Clarke-Gayther MA

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A COMBINED FUNCTION BEAM EMITTANCE AND PROFILEMEASURING SYSTEM FOR THE ISIS 665 keV H −  PRE-INJECTORMichael A. Clarke-Gayther, Rutherford Appleton Laboratory, Didcot, United KingdomAbstractA description is given of the combined function, beamemittance and profile measuring system, installed on the665 keV H− pre-injector at ISIS, the high intensity pulsedneutron source at the Rutherford Appleton Laboratory.The computer controlled system, consisting of in-vacuumbeam analysing slits, and multiwire detectors, linearactuators, position encoders, motor drives, and signalconditioning electronics, measures the transverseemittance and profile of the 300 µs, 35 mA, 665 keV  H−beam, with high precision.1  INTRODUCTIONThe concept and accurate measurement of  beamemittance are of fundamental importance to the designand operation of particle accelerators. This description ofthe beam, when combined with a knowledge of thoseforces and processes that act on the beam (e.g.acceleration, space charge, scattering), enables theaccurate prediction, and modelling of beam dynamics tobe made, in accelerators and transport beamlines.A particle beam, consisting of an ensemble of particles,each defined in terms of a total energy function, orHamiltonian (H), is characterised by defining beamemittance, as the volume of a hyper-ellipsoid in sixdimensional phase space, with the canonical coordinatesx, px, y, py, z, pz [1].In practise, emittance is measured at constant beamenergy, and by techniques that evaluate the ‘root meansquare’ (rms) density distribution in the six dimensionalphase space, in terms of elliptical area projections in thetwo dimensional  longitudinal sub-space, with phase andenergy coordinates (ϕ, W) , and the four dimensionaltransverse sub-space, with position and divergencecoordinates ( , ' , , ' )x x y y  or ( , ' )r r .Measurement systems, fall into two distinct groups, thosefor the determination of longitudinal emittance (∆ϕ/ϕ,∆pz/pz) [2], and those for the determination of transverseemittance ( , ' , , ' )x x y y or ( , ' )r r [3]. Systems can be non-destructive or destructive to the beam. The former includethe wire-shadow and tomographic methods [3], bothcapable of being implemented in H- beams using the laserdetachment method [4]. The latter systems include thepepper pot, electric sweep, slit-slit, and slit-harp [3]. Acomputer controlled, slit-harp  emittance, and profilemeasurement system has been installed on the 665 keVH- pre-injector at  ISIS.2  EMITTANCE MEASUREMENTThe transverse beam emittance ε is defined as thevolume of a four dimensional hyper-ellipsoid given by :ε πx y dx dx dydy, ' '= ∫∫∫∫1The measurement evaluates εx,y in terms of the twodimensional projections in the x x, ' , and y y, '  planes :ε π πx xdx dx A= = ⋅∫1 1' , ε π πy ydy dy A= = ⋅∫1 1'where Ax and Ay are elliptical areas in thex x, ' , andy y, 'planes, for ‘normal’, or ‘perfect’ beams [1]. For realbeams, the evaluation of rms emittance, defines theellipse area, of a transportable ‘equivalent perfect beam’,that is an invariant of  motion in linear focusing transportsystems [1]. This area is defined in terms of the secondrder moments, y2 , y'2 , y y⋅ ' , of the density distribution( )ρ2 y y, ' as : εrms y y y y= ⋅ − ⋅2 2 2' ( ' )Evaluation of εrms, and the related Twiss parameters, atne point in a beam transport system, enables an accurateprediction of the ‘transported’ ellipse parameters  to bemade at any other point in the system, by means of  a‘sigma’ matrix manipulation [5].YsiYs Yh YhYsYsiYsiIw(Ysi,Yhj)Iw(Ysi,Yhj)YhjYhj+nYh j-nYsi,Yhj+nYsi,Yhj-nε βεγε β−αβε−α ε γY'j+nY'j-nXZY'YY'YYX(a)(c) (d)(b)PLANESLITPLANEHARPMOTIONFigure 1. (a) Slit-harp measurement schematic. (b) Beamdensity distribution in (Ys,Yh) plane. (c) Phase spacecontour for a gaussian beam with a y-plane waist.