A method and apparatus for treating a work surface, wherein there is provided a chamber having a longitudinal axis and longitudinally extending electrically conductive sidewalls, at least one sidewall having at least one longitudinally extending gap that interrupts a current path through the sidewalls transverse to the longitudinal axis, and wherein the chamber is sealed to allow pressure inside the chamber to be controlled. Also provided is an axially-extending array of current-carrying conductors which at least partially encircle the chamber, are transverse to the longitudinal axis, and establish a magnetic field parallel to the longitudinal axis of the chamber, and a power supply connected to the conductor array and adapted to provide high-frequency current in the conductors to magnetically induce ionization of the gaseous material in the chamber and form a plasma sheath that surrounds and extends along the longitudinal axis and conforms to the sidewalls of the chamber, and wherein the work surface is exposed to the plasma sheath and extends in the direction of the longitudinal axis.