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Plasma can be formed by an altogether combination of ions, free electrons, atoms, and molecules. Searching a little bit on the internet I found that plasma can't be formed entirely by electrons, because they will fly apart. I wanted to know if plasma can be formed by ions only, as in, cations and anions, but no free electrons or neutral atoms/molecules.
Edit: Seems like electron plasma are also possible.
In principle this is achievable, and is known as an ion-ion, or simply an ion plasma. In practice (that is, in the lab), however, there is always some small level of contamination, which may or may not be meaningful. Have a look at this paper, where a hydrogen ionic plasma with a fractional electron concentration of $n_e/n_{+} \sim 10^{-2}$ is produced via the use of a control grid for electron removal. The resulting ionic plasma is composed almost purely of negative and positive ions, containing molecular ions.
