Laser pulse energy in simulation

[dmargarone]

Hi,

If my laser pulse is somewhere 2 mm inside the plasma (LWFA) is there any way to calculate its energy ? or, can we calculate the laser pulse energy at any time in the simulation ?

Thank you. DM

[Status-Mirror]

Hey DM,

You can use the total_energy_sum key in the output block. This will tell you the total energy present in the EM fields of the full simulation.

Note that this will include all fields, as EPOCH cannot distinguish between laser-fields and plasma-fields once the laser has entered the simulation window.

Hope this helps, Stuart

[dmargarone]

Hi @Status-Mirror

Thanks for your reply. May be we can do it using Poynting vector ?
If you have time, please, could you see the attached PDF file and the expression under the red rectangle. Do you think this expression could be used to estimate laser pulse energy ?

Thanks a lot. DM

Poynting_vector.pdf

[Status-Mirror]

Hi @dmargarone,

You can output the Poynting flux in each cell using the output block key: poynt_flux.

This describes the energy density of all electro-magnetic waves in the system, which would include the laser. However, in some systems, you can get laser-plasma instabilities, which create secondary EM waves. The Poynting flux would include the initial laser energy, and the energies of any secondary waves created over the course of the simulation.

To eliminate these secondary waves, some people would output the Poynting flux over many close time-steps, which gives you a time-series of the Poynting flux in each cell. This allows you to take a temporal Fourier transform of the time-series, and you can filter out EM waves oscillating at the laser frequency. I've seen this technique used before to isolate the energy of the initial laser pulse.

Hope this helps, Stuart

[dmargarone]

Hi @Status-Mirror,

Thank you so much for enlightening my understanding on the printing flux in the simulation. I appreciate it very much.

Thanks again. DM

[DanRRRR]

I have related question here.

Integrating the pointing flux from each cell to see what is radiation from the plasma makes me a bit uncomfortable. Yes, that's OK for the boundary cells. Probably this is OK also for very low density optically transparent plasma. But what about that flux from the internal cells inside dense plasma which should be reabsorbed and reemitted several times before it gets out of plasma? Looks like we should get the problem here, isn't it?

I see EPOCH tracking for total radiation energy and particles energy in the simulation box (good would be to output this separate for each species and also for E and B fields individually). It also tracks for escaped particles between outputs. But does EPOCH track for escaped radiation to make sure that the energy balance is OK ?

[Status-Mirror]

I'm not sure what you mean in the first question. EPOCH calculates the instantaneous Poynting flux in each cell, not the integrated flux. It's up to the user to decide how to interpret this.

EPOCH does not automatically track escaped radiation, as this would be difficult with our boundaries. For users interested in this, I'd advise using the subset block to output the fields in cells near the boundaries at a rapid rate. These can be used to obtain a Poynting flux through boundary cells, and since the output is rapid, you'll have a good time-series of Poynting flux values near the boundary. A Fourier transform on this time series will tell you the frequencies of EM waves passing through the boundaries, so you can estimate total outgoing radiation, or filter by frequency to track the laser.

Cheers, Stuart