I looked more carefully at the Schematic D-2151 and just noticed that the Transzorb is actually a 5KP6.0A which has a breakdown voltage between 6.67 and 7.37. The scope traces that you sent show the Imon peaks clipping at about 6.7 volts and so this all makes sense now. This explains to me why the current peaks have always looked near constant in amplitude and yet the Emon voltage charges exponentially. The "true" Imon pulses are probably decaying exponentially in time as I have calculated in previous models. This also means that the circuitry of schematic C-2154 is not driven by what the Imon current is really doing; the Imon pulses are acting more like an "enabling" gate. This circuit is not as smart with Imon feedback as I had thought but nevertheless I still need to understand what it does in more detail.
In summary, I am looking at this circuit (C-2154) more carefully to understand what it really does. I will also model and make some comments on what the Eurotherm Controller should do for a DC command input and how the system should charge versus various but constant phase angles (this is very simple to implement). In my mind, the ideal Firing Angle input should have a "soft start" ramp for about 8 AC cycles followed by an "average" Phase Angle that has an upward ramp to somewhat increase the area of the charging pulses and therefore enable the Thyratron Capacitor Bank to charge more near linear versus exponential. Although this process can be more efficient and quicker in time, the actual charging of the banks is of little consequence as long as no SCR or Diode ratings are exceeded. The ratings are safely limited by the 100k resistor charging string other than for the initial turn on that is governed by the magnetizing current of the transformer core.