Bharadwaj, V.

Buon, J.

Burke, D.

Halling, M.

Hartman, S.

Helm, R.

Holt, J.

Irwin, J.

Iverson, R.

Jeanjean, J.

Le Diberder, F.

Lepeltier, V.

Massol, N.

Oide, K.

Puzo, P.

Raimondi, P.

Shintake, T.

Spence, W.

Yamamoto, N.

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Beam-based optical tuning of the final focus test beamN. Massol, D. Burke, S. Hartman, R. Helm, J. Irwin, R. Iverson, P. Raimondi,W. Spence, V. Bharadwaj, M. Halling, et al.To cite this version:N. Massol, D. Burke, S. Hartman, R. Helm, J. Irwin, et al.. Beam-based optical tuning ofthe final focus test beam. Biennal Particle Accelerator Conference.16,Pac 95,InternationalConference On High-Energy Accelerators 16, May 1995, Dallas, United States. Ieee Press,pp.749-751, 1996. <in2p3-00005377>HAL Id: in2p3-00005377http://hal.in2p3.fr/in2p3-00005377Submitted on 18 May 2000HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.SLAC-PUB-95-6770BEAM-BASED OPTICAL TUNING OF THE FINAL FOCUS TEST BEAM*P. Tenenbaum, D. Burke, S. Hartman, R. Helm, J. Irwin, R. Iverson, P. Raimondi, W. SpenceStanford Linear Accelerator Center, Stanford University, Stanford, CA 94309 USAV. Bharadwaj, M. Halling, J. HoltFermi National Accelerator Laboratory, Batavia, Illinois, USAJ. Buon, J. Jeanjean, F. Le Diberder, V. Lepeltier, P. Puzo**Laboratoire de l’Accélérateur Linéaire, Orsay, FranceK. Oide, T. Shintake, N. YamamotoNational Laboratory for High Energy Physics, KEK, Tsukuba, Japan  In order to reduce the SLAC 46.6 GeV beam to sub-micron sizes, the Final Focus Test Beam (FFTB) must meettight tolerances on many aberrations.  These aberrationsinclude:  mismatch and coupling of the incoming beam;dispersion; chromaticity; lattice errors in the chromaticcorrection sections; lattice coupling; and residual sextupolecontent in the quadrupoles.  In order to address theseaberrations, we have developed a procedure which combinestrajectory analysis, use of intermediate wire scanners, and apair of novel beam size monitors at the IP.  This procedureallows the FFTB IP spot to be reduced to sizes under 100nanometers.I.  INTRODUCTIONIn order to achieve luminosity in the range of 1034 cm-2sec-1, a TeV-scale linear collider will need to reduce the sizeof electron and positron bunches at collision to sizes on theorder of several nanometers.  This vertical size mandates ademagnification from the linac to the IP of a factor of 400.Such a severe demagnification places unprecedentedtolerances on many optical aberrations of the final focussystem, most of which cannot be met ab initio , but only as aresult of beam-based tuning of the final focus.  Any linearcollider must have an algorithm and diagnostics which willallow such tuning to converge in a finite time.The Final Focus Test Beam (FFTB) is a prototype linearcollider final focus, designed to reduce the 46.6 GeV SLACbeam to a size of 2 microns by 60 nanometers.  The FFTB hasthe horizontal and vertical demagnifications required by afuture linear collider, and thus addresses all the same opticalaberrations.  We have developed such an algorithm for theFFTB, which allows the spot to be focused to 70 nanometers.II.  THE FINAL FOCUS TEST BEAMThe optics of the FFTB have been discussed in detailelsewhere[1].  The optical layout consists of:  a 5-quadrupolebeam matching section; a horizontal chromatic correctionsection (CCSX) with a pair of sextupoles separated by a -Itransform; a beta exchanger (BX), which enlarges the verticalbeam size and reduces the horizontal; a vertical chromatic                                                  *Work supported by the Department of Energy, contract    DE-AC03-76SF00515.  **Current address:  Eurpoean Laboratory for Particle Physics(CERN), Geneva, Switzerlandcorrection section (CCSY); a final telescope (FT), includingthe final doublet (FD) magnets; and an extraction line.The primary aberrations which affect the horizontal beamsize are:  waist and magnification errors; dispersion;chromaticity; and a single normal sextupole aberration, for atotal of 5.  The vertical beam size is affected by theseaberrations, plus an additional two skew sextupole aberrations,and two xy coupling effects, for a total of 9 aberrations.III.  INCOMING BEAM MATCHINGBecause of the “stair-step” phase advance properties of alinear collider final focus, the multi-wirescanner technique formeasuring incoming beam emittance, Twiss parameters, andcoupling[2] is not applicable in the FFTB.  Instead, theincoming beam is measured by scanning a quadrupole magnetand measuring the beam size on a downstream wire scanner asa function of the magnet strength.  This technique has beendescribed elsewhere[3], and has been used for many years atSLAC.  In the FFTB, the first quadrupole magnet is scanned,and a wire scanner in the beam-matching section is used[4].For this measurement the beam matching quads are set to aspecial optics which focus both x and y waists on the wire, andthe beam is stopped before entering the CCSX.Because of the sensitivity of the FFTB to xy coupling, thebeam measurement scanner contains 7 micron wires set tomeasure the beam in x, y, and v (one of the diagonals).  