This post covers work from 09/16-09/27. I worked on finishing up all the titan simulations for the paper and I also started to get back into the synthetic data paper work as well.

Titan Simulations

Collisional Ionization

In my simulations, I did not use collisional ionization, because when I did try, the simulations seemed to take too long. This paper talks about the role of collisional ionization in heavy ion acceleration by high intensity laser pulses. They say that the acceleration of gold ions up to 7 MeV/nucleon has been reported up to charge states of Z=69 (Ne-like) in experiments and the new (at the time) collisional ionization model from EPOCH allows for high charge states. They say that the collisional ionization process is the most important reason for increasing charge states well beyond Z=51 and up to Z=77. However, they say that in experiments, dominant charge state decreases with target thickness, while the simulations imply that it should be constant. Since we used a thick target (much thicker than the 0.5 micron and below they explored in this paper), its possible that the collisional ionization module wouldn’t give us higher charge states.

Above, they show the dominant charge states of FI + CI (Magenta), FI (Cyan), and Experiment (Green). It looks like field ionization is limited to go up to a certain charge state. They also mention that recombination is a feature not contained in EPOCH, although they say it is usually not believed to be important during or shortly after the laser-target interaction.

Simulation Setup

Here is the more complete simulation setup overview:

Since last time, I’ve added the position of Probe 2 (the further back probe) and the polarization directions (which are in-plane or p-polarized). I compared the 20 Degree Single (OAP1), 41 Degree Single (OAP2) and both together as double pulse for the flat target without a exponential scale length to start.

Comparing Single and Double Pulse with Flat Target

We can see that the total energy in the simulation is similar for all of them. Actually, OAP1 has slightly higher energy than OAP2, but this isn’t relevant because OAP2 is able to convert a much higher fraction of the laser into both protons and electrons. Additionally,

there is also much more energy in Au as seen by the brighter color in the ionization plots. From Probe 1,

we can see much higher max proton energies for the double pulse and

the emitted proton beam is much more symmetric as well.

Comparing Double Pulse with PP to without

Above, the initial number density is depicted where we can see the exponential scale length pre-plasma. Around 4 microns of the pre-plasma is above critical density and the rest is below (with a cutoff of 0.02 $n_c$). The density is constructed in such a way that the total number of particles remains constant. The exponential scale density has a slightly higher conversion efficiency to protons and electrons, but a lower overall max proton energy.

Another interesting thing to note is in the proton angular energy distribution:

which shows more filamentation in the proton energies of the flat target. Enam mentioned that this is probably due to the pre-plasma acting as a sort of density gradient that guides the protons better similar to LWFA.

Comparing Single and Double Pulse with PP

With the PP, interestingly, the single pulse is able to achieve higher than 30 levels of ionization, which shows that there is not some fundamental limit there.

The Single Pulse actually gets slightly higher max proton energies

but seems to be filamented more

Conversion Efficiency

Below are plots showing laser to electrons and protons conversion efficiency for the double pulse

and just looking at the single pulse will give similar qualitative behavior: higher efficiency with the 2 micron scale target. Below, are plots showing the improvement (double pulse energy divided by single pulse energy in electrons/protons) for the flat target and 2 micron scale target cases

which all show higher conversion energy from double pulse, yet the effects for the 2 micron scale target are not quite as large as for the flat target