Just look at the basic circuit, lv on one side protected by a string of diodes, hv on the other .  This is the basic formula.  Discharge cap looks like a good idea.

Quote* They are not made to withstand sudden discharges over a longer time!!! This means they will not live very long under such conditions!!!
* They have quite a high ESR, Even the low ESR types!!! This means, that even if you short them, the discharge current will not be very big, compared to a discharge cap. This means a good Electrolyte cap discharges maybe at about 10-40Amps and a discharge cap at about 1-4kAmps (1000-4000Amps)!!! Sure the discharge caps do cost a lot more. If you would like to get a good one which can also withstand high frequency discharges in the range of about 300uF they cost about $200 (e.g. 3x100uF of the WIMA  GTO Caps, which would be able to yield a 12'000Amp discharge)

This is true my 330uF capacitor is now become faulty, internally shorted . But diode on LV discharge side withstand for such high current....

Diodes are not for protection, diodes helps to pass HV towards spark plug,  without diode it works if you manage to generate plasma channel at spark plug tip.
HV given to secondry of coil does not affect it, coil resistance is very high. and High voltage spike will not damage diodes and capcitor.

SCR based desing problems
1. It requires proper voltage to make Igt=1.3mA to 5mA
2. Use of Low value of gate Resistance, creates problem of HV goes to LV side, and no plasma.
3. It is not sutaible for engine, once engine gets started voltage required to run goes low, and also current requirement becomes less. It is difficult with SCR.
4. I have not tried it with diodes 1N4148(use 4007), It may help much to solve this problem.

I raised this on the "break-away" thread and got little response so I'll try here as well - granted, we all want our cars running exclusively on water, but given the idiotic fuel prices now, how about trying the plasma-spark idea on an otherwise unmodified car engine - if this were to increase my mpg from its current 30mpg to 60mpg, (hey, I'm an optimist!) I for one would be elated!  There is far, far more work put into HHO projects, with little hard data available that it actually provides a significant mileage increase...

Sprocket, I believe with the plasma system you should be able to tune your fuel system to extremly lean values and still keep the engine running smooth.  This is the same as the firestorm plugs where they could get ridiculously high air fuel ratios and thus save a ton of gas.  So with a little tuning (efie?) you should be bale to lean out your engine and get great results.  I have not tested this, someday I will.  But for now this is untreaded territory.  It may be worth it, though.  Good idea.