thermal conductivity - Copper gas line / Fresnel lens?

I’m working on ‘solar oven’ that uses a large Fresnel lens as the power plant for heat. The idea is to have a cooking chamber and a heating box on the outside. The Fresnel lens will generate about 1400 F with a .3 inch spot beam. The beam will be focused on a bundle of 6, ¼ inch copper gas lines. The lines go into the cooking chamber in simi-circles of 1 foot each in length . I’m wondering what the heat transfer and estimated heat output of the inner copper lines would be. Not sure how to figure this and have no clue how to work the formulas I did find – Thanks for any halp.

The cooking chamber will be brick / concrete about 9 inches thick with an internal cooking space of 8in x 8in x 11in