I decided to persevere with the 24v supply and dug up another globe and added in parralell the 0.6ohm load resistor bank from my astratech battery tester.
This allowed allowed around 25A of current to flow half way to my goal of 50A.
But I still suffered a failure of the main FET. The FET is a PSMN8R5 100PS good for 100V and 100A
As a current limiting device I'm using 2 halogen headlights globes in parallel their cold resistance is approximately 0.1 ohm which gives a inrush current of 120A coupled with the 0.6 Ohm resistive load which means the FET has to deal with 140A
I'll have to connect a globe to the scope and have a look at the length of time it takes to heat up and increase its resistance but given i can see the globe intensity increase it is a relatively long time
The PSMN8R5 has a maximum pulsed current of 429A but ONLY for a maximum of 10uS im fairly certain my 140 for what would be 100's of mS is pushing the boundaries of the capability of the FET.
I'll need to purchase some big power resistors and make a better current limiting stage to eliminate the surge currents which just won't occur in the real world application.