From fusion and astrophysical plasmas to spacecraft propulsion, Princeton Plasma Physics Laboratory’s Dr Fatima Ebrahimi discusses her work across a variety of areas within plasma physics
Plasma physics sits at the heart of some of the most fundamental and ambitious questions in modern science, from how stars and black holes behave to how we might harness fusion energy and propel spacecraft through the solar system. For Fatima Ebrahimi, Principal Research Physicist at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory’s (PPPL) Theory Department and an Affiliated Research Scholar at the Department of Astrophysical Sciences, Princeton University, these complex problems are central to her broad portfolio of work across the wide spectrum of plasma physics.
Having worked in magnetohydrodynamics for over 20 years, Dr Ebrahimi has worked on a wide range of topics and written papers published in a number of leading peer-reviewed journals, including Physical Review Letters, Nuclear Fusion, Physics of Plasmas, Astrophysical Journal, and Monthly Notices of the Royal Astronomical Society. A significant achievement for Dr Ebrahimi was inventing the first concept for a plasma thruster that uses fast magnetic reconnection.
Georgie Purcell, Senior Editor of The Innovation Platform, spoke with Dr Ebrahimi to find out more about her career, the multi-faceted world of plasma physics, and her current focuses for the near future.
What are the main focus areas of your work in plasma physics?
My work is concentrated in the areas of fusion plasma physics, astrophysical plasmas, and plasma propulsion. My background is in fusion plasma physics, and I first trained in this at the University of Wisconsin-Madison.
During my PhD in Wisconsin, I worked on the reversed-field pinch (RFP) – a very low-field type of magnetic confinement fusion device with interesting plasma physics aspects, such as the generation of magnetic fields and occurrences of magnetic reconnection. This experiment developed my interest in common subjects within plasma physics, including reconnection, dynamo, and magnetic field generation.
After my PhD, I worked at the University’s Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas (CMSO), where I broadened my research portfolio to study plasma astrophysics. Here, I investigated astrophysical problems mostly relating to plasma physics, including things like black holes, matter accreting into black holes, and various instabilities happening around compact astrophysical plasmas.
When I came to Princeton University, I brought an interdisciplinary way of thinking about plasma physics. I started working with the National Spherical Torus Experiment (NSTX) – a spherical tokamak at Princeton Plasma Physics Laboratory (PPPL). Here, I worked on the startup of plasma current formation. As spherical tokamaks are really compact, there isn’t much space to put a lot of magnets in the core of the tokamak. You therefore need to utilise innovative techniques to start the plasma current. For NSTX, we used something known as ‘helicity injection’, where we injected magnetic helicity into the spherical tokamaks to create plasma current. This work led to my invention of plasma propulsion using magnetic reconnection.
I also lead a project which is part of the Scientific Discovery Through Advanced Computing (SciDAC) programme at the U.S. Department of Energy (DOE). This is a multi-institutional work studying the stability of plasma, especially near the edge of it in a tokamak. Sometimes these instabilities, known as ‘edge-localised modes’, are unwantedly triggered at the edge of the plasma in a tokamak. Therefore, you want to control them, predict them, or find a way to avoid them.
Part of my work with the SciDAC programme is to use advanced computation to validate our codes against experiments to see how we can avoid these edge-localised modes. I also work in astrophysical plasmas and plasma propulsion.
How has your work progressed to where it stands today?
As previously stated, I worked on current generation techniques using helicity injection at NSTX. When using computation to help accelerate understanding of the experiment, I discovered that fast magnetic reconnection happens in NSTX during this plasma startup. Fast magnetic reconnection is a physical process in a plasma where oppositely directed magnetic field lines snap, break, and rejoin, rapidly converting stored magnetic energy into kinetic energy, thermal energy, and high-speed particle outflows. Through my simulation, I predicted that many plasmoids (plasma objects made of plasma and magnetic fields) are produced in NSTX when current sheets that form between the two regions of oppositely directed magnetic field lines break into bubbles, triggering magnetic reconnection. This discovery led me to question why we hadn’t yet seen this in the experiment. We reanalysed our various discharges and found that these objects had indeed been captured in the camera images and that they move relatively fast and with high velocity. This theory was then experimentally verified, and the results were published in a paper in 2015.
