The Demonstration: A strong magnet is used to move a plasma around inside a tube. The Physics: The plasma tube is used to show you the unique behavior of plasmas. Plasmas are extremely hot gases, at least 10, degrees! When gases get this hot, the atoms are moving very fast and bumping into each other all over the place.
They bump into each other so hard that the electrons are knocked off of the atoms leaving a mix of negatively charged particles electrons and positively charged particles ions. Whenever you see a spark or a stream of electrons, you are seeing a plasma. The plasma tube is a glass tube filled with air. Because it takes so much energy to make a plasma, we have to first pump out some of that air using a vacuum.
But controlling nuclear fusion is not technically simple. Its achievement requires confining these reagents with the adequate density and for a sufficient time, at a temperature over million degrees Kelvin, in a plasma state. No material is capable of withstanding such conditions! How, then, can we build a container for this plasma?
However, attaining a magnetic field capable of effectively confining plasma is a great challenge for controlling nuclear fusion. How can we prevent the highly energetic particles in plasma from leaking from the reactor?
It is well known that, within magnetic fields, charged particles follow curved trajectories. We must therefore ensure that the magnetic force acting on the charged particles in plasma stably balances the force with which they tend to escape.
Due to its higher degree of development, we will focus on the tokamak, a type of closed magnetic configuration in the shape of a doughnut mathematicians call these shapes toroids. There are other concepts of toroidal containment with operational advantages over the tokamak. Three of them are:. This approximation works well for very diluted plasmas, at very low gas pressures. Indeed, when the pressure rises, the particles undergo more and more collisions in their motion around the magnetic flux lines, and each collision makes them "jump" from one "circle" to another, causing them to diffuse perpendicular to the magnetic field, a thing that was impossible without collisions because of the constraining property of the magnetic field.
Nevertheless, this property of the magnetic field is used in Tokamaks to search for controlled fusion, AND in magnetron sputtering reactors, where, although the electrons are not completely confined, their density in the region of containment is still orders of magnitude higher than outside that region.
To give a possibly final answer, basically when you increase the pressure, you lower the mean free path of atoms in the gas less than nm in air at atmospheric pressure , and also the mean electron-neutral inelastic free path depends on the electron energy, typically some microns , which are the characteristic parameters of the plasma. This will make your plasma very small - hence the size of sparks in an electric lighter.
Unless you have tremendously high magnetic fields, or impossibly small speeds not our case since, to ignite a plasma, electrons need speed, or they recombinate fast , this gyroradius will be a lot more than microns.
This means, finally, that your particles are not at all contained by the magnetic field, but rather by the speed of the decay of the plasma: they will usually recombine faster than they will escape the field. Note that in today's applications of plasmas, this fast recombination is not always the case: hence, the Atmospheric Pressure Plasma Jet, which is not a "plasma torch", because it is bi-temperature, non-equilibrium plasma, with cold gas temperature.
This plasma consumes much less energy than plasma torches, and have proven plasma lifetimes high enough to observe "balls" of plasma, drifting together with the gas flow simple mechanical flow , until a few tenth of centimeters don't have the link to the article at hand.
These "balls" of plasma are mostly rich in radicals, i. These radicals play a key role in plasma chemistry, which is why we use this kind of "cold plasma jets": to treat anything that can be "treated" by such radicals embedded in a cool gas flow, including living tissue. Ok, I stop here the speech, just for you to know that, no, there is no "blade of plasma at atmospheric pressure that would be efficiently confined by magnetic fields", but that there are a lot of other types of plasma that would approach, in various ways, the "cool" object you probably had in mind Sign up to join this community.
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