SLAC Klystron Lectures

Lecture #12June 2, 2004

Klystron Power Supplies, Modulators and TestingSaul GoldStanford Linear Accelerator Centerslg@slac.stanford.edu

What have we covered?History of KlystronsVelocity modulation, Kinematic theory and Space-charge theoryDesign of the electron gunDesign of the electron beam and focusingGain-Bandwidth calculations and simulationsOther microwave amplifiersKlystron fabrication, vacuum and processing

Whats Next?More ProcessingVoltage processingBurn off whiskersElectro polish surfacesBeam processingMore outgassing, beam interceptionCathode surface cleanupObtain even emission amps/cm2RF processingMore outgassing, beam interceptionBurn off whiskers in Cavities

Whats Next? (cont.)Test- Verification of performancePower output, peak and averageGain CurvesEfficiencyCathode roll-off (Emission curve)Best heater power settingRF Breakup checkBandwidth

Prepare Tube for TestDressCollector water jacket and Body water fittingsFocus MagnetElectro-magnetPermanent magnet (Single or PPM)Separate gun coilTemperature monitorsCorona ringsLead shielding

Examples of klystrons5045 in Final Assembly5045 on the Test Stand

Examples of klystronsPPM3-5 with PPM FocusingPPM3-5 on the Test Stand

Examples of klystronsSLAC PEP II KlystronSLAC PEP II Klystron in its magnet

5045 Dress Checklist

5045 Interlock Checklist

XL Klystron Data Sheets

XL Klystron Data Sheets

XL Klystron Data Sheets

XL Klystron Data Sheets

XL Klystron Data Sheets

Test PhilosophyPulsed KlystronsBeam Process onlyNarrow Pulse widthLow Rep RateSlowly raise beam voltage as function of time and pressureLower voltage, Raise Rep Rate and repeat Add RFLow Rep Rate, Narrow RF Pulse WidthIncrease RF drive to saturate Klystron as function of time and gas pressureLower Drive, Raise Rep Rate and repeatLower RF Drive and Rep Rate, increase RF pulse width and repeat

Test PhilosophyWiden Beam Pulse WidthBeam process only as before with voltage and Rep RateAdd RF (starting at previous width) as before slowly process width RF Drive, Rep Rate and Pulse widthContinue until full Beam and RF Pulse width with Highest Rep Rate and Klystron saturated

Processing is a function of time and gas pressure

Test Philosophy (cont.)XL4 Processing ExampleStart at ~0.5usec Beam Pulse at 10 Hz.Raise Beam voltage from minimum ~50kV to a maximum of 440kVRaise Rep Rate in steps of 10Hz, 30 Hz, 60 Hz.Start RF at 100 to 200nsecRaise Drive to saturate at 55 to 60MWRaise Rep Rate in steps of 10Hz, 30Hz, 60HzWiden RF Pulse width 100, 200, 300, 500nsec

Test Philosophy (cont.)XL4 Processing Example (cont.)Widen Beam Pulse in steps of 0.5, 1, 1.5usecRaise Beam voltage from minimum ~50kV to a maximum of 440kVRaise Rep Rate in steps of 10Hz, 30 Hz, 60 Hz.Start RF at 0.500 or 1usecRaise Drive to saturate at 55 to 60MWRaise Rep Rate in steps of 10Hz, 30Hz, 60HzWiden RF Pulse width in steps

Test Philosophy (cont.)CW KlystronsHi-Pot electron gun w/ cold cathodeBeam Process only Slowly raise beam voltage as function of time and pressure within collector dissipation limitAdd RFIncrease RF drive to saturate Klystron as function of time and gas pressure

Klystron ProtectionGun arcsLimit peak current and peak energySense arc and turn off pulse (next pulse)Beam interceptionSense current and turn off pulse (next pulse)Sense with current, sense with temperature,Sense with delta temperatureGas PressureGun or collector pressure- turn off beamOutput or window pressure- turn off RFPulse klystron can stop pulse for gun arcs, etc.CW klystrons require a crowbar on the P.S.

Klystron Protection (cont.)Basic InterlocksKlystron Water or air flowLow heater currentModulator faultLow Tank oilMagnet current (over/ under)Magnet Over tempMagnet waterTurn off beam, add time delay before magnet offAll these interlocks turn off beam

Klystron ArcsAmerican tube companiesArc Energy 10 joules1000 Amps/msec max. rate of riseRemove current in less than 10msecThales (France)40 joule max.For High Power devices below 200kVNewer Klystrons above 500kVMay run more than 1 klystron per modulator

Arcing in a Klystron GunOperate in excellent vacuum10-8 to 10-9 torrDesigned not to arcFields are well below breakdownNo over voltage conditionPlasma createdMoves at 2-3 cm/ msec

Klystron Arc Waveforms

Klystron ArcsKlystron protection will always be an issueGun Vacuum criticalLine-type modulators have been successful at high peak powers for 1 & 2 klystron operationArc formation much slower than originally believedHundreds of nanosecondsLine modulators have dumped ~70 joulesInduction modulator has dumped ~ 200 joulesKlystrons have survived this higher energy

ModulatorsMost high peak power klystrons operate on Line-Type ModulatorsSLAC has close to 250 Line-Type Modulators on the LINACAdvantagesRelatively simple electronicsNatural Protection with current limiting to 2 times operatingDisadvantagesFixed Pulse widthMatched impedance w/ klystronPulse shape load dependentNeeds to be tuned for flat pulseLimited Rep Rate

Basic Line Type ModulatorHeater SupplyVariable DC Power SupplyThyratronTriggerLchL1L2LnC1C2CnRc1:N

Line-Type Modulator FormulasLt = L1+L2+..LnCt = C1+C2+..CnLt= total PFN InductanceCt= total PFN CapacitanceZpfn = Lt / CtZpfn = Zkly / N2T = 2 Lt Ct Ct = T / 2 ZLt = T Z / 2

Line-Type Modulator Formulas (cont.)N = Vpeak max / Vps max Pulse Transformer Ratio# PFN sectionsDependent upon pulse ripple More sections = higher frequency ripple, more tunabilityRise time of PFNtr ~ T / 2 n n = # sectionsValue of components : L & C

Line-Type Modulator Waveforms

Other ModulatorsDirect SwitchVariable DC Power SupplyCHeater SupplyHV IsolationLow CapacitancePulse droop: C E = I T C is filter cap, T is pulse width, I is beam currentRise Time: C E = I T C is load stray cap, T is rise time, E is beam voltage, I is peak current

Other ModulatorsHybrid Modulator

Variable DC Power SupplyCHeater SupplyPrimary C droop: C E = I TRise time of pulse is mainly a function of Pulse Transformer1:N

Other ModulatorsInduction adderStacked cores with a common secondaryHeater SupplyVariable voltage DC Power SupplyUsually single turn primary and secondaryCan use multi-turn secondary# Sections function of switch voltage

Other ModulatorsMarx ModulatorCharge in parallel, discharge in seriesVariable DC Power Supply--+---+++1. Standard- On switch, full discharge2. On switch with PFNs in place of capacitor3. ON/ OFF Switch with Partial discharge of capacitor

ReferencesG.N. Glasoe, J.V. Lebacqz, Pulse Generators, McGraw-HillJ.Millman, H. Taub, Pulse, Digital and Switching Waveforms, McGraw-HillR.B. Neal, The Stanford Two-Mile Accelerator, W.A. Benjamin Inc.P.W. Smith, Transient Electronics, John Wiley & Sons Ltd.S.L.Gold, Klystron Gun Arcing and Modulator Protection