Althoff, K H

Dreist, A

Husmann, D

Neckenig, M

Nuhn, H D

Schauerte, W

Schillo, M

Schittko, F J

Von Drachenfels, W

Wermelskirchen, C

CERN Document Server

Particle Accelerators, 1990, Vol. 27, pp. 101-106Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaD.HUSMANN,M.SCHILLO,ELSA - ONE YEAR OF EXPERIENCE WITH THE BONN ELECTRONSTRETCHER ACCELERATORK.H.ALTHOFF, W.v.DRACHENFELS, A.DREIST,M.NECKENIG, H.-D.NUHNCI, W.SCHAUERTEb ,F.J.SCHITTKO, C.WERMELSKIRCHENcUniversity of Bonn, Physikalisches Institut, Nufiallee 12, D - 5300 Bonn 1(J Now at Stanford Synchrotron Radiation Laboratory"Geodatisches Institut, Univ. BonncNow at SSRL on leave from GMD BonnAbstract One and a half year ago the Bonn ELectron Stretcher AcceleratorELSA came into operation. Since then detailed machine studies have been per-formed between 0.5 and 2 GeV. The control system proved to be a valuable toolfor operating the machine. Injection into ELSA including the fast extractionout of the 2.5 GeV booster-synchrotron has been investigated. The adjustmentof dipoles and quadrupoles has been checked by closed orbit measurements.The slow extraction at a third integer resonance has been studied in detail.Extraction times up to 200 msec with a duty factor of about 35 % are possible.For synchrotron radiation experiments the accumulation and storage of highcurrents up to 275 mA in ELSA was tested. The beam lifetime (lie point) at30 mA is in the order of 15 min. (due to vacuum limitations). Since one yearthe three experiments have been supplied with external beams.IntroductionThe electron stretcher accelerator ELSA is the first continuous beam machine whichprovides electron beams between 0.5 and 3.5 GeV. As a booster for the stretcherserves the 2.5 GeV electron synchrotron that came into operation in 1967.ELSA is operated in three different modes. Up to 2 GeV the pure stretchermode is applied. Immediately after injection from the synchrotron the slow extrac-tion at a third integer tune starts and continuously supplies the experiments untilthe next injection with electrons. Above 2 GeV ELSA works as a post accelerator.After injection of one or several pulses, the electrons are accelerated to the to therequired endpoint-energy (at maximum 3.5 GeV). Then the electrons are slowlyextracted at constant energy. The "macroscopic" duty factor depends on the ramp-ing times, the flat top time and the time used for injection and is lower than the[347]/101102/[348] K. H. ALTHOFF ET AL.duty factor reached in the pure stretcher mode. The third possibility is the use ofELSA as a storage ring for synchrotron radiation. Here up to hundreds of pulsesare injected into ELSA and stored. With a circulating current of 30 rnA a beamlifetime (lie) of up to 15 min. has been achieved. To keep a constant high intensityin ELSA we inject from the synchrotron which operates at a repetition frequencyof 50 Hz every tenth pulse only. The synchrotron radiation leads to evaporationof surface contaminations from the 'vacuum chamber, therfore the pressure risesand the lifetime is limited. At 275 rnA circulating current - our maximum valueobserved so far - the pressure increases from some 10-9 mbar to about 10-6 mbar.We expect that after longer lasting storage runs the cleaning effect will increase thebeam. lifetime considerably. Details of the machine's design and construction canbe found elsewhere!. Some relevant parameters of ELSA are shown in the table(the energy dependent values are calculated for 3.5 GeV).circumference 164.4 m dipole field strength 1.073 T# of dipoles 24 energy loss per turn 1.22 MeVbendin'g radius 10.88 m mom. compo factor 6.3 %# of quadrupoles 32 natural emittance 0.9 mm mrad# of sextupoles 12 natural energy width 0.09 %# of extraction quadrupoles 4 revolution frequency 1.82 MHzharmonic number 274 RF frequency 499.667 MHzSynchrotron Extraction and Injection into ELSAThree septum magnets, 3 fast kickers and 3 slow kickers are used for a three turnshaving extraction from the synchrotron. The slow kickers produce a closed bumpto shift the beam towards the first septum magnet. The fast kickers then arefired individually at different times (time-steps of 25 ns are possible) to produce anextracted beam over about 600 nsec to match approximately the ELSA revolutiontime (548 nsec).Two septum magnets and 3 fast kickers are used for the injection into ELSA.The kickers produce a closed bump for two revolutions in ELSA. The only pa-rameter to be pretended is the bump amplitude at the end of the second septum,the control system inspects the realtime database for the energy, the quadrupolestrengths and the strengths of the sextupoles within the bump to automaticallycalculate and adjust the voltages of septum magnets and kickers. The first threerevolutions after injection into ELSA are shown in Figure 1. Whether several shotsof the synchrotron are accumulated in ELSA or not depends only on the amplitudeELSA-ONE YEAR OF EXPERIENCE [349]1103of the bump. A switch between stretcher and accumulation mode is thus possiblewithin seconds.ELSA Control