NSDI Pulse

Introduction

This is an extension of a previous NSDI code using the strong-field approximation that was valid for monochromatic fields. This code is valid for a sin-squared laser pulse.

Prerequisites

To install this code you will need the following libraries installed:

• CMake (to compile)
• GSL (GNU Scientific Library)
• OpenMP (parallelization library)
• Catch2 (if you want to use the test features---if not you can simply not build the text executable) Now CMake will find and install Catch2 for you but only if you decide to build the tests.

Install and compile

First download the code from GitHub however you prefer. Then compile using CMake. If you do this over the command line go to the root directory and run the following:

cmake -S . -B build

This will make a build directory and configure the CMake file. If you get errors hee it is likely you need to install dependencies or set up CMake properly. Then, to compile and generate an executable run:

cmake --build build

Note this builds both the main project NSDI_Pulse and the tests test_NSDI_Pulse. If you only want to build the main project for now instead run:

cmake --build build --target NSDI_Pulse

This will only build NSDI_Pulse, the executable can then be found in build/src/NSDI_Pulse. It may be easiest to import the project as a CMake project into Eclipse, Visual Studio Code, or another C++-friendly IDE.

Running Tests and Test through GitHub actions

There are a number of unit test for functions and class methods. These are run using the testing library Catch2, which CMake can get for you if you compile the tests, which are in the folder test/. To run the tests, from the build/ directory run the command ctest --build-config Release. Alternatively, this project has continuous integration build and test setup on GitHub Actions, so every time a change is commited to the main branch the code will be compiled and tests will be ran on GitHub servers.

Structure of code

The main function is located in the file src/NSDI_Pulse.cpp, which is where the code begins its execution, this file is what makes the final executable. Here, we have code that makes a Data/ folder for export and sets some parameters. a parameter file could be used instead but due to the small size of this code, I didn't implement that yet. One of the parameters set is the field type and laser field class, this can be switched between monochromatic and sin2 (sin-squared).

An important class is speGrid, which defines a grid of saddle point solutions. Here saddlePointGrid is declared, this is where the saddle point equations are solved. The saddle point equations are solved over the entire grid by first picking a single point in momentum, and then solving this point randomly. What this means is many (currently 2000) random complex times are generated for the single point in momentum. These times are used as guesses for a rootfinding method to solve the saddle point equations. This will exhaustively provide all solutions for a specific momentum. This is a slow way to find solutions but we only need to do it for a single point. After that the solutions for that one point are used as guesses for all the adjacent points, and this can be used iteratively to propagate the grid with all the solutions. The number of solutions can vary from point to point, this means you should choose a starting point that has many solutions so you don't miss any.

After this, the computation of the Action is straightforward, as it is just evaluating functions with the saddle point solutions.