Crescenti, M

Primadei, G

Susini, A

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHORGANISATION EUROPEENNE POUR LA RECHERCHE NUCLEAIRECERN - PS DIVISIONPS/DI/Note 99- 14 (Tech.)PRELIMINARY DESIGN OF THE RF AMPLIFIER FORTERA/PIMMS  MEDICAL SYNCHROTRONM. Crescenti1, G. Primadei2 and A. Susini2AbstractA single gap push-pull RF cavity is designed to accelerate the therapeutic beams in theTERA/PIMMS synchrotron. In this note the RF amplifier, which is foreseen to power thecavity, is described. The main part consists of two tetrodes tubes operating in push-pull.The power to be installed is about 2x50 kW, and the power gain is more than 13 db. Theinput signal comes from a commercial RF pre-amplifier.The actual status of the mechanical design, of the electrical set-up and of the interlockcontrol system is presented. The first two items are in an advanced design stage. Thelatter exists and is fully operational.Geneva, Switzerland3 June 19991  M. Crescenti is a member of the TERA Foundation and Unpaid Associate of the CERN EP Division,currently working on the Proton Ion Medical Machine Study (PIMMS) hosted by the CERN PS Division.2 G. Primadei and A. Susini are consultants to the TERA Foundation and Unpaid Associates of the CERNEP Division.open-2000-16610/Aug/20001INTRODUCTIONThe final design of the RF cavity is documented in reference [1]. Th  design of theamplifier follows the specifications and constraints already established by the RF cavity.In particular, the maximum available space is limited by the dimensions of the cavity andby the height above the floor of the nominal beam line, that has been fixed as 1.2 m.Two coaxial push-pull operated transmission lines with a single gap in the middleconstitute the resonator. The power to be installed is about 2x50 kW.The tetrode tubes SIEMENS RS 1084 have been chosen due to their availability at CERN.Indeed, some tubes can be borrowed for low power tests for a limited time. The datasheets can be found in reference [2]. The electrical circuitry is in agreement with the operating characteristics recommendedby the manufacturer. The configuration is grounded cathode. The operation mode is classA. Despite low efficiency, this mode has the advantage to produce low distortion. Lowdistortion is needed because the resonator has low Q (see reference [3]), and a yharmonic distortion introduced by the amplifier is found almost unattenuated at the gapvoltage. A two-channel 2x500 W commercial pre-amplifier available at CERN gives theRF input to the control grid. For a 5 kW (2x2.5) peak gap voltage on 300 a impedance,the gain of the amplifier is about 13 db.The controls and interlock system is based on a Programmable Logic Controller (PLC).The start-up procedure is automatic, but can be manual for specialist checks. The systemhas been developed in collaboration with a commercial firm3 and is fully operational.Since the allowable volume is quite small (less than 0.9 m3), the mechanical design has todeal with a precise distribution of the space, to allow location of all the elements. Thevolume has been separated in three functional zones, the accesses have been carefullystudied in order to minimise the efforts for assembly and maintenance, and particular carehas been given to the choice of the components. The result is a compact, robust andfunctional assembly. The conceptual design is completed, whereas some details have notyet been fixed.1. ELECTRICAL SET-UPThe electrical configuration of the power amplifier is given in Figure 1. To increase thelifetime of the tube, thermal stresses to the filament when switching on and off have to bereduced. Since the filament resistance increases with temperature, the voltage increasehas to be slow at start-up, and then progressively increased to reach the voltage workingpoint. Vice-versa at shut down. This is obtained by controlling the voltage at the primaryof the filament transformer by a two speed motorised VARIAC.3   C.M.A. Electronics Sas, Via  dell’Artigiano 8/2 I-40016 San Giorgio di Piano (BO).An anti-hum system is installed connecting directly the positive of the grid to the cathodevia a variable resistor. RF input is fed to the grid via a matched low-pass filter, with abandwidth of 0.2-10 MHz. The 