A plasma state is a plasma. We call plasma the fourth state of matter, after the other three: liquid, solid and gaseous. It really is an ionized gas. We have a stable and neutral gas and we provide it with energy, usually in the form of an electrical discharge. What happens then is that part of the atoms of that gas are “broken”. The electrons on the outermost part of the atom get enough energy to separate from the nucleus and move freely through the gas. That is an ionized gas, that is, a plasma or a plasma state.
We call plasma the fourth state of matter, after the other three: liquid, solid and gaseous.
In daily life we find these ionized gases or plasmas: phenomena of nature such as the northern lights in which part of the atmosphere, which is a gas, is ionized by particles that arrive from outer space or the rays of storms in the that the potential difference between heaven and earth causes that when the circuit is closed and connected, the air is ionized and that is the glow that we see as lightning. Outside of nature, but more within reach, we have more technological plasmas such as fluorescent tubes that are tubes with a gas inside to which an electrical discharge is applied, that is, the circuit is closed, and that generates a plasma. And there are also the plasma balls, those toys from which beams shoot out, which you bring your hand closer and the beam goes towards the finger. Well, they are the same, a gas inside that ball to which a small voltage is applied and the result is, again, a plasma.
If we go to other less everyday applications there are some of normal use in the industry such as welding with plasma arcs, some disinfection processes or also in medicine, for example electric scalpels are exactly the same, a plasma that cuts and cauterizes . And we also have plasmas under investigation. The most famous of all is the ITER reactor that is being built in the south of France. What we want to achieve in ITER is fusion energy, which is the opposite process to fission with which we now obtain energy in nuclear power plants. Fusion consists of joining two atoms through tremendous collisions. In this process, a huge amount of energy is released that we want to take advantage of. To achieve this fusion, it is necessary to start from a highly ionized plasma, that is, with many of its “broken” atoms. Through magnets and magnetic fields we bring them closer to each other until in the end they have no choice but to merge. Plasma is used because if it is not very difficult to have so many electrons and so many ions, (atoms that have lost electrons), that travel free.
There are many parameters that can be measured
To your question about how a plasma can be measured, I have to start by telling you that there are many parameters that can be measured. We can measure the degree of ionization, how many atoms are “broken”. If you apply little energy, few will break, but if you contribute a lot of energy, many of them will be released. This is called the degree of ionization and is linked to the first parameter that we can measure, which is density. We can measure ion density and electron density. And it is measured like any other particle density in cm-3.
We can also measure plasma temperature, although when we talk about temperature in a plasma we mean the energy in the different particles. What you measure are the speeds, and with kinetic energy you relate it to temperature. If we are measuring the energy of ions we speak of an ionic temperature and if it is electrons we speak of electronic temperature. The units of these energies are called electron volts, the usual joules are not used, nor the degrees because it is not a real temperature.
You can also measure the distribution of the particles, that is, how they are organized within the plasma because depending on whether we apply a positive voltage to it, we will attract all the electrons and on the contrary, the ions will go to the negative side. You can also measure the electric field, that is, the voltage distribution within the plasma that the structure actually gives you.
But measuring a plasma is very complicated. We cannot reach in with a device to measure what we want to measure. For starters, they are in vacuum chambers, so it is difficult to access there. Then there is the question of pressures and temperatures, which can be very low, as well as enormous, hundreds and even thousands of degrees. Despite this, different measurement methods have been developed. One of the most classic is that of metallic probes that are introduced into the plasma to apply a small current. From there you can get information. The problem is that this method is invasive, it disturbs the plasma. To have better resolution and not disturb the plasma, there are methods based on optics, especially in emission spectroscopy, that is, collecting the light that this plasma emits and analyzing it helps us to know things such as its density or temperature. There are also active spectroscopy methods in which they can introduce, for example, a laser radiation focused on what you want to measure and you can interact with the plasma in a very local way so that the disturbance is minimal. And this method allows obtaining measurements of very high spatial and temporal resolution. I dedicated my thesis precisely to this.
Veronica Gonzalez Fernandez She is a doctor in physics, professor and researcher in plasma physics and optics at the Complutense University of Madrid.
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