(d) Definition of Twiss parameters1491A schematic showing the essential features of the slit-harp measurement system is shown in figure 1(a). Abeamlet, selected by a moveable slit at Ysi , i  interceptedon a downstream multi-wire detector (harp), as a currentdensity distribution Iw(Ysi ,Yhj ), centred on Yhj. Theprofile for each slit position is stored in a twodimensional array, Ysi,Yhj. The right ellipse, shown infigure 1(c), represents an rms contour in the Y Y, ' plane,for a beam with a gaussian density distribution, and a Y-plane waist. The input data ellipse in the Ysi, Yhj plane,shown dotted, indicates how the input data is sheared  togenerate the Y Y, '  phase space data. 3  CONTROL PROGRAM STRUCTUREThree measurement cycles, beam profile (x, IBEAM, andy, IBEAM), and emittance in the two transverse planes( , ' )x x , or ( , ' )y y , are initiated and run under programcontrol on an ISIS control system computer.INITIALISEUNPARKPARKCLEAR ARRAYS / SET PLANEREAD ACTUATORS STATUS / POSITIONREAD BEAM STATUSDISENGAGE LOCKOUT LATCHESLOAD DISTANCE / SPEED DATAHARP AND SLIT TO START POSITIONSFOR N=1 TO 10STORE DATA FROM 10 HARPWIRESINCREMENT HARP POSITION BY 1 mmIS C=42 ? IF NOT :INCREMENT SLIT POSITION BY 1 mmC=42?PARK ACTUATORSCHECK LOCKOUT LATCHES ENGAGEDNOMEASUREBEAMPROFILEDISPLAY CONTOURS IN PHASE SPACEFIT ELLIPSE TO 95% CONTOURCOMPUTE AREAS IN PHASE SPACECOMPUTE TWISS PARAMETERSGRAPHICSSET C=C+1Figure 2: Emittance measurement code structureThe emittance measurement code structure is shown infigure 2. The measurement stores harp data in a twodimensional (42,100) input array, where, for each of42 x 1 mm slit positions, a beam profile consisting of 100x 1 mm measurements is made. Input array data isprocessed (sheared, noise rejected, amplitudenormalised), to produce the phase space data array.Figure 3: ISIS 665 keV H-  Y- plane emittance plotThe phase space data is processed to produce plottingcoordinates for a 2D contour map, with contours at 5, 10,30, 50, 70, and 90% of the normalised peak beamintensity. The area inside the coordinates of the outermostcontour is computed, by summing the areas of all thetriangles formed by the centroid and successive pairs ofcontour coordinates. An ellipse is fitted to the coordinatesof the outermost contour by first performing a leastsquares fit, to determine the slope of the major axis, andthen, setting the length of the major axis, equal to themaximum width of the contour in the x direction, and thelength of the minor axis equal to the maximum width ofthe contour in thex' direction. Computation of rmsemittance is currently performed off-line. 4  MECHANICAL DESIGNA plan view of the ISIS 665 keV H- beam transportarea, showing the location of the emittance measurementinstallation, is shown in figure 4.Y PLANESLITACTUATORY PLANEHARPACTUATORI I I IS S Q QX PLANESLITACTUATORX PLANEHARPACTUATOR665 keV DCCOLUMN70 MeVLINACTANK 1BEAMSTOP BUNCHER0KEY ISQINTENSITY MONITORSTEERING MAGNETQUADRUPOLES::::1 m Figure 4: Plan view of ISIS 665 keV H- Beam Transport The x and y plane harp actuators are located 980 mmd wnstream from the x and y plane slit actuators. The 1mm wide, beam analysing slits are formed from  copperplate, and are water cooled. ISIS beam prf is reducedfrom the operational 50, to 50/32 p.p.s. for themeasurement duration, resulting in an average beampower of ~ 12 Watts (~ 25 kW peak). The harpassemblies are designed to withstand beam heating duringthe profile monitoring function at this reduced prf.  Theyconsist of ten, 1mm diameter tungsten wires, on a 10 mmpitch, supported by beryllium-copper spring fingers.These are attached to machineable ceramic (macor)blocks,  mounted on a stainless steel ‘C’ frame. Themechanical construction of a linear actuator (y- planeharp) is shown in figure 5. The precision mechanism hasa setting accuracy of +/- 1/64 mm. The static loadimposed by the vacuum is balanced by an equal andopposite force exerted by a constant tension spring.Reduction of  this load has permitted the use of lighterweight, lower inertia components in the drive train. As atypical measurement cycle consists of many smallmovements, the reduction in drive train inertia, results ina significant reduction in measurement time.1492 TOOTHED DRIVE