SystemThe ELSA control system is based on a system of distributed front-end microproces-sor modules, connected to the ELSA host computer cluster by two high speed seriallinks. Each microprocessor module runs dedicated software to control its assignedELSA device. The host cluster consists of two VAX-750s and one WorkstationVAX-3200.The current ELSA operator interface is based on a menu driven graphicspackage running on two consoles equipped with high resolution color monitors.Beside standard keyboards, trackballs and push-buttons are used as the main inputdevices. The interface allows full interactive access to all machine parameters andcan be easily adapted to the special requirements of the operator. A first version of asimple control language is already used to automate machine physics measurements.The ELSA control system proved to be reliable and flexible during one yearof operation. The adaption to changing accelerator hardware turned out to berather simple. The system showed extremely few failures, those were mainly dueto host computer shutdowns. It has reached a high degree of acceptance amongexperimentalists as well as machine physicists.At present, major improvements and extensions are under development, aim-ing at an interactive and programmable operator interface which will combine ma-chine steering, diagnostics and data analysis in one integrated environment. It willprovide a high level programming language for accelerator physics and for control.For the experimentalists, the new system will offer an effective, easy to use tool.Measurements with the Beam Position MonitorsThe ELSA beam position monitor system consists of 25 capacitive pickups (sameas used at DESY II) installed around the ring. Since ELSA works in a multi-bunch-mode with every bucket filled, the BPM has to be sensitive to the same frequencyas the accelerating RF. The average signal strength of this component as measuredwith a spectrum analyser at the pickup buttons having a 50 n termination is 1.1mV per mA beam current. Horizontal and vertical difference signals and the sumsignal are generated out of the four button signals with RF hybrid junctions. Oneof the difference signals can be selected with a GaAs RF switch, then it is amplifiedand sent to the homodyne 500 MHz receiver2as well as the amplified sum signal.To calibrate the whole circuit it is possible to switch a constant RF signal to bothchannels, enabling the correction of phase differences or unequal amplification ofthe two channels.104/[350] K. H. ALTHOFF ET AL.The dynamic range of the receiver amounts to 30 dB, but can be shifted withswitchable amplifiers in a range of 50 dB to fit the actual intensity. The bandwidthof the receiver is 20 MHz, so it is fast enough to perceive a single shot from thesynchrotron (232 nsec) or to resolve the intensity distribution around the ring.In the present state these signals are sampled by a digital oscilloscope at arate of 100 MHz. Six prototypes of the described RF-electronics are in operationalready. In the near future, however, a readout system for each BPM (samplingrate: 10 MHz to 78 kHz, buffer depth: 8192 values) an,d the final electronics foreach BPM (resolution: 0.2 mm) will be installed. This system will make it possibleto record the beam position at each monitor covering a period of 0.8 to 105 msec.The position monitor system was designed as a multi purpose beam diagnosesystem. All measurements described in this paper have been carried out usingthe 6 already operating prototypes, except for intercepting screen monitors usedin the transfer beamline and a dc-current-transformer, used as intensity monitorin ELSA. During the setup of the injection the first monitor in ELSA was usedas a fast :elative intensity monitor to see the temporal distribution of the injectedbeam's intensity. The number of turns seen by the monitor has often enabled us todraw conclusions concerning faulty devices.By spectral analysis of a betatron oscillation excited with a fast kicker magnetand sampled at one BPM we determined a variety of parameters - the horizontaland vertical betatron tune and the synchrotron tune; by varying the acceleratingRF frequency and amplitude and measuring the betatron and synchrotron tune wecalculated the chromaticity and the RF over-voltage factor.In the storage mode horizontal coherent betatron oscillations were excitedby firing one kicker. The damping of these oscillations ("Landau damping") wasstudied under different conditions (different betatron tune, sextupole strengths anddifferent intensities). With the help of a tracking program we were able to simulatethis Landau damping process and to estimate the integral octupole strength inthe ring (approx. 