From here, I devised the idea of using this method for accelerating plasma for space propulsion in a simplified thruster configuration. This involved configuring coils to be able to create the current sheets that then break into the plasma objects, allowing plasma to be accelerated to high velocity to produce thrust. Princeton University now has the patents for this invention.
We are currently building an experiment at PPPL called Plasmoid Magnetic Reconnection Thruster (PMRT), which will expand on this concept to propel plasma at high velocity and produce net thrust.
There are also additional experiments and facilities at PPPL, called the Magnetic Reconnection Experiment (MRX) and the Facility for Laboratory Reconnection Experiments (FLARE), which are dedicated to studying the process of magnetic reconnection. This is mainly academic research to study how reconnection is a multi-scale, multi-physics problem. I’m involved in those experiments too. However, those experiments are not designed to create net thrust because, when you have reconnection, typically your plasma moves both left and right – creating bidirectional outflows cancelling the net thrust.
How is the construction of the PMRT experiment progressing so far?
PMRT is supported by internal R&D funding from the Laboratory Directed Research and Development (LDRD) programme. It is a mid-sized experiment, and its construction process spans a couple of years. So far, we have completed the detailed design for it and have procured the experiment’s main components.
We now need to energise the device. We have a large capacitor bank system and power supply system at the lab. We have procured our coils and are in the process of procuring other components, such as vacuum vessels. Assembly will take place this year, and we expect to get the experiment running within the next 12 months.
What would you have to demonstrate in this experiment to take it from a concept to a viable technology?
For it to be a viable technology, we must show several things.
We must demonstrate that we can operate the system. In the NSTX experiment, we saw the unidirectional plasma moving at high speed. We want to see similar plasma movement in the PMRT experiment. We must measure the velocity of the plasma object and the force of the thrust through various diagnostics. We also want to test the flexibility of the device, as flexibility will enable us to operate at various magnetic field strengths. We want to see how the thrust scales with magnetic field to allow us to control it. We also plan to measure how efficient the thruster is and test its power scalability. If the system is very power scalable, it will enable us to go to lower power at a lower magnetic field. The experiment is fairly flexible, as we can tune the magnetic fields in the coils and then see how low or high we can go in terms of power. The advantage of doing that is that, at low power, you could use the thruster powered by solar panels. At higher power, the thruster needs to be nuclear-powered. With the higher power, you produce more thrust. With the solar-powered version, the short-term goal in terms of viability is to use this for transport from low-Earth orbit to the Moon. There are thrusters doing this already, but the advantage of the PMRT is that it is fuel-flexible. Today’s Hall and ion thrusters typically rely on noble gases such as xenon that must be carried from Earth, whereas this thruster can use fuels obtained from local resources found in space. Then, at higher power, you can travel to Mars and beyond.
What could success in this experiment mean for real-life applications?
For real-life applications of the PMRT, we are looking for a simple and compact system. If the system includes external components for things like field variation, pulsing, and radio frequency waves, it will become very complex, and there will be wear and tear on the thruster itself. PMRT is simplified and compact as we only use DC magnetic field and DC electric field to ionise the gas.
Another real-life application of the PMRT is for fusion. In principle, two plasmoids could be merged and heated through reconnection — a possible route to fusion conditions we may explore in the future.
What are the main challenges when working with plasma? How can they be overcome?
Plasmas can be unpredictable sometimes, in terms of stability and turbulence. This means that further experimentation may be required if we don’t understand something at first. Today, high-performance computing and artificial intelligence (AI) can significantly aid in modelling and understanding the details of the plasma, as they enable us to see what’s happening in the entire plasma.
Another difficulty can be plasma-material interactions for all the applications of plasmas. Wherever you have plasma, the plasma is eventually going to interact with material walls. This is a difficult problem in plasma physics, as it can be hard to predict how the plasma will interact with material.
Whilst we encounter significant challenges when working with plasma, they are not detrimental to the experiments. They can be mitigated, and it is possible to find solutions. The innovations available today, such as AI and advanced computation, make it easier to solve such issues.
Please Note: This is a Commercial Profile
Please note, this article will also appear in the 28th edition of our quarterly publication.