Twoalgorithms have been developed to measure the fully-coupledsigma matrix using this wire scanner and quadrupole strengthscans.  It was determined that the incoming beam coupling isdominated by a single term, which can be eliminated using askew quadrupole upstream of the first normal FFTBquadrupole.  When this is done, the measured verticalprojected emittance agrees with the emittance measured at theend of the SLAC linac by the multi-wire system there ( g e y =2 x 10-11 m.rad), and is below the FFTB design value.The five normal quadrupoles of the beta matching sectionare then employed to match the incoming Twiss parametersonto the desired IP parameters.  This allows us to adjust the IPdivergence (and hence the focused size) up or down,depending on requirements of the experimental program.   Thebeam is then allowed to travel to the dump.The BX section contains horizontal and vertical“intermediate waists,” with respective beta functions of 8.3 cmand 2.5 cm (design), separated by 2.85 meters.  Special wirescanners[4] with 4 micron wires at angles optimized forSLAC-PUB-95-6770measuring flat beams are used to verify and tune the betamatch (both magnification and waist).  The beam size on thehorizontal wasit wire is 5 microns by 80 microns, and on thevertical waist wire is 200 microns by 710 nanometers.IV.  LOCAL TUNING AND DIAGNOSTICSWherever an aberration can be traced to an error in a singlemagnet, it is preferable to correct the error in that device ratherthan to apply a global tuning correction.  The primarylocalized aberrations arise from misalignments of quadrupolesand sextupoles, and errors in the strengths or roll angles ofquadrupoles.  To maximize tunability, all quadrupolesupstream of the IP are powered by separate power supplies,and all quadrupoles and sextupoles are mounted on remote-controlled magnet movers with positioning accuracy of under1 micron in x and y[5].  All quadrupoles up to the first doubletmagnet contain stripline beam position monitors (BPMs) withresolutions of 1 micron[6].The procedure for quadrupole and sextupole alignment isdescribed elsewhere[7].  The technique uses a shunt techniquefor the quadrupoles, and scans of the mover positions vsdownstream bpms for the sextupoles.  The overall tuningprocedure is guaranteed to converge if the RMSmisalignments of quadrupoles and sextupoles is below 100microns in the horizontal and 30 microns in the vertical, andthese tolerances are met by the alignment algorithm.  Thealignment is done with the IP divergence low, and this reducesthe beam size in all the limiting apertures of the FFTB.Because the procedure does not rely on beam size diagnosticsat any time, the specifics of the beam matrix are not important,and in fact alignment is usually performed before incomingbeam reconstruction for this reason.In order to minimize sextupole aberration, it is necessary totune the -I  transforms of the CCSX and CCSY as thoroughlyas possible.  This is done by introducing closed orbitoscillations, generated by moving quadrupole magnets on theirmovers in appropriate linear combinations to probe all phasesof oscillation[8].  The technique allows quadrupole strengthmeasurements of 1 part per thousand, and roll measurementsof 1 milliradian.  These are adequate for guaranteeingconvergence of the overall algorithm.V.  IP BEAM SIZE MONITORSBecause the IP beam size is smaller than at any other pointin the beam line, it is subject to aberrations which cannot bemeasured elsewhere.  Consequently, once incoming beam andFFTB lattice properties have been tuned to the limits of theupstream measuring devices, it is necessary to usemeasurements of the IP spot size itself.Initial tuning of the beam can be accomplished using wirescanners set in the IP region.  These scanners use 4 microncarbon wires, and are useful down to a beam size of about 1micron, although wire damage becomes a likely occurrence atthis point[4].  The spot can be measured down to its designsize by a pair of novel beam size monitors (BSMs) set 52 cmapart in the IP region.A.  Gas Time of Flight Beam Size Monitor[9]A Gas Time-of-Flight BSM injects a small amount ofHelium gas into the beam as it passes through the device.  Thebeam produces ions, which are then accelerated transverselyby the electric field of the beam.  The maximum velocity ofthe ions is proportional to the maximum field.  Additionally,the vertical beam is small enough that the ions are trapped byits intense electric field and oscillate with horizontal andvertical amplitudes proportional to the coordinates of theircreation point inside the beam.  In average, horizontalamplitudes are larger than vertical ones for horizontally flatbeams, leading to an anisotropy in the distribution that scalesas the beam flatness.By measuring the velocity and angular distributions ofescaped ions (via a ring of multi-channel plates surroundingthe IP), the beam size in both planes can be measured.  Figure1 shows a typical distribution, which indicates a beam size of1.6 microns by 80 nanometers.0Count040080012001600100 200Azimuth   (deg) 7952A14–95300Figure 1.  Angular distribution of He+ ions produced by beamionization of Helium gas.  