An apparatus for the deposition of a variable thickness coating onto the inside of a cylindrical tube comprises a variable pressure gas, an cathode coaxially positioned within the cylinder, and a voltage source applied between the cathode and cylindrical tube, which functions as an anode. A radial plasma arc is generated between the anode and cathode at a starting point on the cylinder, and the plasma arc travels down the central axis of the cylinder, providing a helical deposition region on the inside of the cylinder. Selection of the combination of cathode material and gas enable the plasma to generate ionic material which is deposited on the anodic cylinder in the region of the plasma. By varying the pressure of variable pressure gas for each helical path, it is possible to vary the composition of this deposition film.