A microwave plasma generating apparatus (10) has a microwave cavity (20) coupled to a microwave source (22) by a wave guide (24). Within the cavity (20) is a reaction tube (30) defining a plasma cavity (40). A gas inlet manifold (50) is provided at the top of the reaction tube (30), which is formed so as to introduce plasma gas tangentially to the longitudinal axis of the plasma. Plasma gas is thus injected into the reaction tube (30) so that it flows in a swirled manner, that is, in the form of a vortex. This prevents overheating of the reaction tube (30).
