440kV Marx Generator with 120Hz repetition rate

[davekni]

Retrospective post of my 440kV Marx generator built in 2017.
    12 stages
    2.7nF per stage, made from 8 series-connected 22nF 6kV WIMA FKP1 caps.
    2 * 68meg 10kV resistors across each 22nF cap
    170mH inductors instead of resistors.  Each made from series connection of 119 x 1mH inductors.  Magnetic proximity of cores increases total inductance from 119mH to 170mH.
    Spark gaps made from 3/4" copper pipe end caps.  Ends made spherical by cold forming, pounding bearing ball inside copper end cap into hollow recess carved into wood outside end cap, repeated for each cap.
    Gaps adjusted for 42kV breakdown.  Line-of-sight between gaps allows UV light to help synchronize triggering.
    Air flow directed at each gap.  Could achieve only ~40Hz repetition rate without forced air.
    Charged by +-21kV supply.  4-stage voltage multiplier (+- 2 stages) from +-11kV 30kHz ferrite transformer output.
    Resonant H-bridge drive to transformer primary.
    Charging supply shuts off for 1-2ms after each firing.  Allows time for arcs to extinguish.  This off time varies (increases) to control repeat rate.  No explicit triggering of spark gaps.  Charges to ~42kV breakdown voltage in ~7ms.  Each firing occurs ~7ms after charging starts.
    About 6kW average power.

Schematic:


Inductor/capacitor side:


Spark gap side:


Video of initial build with max repetition rate of 100Hz.  Updated PFC unit with higher power allowed 120Hz for 2018.
   

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Initial version used hand-wound inductors, made like tapped TC secondaries on pipes up each side.  Too much corona and arcs between windings (racing sparks).  Second version used commercial automotive spark coils (secondaries only) rated for 50kV.  Fast rise times (much faster than normal engine spark generation) made internal voltage distribution uneven.  Worked for a few short demos (with replacing a few spark coils after every few minutes of operation).  Final version is as shown, using 24 170mH inductors each containing 119 small inductors.  Physical layout makes capacitive voltage division roughly even across each inductor.  No more inductor failures with this version.

[huntergroundmind]

Great, and interesting build! I really like the idea with making air low inductance capacitor

[bobfrancis1980]

Dave, I noticed the resistors you used across your capacitors were 2 x 68MOhm but only rated to 10kV. Is the high voltages used not a problem for them? I was looking to use 1 10kV 1GOhm 3 watt resistor across each capacitor mainly because my Marx stages are limited for space. The higher voltage rated ones are significantly longer and I would have to do a major redesign if I used them.

Edit: Nevermind I was able to redesign my Marx base plate by making it a 1/2" thicker and will router out the area and create a string of six 3Watt 10kV resistors in series at about 200MOhm it should be around 1.2GOhm and have only a little wattage going through them while in operation (slightly over 2Watts).