PLASMA. It’s the fourth state of matter, and is generated when gas molecules get so hot that they can’t hold on to their electrons anymore and become charged particles. When that happens, electrical effects become increasingly important to the physics of the flow. In other words, as plasma gets hotter, it becomes one with the (Lorentz) force.
This all sounds pretty exotic, but plasma is more common than you might think. In fact, plasma is the most abundant state of ordinary matter in the universe – it is what stars are made of. The only thing that may rival the mass of plasma in the universe is of course… DARK MATTER.
Here on earth, there are plenty of examples of plasma also: Aurora Borealis, lightning, fluorescent lights, plasma displays in TVs, and spark plugs just to name a few. Here at Alden, we’ve been doing some work with plasma lately, but unfortunately for you dear reader, due to proprietary information agreements, we are not at liberty to discuss these supremely awesome projects. Instead, since Star Wars Day is upon us, here is an introduction to simulating plasma physics using computational fluid dynamics (CFD) by way of lightsabers.
As you are undoubtedly aware, lightsabers are fictional energy swords wielded by Jedi in the Star Wars movies, and their defining feature is a plasma blade that is about 3 feet long. The first thing that is needed to create plasma is enough energy input to heat the gas up to the point where it will start to ionize. Obviously, Jedi use the force for this, but for us normal folk, this requires a strong enough electric potential (voltage) field to maintain the plasma, and an ignition source to get things started.
Lightning and spark plugs are examples of direct current (DC) plasma. They are initiated by a large enough voltage difference that there is a spontaneous electrical breakdown that causes a spark. Once the spark occurs, the hotter gas in the spark is a much better conductor, causing more current to flow, and the increase in electrical current heats the gas further in a positive feedback that grows into a whole lightning bolt. In the case of lightning and spark plugs, the bolt quickly uses up the potential energy embodied in the initial voltage difference, and the whole magnificent affair is over in short order.
Plasma can also work with alternating current (AC) such as in fluorescent light bulbs. These are usually filled with a low pressure noble gas such as Neon or Argon, which are very good at ionizing with low power requirements. The key here is that the gas needs to get up to a hot enough temperature – thousands of degrees – so that the gas molecules can shed their electrons and become good conductors. The heat is generated from the friction of ripping electrons away from molecules, only to have the molecules grab them back again. As the gas heats up, the ions are not as good at grabbing the electrons back, and the gas becomes a better electrical conductor. You may have noticed this with fluorescent light bulbs taking a minute or two to get to full brightness – they need to heat up the gas first.
Of course temperature, gas composition, and flow patterns also play a role in defining the appearance and behavior of plasma, and how it manifests in natural and engineered environments. Just for fun, here are a few pictures from a CFD simulation of Rey with her lightsaber, seeking a deeper understanding of the mysteries of the universe. May the 4th be with you.
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