Brown, J C

Bunge, P J P

Burton, R J

Cracknell, A G

Craig, S T

De Jong, M S

Funk, L W

Hood, A B

Hutcheon, R M

Tran Ngoc, T

CERN Document Server

Particle Accelerators, 1990, Vol. 29, pp. 71-76Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of America52 MHZ RF SYSTEMS FOR PETRA II AND HERA PROTON SYNCHROTRONSL.W. FUNK, J.C. BROWN, R.J. BURTON, S.T. CRAIG, M.S. DE JONG,R.M. HUTCHEON, P.J .P. BUNGE, A.B. HOOD, A.G. CRACKNELL,T. TRAN NGOCChalk River Nuclear Laboratories, Chalk River, Ontario, CANADAAbstract 52 MHz rf systems, including cavities, higher ordermode dampers, amplifiers, feedback systems, resonance controllers,tuners and computer control systems are being provided by CRNL forthe PETRA II and HERA proton synchrotrons, at DESY, in Hamburg,West Germany. The PETRA II systems have been installed in the ringand accepted by DESY. The 52 MHz HERA systems are undergoing pre-acceptance tests at CRNL and'will shortly be shipped to DESY, forinstallation at the conclusion of the current test run of theelectron machines. We review the specifications for the twosystems, describe what was achieved, and report on commissioningand operating experiences.INTRODUCTIONThe requirements and specifications of these rf systems have been pre-viously described1 , 2 and are briefly summarized in Table I. Since theprevious report, the PETRA II systems have been installed in the PETRAtunnel, commissioned and tested with electron beams. The HERA 52 MHzrf systems have been built, and initial high power testing has begun.OPERATING EXPERIENCE - PETRA IIThe systems were installed, commissioned and tested at DESY, initiallyin the hall immediately adjacent to their designated ring location. Allspecifications of the system were met, except for a persistent problemof sparking across the gap ceramic in one of the cavities, and an excessof mains noise in the band 200-400 Hz from the fundamental. Severalattempts were made to eliminate the sparking, assuming that it occurredthrough charging of the ceramic, with the supporting bellows assemblyacting as an electron source when heated by rf currents.[773]/7172/[774]TABLE IL. W. FUNK ET AL.52 MHZ SYSTEMS SPECIFICATIONSPETRA II HERA 52Rf frequency 51.63-52.03 52.03-52.05 MHzTotal CircumferentialVoltage - Capture 50 30 kV- Acceleration 50 60-100 kV- Compression 190 290 kVHarmonic Number 400 1100Average Beam Current 0.17 0.17 ANumber of Cavities 2 2Voltage Stability ±2%, ±5° ±2%, ±5°Open Loop Feedback Gain ~50 ~50Feedforward Compensation available ~90%Sparking caused by tracking or simple thermal effects was ruled out bythe essentially random spatial distribution of visible sparks, and thetime distribution of second and third sparks. The "cures" always workedfor a short time, but sparking would recur after extended operation.Finally the ceramic in this cavity was replaced with one known to havesignificantly lower dielectric loss tangent. No further sparkingproblems have been observed. No satisfactory explanation of theconnection between loss tangent and sparking has been developed.Mains noise was eliminated by re-routing of some cables.The layout of the PETRA II ring, which must accelerate electronsand positrons as well as protons, includes a by-pass line, so that theprotons are shielded from the electron cavity impedance. The smallproton cavity impedance is not expected to affect the electron beam, sothe e-beam will traverse the 52 MHz cavities. The cavities wereinstalled in the ring in October 1988, and very shortly thereafter wereexposed to electron beams. Special instrumentation was installed toobserve wake fields excited in the cavities by the short electronbunches. Both single- and multi-bunch runs were made, at equivalentcurrent levels higher than those expected when filling HERA forluminosity runs. As expected, many higher order modes were excited, butno feedback to the beam could be. observed. Heating of gap ceramic andbellows was observed, but at a much lower level than that experienced52 MHz RF SYSTEMS [775]/73during normal operation at 52 MHz. First operation with protons isanticipated in October or November of this year.The early decision to rely on computer-mediated interlocks forcomponent protection put a premium on computer reliability and soft-warequality. These were achieved with standard industrial computer systemsand high level languages, so that this approach has worked very well.Response time is comparable to that for relay inter-lock systems and thecomputer hardware/software has been the most reliable subsystem. Mostimportant of all, the interlock configuration could be quickly andreversibly modified as needed during system integration, so thatoperation of a partial system was possible with maximum safety. Thisgreatly smoothed integration and commissioning.Communication with DESY is via their custom SErial ~ata ACquis-ition system, SEDAC. Special hardware and software had to be developedfor the interface. Initial difficulties in defining and implementingsuitable protocols have been overcome, and DESY is writing thesupervisory software for their control computers.The most demanding amplifier specification is the ability to copewith transient beam loading. The high average current, and the fact thatthe proton bunches cross the accelerating gap at, or near, the zero inthe gap vol tage, means that large amounts of compensating power aredelivered into the final amplifier plate. Since test beams were notavailable, it was decided to try to simulate this situation by frequencymodulating the cavity drive at the beam revolution frequency, with adeviation which resulted in the same rate of change of phase as thatexpected from the beam. The amplifiers were able to handle thisdissipation at all specified gap voltages, with an adequate safetymargin.OPERATING EXPERIENCE - HERA 52The system is undergoing pre-acceptance testing at CRNL. Witnessedacceptance tes ting is planned for September. Installation, commis-sioning and acceptance testing at DESY is to take place in October.HERA 