Alerted by impedance measurements of ferrite kicker magnets and by apparent beam induced pressure increase in the neighborhood of window frame kicker magnets, bench measurements of magnet heating have been done. They confirmed the necessity of interrupting the ferrite yoke. Another method, which can be applied for existing magnets, will be described. 1 ref., 4 figs

Lee, Y. Y.

van Asselt, W. K.

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1991.Particle Accelerator Conf.,San Francisco, CA, 5/6-9/91BEAM INDUCED HEATING OF FEEB.ITE MAGNETS*W.K. van Asselt and Y.Y. LeeAGS Department, Brookhaven National LaboratoryUpton, NY 11973BNL—45417DE91 011717ABSTRACTAlerted by impedance measurementsof ferrite kicker magnets[1] and byapparent beam induced pressure increasein the neighborhood of window framekicker magnets, bench measurements ofmagnet heating have been done. Theyconfirmed the necessity of interruptingthe ferrite yoke. Another method, whichcan be applied for existing magnets,will be described.OBSERVATIONS AND MEASUREMENTSFigure 1 shows a vacuum scan inthe AGS at the A-10 straight section,where window frame kicker magnets arelocated. The scan was taken during, ahigh intensity running period andincludes a scheduled maintenance shut-down. The horizontal line indicates theFigure 1.12Vacuum scan at A-10 straightsection in the AGS ring.*Work performed under the auspices ofChe U.S. Department of Energy.period without circulating beam. At thebeginning of that shutdown, it was seenthat the vacuum decreases by an order ofmagnitude and that it increases againwhen the AGS resumed running. Measure-ments of beam impedance of ferrite mag-nets [1] led to the assumption that theferrite is cutgassing due to beaminduced heating.To investigate this phenomenafurther, a bench measurement was set upin which an rf power amplifier (ENIA-500) was used to simulate the beam.Ferrite bricks (C2050), similar to theones in the kicker magnets, were stackedto form a one-foot long window framemagnet with 8" x 5" outside and 6" x 3"inside dimensions and the power ampli-fier was connected via a copper rodthrough the magnet into a dummy load.The transmitted current was monitoredwith a.- current transformer. The temper-ature of the ferrite bricks was measuredwith thermocouples. Figure 2 shows thetemperature of the ferrite as a functionof time after the power amplifier wasturned on for two different frequencies.The starting temperatures are slightlydifferent, reflecting that the curveswere obtained on different days. Theheating of the ferrite is seen toincrease strongly with frequency. Espe-cially the behavior at the higherfrequency explains the observed pressurerise in the vicinity of the kicker mag-net in the AGS, since Fourier analysisof the circulating beam shows that allharmonics of the rf frequency show up inthe spectra.The measurenen ts have beenrepeated for the case in which the fer-rite yoke has been interrupted by a 1 mmMf&DISTRIBUTION OF THIS DOCUMENT IS UNLIMITEDcopper sheet as a diamagnetic barrier inthe magnetic path. Even with a currentof 3.5 A at 12 MHz through the magnetfor many hours, no temperature change ofthe ferrite has been observed.HOURSFigure 2. Temperature of ferrite as afunction of time at two rffrequencies.Another way to prevent flux beingcoupled into the ferrite is to wrap ashorted coil around the ferrite yoke.Such a coil will generate a current suchthat the next flux remains zero to ful-fill the boundary condition, that therecannot be any voltage drop across theshort circuit. Actually due to thefinite resistance of such a coil, someresidual flux may flow. With R theresistance of the coil and L the induc-tance of the magnet, it can be shownthat with a current I x sin (ut) in theprimary winding, the induced current inthe coil is within the limit uL » R:I x [R/uL cos (wt) + sin (ut)].With a current of 3.5 A at 12 MHz,the experiment was repeated. A tempera-ture increase of 4°C was observed aftera period of 8 hours. Apparently someflux leaks around the short circuit.Nevertheless, since the test showed somuch improvement and since a shortedcoil can be installed so easily on theexisting magnets, it was decided toinstall shorted coils in the symmetryplane of the kicker magnets in the AGSduring the summer shutdown of 1989.Figure 3 shows a vacuum scan similar toFigure 1 during the high intensity runfollowing the installation of theshorted coils on the kicker magnets. Itis seen that there is still interactionbetween the beam and the vacuum, but itshould be noted that the operating pres-sure in the AGS improved by an order ofmagnitude, thereby significantlyincreasing the sensitivity for outgass-ing.10'Figure 3. Vacuum scan during highintensity running afterinstallation of shortedturns.The ferrite (CMD5005), which hasbeen selected for all the fast kickerFor the magnets under investigation, the magnets for the AGS Booster, has goodcontribution to the flux due to this vacuum properties and a high permeabil-effect is negligible. ity. A sample of the ferrite wasobtained and tested in a similar way.Figure 4 gives the temperature measure-ment for a current of 1 A at three dif-ferent frequencies. It is seen that inall three cases, the surface temperatureof the ferrite rises to above 120*C,which is so close to the Curie tempera-ture of the ferrite that the magneticproperties of the ferrite will havechanged drastically. Although thedimensions of the sample were different,4" x 4" outside and 2" x 2" inside, thehigher permeability leads to anincreased sensitivity for induced heat-ing.of many hours. Under the same condi-tions, a temperature rise of 1-2 degreeswas measured at the end of a 5-hourperiod for the case of a shorted coil.CONCLUSIONSTo reduce the impedance t o thebean, a diamagnet ic b a r r i e r i n windowframe f e r r i t e magnets has proven t o bevery effective and i t is shown that i tis necessary for two other reasons;namely, for maintaining the quality ofthe vacuum and for preserving the mag-netic properties of the ferrite.12010020Equipping window frame magnetswith a shorted coil, which can beexecuted rather easily on existing mag-nets, has been shown to serve the samepurpose, although not as effectively asthe diamagnetic barrier.REFERENCE[1] F. Voelker, G. Lambertson, Proc.1989 PAC, p. 851.5 c i S s tfmnmtFigure 4. Temperature measurementfor a current of 1 A atthree d i f f e r e n t rffrequencies.With the sample CMD5005 ferrite,the tests with the copper interruptionand the shorted coil have been done.With the copper interruption, no tem-perature rise has been detected with acurrent of 2 A at 8 MHz during a periodW ^ c j " " s-Sa Ililgf§|.si3 to t. - -~ 15X" "8 - 5 -llffflllflS1^ S.J.= :gf Ij

Beam induced heating of ferrite magnets

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Beam induced heating of ferrite magnets

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