8
$\begingroup$

Plasma is ionized gas which as far as I know only occurs at high temperatures. When plasma cools down it tends to recombine with the electrons present and turn back into gas. But what if the disassociated electrons in the plasma were removed and the plasma were allowed to cool down in a vacuum, while being held in place by a strong magnetic field. Would this substance still be plasma? Is this possible?

$\endgroup$
4
  • $\begingroup$ If you remove the electrons then the resulting gas would have positive charge. Positively charged particles repel each other. So do you mean 'allowed to cool in a vacuum yet held in place somehow by XXXX or YYYY' ? $\endgroup$
    – Hennes
    Commented Feb 26, 2013 at 1:20
  • $\begingroup$ for the sake of this thought experiment, yes lets suppose its held in place by a strong magnetic field. But shouldn't one molecule of plasma still be plasma? $\endgroup$
    – Loourr
    Commented Feb 26, 2013 at 1:24
  • $\begingroup$ I think I would view it as an atom core, but I really feel out of my depth at this. Which means I will be watching this question and hopefully enjoying the upcoming answers. $\endgroup$
    – Hennes
    Commented Feb 26, 2013 at 1:30
  • 4
    $\begingroup$ Related: What is non-thermal plasma? and Plasma and Plasma Globes. $\endgroup$ Commented Feb 26, 2013 at 1:30

4 Answers 4

6
$\begingroup$

By definition, a plasma is neutral.

Plasma is loosely described as an electrically neutral medium of positive and negative particles (i.e. the overall charge of a plasma is roughly zero).

There is such a thing as a non-neutral plasma. You can construct these by storing charged particles in a Penning trap. These don't display the kinds of properties that you expect from neutral plasmas. For example in neutral plasmas there is a "screening" effect where, because the charge carriers are fairly mobile, any concentration of charge is immediately surrounding by opposite charge, limiting the radius over which its influence is felt.

Note that the electric force is incredibly strong. Depending on how you measure it it's between around $10^{39}$ and $10^{42}$ times as gravity. So even a tiny amount of completely charged plasma would be very hard to contain.

$\endgroup$
2
  • $\begingroup$ So I suppose indirectly what your saying is that while it may be possible to achieve this, it would require lots of force and then it may no longer even be plasma? And I'm assuming that this implies that once this the positively charged part is isolated it may be relatively easy to cool down? $\endgroup$
    – Loourr
    Commented Apr 11, 2013 at 2:18
  • $\begingroup$ @Loourr I don't see any ground for your assumption that it implies that positively charged part, once isolated, may be relatively easy to cool down in Dan Piponi's answer. This looks like pure wishful thinking on your part. $\endgroup$
    – Alfred
    Commented May 16, 2022 at 0:38
3
$\begingroup$

If the density is extremely small, the recombination rate of free electrons with ionized atoms (or just protons) can exactly balance the thermal ionization rate even at reasonably low temperatures. It depends on what one calls "cold", and also what fraction of neutral atoms (a few percent, or only one part in a million ?) one accepts and still calls it a plasma. But yes, in principle, a plasma can be cold if it is extremely dilute. Choose a temperature, a given fraction of atomic hydrogen, and there will always be a density low enough that the equilibrium of the plasma will be reached at your chosen temperature and you chosen fraction of atomic hydrogen. With one electron and one proton per cubic parsec, I guess you can get only 0,01% atomic hydrogen for pretty low temperatures !

$\endgroup$
1
  • 1
    $\begingroup$ In particular, a "cold cathode gauge" measures vacuum pressure by measuring the conductivity of the residual gas in the vacuum chamber. Cold cathode gauges only work below the plasma transition pressure, which is low at room temperature, but quite accessible. $\endgroup$
    – rob
    Commented May 5, 2022 at 20:25
2
$\begingroup$

There is also plasma in outer space, mainly Hydrogen and Helium nuclei, with a very large range of temperatures. Much of it is very hot, but much is also cold.

See for example http://en.wikipedia.org/wiki/Interstellar_medium

Meanwhile, on Earth, experiments have been done on trapped particles, some of which are ionized. It would be a bit misleading to call this a plasma, as they are being held apart (as you said in your comment) by an e-m field, but they also can sometimes strongly interact with each other.

$\endgroup$
-3
$\begingroup$

Plasma is a special form of gas that only exists at temperatures above 6000 degrees Celsius.

Pearson Science

$\endgroup$
1
  • $\begingroup$ This schoolbook definition is different from the definition used in vacuum engineering; see this other answer on this page. $\endgroup$
    – rob
    Commented Apr 29, 2023 at 4:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.