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THE PROMTYPE RF SYSTEM FOR THE 200 MeV LINAC FOR THE ALTERNATING GRADIENT SYNCHROTRO!?J. Keane, R. Lankshear, J. Sheehan and R. WitkoverBrookhaven National LaboratoryUpton, New York

IntroductionThe 200 MeV proton linac presently under con-struction at Brookhaven National Laboratory is partof the program to increase the beam intensity ofthe Alternating Gradient Synchrotron. The RF sys-tem for the linac will power nine accelerator cav-ities at a frequency of 201.25 MHz with a totalpower output of over 40 MW.

Design FeaturesThe basic arrangemen t of the major RF systemcomponents is shown in Fig. 1. One complete RFstation will supply power to one accelerator tanksection. Each RF station comprises seven unitsand can be operated independently of the other RFstations. The power supply provides 60 kV 2 A viathe charge control amplifier to charge the capac-itor bank. The modulator controls the anode volt-age delivered to the 7835 power amplifier. RF griddrive power at 350 kW is provided by the driveramplifier. Control of the RF system is exertedfrom a local control station (L.C.S.) where thenecessary fault diagnosis and performance monitor-ing circuitry is located. The power amplifieroutput power of 5 Mw is delivered to the acceler-ator tank via a 12 in. 50 s2 coaxial line.Due to the large number of system componen tsreliability is a major design consideration. Moreimportant is the "availability" or percentage oftime the equipment is in an operating condition,which is stron F y influenced by the mean time torepair (MTTR). For this reason an MTTR of one hourwas the design goal placed on each unit.A 10 ton crane is installed in the RF equip-ment bay for handling purposes. Standardizationof hardware has been one of the design objectivesand system standards have been established. Thisconcept has been extended to include complete sub-assemblies as far as is practicable.

Cabinet Design and tinstructionThe charge control amplifier, modulator , cap-acitor bank, and driver are housed in RF1 shieldedcabinets. In keeping with the concept of fast re-placement , each cabinet is removable in itsentirety.High voltage compar tments of the cabinets areaccessible only after the operation of the manualgrounding switch which grounds high voltage ter-minals and allows keys to be obtained for the cab-inet doors and removal of HV connectors. RF1

*Work performed under the auspices of theU. S. Atomic Energy Commission.

gasketing is applied to all panels and doors andwindows to provide a min imum of 70 dB shieldingat 200 MHz.

High voltage interconnection of the modulesis via a 60 kV rated triaxial cable with "moldedon" terminations keyed for the particular locations.Eyelets on the top corners of the cabinetshave been arranged for lifting the fully equippedunits. (Maximum all up weight approx. 6 tons.)Blower and manifold systems provide filtered cool-ing air for components in the cabinets.

Contro 1 Sys ternThe control of an RF station is exerted froma local control station (L.C.S.) where monitor ingand measpr ing equipment will be available for faultdiagnosis and performance checking. Control of theac logic, HV logic and pulse logic systems isavailable at this location.All interconnections between the various RFsystem units will be routed via the local controlstation. Each L.C.S. will be connected directlyto the Injector Control Room (I.C.R.). Each unitwill contain its own control system enabling it tobe run and tested.

High Voltage Power SupplyThe high voltage dc is obtained from a 460 Vinput, three phase, full wave transformer-rectifierunit, rated at 60 kV, 2 A, full load output. Fullload to no load regulation is 15%. Compensatedsilicon diode str ings with 3 to 1 PIV rating areused to provide rectification. The diode stacksare designed to be removable independent of thetransformer. These transformer-rectifiers arehoused in 4 ft. x 6 ft. x 6 ft. oil-filled steeltanks. The tanks are located outside the linacbuilding in three groups of three. The ac switch-

gear, a 3-phase 460 V vacuum contactor, is locatedinside the building. The HV dc is brought fromthe units to the RF system located on the secondfloor by the means of triaxial high voltage cableswhich have factory-prepared connectors to matewith those on the equipment cabinets.Charge Control Amplifier

The high voltage dc from the transformer-rectifier is applied to the plate of a MachlettLPT32 (a high p version of the ML80411 whichserves as the series charging element for the cap-acitor bank. This unit is called the charge con-trol amplifier' and is shown in Fig. 2.

