A gas injection system includes (a) a side gas plenum, (b) a plurality of N gas inlets coupled to said side gas plenum, (c) plural side gas outlets extending radially inwardly from said plenum, (d) an N-way gas flow ratio controller having N outputs coupled to said N gas inlets respectively, and (e) an M-way gas flow ratio controller having M outputs, respective ones of said M outputs coupled to said tunable gas nozzle and a gas input of said N-way gas flow ratio controller.