A proton and light-ion medical synchrotron is characterised by a large frequency swing for the RF between the injection and the top energy. For this purpose, a VITROVAC®-loaded RF cavity has been developed for the Proton-Ion Medical Machine Study (PIMMS) at CERN, and for TERA, the Italian project of a proton and light-ion synchrotron for cancer therapy, based on the PIMMS study. The main features are a large frequency swing, particularly extended to the low frequency range, a very large relative permeability and a low Q factor. The total power needed is less than 100 kW, while a very small bias power is required for the frequency tuning. The main mechanical characteristics are compactness (less than 1.5 m), and simplicity of construction. As a result, the requirements of the medical synchrotron are comfortably satisfied, namely: 0.4 to 3 MHz swing, 3 kV peak voltage at a repetition rate of less than 1 s

Crescenti, M

Etzkorn, F J

Fougeron, C

Primadei, G

Schnase, A

Susini, A

Etzkorn, FJ

CERN Document Server

The Vitrovac® Cavity for the TERA/PIMMS Medical Synchrotron

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN – PS DIVISIONCERN-PS-2000-031 (DR)THE VITROVAC® CAVITY FOR THE TERA/PIMMS MEDICALSYNCHROTRONM. Crescenti, TERA FoundationA. Susini, G. Primadei, Consultants to TERA Foundation.F.J. Etzkorn, A. Schnase, Forschungszentrum JülichC. Fougeron, LNS-CEA SaclayAbstractA proton and light-ion medical synchrotron is characterised by a large frequency swingfor the RF between the injection and the top energy. For this purpose, a VITROVAC®-loaded RF cavity has been developed for the Proton-Ion Medical Machine Study(PIMMS) at CERN, and for TERA, the Italian project of a proton and light-ionsynchrotron for cancer therapy, based on the PIMMS study. The main features are a largefrequency swing, particularly extended to the low frequency range, a very large relativepermeability and a low Q factor. The total power needed is less than 100 kW, while avery small bias power is required for the frequency tuning. The main mechanicalcharacteristics are compactness (less than 1.5 m), and simplicity of construction. As aresult, the requirements of the medical synchrotron are comfortably satisfied, namely: 0.4to 3 MHz swing, 3 kV peak voltage at a repetition rate of less than 1 s.7th European Particle Accelerator Conference, 26th-30th June 2000, Vienna, AustriaAlso available on WWW, http://preprints.cern.chGeneva, Switzerland17 July 2000THE VITROVAC®1 CAVITY FOR THE TERA/PIMMS MEDICALSYNCHROTRONM.Crescenti, TERA Foundation, Via Puccini 11, I-28100 Novara, CERN, Geneva,Switzerland.A.Susini, G.Primadei, Consultants to TERA Foundation.F.J.Etzkorn A.Schnase, Forschungszentrum Jülich Postfach 1913, D-52495 Jülich.C.Fougeron, LNS-CEA Saclay, F-91191 Gif-sur-Yvette Cedex.                                                          1 Produced by: VACUUMSCHMELZE GmbH, Grüner Weg 37, P.O. Box 2253, 6450 Hanau 1, Germany.AbstractA proton and light-ion medical synchrotron ischaracterised by a large frequency swing for the RFbetween the injection and the top energy. For thispurpose, a VITROVAC®-loaded RF cavity has beendeveloped for the Proton-Ion Medical Machine Study(PIMMS) at CERN, and for TERA, the Italian projectof a proton and light-ion synchrotron for cancertherapy, based on the PIMMS study. The mainfeatures are a large frequency swing, particularlyextended to the low frequency range, a very largerelative permeability and a low Q factor. The totalpower needed is less than 100 kW, while a very smallbias power is required for the frequency tuning. Themain mechanical characteristics are compactness (lessthan 1.5 m), and simplicity of construction. As aresult, the requirements of the medical synchrotron arecomfortably satisfied, namely: 0.4 to 3 MHz swing,3 kV peak voltage at a repetition rate of less than 1 s.1 INTRODUCTIONThe required frequency swing for acceleration insynchrotrons is usually obtained by RF cavities filledwith ferrite. The frequency variation is obtained byvariation of the inductance of the ferrite by a biasfield. The alternatives are amorphous metals or nano-cristalline materials. A cavity filled with suchmaterials keeps in tune with the driving frequencywith no or just a small bias current. Instead, hundredsof amperes are required for ferrite. The advantages aresmall length, large frequency