The existence of a welding arch is provided with the plasma conductivity thanks to which through the arch there passes welding current. In turn, the plasma conductivity is caused by existences of the electrons and ions which are formed of neutral atoms by their ionization. The intensity of the atoms ionization in specific conditions is provided with the size of the energy demanded for ionization of atom (Wi). Therefore, the knowledge of the size Wi and its dependence on concrete features of a welding arch is very important parameter.
In this article we will speak about the dependence of the size Wi on plasma temperature and the dependence of the size Wi on the atoms location connected with it in the welding arch. The thermal ionization of atom in plasma happens thanks to its collisions with other particles of plasma. The number of collisions in unit of time (ν) can be defined by the formula:
ν = π√ 2d2 nV ,
where d — the atom diameter, m; n — the particles concentration, 1/m3; V — the thermal speed of atoms, m/s.
The n and V sizes are defined by formulas:
V = √2*( kT∕m),
where р — the plasma pressure, Н/m2;
k = 1,38*10-23 J/K — The Boltzmann constant; Т — the plasma temperature, К; m — the atom mass, kg.
Let’s determine the ν size for the welding arch burning in argon with the atmospheric pressure for which the average temperature is equal to 104 К. For argon: d = 3,84*10-10 m, m = 66,3*10-27 kg, then ν = 9,7*108 1/sec.
The residence time of atoms in the excited state is at least 10-8 sec. Consequently, in the welding arc during the time the atom remains in the excited state, it undergoes no fewer than 10 collisions with other particles with approximately equal energy, which does not allow the atom to leave the excited state. Since all the atoms in the plasma are in the same conditions, the above applies to all the other atoms of the plasma, so that all of them at high T are excited and have an excess internal energy Wи. To remove electrons from the excited atoms requires less energy than to remove electrons from atoms in an unexcited state, which can be written as follows:
Wi = Ui – Wи,
where Wi — the ionization energy of the excited atom, eV; Ui — the ionization potential of the atom, eV; Wи — the internal excess energy of the excited atom, eV.
Moreover, the higher the plasma temperature, the greater atom energy will receive from other particles and, consequently, the greater will be its value Wи and less Wi. Since all the atoms in the plasma for given T and p are in the same conditions, this applies to all plasma atoms. What follows from that with increase in T at the set size р Wi of atoms of plasma decreases. This conclusion is confirmed by the fact that with increase in T of plasma extent of the ionization increases.