Check the attachments.
Just make a long narrow plane, add a bend modifier.
Use a rainbow gradient or texture to assign to a Corona Light Material, depending on your scene brightness youll have to adjust the intensity of it.
Plug a gradient into the opacity slot, a Photoshop made one may do better than 3ds maxs gradient ramp.
Dont collapse the plane or covert it to editable poly, then you can easily just adjust the size of the rainbow whenever.