Gardner, I S K

Gray, D A

Planner, C W

Rees, G H

CERN Document Server

Particle Accelerators, 1990, Vol. 31, pp. 227-234Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaTHE RELEVANCE OF ISIS EXPERIENCE TO THE DESIGN OF FUTUREHIGH INTENSITY SYNCHROTRONSI S K GARDNER, D A GRAY, C W PLANNER, G H REESRutherford Appleton Laboratory, Chilton, Didcot, Oxon UKAbstract The relevance of the experience obtained in the design andoperation of the high intensity synchrotron ISIS is discussed in relation to thedesign of future high intensity machines.1. INTRODUCTIONThe ISIS 50 Hz, 800 MeV proton synchrotron (1) now operates at an average outputcurrent of 100 JlA, a level markedly above that of other operating synchrotrons.Experience gained in the design and operation of the ring is relevant to many designareas of future high intensity machines. Of most relevance are the civil andmechanical engineering design details, the interlock and beam loss protectionsystems, the level of beam losses and damage, the performance of the ceramic vacuumchambers with their associated radio frequency shields, the H- injection systemperformance, and the observations and studies of beam instabilities. These topics arediscussed in sequence in this report.2. CIVIL AND MECHANICAL ENGINEERINGThe ISIS synchrotron has been constructed in the magnet hall that previously housedthe weak focusing, 7 GeV proton synchrotron, Nimrod. Many advantages haveaccrued from the use of this large magnet hall. The large space available, both insideand outside the magnet ring, see Figure 1, has eased the procedures adopted forhands-on-maintenance and simple remote-handling.Nowadays the cost of such a magnet hall would be prohibitive. Instead a tunnel issuggested, but one with enlarged by-pass regions for one or two areas of the ringdesignated for beam loss collection. Such a beam loss concentration system hasworked well at ISIS. The by-passes would be equipped with movable shielding and aworking area for the repair of active, damaged components. For access into andaround the ring, shielding would be moved to lie adjacent to the components of highactivation. Maintenance in the low radiation regions is then conventional and onlythat in the highly active regions requires special procedures.The important civil engineering features are the sizes of the tunnel and by-passregions. The tunnels may be required to house more than one magnet ring and theease of removal and installation of components should be a major consideration.[1435]/227228/[1436] I. S. K. GARDNER ET AL.3. PROTECTION AGAINST LOST BEAMThe beam power in ISIS is such that protection against lost beam must be provided toprevent damage to the accelerator components. The protection takes two forms.Firstly, most of the lost beam is picked up on water cooled graphite collectors,appropriately placed in the ring. Separate collectors are provided for horizontal andvertical loss, the former being the more important for ISIS. Most of the collectors areremotely adjustable and are positioned in long straight 1 of the synchrotron, seeFigure 1. This serves to concentrate the loss in one area. The important feature of theprimary horizontal collector is its location at ,a high dispersion point in the ring at aninside machine radius, so that it intercepts non-accelerated beam that spirrals inwards.Separate collectors are provided to gather the partially stripped H O beam producedduring injection and also to shield the septum of the extraction magnet.Figure 1 ISIS sytlchrotron superperiod 1 showing the beam collector straightand a dipole in the maIntenance area (arrowed)Secondly, action is taken dependent on the magnitude and frequency ofoccurence of the beam loss. This is measured in all areas of the machine every pulse bymeans of beam intensity and beam loss monitors. The action varies from turning thebeam off to writing warning messages on the display at the main control desk.ISIS AND FUTURE HIGH INTENSITY SYNCHROTRONSTABLE 1 Intensity Monitor Operated Protectiona) 3 PULSE TRIP[1437]/229Interlock broken only if excessive beam loss occurs on three consecutive beampulsesLEVELS InjectionnappingAccelerationExtractionEPB LineTotal Lossb) AVERAGING TRIP4.9 X 1012 (per pulse)4.9 x 10122.0 X 10121.0 X 10121.0 X 10124.9 x 1012Interlock broken when excessive beam loss occurs for an averaged loss during theprevious 25 machine pulses (updated each machine pulse)LEVELS InjectionTrappingAccelerationExtractionEPB LineTotal Lossc) WARNINGS1.1 X 1012 (per pulse)4.0 x 10125.0 x 10115.0 x 10115.0 X 10114.8 x 1012No beam trip produced. A visual warning is displayed on the control desk if theaverage beam loss during the previous 30 seconds or 20 minutes exceed pre-setvaluesLEVELS InjectionTrappingAccelerationExtractionEPB LineTotal Loss30 sec1.0 x 10123.3 x 10121.5 X 10112.0 X 10113.5 X 10114.0 x 101220 min8.5 X 1011 (per pulse)2.8 x 10121.0 X 10111.0 X 10113.5 X 10113.8 X 1012230/[1438] I. S. K. GARDNER ET AL.Toroids are used as intensity monitors, and long ionisation chambers filled with Argonas