<p>The arc chamber (13) has a bottom liner (34) spaced from the base (26) of the housing (25) and side liners (36) spaced from the side walls (27) to direct gas entering from an inlet port (32) along the base (26) and up the side walls (27) of the housing (25). The bottom liner (34) has notches (35) at two side edges to direct the gas upwardly into the arc chamber (13) and the side liners (36) have slots (3) to direct the gas horizontally into the arc chamber (13) to create a uniform distribution of the gas for gas ionization in the arc chamber.</p>
