Sidlow R

Thomason J W G

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ISIS ION SOURCE OPERATIONAL EXPERIENCEJ.W.G. Thomason, R. Sidlow, CLRC RAL, Didcot, Oxon, UK.AbstractThe ISIS ion source is a surface plasma ion source ofthe Penning type, and routinely produces 35 mA of H−ions during a 200 µs pulse at 50 Hz for uninterruptedperiods of up to 40 days. A new computer control systemwas installed in 1995, which, in addition to providingmuch tighter control of ion source parameters, hasallowed these parameters to be logged every fifteenminutes and then averaged on a daily basis. A detailedanalysis of these data from every ion source run on ISISduring the last five years shows the range of regimesunder which operation is possible, but highlights theadverse effects that certain modes of operation may have.Evidence is provided that apparently small mechanicaldefects can have a marked impact on how an individualion source behaves. Practical experience has determinedthe parameter changes necessary to maintain optimumperformance. These typical changes are seen in the loggeddata and explained in terms of ion source physics. Thisdesign of ion source will be used on the RFQ test facilityat Rutherford Appleton Laboratory (RAL), and animproved version may be viable for the EuropeanSpallation Source project.1  INTRODUCTIONPrevious papers have described the operation of theISIS ion source at RAL [1] and the installation of itscomputer control and monitoring system [2]. A schematicdiagram of the source is shown in figure 1.A Penning discharge is struck in a mixture of hydrogengas and caesium vapour by applying a pulsed (≈ 500 µs,50 Hz) bias of ≈ −150 V to the cathode, which iselectrically isolated from the other source components.H+, H2+, and H3+ ions from the hydrogen discharge arebackscattered from the cathode surfaces and can captureelectrons to form H− ions. The donation of electrons isgreatly enhanced by adsorption of caesium onto themolybdenum cathode surface, which reduces its workfunction from 4.5 eV to 1.2 eV for an optimal surfacedensity of caesium. These surface reactions are efficientfor production of 10−100 eV H− ions, but these wouldgive too high an emittance in the extracted beam. HenceH− ions from the cathode surface are physically obstructedfrom passing directly into the extraction aperture and mustfirst undergo resonant charge exchange with slow (<1 eV)H atoms in the recess adjacent to the aperture. The slowH− ions produced are extracted by a 17 kV potentialapplied to the extractor during the last ≈ 200 µs of thedischarge pulse and then pass through a 90° sector magnetin order to separate out electrons extracted from thesource. The magnet pole pieces are set into the sides of astainless steel ‘cold box’, which is maintained at about−5 °C to condense excess caesium emitted from thesource. The H− ions then traverse a 665 kV DC mediumgradient accelerating column to provide a high brightness35 mA ion beam for the ISIS linac.2  OPERATIONAL PARAMETERSOver the last five years ≈ 60 ion sources have been runon ISIS. There are six operational ion source assemblies,which supply hydrogen via a pulsed piezo-electric gasvalve and caesium from a boiler, as well as providingthermocouple monitoring of source componenttemperatures. Before use each source is fitted with a newcathode, aperture plate and ceramic. Nine source bodies,into which anodes are press fitted, are reused, withindividual anodes being replaced when there is evidencethat the thermal contact between the anode and sourcebody is poor or that the anode has become unduly worn.The computer control system, which operates over afibre-optic link, allows discharge current (ID), extractvoltage, sector magnet current, gas pressure, anodetemperature (TA), cathode temperature (TC), source bodytemperature (TS), caesium boiler temperature (TCs), H2 gaspulse length, discharge pulse length (τD), extract pulselength and ion beam current (IB) to be logged every fifteenminutes and then averaged on a daily basis. The databasederived from these parameters has been analysed toinvestigate the behaviour of the operational ISIS ionflangemicacopper disccathodeanodeceramicsource bodyPenning B fieldextractordischarge regionaperture plateion beam10 mmFigure 1: Schematic of the ISIS H− ion source.1625Proceedings of EPAC 2000, Vienna, Austriasource. It should be noted that the source is always tunedto give IB ≈ 35 mA; other operating modes could probablybe established for different ion beam currents. Manyinteresting correlations have been found between thelogged parameters, the most significant of which arepresented in figure 2.In order to reduce the ≈ 1000 data points that wouldotherwise appear in each part of figure 2, data have beenaveraged in suitable intervals. For instance in figure 2a)5 A intervals in ID have been chosen to give reasonablestatistics, with bars representing one standard deviation ofthe values being averaged. In each case a weighted leastsquares fit has been applied to produce the dashed line andemphasise the trend in the data.Figure 2a) shows the interdependence of ID and τD. Thesource is normally set up at a particular value of TC,established by expedience, and then TC is maintainedthroughout the run. This ensures that the amount ofcaesium on the cathode surfaces remains optimal. It canbe seen from figure 2d) that if TC is raised increasedthermal desorption of caesium must be overcome byincreasing TCs and hence the flux of caesium onto thecathode surfaces. The value of TC is determined by thetotal power fed into the discharge, explaining why anincrease in ID is accompanied by a decrease in τD.However, it should be noted from figure 2b) that anoperating regime with