Townsend coefficient,escape curve,and efficiency of runaway-electron beam formation in argon

The ionization and drift characteristics of electrons in argon are simulated by the method of multi-particle dynamics. It is shown that, in argon (as well as in other gases studied earlier), the Townsend regime of ionization sets in even in strong fields if the electrode distance is much larger than the reciprocal Townsend coefficient. The dependences of the basic ionization and drift characteristics on the reduced field intensity are obtained, and an escape curve is constructed separating the region of effective electron multiplication from the region where the electrons leave the discharge gap having no time to multiply. The formation efficiency of a runaway-electron beam is calculated. It is shown that the dependence of the electrode voltage generating a given fraction of runaway electrons on the product of the pressure by the electrode distance has a form that qualitatively agrees with the runaway curve. When the efficiency is not too high (≤20%), the runaway curves virtually coincide with isoefficiency curves.