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?

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

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.