One of the goals of the low energy 60 MeV section of the HINS H{sup -} linac [1] is to demonstrate that a total of {approx}40 RF cavities can be powered by a single 2.5 MW, 325 MHz klystron. This requires individual vector modulators at the input of each RF cavity to independently adjust the amplitude and phase of the RF input signal during the 3.5 ms RF pulse. Two versions of vector modulators have been developed; a 500 kW device for the radiofrequency quadrupole (RFQ) and a 75 kW modulator for the RF cavities. High power tests showing the vector modulator phase and amplitude responses will be presented

Madrak, Robyn Leigh

Wildman, David

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HIGH POWER 325 MHZ VECTOR MODULATORS FOR THE FERMILABHIGH INTENSITY NEUTRINO SOURCE (HINS)*Robyn Leigh Madrak and David WildmanFermilab, Batavia, IL 60510, U.S.A.AbstractOne of the goals of the low energy 60 MeV section ofthe HINS H- linac [1] is to demonstrate that a total of ~40RF cavities can be powered by a single 2.5 MW, 325 MHzklystron.  This requires individual vector modulators atthe input of each RF cavity to independently adjust theamplitude and phase of the RF input signal during the 3.5ms RF pulse.  Two versions of vector modulators havebeen developed; a 500 kW device for the radiofrequencyquadrupole (RFQ) and a 75 kW modulator for the RFcavities.  High power tests showing the vector modulatorphase and amplitude responses will be presented.                       INTRODUCTIONThe first high power microwave phase shifters usingferrite-loaded coaxial structures were first described morethan forty-five years ago [2,3].  Recently, there has beenrenewed interest in the field with advances in technologyand materials.  The possibility of powering multiple RFcavities from a single RF source has become a viablealternative to the traditional single RF source per cavity[4,5].  At Fermilab we are in the process of building a 60MeV H- linac to be used as the front end of a future HighIntensity Neutrino Source (HINS).  The linac is designedto accelerate 20 mA of beam current in either 1 ms pulsesat a 10 Hz rate or 3 ms pulses at a 2.5 Hz rate.  The first30 MeV section of this new linac will have a 2.5 MeVRFQ, two bunching cavities, 16 room temperature CHtype (3 & 4 spoke) cavities, and 18 superconductingsingle spoke cavities all being driven by a single 2.5 MW,325 MHz klystron (Toshiba E3740AFermi.)  Here therewill be one vector modulator between the klystron andeach cavity to independently control the phase andamplitude of the RF drive signal to its coupling loop.  Forthe room temperature cavities these vector modulatorswill serve two purposes.  First, they will be used to correctfor cavity tuning errors caused by cavity heating andcooling water temperature fluctuations. Second, they willprovide a means of compensating the effects of beamloading.  To meet these goals we have developed twoversions of high power  vector modulators using fastferrite loaded coaxial phase shifters.  A 75 kW versionwill be used for all of the room temperature RF cavities.The RFQ which requires a larger 500 kW model.VECTOR MODULATORSA photograph of one of the 75 kW vector modulators isshown in Figure 1.  The 325 MHz signal from theklystron enters port 1 of a –3 dB quad hybrid (DielectricCommunications) where it is equally split with a 90°phase difference between ports 2 and 3.  Two shorted1.625!  OD coaxial, ferrite-loaded phase shifters,described below, are attached to ports 2 and 3 and providefull reflections with phase shifts "2 and "3 respectively.The desired phase shift is produced by a variablesolenoidal magnetic field along the axis of the coaxial linewhich is used to adjust the permeability, µ, of the ferrite.These reflected signals are recombined at the output (port4) of the quad hybrid with a resulting output phase of ("2+ "3)/2 and power proportional to cos2 ("2 - "3)/2.   A 3port 75 kW circulator (D & M Co. Ferrit-Quasar) with a 5kW water cooled RF load (Altronic) is connected betweenthe quad hybrid output and the RF cavity to isolate thephase shifters from any power being reflected from thecavity during the cavity filling time or under cavitydetuning conditions.The higher power vector modulator to be used with theRFQ is similar to the one described above except that thecomponents are physically larger.  The –3 dB quad hybrid(Micro Communications) has a coaxial design with 6!EIA flanges and is filled with SF6 to prevent sparking.The two phase shifters are 3.125! OD, ferrite-loaded,shorted coaxial lines and are also filled with SF6.  A highpower 3 port coaxial/stripline circulator (D & M Co.Ferrit-Quasar) is used to prevent the reflection of powerback to the phase shifters.Figure1: Photograph of a 75 kW vector modulator.* Operated by Fermi Research Alliance, LLC under contractwith the U.S. Department of EnergyFERMILAB-CONF-08-383-APCFigure 2: Phase shifter attenuation and phase shift vs.internal bias field, H, inside the garnet cores.Figure 3: Time response of the 75 kW phase shifterbiasing solenoid current (CH1 @ 50A/div.) and reflectedphase (CH3 @ ~30 deg./div.) to a fast step (risetime< 1µs) in the current program.  The time scale is 20µs/div.PHASE SHIFTERSThe phase shifters are OFHC copper shorted coaxiallines filled with a 5! long section of aluminum dopedyttrium-iron garnet (TCI Ceramics type AL-400) whichhas a saturation magnetization (4"Ms) of 400 gauss.  The75 kW version uses a 1.5! OD x 0.65! ID x 5! long garnetcylinder at the shorted end over a 0.65! diameter centerconductor.  The center conductor diameter is thenincreased to 0.880! for a quarter of a wavelength (9.08!)to form a Z0 = 33 # matching section between the garnetfilled region and the standard Z0 = 50# port of the quadhybrid.  