slac klystron lectures lecture #12 june 2, 2004 klystron power supplies, modulators and testing saul...
TRANSCRIPT
SLAC Klystron Lectures
Lecture #12June 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/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
SLAC 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 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/sec max. rate of rise– Remove current in less than 10sec
• 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/ 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
Var
iabl
e D
C
Pow
er S
uppl
y
Thy
ratr
on
Trigger
Lch L1 L2 Ln
C1 C2 Cn
Rc
1:N
Line-Type Modulator FormulasLt = L1+L2+…..Ln
Ct = C1+C2+…..Cn
Lt= total PFN Inductance
Ct= total PFN Capacitance
Zpfn = Lt / Ct Zpfn = Zkly / N2
= 2 Lt Ct
Ct = / 2 Z Lt = 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
Var
iabl
e D
C
Pow
er S
uppl
y
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
Var
iabl
e D
C
Pow
er S
uppl
y
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
Var
iabl
e D
C
Pow
er S
uppl
y
-
-
+
- - -
+ + +
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”