50 IR resistive matching load is water cooled, allowing a1 kW maximum dissipation.2. CONTROLS, INTERLOCKS AND START-UP PROCEDUREIn Figure 2 a global view of the controls and interlocks is presented, with each powertube belonging to a control and interlock chain. The system is completely automatic. Thestart-up procedure is divided in three levels. Level one starts all the services: coolingwater, fan, and control grid power supply. There is then a one-minute wait to allow theworking conditions to be properly established. Then level two starts and the anode andscreen power supplies are switched on. For tube protection the screen power on isdelayed by a few seconds. Finally level three gives the RF input to the power tubes.A PLC controls the whole system. The procedure can be automatically started by asimple push button by the operator, or can be manual. In this case, the operator canfollow and check level by level on the PLC screen the set-up of the system.The interlock system is also automatically set. If there is a fault, it appears directly on thescreen for an immediate check and recovery. If the fault refers to an element belonging tolevel two or three, the services of level one still remain in function. If the fault appears onlevel one and is not recovered within one minute, all the procedure has to be restarted.In Figures 3, 4, and 5 the procedure is explained with some detail.3. MECHANICSThe base support for the cavity is shown in Figure 6. The 80x80 mm iron bars form theskeleton. Two lateral bars and another bar at the rear are added, to give sufficientstiffness to sustain the weight, estimated to about 600 kg. The spatial positioning withrespect to the beam axis is not too critical for the RF cavity. A precision in position of+l mm in the three axes is sufficient.Externally, the amplifier appears as a closed structure on wheels, to be inserted below thecavity. Internally it is divided in three parts: the RF well, the bottom well and the rearemplacement.Once in position the structure is lifted and then fixed by special hooks to keep it incontact with the bottom part of the base plate of the cavity. The ground return is insuredby means of special pressure RF contacts via a thin copper layer._ The RF well (see Figure 7).This contains the power tubes, the tuning capacitors and the water tube connections. Toguarantee the confinement of the electromagnetic field inside the cabinet and a controlledpath of the anode current to ground, it is important to have a closed cabinet for the RFwell.A silvered brass cover enclosing the anode and fixed to it ensures the HV contact. Theupper contact with the blocking capacitor plate is made with special elastic RF contacts.The total blocking capacitance is obtained with twelve capacitors in parallel for a total ofabout 25 nF for each tube.Two tuning capacitors (Jennings) are fixed between the two anodes. The capacitance isvaried by two helipot-like turn-buttons on the front panel.The cooling water comes from the distribution system. The needs per tube are about35 l/min for an inlet temperature of 35°C at 5 bar, see Figure 8. Additional space has tobe allocated for the water tubes and connections. The input and output tube length isabout 1 m to insure high impedance (the water conductivity should be better than100 ksWcm).The bottom well.This locates the air-cooled socket. The required flow per tube is 0.6 m3/min at 1.5 mbar.This flow is generated by a fan and directed to the socket from the bottom.Beneath the socket there is a variable rheostat for hum suppression, an RF filter for thecontrol grid RF input and a 50 Q water cooled matching load.The rear emplacement.All the services are placed here. There is room for the filament transformer, the motorisedVARIAC, the fan, a box containing the sockets for interlocks and controls, and a box forthe main power distribution. The HV cables come directly on the rear side of the cavity,via a low-pass filter. In Table 1 the list of the fundamental elements is presented.Table 1. List of the fundamental elements composing the RF amplifierElement Num. Type Dimensions x,y,z UseRheostat 2 LCC 1OQ L=lOO, 0=80 Hum suppressionFilament transformer 2 Step down 220-12SV, 2SkW 220x240~200 Power filamentRF