Achieving non-contracted and non-filamentary rare-gas tubular discharges at atmospheric pressure

At gas pressures >1 kPa, tubular rare-gas electrical discharges can contract radially yielding a single axially centred bright filament. When sustaining the discharge with microwave fields of a high enough frequency, instead of a single filament, two or more smaller-diameter and off-centre plasma filaments can be formed. When a specific percentage (<1%) of a rare gas having a lower ionization potential is added to a pure rare-gas atmospheric-pressure discharge, the single-filament contracted discharge fully expands radially or the initially multi-filament discharge becomes homogeneous. Experimental characteristics of this phenomenon and the required operating conditions are reported here for the first time.