A rotor for driving an implement of a dental preparation instrument, having a turbine wheel mounted in a turbine housing, which turbine wheel has a plurality of drive blades, wherein the turbine housing is formed from an inner wall radially orbiting the turbine wheel, and cover surfaces enclosing the inner wall above and below, at least one gas inlet channel with a gas inlet opening for admitting a propellant into the turbine housing for driving the turbine wheel and at least one gas outlet channel with a gas outlet opening for leading the propellant away from the turbine housing once the turbine wheel has been driven, wherein gas inlet opening and gas outlet opening are arranged in the inner wall within a segment of a circle of a maximum of 180°, wherein the inner wall in the segment of a circle comprises at least one return flow channel extending at least partially from the gas inlet opening as far as the gas outlet opening, wherein the return flow channel allows at least one part of the propellant to flow back to the gas outlet opening through the return flow channel after impinging on at least one of the drive blades counter to the direction of rotation of the turbine wheel, and wherein, in the axial direction, the turbine housing has at least one cross-sectional extension between drive blades and the cover surfaces, said cross-sectional extension is formed by a recess in one of the cover surfaces, with a first distance in the axial direction between drive blades and the cover surface, which extends at least over the region of the return flow channel viewed in the direction of the propellant flowing in the return flow channel at least as far as the outer end of the gas outlet opening in the direction of the returning propellant, and with at least one second distance in the axial direction between drive blades and the cover surface in an other region, wherein the second distance is shorter than the first distance. a turbine wheel mounted in a turbine housing, which turbine wheel has a plurality of drive blades, wherein the turbine housing is formed from an inner wall radially orbiting the turbine wheel, and cover surfaces enclosing the inner wall above and below, at least one gas inlet channel with a gas inlet opening for admitting a propellant into the turbine housing for driving the turbine wheel and at least one gas outlet channel with a gas outlet opening for leading the propellant away from the turbine housing once the turbine wheel has been driven, wherein gas inlet opening and gas outlet opening are arranged in the inner wall within a segment of a circle of a maximum of 180°, wherein the inner wall in the segment of a circle comprises at least one return flow channel extending at least partially from the gas inlet opening as far as the gas outlet opening, wherein the return flow channel allows at least one part of the propellant to flow back to the gas outlet opening through the return flow channel after impinging on at least one of the drive blades counter to the direction of rotation of the turbine wheel, and wherein, in the axial direction, the turbine housing has at least one cross-sectional extension between drive blades and the cover surfaces, said cross-sectional extension is formed by a recess in one of the cover surfaces, with a first distance in the axial direction between drive blades and the cover surface, which extends at least over the region of the return flow channel viewed in the direction of the propellant flowing in the return flow channel at least as far as the outer end of the gas outlet opening in the direction of the returning propellant, and with at least one second distance in the axial direction between drive blades and the cover surface in an other region, wherein the second distance is shorter than the first distance.