Setting the Laser Initialization Cut-Off

Section author: Axel Huebl

Laser profiles for simulation are modeled with a temporal envelope. A common model assumes a Gaussian intensity distribution over time which by definition never sets to zero, so it needs to be cut-off to a reasonable range.

In laser.param each profile implements the cut-off to start (and end) initializing the laser profile via a parameter PULSE_INIT \(t_\text{init}\) (sometimes also called RAMP_INIT). \(t_\text{init}\) is given in units of the PULSE_LENGTH \(\tau\) which is implemented laser-profile dependent (but usually as \(\sigma_I\) of the standard Gaussian of intensity \(I=E^2\)).

For a fixed target in distance \(d\) to the lower \(y=0\) boundary of the simulation box, the maximum intensity arrives at time:

\[t_\text{laserPeakOnTarget} = \frac{t_\text{init} \cdot \tau}{2} + \frac{d}{c_0}\]

or in terms of discrete time steps \(\Delta t\):

\[\text{step}_\text{laserPeakOnTarget} = \frac{t_\text{laserPeakOnTarget}}{\Delta t}.\]

Note

Moving the spatial plane of initialization of the laser pulse via initPlaneY does not change the formula above. The implementation covers this spatial offset during initialization.