SLAC Klystron Lectures Lecture #12 June 2, 2004 Klystron Power Supplies, Modulators and Testing Saul Gold Stanford Linear Accelerator Center

What have we covered? History of Klystrons Velocity modulation, Kinematic theory and Space-charge theory Design of the electron gun Design of the electron beam and focusing Gain-Bandwidth calculations and simulations Other microwave amplifiers Klystron fabrication, vacuum and processing

What’s Next? More Processing –Voltage processing Burn off whiskers Electro polish surfaces –Beam processing More outgassing, beam interception Cathode surface cleanup –Obtain even emission – amps/cm 2 –RF processing More outgassing, beam interception Burn off whiskers in Cavities

What’s Next? (cont.) Test- Verification of performance –Power output, peak and average –Gain Curves –Efficiency –Cathode roll-off (Emission curve) Best heater power setting –RF Breakup check –Bandwidth

Prepare Tube for Test Dress –Collector water jacket and Body water fittings –Focus Magnet Electro-magnet Permanent magnet (Single or PPM) Separate gun coil –Temperature monitors –Corona rings –Lead shielding

Examples of klystrons 5045 in Final Assembly5045 on the Test Stand

Examples of klystrons PPM3-5 with PPM FocusingPPM3-5 on the Test Stand

Examples of klystrons SLAC PEP II Klystron SLAC PEP II Klystron in its magnet

5045 Dress Checklist

5045 Interlock Checklist

XL Klystron Data Sheets

Test Philosophy Pulsed Klystrons –Beam Process only Narrow Pulse width Low Rep Rate Slowly raise beam voltage as function of time and pressure Lower voltage, Raise Rep Rate and repeat –Add RF Low Rep Rate, Narrow RF Pulse Width –Increase RF drive to saturate Klystron as function of time and gas pressure –Lower Drive, Raise Rep Rate and repeat Lower RF Drive and Rep Rate, increase RF pulse width and repeat

Test Philosophy Widen Beam Pulse Width –Beam process only as before with voltage and Rep Rate –Add RF (starting at previous width) as before slowly process width RF Drive, Rep Rate and Pulse width Continue 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 Example –Start at ~0.5usec Beam Pulse at 10 Hz. Raise Beam voltage from minimum ~50kV to a maximum of 440kV Raise Rep Rate in steps of 10Hz, 30 Hz, 60 Hz. –Start RF at 100 to 200nsec Raise Drive to saturate at 55 to 60MW Raise Rep Rate in steps of 10Hz, 30Hz, 60Hz Widen RF Pulse width 100, 200, 300, 500nsec

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

Test Philosophy (cont.) CW Klystrons –Hi-Pot electron gun w/ cold cathode –Beam Process only Slowly raise beam voltage as function of time and pressure within collector dissipation limit –Add RF Increase RF drive to saturate Klystron as function of time and gas pressure

Klystron Protection Gun arcs –Limit peak current and peak energy –Sense arc and turn off pulse (next pulse) Beam interception –Sense current and turn off pulse (next pulse) –Sense with current, sense with temperature, –Sense with delta temperature Gas Pressure –Gun or collector pressure- turn off beam –Output or window pressure- turn off RF Pulse klystron can stop pulse for gun arcs, etc. CW klystrons require a crowbar on the P.S.

Klystron Protection (cont.) Basic Interlocks –Klystron Water or air flow –Low heater current –Modulator fault –Low Tank oil –Magnet current (over/ under) –Magnet Over temp –Magnet water Turn off beam, add time delay before magnet off –All these interlocks turn off beam

Klystron Arcs American tube companies –Arc Energy 10 joules –1000 Amps/  sec max. rate of rise –Remove current in less than 10  sec Thales (France) –40 joule max. For High Power devices below 200kV Newer Klystrons above 500kV –May run more than 1 klystron per modulator

Arcing in a Klystron Gun Operate in excellent vacuum –10 -8 to torr Designed not to arc –Fields are well below breakdown –No over voltage condition Plasma created –Moves at 2-3 cm/  sec

Klystron Arc Waveforms

Klystron Arcs Klystron protection will always be an issue Gun Vacuum critical Line-type modulators have been successful at high peak powers for 1 & 2 klystron operation Arc formation much slower than originally believed –Hundreds of nanoseconds Line modulators have dumped ~70 joules Induction modulator has dumped ~ 200 joules Klystrons have survived this higher energy

Modulators Most high peak power klystrons operate on Line-Type Modulators –SLAC has close to 250 Line-Type Modulators on the LINAC Advantages –Relatively simple electronics –Natural Protection with current limiting to 2 times operating Disadvantages –Fixed Pulse width –Matched impedance w/ klystron –Pulse shape load dependent –Needs to be tuned for flat pulse –Limited Rep Rate

Basic Line Type Modulator Heater Supply Variable DC Power Supply Thyratron Trigger LchL1L2Ln C1C2Cn Rc 1:N

Line-Type Modulator Formulas Lt = L1+L2+…..Ln Ct = C1+C2+…..Cn Lt= total PFN Inductance Ct= total PFN Capacitance Zpfn = Lt / CtZpfn = Zkly / N 2  = 2 Lt Ct Ct =  / 2 ZLt =  Z / 2

Line-Type Modulator Formulas (cont.) N = Vpeak max / Vps max Pulse Transformer Ratio # PFN sections Dependent upon pulse ripple – More sections = higher frequency ripple, more tunability Rise time of PFN tr ~  / 2 n n = # sections Value of components : L & C

Line-Type Modulator Waveforms

Other Modulators Direct Switch Variable DC Power Supply C Heater Supply HV Isolation Low Capacitance Pulse droop: C E = I T C is filter cap, T is pulse width, I is beam current Rise Time: C E = I T C is load stray cap, T is rise time, E is beam voltage, I is peak current

Other Modulators Hybrid Modulator Variable DC Power Supply C Heater Supply Primary C droop: C E = I T Rise time of pulse is mainly a function of Pulse Transformer 1:N

Other Modulators Induction adder –Stacked cores with a common secondary Heater Supply Variable voltage DC Power Supply 1.Usually single turn primary and secondary 2.Can use multi-turn secondary 3.# Sections function of switch voltage

Other Modulators Marx Modulator –Charge in parallel, discharge in series Variable DC Power Supply Standard- On switch, full discharge 2. On switch with PFN’s in place of capacitor 3. ON/ OFF Switch with Partial discharge of capacitor

References G.N. Glasoe, J.V. Lebacqz, ”Pulse Generators”, McGraw-Hill J.Millman, H. Taub, “Pulse, Digital and Switching Waveforms”, McGraw-Hill R.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”