swing, simplemechanical design and a simplified control system. Adisadvantage, if very large gap voltages are required,is the low shunt impedance: some hundreds of :against k: for ferrite. The TERA/PIMMS medicalsynchrotron requires 3 kV peak voltage and a 0.4 - 3 MHz frequency variation [1]. Due to the lowinjection frequency, it is difficult to find a ferrite ableto cover the full swing [2]. Therefore, to avoidharmonic number changes during acceleration, twocavities are needed, with a more than 3 m ringoccupation. One cavity filled with a material able tocover the full swing would occupy only 1.5 m. In1996 the LNS experimented VITROVAC 6025, anamorphous metal Co-based alloy produced byVACUUMSCHMELZE. In low power tests a swing of 0.2 to10 MHz has been reached. In high power tests a 1.5 kV peakvoltage was obtained at 1 MHz. In 1997, TERA acquired theprototype and continued the experiments at CERN. A2x50 kW amplifier from COSY was modified by TERA byimproving the shielding and developing new RF filters. Theinstallation is shown in Fig. 1.Fig.1 Set-up of the system in the LNS early version, with the2x50 kW COSY amplifier based on Thomson TH120 tetrodes.2 THE MATERIALIn Table 1 the main parameters of the material are given. It isdelivered as a 25 Pm thin ribbon wound in the shape of atoroidal core. Each turn is electrically insulated from the nextone by a 0.2 Pm thin film. The maximum permeability is 105at very low frequency, and is 1.5*103 at 10 MHz. Thesensitivity to the bias field is high: 0 - 50 A for 0.2 - 10 MHz.A water cooling copper ring is sandwiched between twocores, forming a so-called magnetic module. The veryefficient cooling assembly, untouched from the originalstructure, can dissipate up to 1.35 kW per core.Table 1 Main parameters of VITROVAC 6025F cores.Disk internal diameter [mm] 355Disk external diameter [mm] 510Thickness [mm] 25Pr at 0.2 MHz 3*104Pr at 10 MHz 1*1033 MECHANICAL DESIGN OF THECAVITYThe LNS structure has been modified and completed,(see Fig.2), to allow high power operation, which wasnot possible before. The new shape simplifies theinternal access and the coupling with the amplifier.Fig.2 Cavity technical drawings. Top: longitudinalsection of the assembly. Bottom: schematic view of thevacuum pipe. The gap (1), the bellows (2) the copperenvelope (3), the vacuum pipe (4), a magnetic modulesection (5) and the support plates (6) of the biaswindings (not represented).Two face-to-face O /4 transmission lines operate inpush-pull. Resonance at the lowest frequency is set bya 130 pF capacitor. 12 magnetic modules are alignedaround the vacuum pipe, and space is left for other 2modules. 14 modules are represented in Fig.2. Twobellows (2) protect the ceramic gap (1) frommechanical stresses. To avoid heating the bellows, theRF current path to ground is via a thin copper line (3)surrounding the vacuum pipe (4). Sliding contactsallow thermal dilatation between the ceramic gap andthe copper line. The magnetic modules are biased byfour, 4 mm diameter, figure-of-eight windings,repositioned from the original design to avoidbreakdowns, and kept in place by four polypropyleneinsulating plates (6). The measured shunt impedance isshown in Fig. 3. The Q factor has been also measuredand is of the order of one. For a 0.4-3MHz swing, thebias current is less than 20 A. The main parameters ofthe cavity are listed in Table 2.Table 2 Main parameters of the cavityNumber of disks 2x12Total cavity length [m] 1.5Shunt impedance [:] (0.4 MHz) 2x400Q factor 1Frequency swing [MHz] (harmonic number=1) 0.2-10DC bias current [A] (4 windings, 0.4-3 MHz) 1-184 MEASUREMENTSThe requirements of the medical synchrotron, are presentedin Table 3 [3].Shunt Impedance of Half a Resonator0.250.30.350.40.450.5 1 1.5 2 2.5 3 3.5 4Frequency [MHz]Re(Z) [k:] (Im(Z)=0) I bias = 1.8AI bias = 18AFig.3 0.5-4MHz shunt impedance of half a resonatormeasured with HP4815A vector impedance meter. The cavityimpedance is twice this value.Table3 Basic parameters for proton and 12C6+ acceleration.Injection Extractionp + 12C6+ p+ 12C6+Number of particles 6.8x1010 7.9x108 6.8x1010 7.9x108Energy [MeV/u] 20 7 250 400Rev. frequency [MHz] 0.81 0.486 2.445 2.85p + 12C6+Max. RF voltage [kV] 2.9 2.7Max. phase [deg] 16.9 20.6RF harmonic number h 1 1Frequency swing 3 5.9Max. tuning rate [MHz/s] 10.4 6.0Min. time for RF cycle [ms] 167.6 402.4Min. trapping