MickaelGrech
commented
6 years ago Hi!
So SMILEI is built up on relativistic normalized units that appear naturally from the Vlasov-Maxwell system of equations. This is explained here, and also discussed in SMILEI paper which you can find here.
Now, if you are considering a simulation relying on the Vlasov-Maxwell system only, i.e. as long as you do not use additional physics modules such as collisions and/or ionization etc..., you can scale your system by introducing a characteristic (reference) angular frequency $\omega_r$. (1) If you are considering a plasma only (e.g. no laser), the best is to chose $\omegar = \omega{p0}$ with $\omega_{p0}$ the electron plasma frequency for a given density of $n_0$ of your plasma. Then this density :$n_0$ will be equal to 1 in the code, and you will have to normalize your density profile to this $n_0$ density. (2) If you are considering laser-plasma interaction, it is better to consider $\omega = \omega_0$, with $\omega_0 = 2\pi c/\lambda_0$ the angular frequency of your laser (with $\lambda_0$ its wavelength). Doing so implies that all densities in the code will be in units of the critical density for the electromagnetic radiation at angular frequency $\omega_0$.
From your previous issue, I guess that you are in situation (2). That means that all times in the code will be in units of $\omega_0^{-1}$, and distances in units of $k_0^{-1} = \omega_0/c$. Obviously, it might be more tractable for you to express everything in units of the laser wavelength in your input file. This is where SMILEI's python input file becomes very useful. Indeed, you can just introduce the normalized laser wavelength $\hat{\lambda}_0 = k_0\,\lambda_0 = 2\pi$, and similarly for times the normalized optical cycle $\hat{\tau}_0 = \omega_0\,\lambda_0/c = 2\pi$. Then, whenever you need to define a distance in your input file (similarly a duration) you will write it as $\hat{D} = (D/\lambda_0) \hat{\lambda}_0$ ($\hat{T} = (T/\tau_0) \hat{\tau}_0$, with $tau_0=\lambda_0/c$ the duration of an optical cycle).
dsbertini
Hi , I got confused with the SMILEI internal system of Units- So let's suppose i have a 2D domain defines by the standard x_min, x_max y_min y_max points expressed in m, x_min = -90.0e-6
x_max = 60.0e-6
y_min = -80.0e-6
y_max = 80.0e-6
Now i have the following number of cells: nx = 15000
ny = 16000
which defines a space step i.e resolution in x y of 10 nm How should i program this simple domain using arbitrary units in SMILEI ? Which reference value, entity should i use , wavelength, frequency ? And most of all how then can i re_scale the output once i choose such a reference ?