I changed the circuit slightly after finding that pin that was used to drive the IGBT was slowed down slightly by driving the LED indicator on the board. I moved the drive to PB3, and added a 1k resistor to ground eliminate a transient, that would sometimes reset the Arduino Nano. The LED absorbed the transient, in the original connection.
I have updated the schematic. I also found it necessary to use a USB cable, with a ferrite filter, on both ends, to avoid intermittent USB communication failures. The spark is healthy now. The change does not affect the earlier current based measurements. It does change the dv/dt when the coil is released to generate the spark. I hope to find an affordable high voltage probe for my scope.
