Clarification and Assistance Needed on `x_start` Definition and Gaussian Beam Profile in EPOCH

[FeliciaHtt]

Dear developers,

I am currently working on simulating a Gaussian beam using EPOCH and have come across a couple of points where I need your confirmation and assistance.

1. Clarification on x_start Definition:
   In the user manual examples, I encountered the x_start variable. I suppose this refers to the laser focal length. However, when setting x_start to any length within the simulation domain, it seems to only accept the minimum or maximum values of the domain. For instance, with x_min = -20 micron and x_max = 20 micron, I receive an error if x_start is set to any value other than x_min or x_max. Could you please confirm if x_start is indeed the laser focal length and why it only accepts the boundary values of the simulation domain?

2. Gaussian Beam Profile:
   Additionally, I would like to confirm the correct syntax for simulating a Gaussian beam. Should we use:
   profile = gauss(y, 0, w0)
   or
   profile = gauss(y, 0, w_bnd)
   for the Gaussian beam profile?

Thank you in advance for your help and looking forward to your response.

[TomGoffrey]

1. x_start is a synonym for x_min, it has no impact on the laser. See the documentation: https://epochpic.github.io/documentation/input_deck/input_deck_control.html
2. I can't answer this without knowing what w0 and w_bnd are, but you might want to consult the documentation on the gauss function: https://epochpic.github.io/documentation/code_details/maths_parser.html

[FeliciaHtt]

Thank you for your reply and sorry for not clarifying what w0 and w_bnd represent.
w0 is the beam waist and w_bnd is the beam waist at boundary.

[TomGoffrey]

As the profile is evaluated at the boundary I would think the boundary value is the correct one to use. You could always perform a quick test run in a vacuum to check if the laser profile is as expected.

[FeliciaHtt]

Thank you very much for the suggestion.