beam loss monitors, for linac, synchrotron and extracted proton beamline (EPB).The beam intensities of all three are continuously displayed along with the injection,acceleration and extraction efficiencies. Also on continuous display are the outputs ofthe beam loss monitors. Thbles 1 and 2 show the levels of beam loss which are used inthe protection algorithms. Where the cause of loss is uncertain, the repetition rate ofthe accelerator is reduced by a factor of 32 and a high intensity byam is thenre-established. In the case of loss of a high percentage of the 70 MeV injector beam,the intensity of the pre-injector is reduced by means of a beam diluter.One special consideration for ISIS is that the spallation target, which, runs at hightemperature, suffers a severe thermal shock if the beam is turned on or off suddenly.This shortens the target life-time and so the protection system has been developed toallow some intermittent beam loss conditions to exist, for example the occasionalextraction kicker mis-fire.Even with all the protection provided, there have been a few operational mishaps.In the beamlines, vacuum leaks have developed due to the distortion of joints by beamheat deposition. In the synchrotron there has been some vacuum chamber componentdamage, and this is described in section 4. High levels of induced activity havesometimes been found away from the beam loss collector region. However, in generalthe protection has worked well and has been an indispensible feature of the machine.It has allowed the machine to continue operating at times with some systems workingnon-optimally. The general level of induced activity has been acceptable andmaintenance has proceeded with no member of staff receiving an annual dose higherthan one tenth of the permitted maximum level. Figure 2 shows the output of the 40synchrotron beam loss monitors and Figure 3 shows the induced activity levels aftershut down.TABLE 2 - Beam Loss Monitor Operated ProtectionInjectorBeam trip occurs if any 1 of 19 beam loss monitor outputs exceeds its pre-setlevel or if the summed output of the 19 monitors exceeds a pre-set level on 2machine pulses out of 4. Typical levels are below 4 x 1010 ppp.ExtractionBeam trip occurs if the output of the ring beam loss monitor closest to theextraction septum magnet exceeds a pre-set level on 2 machine pulses out of 4.1)rpicallevel is less than 4 x 109 ppp.EPBWarnings only are provided. The warning is displayed when the output of any 1 of10 monitors exceeds its pre-set limit. Typical levels are less than 1 x 109 ppp.ISIS AND FUTURE HIGH INTENSITY SYNCHROTRONS [1439]/2311000OUTPUTmv 10010o01234687898UPEAPEAIODFIGURE 2 - Output of 8ynchrotron beam 101. monitors.10'ACTIVITYJ.L Iv/hl-I-10' ~~~~"""~~I-III III10· U I II0 1 234 !5 8 7 8 08UPERPEAIODFIGURE 3 - Induced aotivlty in eynchrotron.232/[1440] I. S. K. GARDNER ET AL.The importance of a protection system for any future high intensity machinecannot be stressed too highly. As at ISIS, the magnet lattices should be designed fromthe outset with appropriate straight section spaces available for beam loss collectors.A finite dispersion straight is important for longitudinal beam loss collection.4. OPERATIONAL PERFORMANCE OF VACUUM CHAMBERCOMPONENTS.The alumina vacuum chambers with glazed joints developed for ISIS have proved tobe extremely robust and reliable in operation. To date only one small leak has occuredat a glazed joint and this was easily cured. Accidental beam spill has not damaged anyceramic chamber although it has caused discolouration which gradually fades. Radiofrequency shields are required inside such chambers to improve beam stability byproviding a low impedance for beam induced r.f. currents. The shields in ISIS arestainless steel wire cage structures, as illustrated in Figure 4, which are contoured tofollow the beam profiles to minimise the space-charge reactance. The' shields arecoupled by capacitors presenting a low impedance at r.f. but a high impedance to thesynchrotron cycling frequency. IThere are developments at other laboratories to integrate the rf shield andcoupling capacitors with the ceramic chamber using printed circuit techniques. Itwould be difficult to follow the profiles with such a shield, but this is of lessl' importancein higher energy machines. However, damage of such a shield by the beam wouldmean the replacment of the ceramic chamber.In two separate incidents on ISIS, r.f. shields have been hit by the beam resultingin melting of the stainless steel. With the non-integral construction it pro¥ed possibleto remove these damaged shields from their ceramic chambers without difficulty andreplace them with new ones. This was done without the need to re-surveycomponents, which would not be the case if a vacuum chamber had to be replaced.The only other incident involving beam damage to components, apart fromdamage to vacuum seals, was to one of the three extraction kicker magnets. Extractionfrom ISIS is vertically upwards and the kicker magnets are relatively simple, lowimpedance, single