The distribution is peaked in thehorizontal direction (90˚ and 270˚), indicating a flat beam.B.  Laser Interferometer Beam Size Monitor[10]A laser interferometer BSM splits a Nd:YAG laser pulseand crosses the two beams thus produced at an angle in thepath of the electron beam.  This produces an interferencepattern with a characteristic modulation spacing.  When thebeam encounters the laser, the laser photons are Compton-scattered forward into a detector.  The amplitude of the signaldepends upon the relative transverse position of the electronbeam and the laser pattern, and also upon the relative size ofthe electron beam and the modulation spacing.  Scanning theelectron beam across the interference fringes gives asinusoidally-varying Compton signal, whose modulation depthgives the beam size.  Figure 2 shows such the results of such ascan, and indicates a beam size of 73 nanometers.SLAC-PUB-95-6770 250200500100150–0.8 –0.4 0 0.4Electron Beam Vertical Position (µm)Compton Signal4-956700A186Figure 2.  Laser-Compton Beam Size Monitor Comptonphoton signal vs. beam position, showing the sinusoidalmodulation expected.  The beam size indicated is 73 nm, witha statistical uncertainty of 4 nm.VI.  GLOBAL TUNING ON IP BEAM SIZEIn order to minimize the IP beam size, it is necessary toprepare linear combinations of tuning elements (“knobs”),which are properly orthogonal.  Knobs which cause more thantrace alterations in secondary aberrations require additionaliterations, and may prohibit tuning altogether.The knobs of greatest concern are waist, dispersion, andthe single coupling term (x’y) which arises from any rolledquadrupoles in the FFTB line, including the doublet.  Becausethese knobs are scanned repeatedly, and need to beincremented by small amounts, magnet hysteresis and powersupply granularity are significant issues.  These issues areaddressed by moving the CCS sextupoles, via their movers, inpatterns which generate orthogonal waist shifts, dispersionshifts, and coupling at the IP.  Because the mover positions aremonitored by a sensor system independent of the steppermotors, the positions are not subject to hysteresis, and havethe step size needed to change aberrations by the smallincrements required.  In order to minimize the sextupoleoffsets from their aligned positions, only small changes inthese aberrations are implemented via the mover knobs.  Largechanges are converted to an equivalent magnet knob.  Thetuning elements for each aberration are as follows:A.  Waist PositionThe waist positions are moved by changing the strengths ofthe final doublet quadrupoles.B.  DispersionsThe IP dispersions are changed by steering the beam off-axis across the final doublet magnets.C.  Coupling (x’y)The “Rolled Quadrupole” coupling term is changed via askew quadrupole close to the final doublet.D.  ChromaticityThe lattice chromaticity is changed by changing thestrengths of the CCSX or CCSY sextupoles.  In order to keepthis correction orthogonal from the linear aberrations above,the sextupoles must be near their aligned positions, thus theuse of magnet knobs for coarse changes.E.  Geometric SextupoleThe geometric sextupole aberrations are changed via twonormal and two skew sextupoles in the dispersion free regionof the FT.  Of the four aberrations these magnets generate, oneeffects only the horizontal beam size, one effects only thevertical, and two have combined effects.  These magnets arescanned in combinations which excite aberrations singly.VII.  RESULTSThe scheme described above has been used repeatedly totune the beam size of the FFTB, and 70 nanometers has beenregularly achieved.  Further improvements in the spot size areanticipated as unexpected effects are fully understood.VIII.  ACKNOWLEDGMENTSThe authors would like to thank the SLC Beam DeliveryTask Force for pioneering many of the approaches used here,and the SLAC Operations staff for their hours of vigilance.IX.  REFERENCES[1] G. Roy, “Analysis of the Optics of the Final Focus TestBeam Using Lie Algebra Based Techniques.”  SLAC-Report-397 (1992).[2] K.D. Jacobs et al, “Emittance Measurements at theBates Linac,” Proc. 1989 IEEE Part. Acc. Conf,. 1526 (1989).[3] M.C. Ross et al, “Automated Emittance Measurementsat the SLC,” Proc. 1987 IEEE Part. Acc. Conf., 725 (1987).[4] C. Field, “The Wire Scanner System of the Final FocusTest Beam,” SLAC-PUB-6717 (1994).[5] G. Bowden et al, “Precision Magnet Movers for theFinal Focus Test Beam,” SLAC-PUB-6132 (1994).[6] H. Hayano et al, “High Resolution BPM for FFTB,”Nucl. Inst. Methods A320:47-52 (1992).[7] P. Tenenbaum et al, “Beam-Based Magnetic Alignmentof the Final Focus Test Beam,” these proceedings.[8] V. Bharadwaj, “Fermilab Contributions to the FFTB,”these proceedings.[9] J.  Buon et al, “The Orsay Spot Size Monitor for theFinal Focus Test Beam,” Proc. 1992 Int. Conf. on High-Energy Accelerators, 219 (1992).[10] T. Shintake et al, “Design of Laser-Compton SpotSize Monitor,” Proc. 1992 Int. Conf. on High-EnergyAccelerators, 215 (1992).

Beam-based optical tuning of the final focus test beam

Beam-based optical tuning of the final focus test beam

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