voltage [V] 29 35Max. trapping voltage [V] 290 350The tests have been performed without beam. The schematicset-up is presented in Fig.4. The low–level signal generationis based on the digital function generation techniquesdeveloped at COSY [3]. The computer sets the frequencyword, which is fed to a numerical oscillator (NCO) and aDAC outputs the driving frequency at constant amplitude.The signal is split into two 180º phase shifted signals, whichare the inputs to a 2x500 W commercial driver amplifier. A12-bit DAC 0 – 10 V is programmed to output the requiredvoltage for pre-regulation of the bias current. The voltagefollows a cubic law set by interpolation of the fixedfrequency values at resonance. A feedback loop then controlsthe final tuning voltage. Another feedback loop sets thevoltage amplitude at the gap with respect to a programmablereference value. The gap signal is measured with a capacitivedivider (1:1000). The 0.4 - 4 MHz fixed frequencymeasurements show a signal-to-noise ratio better than 60 dBat the accelerating gap when the system is in tune. This istrue for high voltage (3 kV) as well as for low voltage levels(~30 to 300 V) required for beam trapping. In Fig.5 ispresented an example of the tested RF cycles. The first signalrepresents the tuning current to the cavity from the biaspower supply. The second represents the voltage at the gap inkV. The voltage reaches 3 kV before the end of the sweep, asrequired by the theoretical RF cycle.splitterDAC2x500Wsolid state2x50kWtetrode TH120NCOV refPhasecontrol+-GridGridGap GapFeed-back Feed-backI biasFig.4 Scheme of the measurement set-up.Accelerating sweeps below 100 ms can be simulatedwith a repetition rate below 0.5 sec. The phasestability between gap and grid of the power tubes isbetter than 2q. The maximum peak voltage obtainedfor a sweep of 0.4 – 4 MHz is 4 kV.0123450 200 400 600 800 1000Time [ms][V]I bias [1div = 4A]Gap voltage [kV]Fig.5 Example of a 0.4 - 4 MHz frequency sweep, witha sweep time of 170 ms. The large signal representsthe bias current (1 V = 4 A), the other represents thegap voltage in kV.5 CONSTRUCTION OF THE NEWAMPLIFIERAt the end of 1999 the amplifier was returned toCOSY and TERA started the construction of a newamplifier. The new design is based on 70 kWSIEMENS 1084 tubes. Two symmetric, independentmodules will be installed below the resonator. Theconfiguration, presented in Fig.6, is a groundedcathode, class A, push-pull operated. Despite lowefficiency, this mode produces low distortion. Lowdistortion is needed because there is no filtering by thewide bandwidth resonator. The noise from the tubeheating is reduced by optimising the control gridworking point via a variable resistor. To increase tubelifetime, thermal stresses to the filament at switch onand off are reduced by slowly reaching the voltageworking point with a motorised VARIAC. In Fig.7one amplifier module is shown during construction.The performances expected from the new amplifier areof 0.3 – 10 MHz bandwidth and 8 kV peak voltage.Fig.6 Configuration of the new amplifier module based onthe SIEMENS RS 1084 tetrode.Fig.7 The amplifier module during construction. On the rightthe RS 1084 power tube mounted on its socket, on the left themotorised VARIAC and an air-cooler fan. Dimensions: width530, height 690, depth 720 mm.6 CONCLUSIONThe cavity satisfies all the requirements of the TERA/PIMMSRF cycle. The construction of the new amplifier is to befinalised. Installation and tests are planned in summer 2000. Iffinancing continues, the RF system can be commissioned forthe end of the year 2000.ACKNOWLEDGEMENTSMany thanks are due to CERN/EP for providing the space forthe laboratory, and to CERN/PS for the instrumentation loan.References[1] Accelerator Complex Study Group supported by the Med-AUSTRON,Onkologie-2000 and the TERA Foundation and hosted by CERN, Proton-Ion Medical Machine Study (PIMMS) Part I, CERN/PS 99-010 (DI)(December 1999), Part II in preparation.[2] A.Schnase, H.Rongen, H.Meuth, A Digital Synthesizer and PhaseControl System for RF Acceleration in COSY, EPAC 1992.[3] M. Crescenti, P. Knaus, S. Rossi, The RF Cycle of the PIMMS MedicalSynchrotron, These Proceedings.

The VITROVAC Cavity for the TERA/PIMMS Medical Synchrotron

The VITROVAC Cavity for the TERA/PIMMS Medical Synchrotron

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