The OFHC copper center conductor/garnetcylinder is assembled using a shrink fit technique wherethe center conductor is first cooled to LN2 temperature andthen inserted into the garnet cylinder.  The phase shiftercoaxial line outer conductor is a standard 1.625! OD, 50#line (1.527! ID) whose OD has been reduced to 1.567!over the 9! length closest to the short.  Both this 9!section and the bottom copper shorting plate have a0.0197! wide slot machined through the copper to reduceeddy current effects.  The shorted end of the phase shiftercontaining the garnet cylinder is surrounded by a solenoidwound from 47 turns of 12 AWG stranded copper THHNwire on a 1.812! OD G10 form.  A magnetic flux returnmade from 0.75! thick ferrite blocks (TCI Ceramicsmaterial G4) helps to increase the phase shifter frequencyresponse.The larger version phase shifter uses a 3.0! OD x 0.65!ID x 5! long AL-400 garnet cylinder along with a Z0 =46# matching section. Here also, the outer conductor hasa 0.020!  thick wall and a 9! long slot. The biasingsolenoid is 62 turns of 12 AWG wire on a 3.316! OD G10form with a G4 ferrite flux return.  Each solenoid isindependently powered by a 300V, 300A, 2 quadrantswitching supply with a switching frequency of 250 kHz.A Praeg [6] style filter on the supply output, currently setto roll off at 40 kHz, is necessary to reduce the 250 kHzripple that would otherwise be seen on the vectormodulator outputs.TUNING RANGEFigure 2 shows a low power measurement of the roundtrip phase shifts and RF losses at 325 MHz for the 1.625!OD and 3.125! OD shifters plotted as a function of theinternal biasing field, H.  The maximum solenoid currentfor both cases was 300A during the 4ms pulse. The abruptstep in phase shift corresponds to the garnets goingthrough gyromagnetic resonance.  In Figure 2 the scale ofH is normalized so that the gyromagnetic resonance at325 MHz corresponds to the theoretical value of 2.8MHz/oersted or H = 116 oersted.  Below and at resonancethe RF losses at 325 MHz are large which makes thisregion unsuitable for high power operation.  However,above H ~ 200 oersted RF losses drop below -0.2 dB(typically < -0.1 dB) and high power operation becomespractical.  Using –0.2 dB loss as an acceptable figure, theuseable phase shift range is seen to be ~120 degrees.FREQUENCY RESPONSEFigure 3 is a scope trace showing the low power phaseshifter response to a fast step used as the input program (0to 300A) to the solenoid pulsed power supply.  The rate ofrise of the solenoid current is seen to be limited by the300V maximum output of the supply and the ~45 µHinductance of the solenoid coil.  The measured phase shiftwas obtained with an HP 86205A directional bridge andHP 10514A  mixer, used as a phase detector at 325 MHz,comparing the input and reflected RF signals.  Aboveresonance the average phase slew rate is ~ 6°/µs.The small signal (+/- 10°) frequency response of the1.625! OD and 3.125! OD shifters, centered at midrange,is shown in Figure 4.  The open loop case (triangles) canbe compared to a case with feedback (squares).  Thefeedback error signal was obtained by comparing themeasured reflected phase to a reference.  The error signalwas then amplified and summed back into the biassolenoid current program.  The 1.625! OD shifter openloop bandwidth is seen to be ~ 15 kHz which wasextended to > 35 kHz with the phase feedback.Figure 4: Phase shifter frequency responses with andwithout feedback.        HIGH POWER OPERATIONFigure 5 illustrates the amplitude modulation capabilityof the 75 kW vector modulator during a 2 ms RF pulsewithout any phase feedback.  With a constant 50 kW inputpower to the vector modulator, the output power isstepped from 25kW ( "2 - "3 = 90°) to the full 50 kW ("2- "3 = 0°).  During the first portion of the pulse, the 25kW of unwanted power is directed back towards thecirculator at the output of the 2.5 MW klystron where it isabsorbed in the circulator load.Both the 75kW and 500 kW phase shifters havesuccessfully operated at their design power levels alongwith the 75 kW circulators.  However; we haveencountered some breakdown problems at the ~ 400 kWpower level in the larger circulator.  This circulator iscurrently being modified to reach the full 500 kW level.Figure 5: Vector modulator 325 MHz RF outputamplitude being increased from 25 kW to 50 kW during a2 ms pulse without any phase feedback.  The time scale is400µs/div.ACKNOWLEDGEMENTS   The authors would like to thank the FermilabAccelerator Division EE Support Group, specifically,B. Claypool. S. Hays, and H. Pfeffer for providing thesolenoid pulsed power supplies used in this work.REFERENCES[1] SCRF Linac Proton Driver Study Group, “An 8 GeVSCRF Linac Proton Driver,”http://protondriver.fnal.gov/#Technical_Design_Link[2]  A.S. Boxer, S. Hershenov, and E.F. Landry, IRETrans. Microwave Theory Tech. 9 (1961) 577.[3] A. S. Boxer and R.S. McCarter, J. Appl. Phys. 33(1962) 1263.[4]  R. Madrak et. al, “ New Materials and Designs forHigh-Power, Fast Phase Shifters,” LINAC 2006,Knoxville, FR2003, p.829 (2006).[5]   J.L. Wilson et. al, “ Development of High –Power RFVector Modulator Employing TEM Ferrite PhaseShifters,” LINAC 2006, Knoxville, TUP083, p.451(2006).[6] W.F. Praeg, IEEE Trans. Industrial Electronics andControl Inst., IBECI-1 (1970) 16.

High power 325 MHz vector modulators for the Fermilab High Intensity Neutrino Source (HINS)

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High power 325 MHz vector modulators for the Fermilab High Intensity Neutrino Source (HINS)

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