Derégel, J

Galbraith, Peter

Genevey, P

Henrichsen, K N

Rifflet, J M

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsLHC Project Report 4Large Hadron Collider ProjectPROPOSAL OF A K-MODULATION SYSTEM FOR THE LHC QUADRUPOLESJ. DEREGEL, P. GENEVEY, J.M. RIFFLET, CEA-SACLAYP. GALBRAITH, K.N. HENRICHSEN, CERNAbstractA method for the precise measurement of the beam position with respect to themagnetic centre of quadrupole magnets installed in particle accelerators has beendeveloped over the last decade.  The procedure consists of modulating the fieldstrength in individual lattice quadrupoles while observing the resulting orbitoscillations.  This so-called K-modulation provides the precise information on thelocation of the particle beam and is of particular importance in superconductingaccelerators, in which the quadrupole magnets are not provided with a direct opticalreference to the magnetic centre.  Measurements on the first LHC prototypequadrupole showed that sufficient modulation may be obtained up to a frequency of40 Hz and that the existing equipment for the modulation of individual latticequadrupoles in LEP may later on be used in LHC.LHC-MMSAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandGeneva, 21 March 19961. IntroductionA method for the precise measurement of the beam position with respect to themagnetic centre of quadrupole magnets installed in particle accelerators has beendeveloped over the last decade [1,2,3].  The procedure consists of modulating the fieldstrength in individual lattice quadrupoles while observing the resulting orbitoscillations.  This so-called K-modulation permits the absolute calibration of theassociated beam position monitors, thus providing a perfect knowledge of the locationof the closed orbit.  In addition, it may provide other very useful observations foroperation and adjustment of the accelerator.  This is of particular importance forsuperconducting accelerators, in which the quadrupole magnets are not providedwith a direct optical reference to the magnetic centre.  The field geometry is defined bymagnetic measurements and usually transferred to an external reference target.Errors caused by changes of temperature profiles and other phenomena might buildup as time passes.2. Measurement methodSimultaneous K-modulation measurements may be performed in several partsof the accelerator if different modulation frequencies are applied.  A difference ofabout 1 Hz has proved sufficient [2,3].  Local d.c. orbit distortions are then applied inthe search for the magnetic centre.  The method may be improved by synchronousdetection of the oscillation, so that its phase can be identified.  The sensitivity of thedetection is quite impressive.  Experience from LEP shows that a modulation of thequadrupole field of about 300 ppm can be clearly detected.  This means thatmeasurements may be taken continuously during a physics run.3. K-modulation in LHCThe method can easily be applied to the LHC quadrupoles.  A small a.c. powersupply could be connected directly to the quadrupole winding through the wiresforeseen for damping of excitation current oscillations and for the periodicconditioning of the protection diodes.  It would thus be connected in parallel to themain excitation current.  The a.c. excitation circuit must be designed to support therather high terminal voltage and common mode voltage occurring in case of quenchor fast discharge of the magnet string.  This may be obtained by protection resistors,limiting the d.c. current to a few amps.The system for powering in the LHC tunnel could be very similar to that usedfor K-modulation in LEP: Two a.c. power supplies located in each of the undergroundareas may power any of the quadrupoles located in the two adjacent half octants.  Apossible scheme for powering one half octant is shown in Fig. 1.  In fact, the existingequipment for the K-modulation in LEP could be reused for LHC. A multicore controlcable permits the multiplexing of the magnet excitation in each half octant through amatrix consisting of a simple relay and diode installed near each quadrupole.Simultaneous measurements may then take place in all half octants if differentfrequencies are applied.- 2 -4. Measurements of the quadrupole prototypeMagnetic measurements were performed at CEA-Saclay on the second LHCquadrupole prototype [4] in order to determine its frequency response.  The mainpower supply was modulated with a superimposed a.c. signal at frequencies between 1and 24 Hz, the upper frequency being limited by the regulation system of the powersupply.  The harmonic coil used for the systematic magnetic measurements of thequadrupole was placed at a fixed angle providing maximum flux linkage andconnected to an analog electronic integrator in order to measure the resultingmodulation of the quadrupole field.  The amplitude of the modulation was directlycompared to the quadrupole field as measured with the same coil and integratorduring the ramp of the d.c. excitation current.  The result showed that the fullfrequency range can be comfortably used.However, some additional damping might occur due to eddy currents in thethin copper layer inside the beam screen.  The measurements were therefore repeatedat an extended frequency range and at a lower temperature inside an early prototype ofthe beam screen.  This tube had an internal copper layer with a thickness of about 50microns.  The measurements were carried out, using a pair of standard search coils [5]placed on the inside wall of the beam screen.  The magnet was excited by a powerconverter of same type as those used in the LEP K-modulation scheme, including theprotection resistors which would be needed in LHC. No significant damping wasobserved in the frequency range up to 100 Hz. The useful modulation frequency willtherefore be limited only by the protection diode fitted in parallel with the quadrupolemagnet.  So the minimum modulation of 300 ppm peak-to-peak can be obtained in afrequency range up to about 40 Hz.5. Concluding remarksIt would seem useful to repeat these measurements in Test String no. 2.  Thiscould be achieved, using the same search coils which will serve for the field trackingexperiments, again placed on the inside wall of the beam screen.  The functioning ofthe proposed powering scheme, including the protection diodes could be verified andthe possible interference with the main excitation circuit could be studied.References:[1] D. Rice, G. Aharonian, K. Adams, M. Billing, G. Decker, C. Dunnam, M. Giannella,G. Jackson, R. Littauer, B. McDaniel, D. Morse, S. Peck, L. Sakazaki, J. Seeman,R. Siemann, R. Talman, "Beam diagnostic instrumentation at CESR", IEEE Trans.on Nucl. Sci., NS-30 (1983) 2190-2192.[2] I. Barnett, A. Beuret, B. Dehning, P. Galbraith, K. Henrichsen, M. Jonker,M. Placidi, R. Schmidt, L. Vos, J. Wenninger, I. Reichel, F. Tecker, "Dynamic basedbeam alignment", presented at the 6th Beam Instrumentation Workshop,Vancouver, Canada (1994).- 3 -[3] I. Barnett, A. Beuret, B. Dehning, P. Galbraith, K. Henrichsen, M. Jonker,G. Morpurgo, M. Placidi, R. Schmidt, L. Vos, J. Wenninger, I. Reichel, F. Tecker,"Dynamic beam based calibration of orbit monitors at LEP", presented at the 4thInternational Workshop on Accelerator Alignment, KEK, Japan (1995).[4] J.M. Baze, D. Cacaut, M. Chapman, J.P. Jacquemin, C. Lyraud, C. Michez, Y. Pabot,J. Perot, J.M. Rifflet, J.C. Toussaint, P. Vedrine, R. Perin, N. Siegel, T. Tortschanoff,"Design and fabrication of the Prototype Superconducting Quadrupole for theCERN LHC project", IEEE Trans. on Magnetics, vol. 28, no. 1 (MT-12) (1992) 335-337.[5] S. Bidon, J. Billan, F. Fischer, C. Sanz, "New technique of  fabrication of searchcoils for magnetic field measurement by harmonic analysis", AT-MA Internalnote 95-117, May 1995, unpublished.- 4 -

Proposal of a K-modulation system for the LHC quadrupoles

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