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The floating deck circuitry consists of acurrent viewing resistor, amplifiers, optical linktelemetry receivers and the LPT 32.The current reference level is adjusted by anoptical pulse width modulated telemetry link.3 A

voltage developed across the current viewing resis-tor is compared with the reference voltage, ampli-fied and applird to the LPT 32 grid, providing con-stant current charging of the capacitor bank. Theopen loop gain of the floating deck amplifiers is63.0 dB. The charging logic provides gating of thecharging current. If the charging logic is satis-fied, then a signal sent by the optical link to thefloating deck opens a shunt gate allowing theLPT 32 to conduct. Charging cant inues until thebank voltage as viewed by a voltage divider satis-fies a comparator and cuts off the telemeter signal.(10 iJs to cut off.)The optical transmitte r unit consists of aninfrared light emitting gallium arsenide diode and

its associated drive circuitry. The optical outputis transmit ted via quartz flexible fibre optics toa silicon pn photodiode detector. Electronic ad-jusL:wnt . of the current reference level provides ameans of adjusting the charging current on a pulseto pulse basis such that the full interpulse periodis used to charge the bank to the desired level. Inthis way, the min imum allowable current is alwaysused and the variations in power load on the utilityLines is greatly reduced.Capacitor Bank

The energy storage for the plate supply is pro-vided by the capacitor bank. The sizing of the bankwas such tha t the droop produced by a 400 +s, 300 Adischarge, was about 3 kV. This requires a 40 @Fbank. At 60 kV the capacitor bank stores 72 kJ. Toprotect the modulator and power amplifier tube(7835) in the event of a fault, a crowbar system isused to divert the stored energy to an energy dis-sipating resistor. If the 7835 is the source of thefault, the modulator is gated off to provide thefirst line of protection.

If the fault is in the modulator or the modu-lator is unable to protect the 7835, the crowbarsystem is used to dump the capacitor bank. Sensingof an overcurrent is performed locally by means ofa current transforn

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supply to turn off rapidly during the operation ofthe crowbar.The crowbar system is similar in constructionand design to that used in the main capacitor bankwith minor modifications, Screen modulation ofthe 4616 is provided by a 4 C X 1000 bootstrap

amplifier driver by a transistorized amplifiermounted on the floating deck.A fibre optic light link connects the floatingdeck to the ground electronics. Two amplifiershave been in daily use for the past year withlittle or no trouble. One unit is used on the RFdrive chain of the 50 MeV linac, the other as asource of RF power for test purposes in the 200MeV linac deve lopment area.Documenta tion of the driver amplifier is vir-tually complete and a contract is let for the in-

stallation of power supplies, components and wir-ing of the power supply cabinet. The delivery ofthe first unit will be in June, 1969,Power Amplifier

The final amplifier in each of the nine RFsystem utilizes an RCA 7835 super power triode withan RF cavity supplied by Continental Electronics.The tuned outpu t section of the RF cavity can bepressurized (dry nitrogen) to 40 psig. Frequencytuning is provided by remotely controlled slidingshorts with loading variation providud by adjustingthe coaxial output tap (5 in. total movement). Theprototype unit outpu t power was lilrited tc> 4.5 MW,35 kV due to insufficient loading.4Upon investigation it was found that by modify-ing the characteristics impedance and lt,ngth of

transmission line internal to the Continental cav-ity, the tube could be unloaded sufficiently. Afterthis modification, the peak output power into aresistive load was 7 MW with a plate voltage of 37kV and a plate efficiency of 60%.Filamen t power is provided by an Acme powersupply capable of delivering 7500 A at 5.5 volts.A General Electric induction regulator assures safeautomatic run-up for the 7835 filai:l~~nt. In addition,it is the center of the. regulating system which in-sures that the filament current will not vary bymore than 2 1% of its present value. Measurementand feedback signals for the regulating syetell: areobtained f rum a Daytron transducer, Contrail cir-cuits for filament run-up and safety interlock are

housed in the filament supply cubicle which pro-vides a signal routing point to the L.C.S.Transmission Line System

