For producing this kind of energy wave a high current source must feed the primary of the pancake coil and load capacitor.
The inside lead of the pancake coil should be grounded in the earth and not be common with high current power source.
A magnetically quenched spark gap can be used for creating the short duration pulses needed that have zero back flow of energy.
The spark gap and magnets are setting for preventing this back flow by careful adjustment. This is the key for creating the longitudinal waves.
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