turn, lumped element, ferrite magnets. When trying to compensatefor a reduced kick in an upstream kicker magnet by increasing a slow orbit bump,about one third of the area of the upper half of the single turn coil of the downstreamkicker was melted away by the beam. However, the extraction system continues towork at very high efficiency with the magnet in this state.The operational performance of the ISIS vacuum chamber components thusleads to the following main recommendations for future high intensity machines.i) Glaze-jointed ceramic vacuum chambers with integral or separate shieldsmerit serious consideration.ii) Simplicity in design is important to provide rugged components and to allowease of maintenance of radio-active items.5. CHARGE EXCHANGE INJECTIONDirect multiturn charge-exchange injection of H- ions through a stripping foil hasproved to be the most effective way of injecting into a high intensity protonsynchrotron. At the present operating intensity of 100 J.1A on ISIS, the injectionISIS AND FUTURE HIGH INTENSITY SYNCHROTRONS [1441]/233Figure 4 - Ceramic Doublet Quadrupole Vacuum Chamber Fitted withcapacitively coupled r.f. shield and bellows unitefficiency is 98%, with 2% of ions being only partially stripped to HO and collected ona beam dump. Higher intensitites have been injected without showing any sign ofsaturation, but injection efficiency does fall slightly and at 200 J.1A injected, theefficiency is down to 92-94%.The aluminium oxide stripping foils used in the ISIS injection system have beenextremely successful, achieving irradiation lifetimes of around 180,000 J.1Ah injectedcurrent (- 4 Ah total irradiation). Because the injection energy for ISIS is 70 MeV:these foils are only 50 J.1g/sq cm thick and the l()ng lifetimes are obtained by havinginitially stress-free foils, which are now separated from the support frame on threesides.In a suitably designed charge-exchange injection system it is possible to haveconsiderable control over beam brightness, emittance, phase-space density and linedensity distributions. In the ISIS system there is some control over these parameters inthe two transverse phase-planes but for future machines control of all threephase-plane parameters should be possible. Minimising the number of foil traversalsby the circulating proton beam, to reduce beam loss from momentum growth andangular scattering, is of great importance in the design of such injection systems. This234/[1442] I. S. K. GARDNER ET AL.requires a lattice designed to optimise injection, and longlife stripping foils with atleast two free edges which are stable in operation.Experience on ISIS indicates that manufacture of such foils is possible. Themaximum thickness of aluminium oxide foils is limited to around 150 J,.lg/sq em, but ithas been demonstrated that thicker foils can be obtained by folding to producemultilayers. The KEK Laboratory in Japan is also experimenting with irradiation ofsuch folded foils (2), but made of carbon. Thick foils could readily be made in carbonbut multilayer foils may have the advantage that radiation induced stresses may bepartially cancelled between the foil layers, particularly at the unsupported corner.6. OBSERVATIONS AND STUDIES OF BEAM INSTABILITIESFive instabilities have been observed on the ISIS synchrotron: coasting beam verticalresistive wall, coasting beam longitudinal microwave (3), bunched beam vertical headtail resistive wall, bunched beam longitudinal quadrupole mode and, during thetransition from coasting to bunched beam, a high frequent}' vertical instability. Thesecond, third and fourth of these do not conform to theory and the fifth is notunderstood. That three sets of observations should be mis-interpreted appearsunlikely and so doubt is cast on the underlying theories.Longitudinal coasting and bunched beam theories have therefore beenre-examined and a number of doubtful features have been identified. In particular, forthe coasting beam case, it appears that both the form assumed for the initialperturbation and the calculation of the beam-environment interaction are not strictlyvalid and that as a consequence, the related Keil-Schnell criterion (4) is incorrect.Similar invalid features have been identified for the bunched beam case and have beenreported in the literature (5). It is planned to publish the coasting be.am findingsshortly. It is believed that estimates for instability thresholds in future high intensitymachines should thus be treated with some caution.REFERENCES1) B. Boardman, Spallation Neutron Source: RL-82-006, March 19822) A. Yamane, Proceedings of Kaon Factory Workshop, Los Alamos,April 1989,to be published.3) R. Martin, I. Gardner, C. Planner and G. Rees , Symposium on Heavy Ion Fusion,Darmstadt, Germany, June 1988.4) E. Keil, W. Schnell, CERN Int. Rep., ISR-TH-RF/69-48 (1969)5) G. Rees, European Particle Accelerator Conf. Rome, 198 (1988)

The Relevance Of Isis Experience To The Design Of Future High intensity synchrotrons

Abstract

Similar works

Full text

Available Versions

CERN Document Server