Looking Back: Results and Summer Experiences

Looking Back: Results and Summer Experiences
Atoms displaced in a cascade of tungsten. Red area is greater tungsten lattice. PKA = Primary Knock-on Atom

Hello again. This is a little retrospective blog, as my project (2102) winds up and we finalize our report. The Covid-19 pandemic has obviously dictated alot of how the project played out, but despite this I’ve had a brilliant time. I’ve spent it working with the fusion group at the Barcelona Supercomputing Center (BSC), on a project modelling defect cascades in tungsten during nuclear fusion (see previous blog here).

The fusion group were really supportive and let us join their weekly meetings, which really helped us feel part of something other than for reports or presentations! It also allowed me insights into the different directions the group pursued. The project supervisors were Julio Gutiérrez Moreno and the group organizer Mervi Mantsinen and were both fantastic, endlessly patient with our issues and quick with clever solutions.

From the get-go we were introduced to the HPC at the BSC, MareNostrom, and the LAMMPS setup Julio had prepared to get us started. I had never used LAMMPS before so it was fun to get to grips with it as I began to plot out my project. Over the weeks we were exposed to the full gambit of research work from testing code and debugging our simulations to literature review and diving into the current research to situate our work. It gave me a really great insight into the roles individuals play in a large research group.

The other project student Paolo and I have gotten on well which really helped to make the project a happy one. We approached the project from different ends, him working on the calculations of thermal conductivity while I produced defect structures. We were able to support each other quite well when issues arose and the cooperation made the presentations and video we produced very enjoyable.

In terms of results, I was able to establish a successful procedure for the cascade. A picture of it in process is shown below (Figure 1). Initially many atoms are displaced in a wave rippling across the structure but most of these will settle back into usually occupied positions, leaving a lesser number of permanent defects, after having time to settle. In line with literature, the number of defects formed was proportional to the energy of the cascade up to 200 keV. Various potentials for tungsten were tested and showed slight differences in the number of defects formed, but as the differences are small more repeats are being performed to give more statistically validated results. This then allows future work to tie the projects together and calculate thermal conductivity for these cascades, and move onto various alloys of tungsten which are also of interest e.g. WTa.

Figure 1 – Rounded picture of 60,000 atom cascade simulation in the middle of a cascade (left) and after being allowed to settle (right). the larger atom is the initial cascade atom, given high velocity.

A side of things I really enjoyed was getting to grips with the literature on fusion, such as learning about the state of the art in fusion technology and the position of various large experimental reactors in their long-running timetables. The overlap of these massive engineering projects with our atomic level theoretical chemistry, is a fascinating area to study.

Wrapping things up has left me a bit surprised, the months went by so quickly! Overall it was a great way to spend the summer!

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