I've seen that charged particles in plasma are shielded at a distance of a Debye length, so how is it possible that charges in plasma interact on a long range distance too? It seems impossible to me because they should not see each other when they are far. Maybe an example would help
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$\begingroup$ from what I read here sciencedirect.com/topics/earth-and-planetary-sciences/… " the Debye length defines the distance over which individual particle potentials are shielded. Where do you get the idea that there are long range interactions?" The Debye length represents the physical scale of the transition from plasma collectivity to individual particle behavior. It interfaces between the physics of micro and macro scales" $\endgroup$– anna vCommented Aug 5, 2021 at 19:27
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$\begingroup$ How it's possible that collectivity behaviors happen? if all the particles are shielded they should be independent after a certain distance right? $\endgroup$– SimoBartzCommented Aug 5, 2021 at 19:53
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$\begingroup$ they are, the definition becomes classical, no elementary particles detectable, afaik. You should read the article. $\endgroup$– anna vCommented Aug 6, 2021 at 3:30
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$\begingroup$ @annav - The Debye length is considered long-range compared to nuclear interactions and/or neutral-neutral particle collision scale lengths in many contexts. In lab plasmas and inside stars, the Debye length can be tiny, yes, but most plasmas by volume have macroscopic Debye lengths (i.e., meters) which is huge compared to atomic scales. So the long-range is just referring to the Coulomb interactions. $\endgroup$– honeste_vivereCommented Aug 6, 2021 at 12:47
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I've seen that charged particles in plasma are shielded at a distance of a Debye length, so how is it possible that charges in plasma interact on a long range distance too?
Long-range is referring to the Coulomb interaction between free-charged particles within a Debye sphere. A Debye sphere may seem small to you, but compared to atomic interactions or neutral-neutral interactions, the distances can be huge.
How it's possible that collectivity behaviors happen?
The collective behavior of a plasma results from the accumulated effects of all these Coulomb potentials. Electric fields do work to get rid of themselves, thus how Debye spheres form. Because of the long-range interactions of the electric fields, you cannot "push" one particle without it affecting all the others within a given Debye sphere. Thus, the system begins to behave somewhat like a fluid.
Note that most plasmas, by volume, in the universe are in the range of weakly collisional to collisionless.
if all the particles are shielded they should be independent after a certain distance right?
Yes, they are over distances longer than roughly one Debye length. That does not impede collective behavior. Take the water in a glass of water, for example. Any given particle's immediate sphere of influence is defined by its collisional mean free path. In this case we are talking about length scales on the order of sub-microns (it's actually much much smaller than a micron). Unlike a plasma the molecules in water do not care about the collective influence of a surrounding sphere of water molecules, they only care about the next water molecule with which they collide.
Note that when the collide, they don't actually touch. Neutral particle collisions are also mediated by the electric fields from Coulomb potentials but they don't see these fields until the inter-nuclei separation is on the same order of magnitude as the collisional cross-section of interaction. For water-water molecule collisions, the cross-section is on the order of ~10-15 cm2.
In fact, if you are worried about shielding, you can actually think of a monatomic atom as a super simple analogy to a plasma. The nucleus is positively charged while the electron cloud is negatively charged. However, outside the electron cloud, the total electric field from the atom asymptotes to zero rather quickly.
answered Aug 6, 2021 at 12:45
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$\begingroup$ So the collective modes of the plasma should not create electrical fields? because its the motion of neutral particles $\endgroup$ Commented Aug 9, 2021 at 23:42
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$\begingroup$ No, the normal modes of a plasma (i.e., the thermal oscillations that are always present) are electric and magnetic fluctuations. Debye shielding is more about quasi-static fields than oscillating ones. For instance a Langmuir wave typically has a wavelength on the order of an electron inertial length (sometimes called the skin depth). In most plasmas this is much much larger than the Debye length. It's fine because the charges are oscillating, i.e., the electric fields are localized to the wave and are doing work to eliminate themselves. $\endgroup$ Commented Aug 10, 2021 at 13:17
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$\begingroup$ Do you have Langmuir wave and Debye spheres structure at the same time? or does the Debye spheres structure vanish when Langmuir wave starts? $\endgroup$ Commented Aug 10, 2021 at 15:24
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$\begingroup$ @SimoBartz - No, the electrons oscillating along the magnetic field temporarily create a localized electric field. The surrounding plasma is otherwise quasi-neutral but the oscillating electrons act as an oscillating charge imbalance that violates quasi-neutrality, but only locally. It is actually the electric field generated by this displacement of one charge species that acts as the restoring force to try and drive the electrons back into a quasi-neutral state. $\endgroup$ Commented Aug 10, 2021 at 15:39
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$\begingroup$ @honeste_viviere So the electrical field can be generated by collective modes but it's weak because it's due to non neutral regions of a Debye length dimension? $\endgroup$ Commented Aug 10, 2021 at 16:45