Hi Ahmed,
Please see the publications listed under "Linear Fresnel" for details about SAM's Linear Fresnel model:
sam.nrel.gov/concentrating-solar-power/csp-publications.html
To answer your questions:
1. Yes, you can use a ray tracing model to generate data characterizing the optical performance of the field. The NREL publications mention NREL's SolTrace model as an example:
www.nrel.gov/csp/soltrace.html.
2. The fourth-order polynomial is what is available in SAM and should be suitable for most locations. It is up to you to determine if it is sufficient for your needs.
3. The default solar field data in SAM was developed as part of the collaboration between NREL and Novatec Solar in 2012 described in the SAM Virtual Conference 2012 presentation, which you can find here:
sam.nrel.gov/events.html
Best regards,
Paul.