Threshold energy for ruby laser?

[Samuel M. Goldwasser]

I have been asked by someone to estimate the amount of flash energy
required to reach threshold on a 6 inch x 3/8 inch ruby rod with dielectric
mirrors. It's being pumped by a large helical flashlamp. That's all the
information I have. It may not even have a reflector for the flashlamp.

My feeling is that the required energy will be unrealistically huge,
but would appreciate comments from anyone who has built or used a
ruby laser with a similar configuration.

Thanks!

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