power tube 2 SIEMENS RS 1084 CJ, TetrodeH=400,0=200 RF power generatorSocket 2 SIEMENS Q81-X-1852 H=lOO, PI=265 Power tube supportFan l(2) MARELLI 350X350X350 Air cooling filamentVARIAC 2 H=2OO, 0=150 Filament voltage controllerMotor 2 24Vdc 1 turn/mm L=140,0=14O VARIAC controlResistive load 2 BIRD 5On 1kW water cooled L=120,0=15 Control grid matching load4CONCLUSIONThe mechanical design and the electrical set-up design of the RF amplifier are almostcompleted. The controls and interlock system has been designed, developed, and is nowoperational. The final design and the practical mechanical and electrical construction arestrictly connected and need now to proceed in parallel.ACKNOWLEDGEMENTSMany thanks go to CERN EP division who gave a laboratory to host the hardware and toCERN PS division who gave instrumentation for the measurements and hosted PIMMSstudy. A thank goes also to Philip Bryant, head of PIMMS project, who revised themanuscript.REFERENCES[l] M. Crescenti, G. Primadei, and A. Susini. Mechanical Design of the RF Cavity forTERA/PIMMS Synchrotron,  to be published.[2] SIEMENS.  Data Book Transmitting Tubes, Edition 1996 pp.l0l-110.[3] M. Crescenti, G. Primadei, and A. Susini. A New Compact Large Frequency-SwingRF System for Hadron Acceleration: Test Results, CERN/PS 97-60 (DI).Distribution :PIMMS Study Team !I+; --1III----6RF controls & interlocks, global viewwater cavity emperature & limitwater temperature reference (common)cavitywaterflow interlockwaterflow interlockairflow interlockwater temperature & limitheating filament interlockVgl interlock (bias)Vg2 interlockVa interlockblower 10R.F. to resonatorIpower amplifier 11 4I I R.F. to amplifierspower amplifier 2heat exchangerwaterflow interlock \airflow interlock,water temperature & limit.heating filament interlockVgl interlock (bias)Vg2 interlock--.Va interlock-.I Idriver amplifier 42xlkW380 V main power supplypower distributionPLCN.B. all interlocks are free contacts normally open)all temperatures measured with PT100 3 wiresand A / D converter inside PLCb1 ’_--Figure 2. Global view of controls and interlocksstartPlevel 1services on/offR.F. cavity: main start-up procedureNOh wait 60 sec.NO) YESI YESTERA RFG.P.&M.C. 16/6/98Figure 3. RF cavity main start-up procedure8LEVEL 1 on/offlogic chain interlocksTERA RFG.P. & M.C. 16/6/98draw water interlockprimary heat exchangerwater flow interlock ampli 1water flow interlock ampli 2air flow interlock ampli 1air flow interlock ampli 2Vgl interlock ampli 1Vgl interlock ampli 2water temperature reference & limitheating filament interlock ampli 1heating filament interlock ampli 2:< wait 60 sec. then closeready for level 2ILEVEL 2 on/offVanode interlock ampli 1Vanode interlock ampli 2Vg2 interlock ampli 1Vg2 interlock ampli 2overcurrent anode ampli 1overcurrent anode ampli 2ready for level 3LEVEL 3 on/offwater temperature cavitywater temperature ampli 1water temperature ampli 2water cavity interlockvacuum cavity interlockready for R.F.Figure 4. Logic chain interlocks sequence9TERA RFG.P. & M.C. 16/6/98Controls : schematic diagramLEVEL 1OFF ON0,0,L-LEVEL 2heating filament 1heating filament 2blower 1blower 2water pumpVgl ampli 1Vgl ampli 2LEVEL 3 ON / OFF- -is automatically controlled by the R.F. pin switchON / OFF- -  from R.F. generatorVanode power supply (common)Vg2 ampli 1Vg2 ampli 2I bias power supply cavityFigure 5. Controls: schematic diagram --I-tIII.L----___-_-_------22 __I-rj: _-_-_---_- --.-__--ok-i_it\1_’._1\. . ..-\-. T - -- _.*__-- -.- -_“.T_- - - -\>_: . . ._... ._7--.. .____I12IN$epralised-I I I 1 spareIN -60 I/min4OUTTo To magnetic!ower modulesubesfigure 8. Scheme of the water cooling circuit (see Ref. [l])1 Heat exchanger type PO1 ALFALAVAL 1 0 0 k W2 Pump 120 Iimin3 Vessel 2006 bars (iron steel)4 Flowmeter switch type BIBER (WISAG)5 Strainer6 Demineraliser cartridge7 Pressure reducer8 Temperature flow control (DANFOSS)  only if town water is used9 Check valve10 Regulotion valve11 Ball volve (on-off)12 By-poss volve13 Temperature controller PT100/0 deg. (3 wires)14 Water pressure controllerFrom Fromower ma neticF !ubes mo ules

Preliminary design of the RF amplifier for TERA/PIMMS medical synchrotron

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