52 cavities differ from those of PETRA in that the former arefully evacuated. Vacuum envelope assembly and bakeout has beencompleted. System base pressure is expected to be <10-9 mbar with a74/[776] L. W. FUNKET AL.400 l/s ion pump after extended rf operation completes the cleanupprocess. Cavity Qs were about 14,000 unloaded and about 7000 loaded.The change in cavity aspect ratio from the PETRA II design shiftedthe first higher order mode frequency from 125 MHz to about 145 MHz. Theconfiguration of the HOM damper coupling loop and resistor have beenadjusted to achieve maximum coupling at the new frequency. This is stillcomfortably distant from three times the fundamental, so that this typeof damper is still useful.A titanium film has been evaporated onto the inner surfaces of onecavity to suppress multipacting. If successful, the process will beapplied to the second cavity. Simulation with NEWTRAJ 3 predicted theonset of multipacting at about 1 kV gap voltage, occurring mostly in thevolume between the inner conductor and the inside of the intermediatecylinder. These surfaces received particular attention during thetitanium coating process. The coated cavity multipacted at 500 V gapvoltage. After a short period of conditioning this cavity operatedwithout multipacting in pulsed mode. Attempts to achieve cw operationare underway.A major concern during design of the final amplifier and driveloop assemblies was the achievement of minimum reactances, so that theresonant frequency of the circuit formed by the drive loop, tetrode andbalancing capacitor could be kept above the operating frequency. Withcareful attention to compact design, and maintenance of a coaxialgeometry, this goal was achieved. The measured resonances of thesecircuits in the two HERA 52 systems were at 63.5 and 62 MHz. Looppenetration is adjustable, to achieve the correct coupling to the cavity,with a ratio of gap voltage to plate rf voltage of 30. A complex designwas necessary to meet the requirement that this be achieved withoutmodification of the vacuum feedthroughs or disturbance of the water lineswhich run through the loop from the centre to the tube and balancingcapacitor, and back out again. Success was achieved and step-up ratiosof 27.5 and 29 were realized.A folded, broadband, final amplifier input cavity incorporating atube socket, and means to provide cooling air for the screen connectionand water cooling to the filament, was designed to fit into the finalamplifier enclosure. The completed cavity, loaded by the grid-cathodeimpedance of the tetrode, has a bandwidth in excess of 10 MHz. Final52 MHz RF SYSTEMS [777]/75adjustment of the coupling to the desired 5 0 impedance is achieved witha plunger with a tuning rate of 0.8 MHz/em, and the overall cavityfrequency is adjusted by movement of a ceramic load, which has a tuningrate of 1.0 MHz/em.The ten-channel solid state driver amplifier necessitates the useof a power combiner. This compact and efficient unit has input VSWRs of1.6 or less, and prevents failure of any driver module from affecting anyof the other modules. To minimize the effect of input mismatch, theamplifier modules have been connected to the combiner with lengths ofcoaxial line which are two 52 MHz wavelengths long.Performance of the solid-state amplifier modules has beenexcellent. Individual modules provide (at 52 MHz) output of 300 W, gain> 20 dB, 3 dB bandwidth greater than 50 MHz, dc-to-rf efficiency> 70%at full output, input VSWR ~ 2.5, gain linearity better than ± 0.5 dB at1 A bias current, and suppression of the 3r d harmonic by more than 20 dBrelative to the fundamental. As combined into a 3 kW unit, with an inputexciter module, 1:10 splitter and 10:1 power combiner, they achieve>40 dB gain, 35 MHz bandwidth, input VSWR < 1.8, and tolerance of VSWR upto 4 at the output of each module. Total delay from the exciter inputto the ten driver outputs is 33 ns.The lessons learned in the operation of the PETRA II analog anddigital control systems were carefully applied, with the result that thecommissioning of these HERA 52 sub-systems was completed smoothly andquickly. In the digital control system, the decision to utilize standardindustrial modules for signal conditioning proved to be of particularvalue in saving time and money in manufacture, installation andcommissioning. This philosophy had already been applied in the analogcontrol system for PETRA II.Figure 1 shows a Bode plot of the driver, final amplifier andcavity. The left feature is the main cavity response. The right featureis associated with the drive loop, balancing capacitor and tetrode. Thecentral feature represents a two-path interference signal arising fromexcitation of the tuner resonance. Only the loop resonance is less than34 dB below the cavity response, so that it will be simple to adjust thefeedback path length to assure stability.76/[778] L. W. FUNKET AL.NETWORKA: REF-22.54[ dBCintB: REF180.0] [ deg ]J MKRJT/RJ 89 975 000.000 Hz10.2193 dB-36.4163 deg1\~ \~ 1\~N\"<,1 r-,)< D\ "\~ ~ ) 1\ ~1/-. I~~ V -,/ ~-- ~~"~V ~ ....,., '-,~ ~"1,-.<, \o ~~ ,~~DIV DIV START 50 000 000.000 Hz10.00 36.00 STOP 65 000 000.000 HzRBW: 1 KHz ST: 12.8 sec RANGE: R= 10. T= 10dBmFIGURE 1. Bode plot of HERA 52 MHz system response.ACKNOWLEDGMENTSThese contributions were mostly financed by the Natural Sciencesand Engineering Research Council, and the entire Canadian contributionwas co-ordinated by the Institute of Particle Physics.REFERENCES1. L.W. Funk, et al., "The 52 MHz RF System for PETRA II",European Particle Accelerator Conference, Rome, June 1989.2. L.W. Funk, et al., "52 MHz RF Systems for HERA", EuropeanParticle Accelerator Conference, Rome, June 1989.3. Boni, R., et al., "Design and Operation of a Multipacting-Free 51.4 MHz Rf Accelerating Cavity", Nucl. Instr. Meth.,A274 (1989) 49-55.

52-mhz Rf Systems For Petra-ii And Hera Proton Synchrotrons

52-mhz Rf Systems For Petra-ii And Hera Proton Synchrotrons

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