More info on Ricky’s No Laser Simulations

I previously talked about Ricky’s energy conservation simulations without the laser, but I’ll go into more detail about it here.

He used field_ionization = T and use_multiphoton = F in the control block and use_collisions = F in the collisions block. In this way, we keep track of ionization, but since there is no laser to induce multiphoton ionization, we need to turn that ionization mechanism off (or else EPOCH will give an error message).

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In the table above, Ricky showed some of his heating results with collisions on and collisions off and it seems like it doesn’t make an appreciable difference. So, we mind as well turn collisions off just so that we can run more simulations (the heating gain is expressed as a percentage increase from the simulation with 257 electron PPC).

Running More Simulations: Ejected Electrons

I ran a new set of simulations without the laser to try to understand better how we can predict the amount of numerical heating. I ran a bunch of 2D simulations w/o a laser for 500fs and kept track of the electrons currently in the simulation and also the number of electrons ejected from the simulation.

The results were the same regardless of using higher or lower particles per cell or the changing the initial electron temperature. One caveat was that I lowered the resolution 10x to get the simulation to run really fast. If I want to probe this further, I can just re-run this with more a higher resolution. Around 0.1\% of electrons left the simulation at 120fs and 22 left around 500fs. Chris says this may not be physically accurate, the positive ions should prevent the electrons from being ejected. He says its worth our time to try to run the simulations with periodic boundary conditions and see what changes.

Additionally, he said I could try to get an estimate of the turn-around distance of an electron with a certain rms velocity under the influence of a certain e-field. In doing this, I would also need to output the electric fields. Maybe I can just run two or three of these simulations with all the diagnostics and the rest with just the bare minimum energy diagnostics.

Another thing worth doing is finding out not only the amount of electrons ejected, but also their velocities so that I can gain an estimate of how much energy is leaving the simulation through the open boundary conditions.

Heating Simulations

I ran many simulations without the laser varying the PPC and initial electron temperature and here is an example of some of my results

I have a few issues with these results:

1. There is an initial dip in the electron temperature (which we think is related to the sudden transfer of energy from electrons to ions but also the fact that when the carbons/oxygens ionize, they give off slow electrons which lower the average energy) that is not constant when changing the number of PPC or the initial electron temperature. This makes it difficult to use the final electron temperature as a point for comparison
2. For the higher temperature sims, it takes a finite amount of time for the electrons to stop losing energy and to start heating up. This is a limitation of the 120fs simulations, maybe this could be fixed by increasing the simulation run-time.

Refamiliarizing myself with EPOCH

I ran some basic EPOCH simulations and made some animations. First, I simulated a laser interacting with a target at 45 degrees and plotted the electric fields in the x and y direction

Then, I plotted the number density of electrons and protons over time

Things to Do

• Try to figure out an estimate for the turn-around time/distance of electrons and see if that matches up to simulations with tracking ejected electrons
• Run simulations for longer and with periodic boundary conditions which can hopefully be useful.
• Look into what else EPOCH can track (e.g. energy of particles leaving grid)
• Start write up of Jack’s Fuchs V4 Paper on Overleaf.