S/N ratios, such as at high momentum, signalprocessing is performed on high harmonics, thus speedingdata acquisition; at low S/N ratios, low frequencyharmonics must be used. The PSD of a debunchedcoasting beam is peaked at fREV, as shown in Figure 3.2372Proceedings of the 2001 Particle Accelerator Conference, Chicago1.486 1.487 1.488 1.489Frequency, f [MHz]PowerSpectralDensity,PSD[a.u.]PSD 4PSD 2PSD 1PSD 3f REVFigure 3: Effect of stochastic cooling on PSD, for adebunched beam, before (PSD 1) and aftercooling (PSD 4).The number of coasting particles N is found from fREV andthe area APSD under the peak of the PSD vs. f curve:22REVPSDUUfAGN ⋅=α(1)where αU is a calibration constant and GU is the globalsystem gain. From the width ∆f of the PSD curve at a 2σheight, the momentum spread ∆p/p is:ffnppREV∆⋅⋅⋅=∆η1(2)where η = (γTR2-γ 2)/(γTR γ)2, γ = E/E0, γTR = 4.75 and n isthe harmonic number.Figure 3 shows the PSD measured by the system, inarbitrary units, as a function of frequency f. Each PSD isthe average of 60 complex FFTs, and is calculated every1400 ms. The ADC fS is fixed at 40 MHz and n equals 5.The beam cooling is evident from the progressivenarrowing. Thus the beam progress can be followed byexamining a similar sequence of spectra.3.2 Bunched beamFor a bunched beam, the ADC fS is proportional to fREV,thus allowing to track the beam when its revolutionfrequency changes. At each turn m = 0,1,2 .., the intensitytime profile I(t) of a single bunch is approximated, in theinterval [(m-½)/fREV ; (m+½)/fREV], by a parabola:−⋅⋅=22021243)(ττtfItIREV(3)where I0 is the DC current intensity per bunch and 2τ isthe FWHM.The experimental Fourier coefficients Jk (k=1, 2, ...) arecalculated and then fitted with a truncated series of f/fRF⋅∆−⋅⋅=20 2RFk ffhIJ (4)where h = fRF/fREV is the RF harmonic number. Wetherefore obtain I0 and ∆, and thus the bunch size τ :2521 ∆⋅⋅⋅=REVfπτ (5)Integration of the beam profile over time gives the totalnumber N of particles circulating in the machine:REVB fhIN⋅⋅=0α (6)where αB is a calibration constant. Figure 4 shows fREVand the progress of the antiproton beam intensity in termsof N.012340 50 100 150 200Time, t [s]Numberofparticles,N/10701234Revolutionfrequency[MHz].Number of particlesRevolution frequencyFigure 4: Number of particles N in the beam vs. time,computed from beam current amplitudes at fRFand 2 fRF . Bunched-beam data for 3 AD cycles.4 FUTUREWORKAn upgrade to the system is under way, to measuretunes from transverse signals using the Beam TransferFunction (BTF). Other improvements will include usingthe signals from a second LPU optimised for lowfrequencies. In addition, PU to amplifier cables will bereplaced with EMC shielded cables, to reduceenvironment noise pick-up.5 REFERENCES[1] S. Baird et al., The antiproton decelerator: AD, PAC‘97, Vancouver, Canada, 1997.[2] C. Gonzales, F. Pedersen, An ultra low-noise AC beamtransformer for deceleration and diagnostics of lowintensity beams, PAC ’99, NY, 1999.[3] D. Boussard, Schottky Noise and Beam TransferFunction Diagnostics, CERN/SPS 86-11 (ARF), 1986.[4] M. E. Angoletta, Low-level software for the Pentek6510 DRX, in application to the new AD beammeasurement system, CERN/PS/Note 2001-004, inpreparation.[5] M. E. Angoletta, et al.: Antiproton Beam ParametersMeasurement by a New Digital Receiver-BasedSystem, CERN/PS 2001-016 (BD), in preparation.2373Proceedings of the 2001 Particle Accelerator Conference, Chicago

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

Abstract

Similar works

Full text

Available Versions

CERN Document Server