I humbly suggest you forget the torch.

A shape like that would normally be formed on a metal rolls. Your local welding shop or metal railing fabricator probably has one. Rebar may not run through the rollers smoothly, but mild steel round stock is cheap and chances are they already have some handy.

The DIY option is to use a stake and some string to scribe a 6' radius half-circle in the dirt. Pound in some stakes along the arc and then wrap your rebar around them. Let it spring back and guess how much smaller your next circle needs to be.

Stay on the INSIDE of the circle while you're bending and keep other people away. !