30 ~3). However, the observed damping time depends on thecirculating current and is shorter at higher intensities.An orbit correction program doing least square or harmonics correction isoperating already. All 24 main dipoles are equipped with backleg windings as hor-izontal correctors and additional small magnets are used as vertical correctors. Tocheck the function of the orbit correction system we measured the horizontal closedorbit with the 6 existing prototypes (Xrm6 == 3.4 mm) and corrected it successfullyat this monitors using 9 corrector magnets (Xrm6 == 1.25 mm). Note that completeorbit correction requires all monitors, the described measurement could only testthe orbit correction system installed so far.Furthermore we set up a test to estimate the accuracy of the prototype mon-ELSA-GNE YEAR OF EXPERIENCE [351]/105itor electronics: Varying the RF frequency forces the electrons to an dispersionorbit. The standard deviation of the measured positions from the one, which wascalculated using the dispersion function, then yields an estimate of the accuracy.Although there are problems in the calibration unit of the prototypes the accuracyas measured by the described procedure is better than 0.2mm.Extraction from ELSAThe electron beam is extracted from ELSA via a slow resonant ejection near athird integer resonance, driven by four extraction sextupoles in the dispersion freesections. Four ironless quadrupoles shift the machine tune continuously from theinjection value to 4~, the maximum tune shift possible at 1 GeV is t1Q == 0.032.Problems arised from the 600 Hz ripple on all the magnet power suppliesdue to a 12-pulse rectifying system. Recently the ripple on the quadrupoles hasbeen compensated by active and passive filters. In subsequent runs the machine'sbehaviour near resonance has resembled much more the theoretical calculationsthan before. Especially the influences of the three different sextupole families on theresonance strength were as expected. Likewise the time structure of the externalbeam can easily be modified by changing the shape of the control curve for thecurrent of the ironless quadrupoles. An active filter for the dipole power supply isunder construction but first tests have shown that a passive filter is necessary too.In the stretcher mode the extraction time normally is 20 msec. Then upto 40 nA are delivered to the electron beam experiment. Lower intensities arenecessary for the tagged photon facilities and can be obtained, e.g. by expandingthe extraction times. Extraction times up to 200 msec (Figure 2) were tested, thelarge scale time structure exhibits a smooth behaviour and there are no gaps in thespill-out.The effective duty factor was measured by one of the experimental groups bycounting the random coincidences between two counters of their tagging system andusing the formula:NI -N2dell == · -T jN.,.NI and N2 are the individual counting rates, N.,. is the random counting rate,measured as coincidence between NI and N2 with coincidence resolving time T.The used counters have pulse lengths of 10 nsec which leads to a coincidence timeof 20 nsec, which is short compared to the revolution time of ELSA (548 nsec).Thus this method is sensitive to unsymmetries in the filling of the stretcher. Afterinstallation of the filters for the quadrupole power supplies, the achieved values ofthe duty factor are 35 %.106/[352] K. H. ALTHOFF ET AL.ConclusionsThe ELSA control system offers an effective tool to control all necessary machineparameters; thus it has proved to be an essential part of the stretcher.In the storage mode ELSA turned out to be an excellent source for synchrotronradi~tion which will be used in the near future.The duty factor of ELSA has been enhanced remarkably and in addition withthe so far measured spill-out intensities good beam conditiones for the three exper-imental facilities could be achieved.- /'...~ ~~~ I/"""-V 1\ -/'~I f .... 1\""A'r\/ 1\/ ~I 1\/ \ /"LI\ / \ v "-V\I/ -jFIGURE 1Two BPM signals of the first three rev-olutions after injection into ELSA areshown. The shaving extraction methodused for the extraction from the syn-chrotron leads to a complete filling ofELSA with a small variation in inten-sity over the revolution time of 548 nsec(above). Below the relative beam. p0-sition in the horizontal plane is shown.The intensity signal depends slightly onthe beam position what causes the vari-ation in the shape of the intensity signalfrom one revolution to the next. Thevertical axes have linear units.~ 164808T/dlv .2)JSFIGURE 2In the upper picture the intensity sig-nal from a BPM in ELSA is seen overa time of 200 msec. The lower pictureshows the signal of a photomultiplier inthe extracted beam. The vertical axeshave relative units.References T/dlv20ms[1] D. Husmann et al., ELSA - The Continuous Beam Accelerator at Bonn (Pro-ceedings of the European Particle Accelerator Conference, Rome, June 7-11,1988) p.356[2] R. Bossart, J.P. Papis, V. Rossi Synchronous RF-Receivers for BeamPosition and Intensity Measurment at the CERN SPS Collider (IEEE Trans.Nud. Sci. NS-32.) p. 1899[3] K. Unser A Toroidal D.C. Beam Current Transformer with High Resolution(CERN-ISR-OP/81-14)

ELSA: One Year Of Experience With The Bonn Electron Stretcher accelerator

ELSA: One Year Of Experience With The Bonn Electron Stretcher accelerator

Abstract

Similar works

Full text

Available Versions

CERN Document Server