Marx Bank Modulator Development at SLAC

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Marx Bank ModulatorDevelopment at SLAC

• G.E. Leyh, Stanford Linear Accelerator Center

A Marx Bank at the Technical Museum, Munich
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Classical Marx Concept

• Developed by Dr. Erwin Marx in the 1920s
• Vout Vin number of cells
• Originally used carbon resistors, spark-gaps

• Design Evolution
• CuSO4 Charging Lines
• Inductive Isolators
• HV Diodes
• Triggered Gaps
• Thyratrons
• Solid State FETs, IGBTs

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The Solid-State Marx Bank

• NOT a series switch Diodes provide parallel
  path
• Each cell can switch on/off independently
• Staged firing of cells allows contouring of pulse
  shape
• Common-mode chokes provide isolated DC power

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SLAC 500kV NLC Marx Concept
circa 2003

• Total stack output 500kV, 550A
• 30 Marx blocks total, 18kV per block
• Commercial 2.4Ghz RF chipsets
• no wires or fiber-optic cables

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500kV NLC Marx Cell Components
circa 2003
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500kV Marx Block Waveforms
circa 2003
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From NLC to ILC Modulator Requirements
NLC ILC
Pulse Voltage Pulse Current Pulse Length
flat-top Total Pulse Charge Total Pulse
Energy Repetition Rate Average Output Power Total
of Stations
500 kV 120 kV 550 A 140 A 1.6 uS 1370 uS 0.88
mC 192 mC 440 J 23,000 J 120 Hz 5 Hz 53 kW 115
kW 1600 576
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Existing 10MW TTF Modulator

• Developed in the early 90s at FermiLab for use
  with the TTF.
• Currently in use at FNAL and on the XFEL at DESY.
• Uses a passive bouncer circuit to compensate
  for capacitor droop.
• Advantages
• Simple circuit topology
• Proven design 10 years of operation
• Disadvantages
• High stored energy 270kJ
• Massive pulse transformer 6.5 tons
• Single-point failures can damage klystron
• Requires large floor area
• Insulating oil 100s of gallons

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Solid State Marx ModulatorPros and Cons

• Lower IGBT currents
• No magnetic core issues
• losses
• core reset
• acoustic noise
• core saturation
• leakage inductance
• magnetizing currents
• PC board integration
• Mechanically simple, more compact
• Finer waveform control

• IGBT controls float at high voltage during pulse
• DC power flow to cells must be isolated
• Timing signals must cross high voltage gradients

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ILC Marx Design Approach
NLC Marx
ILC Marx

• Inductive isolation of DC not practical for long
  pulse use solid-state switches
• Cell capacitors must be substantially larger
• Active regulation required to maintain pulse
  flatness

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Integrated Charging Regulator / Cell Isolator

• Single IGBT switch performs charging regulation
  and cell isolation functions
• A simple fusible link isolates cells in the event
  of a short circuit fault
• Dual mode charger converts from current mode to
  voltage mode upon reaching setpoint
• Control power provided to each cell through a low
  current diode stack
• Cells charge in sequence, starting at the bottom

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Availability ILC Modulator Considerations
MTBF
Availability
MTBF MTTR
Number of required modulator stations
560 Number of available modulator stations
576 Required steady-state modulator
availability 99.3
Increase MTBF

• Minimize possibilities of single-point failures
• Modulator must automatically work around
  failures

Decrease MTTR

• Repairs must be fast modulator identifies bad
  cells
• Partition stored energy, to isolate catastrophic
  failures

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ILC Marx Design Approach

• Use emerging technology
• High ohm/sq explosion-proof capacitors
• 4500V single-die IGBTs
• Commercial RF chipsets for signal paths
• Avoid transformer oil, controlled materials
• Closed-circuit air cooling, exchanged with LCW
• Modular mechanical design
• Simpler mechanical solution, more compact
• Lower construction costs
• Faster repair times

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12kV Solid-State IGBT Switch

• 5-section modular PC-board design, using 4500V
  Single-Die IGBTs
• Each section has independent gate drivers, delay
  stabilization circuitry, overvoltage protection
  and snubbing networks
• Switch designed to operate at full spec with one
  failed section
• Overcurrent protection with multiple
  threshold/delay setpoints

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ILC Marx Cell Module Layout
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ILC Marx Modulator Core Layout
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Air Cooling State of the Art
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Prototype Development Approach

• Start with the highest technical risk items
  12kV switch, energy storage capacitors.
• Assemble, test, debug a complete cell.
• Work towards developing a short stack.
• Explore stack-level fault scenarios.
• Design, test the active regulation control loop.
• Develop complete modulator, control system, RF
  station. Integrate with L-Band klystron.

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Marx Prototype Schedule

• Test 12kV switch module performance May 05
• Evaluate cell components May 05
• Integrate, test completed Marx cell Aug 05
• Start active control system design Sep 05
• Marx short stack ready for evaluation Nov 05
• Integrate short stack, control system Dec 06
• Short stack fault scenario testing complete Feb
  06
• Start construction of full modulator stack Mar 06
• Marx modulator ready for klystron connection Oct
  06

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Current Status 24 Mar 05

• 12kV Switch v1.0
• Design 60 complete
• Assembly 20 complete
• Capacitors
• First engineering samples have arrived
• Lifetime testing to start in a few weeks
• ILC Marx Test Area
• Design 80 complete
• Assembly 10 complete