high ID requires higher TCs. Thismay not be desirable, as increased caesium flux can leadto excessive extractor currents as well as possiblecontamination of the accelerator column. These effectsmay not necessarily affect source lifetime or performance,but often lead to an increased incidence of extract andcolumn breakdowns which may damage other equipmentor interrupt ISIS running.The data analysis reveals that the difference between TCand TA is the only parameter which appears to have anymarked effect on source lifetime, as shown in figure 2c).Clearly, given the evidence of figure 2d) and thepreceding discussion, it is preferable to achieve highTC−TA by maintaining low TA rather than high TC. TA isusually determined by the thermal conductivity betweenthe anode and source body (which is air cooled internally),which in turn depends upon the interference fit betweenthese two components. This demonstrates the vitalimportance of quality control during the assembly of ionsources and the manufacture of source components.Practical experience has shown that, during the lifetimeof a source, IB can be tuned most effectively by alteringthe values of ID and τD. Generally an increase in IB isobtained by moving from low to high ID and compensatingfor the change in TC with τD as in figure 2a). Figure 2e)shows that ID must be increased gradually with time inorder to maintain an optimal IB.Figure 2 represents the entire database, with no datahaving been excluded. Trends in the data are broadlysimilar when individual ion source bodies and assemblies40 50 60 70450500550600650a)ID (A)τ D ( µs)40 50 60 70160165170175b)ID (A)T Cs ( °C)0 10 20 30 40 50−100−50050 c)Lifetime (Days)T C−T A ( °C)150 160 170 180 190 200450500550d)TCs ( °C)T C ( °C)0 10 20 30 40 504045505560e)I D (A)Day Figure 2: Operational ion source parameters.Proceedings of EPAC 2000, Vienna, Austria1626are considered, but this analysis is beyond the intendedscope of this paper. 3  QUALITY CONTROLRecent experience has shown that apparently smallchanges introduced in the manufacture of ion sourcecomponents can cause unexpected problems. Normalcathode edges are specified as having 0.15 mm radii.However, one batch of twenty cathodes procured throughoutside manufacture was found to have edges muchsharper than this.Figure 3a) shows oscilloscope traces of ID and extractorcurrent (IE), obtained from the ion source via a 0-33 MHzfibre-optic analogue link, and of IB. This is a typicalexample of running with a properly manufacturedcathode. It can be seen that ID (55 A) and τD (520 µs) areclose to the centre of the range of values shown infigure 2a). Noise is evident during the first ≈ 280 µs of thedischarge current pulse, but is damped because there is asufficient density of caesium on the cathode surfaces. Themechanism for this phenomenon is discussed in [3]. Noisedoes not intrude into the extract pulse or the ion beampulse, where IB = 35 mA. The initial spike in IE representsthe charging current for the extractor and cold box, but thedemand rapidly settles to a level of ≈ 0.25 A.Running with a cathode with sharp edges results in theoscilloscope traces of figure 3b). Here ID (68 A) and τD(450 µs) are at the extreme of the operating range. This isattributed to additional discharge current being required tofeed extraneous discharges induced by the enhancementof electric field gradients at the sharp edges. Carefulexamination of burn marks on the used sourcecomponents reveals these discharges to have been asindicated in figure 3c). With such a high ID, TCs must alsobe high, as in figure 2b), and the consequent large flux ofcaesium into the extraction gap results in an excessivelyhigh value of IE (> 0.5 A) and an increased risk of extractbreakdown. In addition it has been necessary to increasethe extract pulse width to compensate for the smallermagnitude of IB (≈ 32 mA), further increasing the demandon the extract power supply. Even with high TCs there isnot sufficient caesium on the cathode surfaces to dampnoise in the discharge effectively, and so noise nowintrudes into the first ≈ 80 µs of the ion beam pulse.During the lifetime of this source large amounts ofcaesium were condensed on the magnet pole pieceshoused in the cold box. While this caused no immediateproblem, in a subsequent run some of this caesium wasliberated by H− ions striking the pole pieces after a suddenincrease in source output. This resulted in contaminationof the accelerator column and a spate of breakdowns.Obviously cathodes with sharp edges have seriousdisadvantages, and the remaining cathodes from theoffending batch were modified to meet the original designspecification.4 CONCLUSIONSThe results presented in sections 2 and 3 indicatefavourable operating regimes for the ISIS H− ion source,and also emphasise the importance of maintaining highstandards in manufacture and assembly. An identical ionsource will be used on the RFQ test facility at RutherfordAppleton Laboratory [4], and an enhanced version may bedeveloped for the European Spallation Source project.REFERENCES[1] R.Sidlow et al., EPAC 96, THP084L.[2] M. Perkins et al., EPAC 96, TUP081L.[3] G.E. Derevyankin and V.G. Dudnikov, Proceedingsof the 3rd international symposium on the productionand neutralization of negative ions and beams, BNL,1983, AIP Conf. Proc. No. 111, pp 376-396, 1984.[4] D.J.S. Findlay et al., EPAC2000, THP4A03.b) IBIEIDa) IBIEIDABc)Figure 3: Oscilloscope traces (100µs/div) fora) typical running and b) abnormal running. c) Detailof the top of the cathode showing normal (A) andextraneous (B) discharge regions.1627Proceedings of EPAC 2000, Vienna, Austria

ISIS ion source operational experience

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