Image 1 Caption: A diagram of a spectral tube (AKA discharge tube) (a special case of a gas-filled tube). A direct current (DC) is used for simplicity. The electrical potential difference (AKA voltage) causes an electric field which causes the free cations (positive ions) to move toward the cathode (negative electrode) and the anions (negative ions) and free electrons (negatively charged particles) to move toward the anode (positive electrode). The ammeter (circle-I) and voltmeter (circle-V) are shown by standard electric symbols.

The electric field in the spectral tube causes an electrical breakdown of the neutral gas in the spectral tube and turns it into a plasma (i.e., an ionized gas).

Explication of plasmas (i.e., ionized gases):

An atom contains a small nucleus made of protons and neutrons: size scale 1 fm = 10**(-5) Å = 10**(-15) m which is 10**5 times smaller than the atomic size scale.
The protons and neutrons make up nearly all the mass and they are bound by the strong nuclear force.
Electrons surround the nucleus in quantum mechanical atomic orbitals (the quantum mechanical analogs of orbits though they are very different from orbits in many ways) and are bound to the nucleus by the electric force. They make up only a tiny bit of the mass, but give the atom its size scale of 1 Å = 10**(-10) m.
Atoms and molecules (which are bound collections of atoms) with equal numbers of protons and electrons are uncharged (i.e., electrically neutral). This is the usual state for atoms and molecules under relatively low temperatures and sufficiently high densities (but NOT extremely high densities).
If an atom or molecule becomes charged by losing or gaining electrons, it is called an ion.
Losing/gaining electrons creates positively/negatively charged ion.
The losing of electrons is called ionization.
In fact, negatively charged ions can only exist if there free electrons, and so there must be positively charged ions for there to be negatively charged ions, but the reverse is NOT true. Highly ionized gases usually have NO negatively charged ions at all.
Negatively charged ions though sometimes important in chemical reactions and opacity are usually a small minority species.
Electrical breakdowns, sufficient heat energy, Townsend discharge, and other ionization process can cause ionization.
A gas with a significant fraction of atoms and/or molecules ionized is a state of matter called a plasma.
Note yours truly like many other astronomers just considers a plasma to be a kind of gas (i.e., an ionized gas) and seldom uses the word plasma.
Most plasmas we encounter in experiments are low-ionization plasmas where the atoms have lost (or gained) only a few electrons.
Deep in the interior of stars, one gets an extreme plasmas where all an atom's electrons are stripped off (i.e., the atom is completely ionized) and one just has a gas consisting of unbound electrons and bare atomic nuclei. The stellar photospheres and adjacent regions are usually only partially ionized.