Switches and Sparkplugs


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Posted by Rezwan on Jan 01, 2011 at 11:59 PM
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This post breaks down the design of FoFu-1’s twelve spark gap switches, which release current from the twelve capacitors to the main electrodes.  The switch plates and spark plugs are undergoing “ruggedization” to overcome the challenges discussed here.

Close up: Cross section spark gap switch

The trigger pulse carried by the tungsten rod ionizes the gas and allows a spark to jump from the lower electrode, which is connected to the capacitor. Very quickly the current path jumps from the rod to the upper electrode. The current then flows through the switch plate into the main circuit of the device.

Pre-firing occurs when a spark spontaneously jumps between the bottom electrode and the rod during charging.

Insulator breakage occurs when the rod suddenly moves due to magnetic forces generated when the spark jumps from the rod to the electrode. Also, if other switches pre-fire, and this one does not, when the pinch occurs, a high voltage can appear between the upper electrode and the rod which can exceed the dielectric strength of the insulator and cause it to break down and vaporize. 

Shorting. Finally the trigger pulse can be shorted before it reaches its peak by a surface breakdown across the end of the insulator to the upper electrode. This may prevent firing of the switch.

The size of all the components is current limited by the hole in the switch plate, which cannot be expanded without building new switch plates, due to the location of many screw holes.

In LPP’s new design, the hole will be made larger in new plates, and the rod, insulator and electrodes will all be expanded. As well the gap between the electrode will be increased to prevent pre-firing up to 45 kV.

Overall cross section of spark gap switch

Note the upper and lower electrodes.  These are the switch electrodes, which are not the same as the core DPF electrodes.  There are twelve switches, moving the charge from the capacitors into the hot plate and down to the main electrodes of the machine.  These switches (and triggers) all have to work together with nano-second timing to deliver the charge correctly to make what we see in the DPF animation happen.  Fusion results are obtained when the charge is delivered without prefiring, shorting, or insulation breakage - the engineering challenge.

Note the above are schematic diagrams.  Of course the switch doesn’t just hang in space.  It’s encased in a container that keeps the gas in, as you’ll see in the photographs below.

Switch Photos

These photos are from the FoFu-1 Design Slide show.  They provide perspective, showing the switches in context and also disassembled.

A switch on FoFu-1


The switch is covered by the transition plates which carry the charge to the main electrodes.  Note the spark plug in the middle.  The first spark plugs were standard automotive spark plugs as shown here, not the tungsten rod spark plugs (pictures pending).  Note also that the spark plug needs to be connected to the trigger generator with the trigger head, as you see in the next image.

Switches:  full and partial assembly

A close up of two switches. The one on the left is fully assembled, with the trigger head in place and gas line connected (white tube connected directly to switch cylinder). The one on the right has the trigger head, transition plate, spark plug and top switch plate removed, showing the lower switchplate with anode visible.

Switches fully assembled

The trigger head, the white elbow shaped PVC piping, covers the spark plug.

Switch, dismantled


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MTd2's avatar

Thank you! :D


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