A twelve inch slluninum coaxial line system de-Livers power from the amplifier t;, the linac tank.Each tank is driven by coaxial loops at the l/i and3/4 points along the tank Length. Power is splitby use of a 3/4 ~1 hybrid. Thsse same hybrids areused for the phase shifter that is in each of thl:branch lines delivering power to the tank (seeFig. 5). The hybrid was developed at BrookhavenNational Laboratory. Pc;wcr splits of 3 dB + 0.1dB were achieved with isolation exceedins 40 dB.Due to higher order moding in the tee sections,the characteristic impedance normally expected fcrthe 12 in. coaxial lines in the hybrid had to bemodif ied. All four arms of the hybrid ended uphaving a characteristic impedance of 34 fi.

Preliminary measurements indicate that thecoupling loops driving the linac tank can be mount -ed on the air side of the vacuum window. Thesetests indicated that the match position for theloop will be approximately 5 in, away from the tankwall. Since the loop is completely in air themechanical design of a retractable Loctp is greatlysimplified. The penetration of the loop will br-varied in order to achieve match at various beamintensities. The present design r>f the vacuw.window uses a 12 inch ceramic disc (Coors aluminal/94?.

Designs of 12 inch elbows and dir-rctionalcouplers have been completed and high powt ti.stshave been performed satisfnctxri ly .Kefct-ences

1. MIL Handbook 21711.2. R. L. Witkover, High V;lLt-ig: C~ilaciti,r--1:aitkCharging Through a Series V~CUUIII Tube; to be pub-lished in Rev. Sci. Instr., April 1969.3. R. L. Witkover, Pulse W idth Modulated OpticalData Link, to be published in RlcNcrney, l:xp.\ rim:ntal Resu! t 5on the RCA 7835 and Initial Operating Paranwtersof the FTH 515, Proceedings of the 1966 Linc,arAccclcrator Conference, Los Alamos (LA-3609),1 * 183.

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3-PHASE440 V AC

LOCAL CONTROL STATION

I I I II I I I I I I I60 kVdc CHARGF 40gFZAMPERE - HARD-TUBECONTROL -CAPACITOR BANK-TRANSFORMER/ MODULATORTUBE AND CROWBARRECTIFIER

I0 5 mWLINAC CAVITY 4 201.25 MHz -

c!-LPT 32V-liK $

I

CCAPACITOBANK

AMPLIFIER

RF SYSTEM BLOCK DIAGRAMFig. 1. Block Diagram of System.

_--- ---- ---%%FIER GAIN: G = 63 db 1I I II 4 DRIVERAMPLIFIER - 1I DECK[REFERENCE GATED I VOLTAGE LOOPOPTICAL1 LOW LEVEL w

AMPLIFIER TELEMETER hI I , RECEIVERIERRORI---+ AMPLIFIER I

I I FIBERCURRENT LOOP OPTICSI REFERENCE INPUTIL------------J TELEMETER

, /VOLTAGEDIVIDER(5OOO:l)

VOLTAGE LOOPCHARGING OPTICAL- TELEM!ZTER JTRANSMITTER

REFERENCE EXTERNAL LOGICINPUTSCHARGE CONTROL FUNCTIONAL BLOCK DIAGRAM

Fig. 2. Charge Control Amplifier .

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I 0 OUTPUT PULSE

FIBER OPTICSRECEIVER NSMITTER

POWERSUPPLY+ ON-OFFINPUTSIGNAL

RF MODULATORFig. 3. Modulator.

RFOUTPUTI 1 I/ 0~ES M p,t~n~!nD CROWBAR

I Ir---tCR r kVCONTROLLER PS1I

I

350 kW FIBRE OPTIC

7651

1 kVPS

I7651

I

4AC INPUT

d8w 200MHz INPUTBLOCK DIAGRAM OF DRIVER AMPLIFIER

Fig. 4. Driver.

v UT tto=/ %P qJ=l - W,N t&g

10~

/ral z&t-- --- ------ -_- __

qy;/ ii&& zo= t- &N I- 902

Fig. 5A. Variable Phase Shifter. Fig. 533. Co-Ijirectional Coupler.

355