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Low Temperature Plasma Applications

Low temperature plasma might seem like a contradiction. All of our previous examples required extreme heat to strip the electrons from the ions. So what is happening to the plasma to make it so cool that, in some cases, you can touch it without burning your hand?
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A plasma is a very particular and energetic state of matter usually described as an ionised gas, made of neutral and charged particles. It is mostly known as a hot gas for nuclear fusion applications – but in fact, a plasma can also exist at low temperatures, even close to room temperature, if it is produced in a clever way. The key is that within these plasmas, the temperature is not at equilibrium – different particles may have different temperatures. In particular, the temperature of the electrons can reach several tens of thousands of degrees, while in some conditions, the temperature of the other particles can stay close to room temperature. The hot electrons efficiently generate
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other important parts of low temperature plasmas, including: positive and negative ions, as well as excited states, which are neutral particles with an excess of energy, radicals, which are particles that induce chemical reactions very quickly, . and not to forget photons, that make the plasma bright. Each of these species play a crucial role in the properties and behaviour of the plasma. Apart from thermal non-equilibrium, another property of low-temperature plasmas is that they are weakly ionised – typically, less than one molecule out of ten thousand are ionised. That might seem really small, almost negligible – but it is a good thing!
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This tiny fraction of energised particles creates the specific properties of plasmas while keeping the plasma temperature and energy cost as low as possible. The presence of charged particles gives the plasma its electrical properties, inducing collective behaviours of the particles. In fact, a plasma is an extraordinarily organised medium, driving endless physical phenomena to study. Sometimes they create very visual effects, like this blue dome due to the propagation of an Argon plasma along a conical shaped electrode in a low-pressure environment. To make a low-temperature plasma from a gas, we need to add sufficient energy to the gas. One of the easiest ways is to provide this energy electrically to the medium, by applying a potential difference between two metal electrodes.
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There are other ways to excite gas molecules – like using microwaves. You can even create a plasma from a solid surface, using a high energy laser to vaporise the solid surface and ignite a plasma in the vapour.

Low temperature plasma might seem like a contradiction. All of our previous examples required extreme heat to strip the electrons from the ions. So what is happening to the plasma to make it so cool that, in some cases, you can touch it without burning your hand?

Thermal Equilibrium

As Dr Alexandra Brisset explains in the video above, the answer lies in a physics concept called thermal equilibrium. Two systems are said to be in equilibrium if, when put into contact, no heat flows between them. Another way to say this is that heat will flow between two systems if they are at different temperatures. Temperature can be a tricky quantity to describe, but for a gas or plasma it is relatively simple. It is just a measure of the average kinetic energy of the particles. The more energy the particles have, the higher the temperature of the gas or plasma. If a system is in thermal equilibrium, we can describe that system as having a single, well defined temperature.

Thermal Equilibrium and Low Temperature Plasmas

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The key to understanding low temperature plasmas is in realising that they are not in thermal equilibrium. When we produce a low temperature plasma we put much more energy into the electrons than we do the ions, through subjecting them to rapidly changing electric fields. This means that the electrons are defined by one temperature, (T_e), and the ions by a much lower temperature, (T_i). Moreover, we might not fully ionise the sample, leaving some neutral atoms (just like in a gas) at a low temperature, (T_g). Low temperature plasmas rely on the fact that the energy is not evenly distributed between electrons, ions and neutral particles.

So, the plasma is not entirely low temperature, but the high temperature portion is limited to the electrons and we have far more cold ions and neutral atoms than we do high energy electrons. A material that touches this plasma will only feel the average low temperature of the plasma. Overall, there is far less energy in the plasma, and so it is far less extreme than the fully thermal plasmas we have met so far, but certainly not less interesting or useful.

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