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SLAC Klystron Lectures
Lecture #12
June 2, 2004
Klystron Power Supplies, Modulators
and Testing
Saul Gold
Stanford Linear Accelerator Center
[email protected]
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/cm2
– 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 Assembly
5045 on the Test Stand
Examples of klystrons
PPM3-5 with PPM Focusing
PPM3-5 on the Test Stand
Examples of klystrons
SLAC PEP II Klystron in its magnet
SLAC PEP II Klystron
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 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 0.500 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/msec max. rate of rise
– Remove current in less than 10msec
• 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 10-9 torr
• Designed not to arc
– Fields are well below breakdown
– No over voltage condition
• Plasma created
– Moves at 2-3 cm/ msec
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
Lch
L1
Trigger
C2
1:N
Thyratron
Variable DC
Power Supply
C1
L2
Heater
Supply
Ln
Cn
Rc
Line-Type Modulator Formulas
Lt= total PFN Inductance
Lt = L1+L2+…..Ln
Ct= total PFN Capacitance
Ct = C1+C2+…..Cn
Zpfn =
Lt / Ct
Zpfn = Zkly / N2
T=2
Lt Ct
Ct = T / 2 Z
Lt = T Z / 2
Line-Type Modulator Formulas
(cont.)
Pulse Transformer Ratio
N = Vpeak max / Vps max
# PFN sections
Dependent upon pulse ripple –
More sections = higher frequency
ripple, more tunability
Rise time of PFN
tr ~ T / 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:
CE=IT
Rise Time:
CE=IT
C is filter cap, T is pulse width, I is beam current
C is load stray cap, T is rise time, E is beam
voltage, I is peak current
Other Modulators
• Hybrid Modulator
1:N
Variable DC
Power Supply
C
Heater
Supply
Primary C droop:
CE=IT
Rise time of pulse is mainly a function of Pulse Transformer
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
-
-
-
-
+
+
+
+
1. 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”