BELTCOVERSTEPPINGMOTORLEAD SCREWCAPTIVE BALL NUTVACUUMBELLOWSLINEAR BALL BEARINGHARP SIGNALFEEDTHROUGHSLIT COOLANTFEEDTHROUGHSHARP SUPPORTFRAME10 X 1mm DIAMETERTUNGSTENWIRESTUBULAR ACTUATORARMRIBBON CABLE ANDCOOLANT FLEXIBLESCONSTANT TENSIONSPRINGZERO BACKLASHGEARBOXROTARY POSITIONENCODERHARP SIGNALJUNCTION BOXSLIT COOLANTFEED AND RETURNLOCK OUT LATCHLIMIT SWITCH0 200 mmBEAMLINE VACUUMBOXHARPX-PLANE Figure 5: Actuator construction (schematic) Limit switches prevent actuator movement beyondpredefined limits. Lock out latches prevent the possibilityof positional ‘creep’ in the parked position, when thestepping motors, in the de-selected plane, aredisconnected from the multiplexed motor drive units. 5  ELECTRONIC DESIGNThe important features of the emittance measuringsystem drive control, and signal conditioning electronics,are shown in figure 6.BEAMLINEINJECTORCONTROL~20m~100mBEAMHOR.SLITVERT.SLITHOR.HARPVERT.HARPDRIVE / POSN.ENCODER / HARP SIGNAL MULTIPLEXSLITDRIVEHARPDRIVEPOSITION400 Hz ENC.REF.HARP SIGNALAND -100V BIASDECODERS AND INTEGRATORSHALLAREAAREAPLANE MULTIPLEX CONTROLAND INTERLOCKSDATAAQUISITIONMICROPROC.ISISBUS I/OTYPE SISISBUS I/OTYPE DISISBUS I/OTYPE EISISBUS I/OTYPE TISISBUS I/OTYPE BSTEPMOTORCONTROLISISCONTROLCOMPUTERVDUISISCENTRALTIMINGFigure 6: Electronic system schematicEach linear actuator is powered by a  16 watt, two phasestepping motor [6]. The output of a programmablestepping motor controller, with a pre-set acceleration /deceleration slope [6], is multiplexed via two motor driveunits [6], to control the positioning of the pre-selectedslit / harp actuator pair. Plane selected slit / harp positionmeasurement signals, derived from rotary, absoluteposition resolvers, are multiplexed to two resolver todigital decoders with 14 bit output resolution (1 LSB =1/64 mm). Signals from limit switches and lock-outlatches provide status information, and are hard wired toprevent damage under fault conditions.Harp signal wires are biased at -100 volts, to repelthermionic and secondary electrons, stripped from the H-beam. Signals from the ten harp wires are planemultiplexed to a ten channel gated integrator. Integrationtime is synchronised to the beam pulse length. Integratedsignals are output to the data acquisition microcomputer,where the analogue signals are stored as 12 bit datawords, that are subsequently transferred to the  controlcomputer under program control. 6  OPERATIONAL EXPERIENCEA routine measurement of beam alignment in the 665keV H- transport line, is made after each ISIS ion sourcechange (lifetime ~ 30 days), and normally, a fullemittance measurement,  taking about 20 minutes, will bemade. The profile measurement, enables a more rapid(~30 s) check of beam position and width to be made. 7  ACKNOWLEDGEMENT Contributions from the following are gratefullyacknowledged : S. L. Thomas (ISIS emittance graphics),A. I. Borden , and D. Wright (data acquisitionmicrocomputer and software), ISIS project engineeringgroup (mechanical design), and many others.REFERENCES[1] C. Lejeune and K. Aubert, ‘Emittance andBrightness’, in ‘Applied Charged Particle Optics’,Ed. by A. Septier, ‘Advances in Electronics andElectron Physics, Supp. 13A, 1980, p.159-259.[2]  R. C. Webber, ‘Longitudinal Emittance’, Proc. of  thefirst Accelerator Instrumentation Workshop, Upton,NY, 1989, AIP Conf. Proc. No.212, p.85-126[3]  O. R. Sander, ‘Transverse Emittance’, Proc. of  thefirst Accelerator Instrumentation Workshop, Upton,NY, USA,1989, AIP Conf. Proc. No.212, p.127-155.[4]  D. R. Swenson, E. P. MacKerrow, H. C. Bryant,‘Non-invasive diagnostics for H- beams usingphotodetachment by a focused laser beam’, Proc. ofthe fifth Beam Instrumentation Workshop, Sante Fe,NM, USA,1993, AIP Conf. Proc. No.319, p.343-352[5]   K. L. Brown, B. K. Lear, S. K. Howry,‘Transport .360’, Stanford Linear Accelerator CentreReport, SLAC-91 (1977).[6]  Sigma Instruments Inc., Braintree, Mass. USA.1493

A combined function Beam Emittance and Profile measuring system for the ISIS 665 keV H- pre-injector

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