Development of a Marx-Generator for the drive beam electron gun B. Cassany K. Pepitone bruno.cassany@cern.ch

Outline Why a new HV modulator is needed ? Schematics & concept Results Conclusion

Existing power supply (CTF3) We need to operate with 5 A x 140 µs = 700 µC With the actual 10 nF capacitor : The stored charge is 1.4 mC The decay on voltage (energy) for 140 µs pulse is 40 % Droop for 4.5A , 140 µs pulse ~ 35 % To achieve 1% droop a ̴1 µF capacitor should be used and charged at 140 kV. The energy stored is ̴10 kJ This option requires a 140 kV opening switch protection in case of diode short circuit.

Technical solutions Direct switch modulators Transformer based modulators C ≈ 1µF at 140kV Switch 140kV   Flat top and overshoot It has been decided to propose a new modulator based on Marx topology. The schematic is derived from a prototype developed at SLAC few years ago . M.A. Kemp & al., Design of the second-generation ILC Marx modulator, SLAC-PUB-14249.

How it works ? Charge Discharge Optional Crowbar Vout Vout Vout t t t

Auxiliary power supply : 48V Previous results Design & Choice of components : 1700 V IGBT, Capacitors, Diodes, Power supply Hybrid auxiliary power supply Tests with 30 stages (6 cards) Main power supply : 1.5kV 1.5kV Auxiliary power supply : 48V 7.5kV GND One card

Evolutions 2016 Ten stages per card / Global size reduction 2015 2016 Ten stages per card / Global size reduction SMD components First industrial prototype IGBT drivers with overcurrent protection Short circuit protection

Evolutions 2016 Previously: 2 Optical Fibers / stage  200 OF for the whole modulator ! Needs to limit the dV/dt for EMC  each card was triggered with few µs delay Now only 2 OF / card (ten stages). The trigger propagates on the card through optocouplers with 0.4 µs delay per stage. Slow down rising time

Results: Pulse to pulse variation Allan Standard Deviation One card test 300 pulses on 1.8 kW resistive load 50 Hz Rep. Rate White noise from digitizer Drift due to load heating Allan Standard Deviation = A metric for stability = Two-sample Standard Deviation taken over variable interval of time or variable interval of pulses

Ideal low pass filter period Results: Flatness Allan Standard Deviation RC decay 1 % White noise Ideal low pass filter period RAW DATA

RC corrector RC in parallel with the load in order to absorb ̴1% of the energy during the first half of the pulse. 50 W 6 nF V 1.8 kW 6.6 kW Without corrector With corrector

Conclusion Modulator Power electronics for cathode (to do) Concept tested in 2015. New improvements in 2016 (drivers, SMD). Tests and mechanical integration are ongoing. The new modulator will be assembled by S1 2017. Power electronics for cathode (to do) Implement a fast correction on pulser (RF mosfet). Implement a new heather power supply. Improve all electrical diagnostics (10-3 stability). Electrostatic analysis

Thank you for your attention

Design Classic Marx Generator Charging resistors replaced by diodes Grounding resistors replaced by switches Bypass diodes added