A diffuser to be disposed between a centrifugal compressor and a combuster of an engine, wherein the diffuser is designed to increase the pressure and reduce the velocity of air leaving the compressor prior to entering the combuster, the diffuser comprising: a first wall of the diffuser having a section, extending radially with respect to a central axis of the diffuser, and a curved section; a second wall having a section, extending radially with respect to said central axis, and a curved section, wherein the radially-extending section and the curved section of the second wall are spaced apart from the radially-extending section and the curved section of the first wall such that the curved section of the second wall is disposed radially inwardly from the curved section of the first wall and a flow path is defined therebetween, and wherein said diffuser is so designed that in use the air leaving the compressor flows in a radial direction between the radially-extending sections (232, 236), and an axial velocity is imparted to the air as it flows between the curved sections (234, 238); said diffuser being characterised in that the curvature radius of said curved section of said second wall is between 0.38 and 1.52 x 10 -2 metres, and in that the radial section of said flow path between said radially extending sections (232, 236) of -2 said first and second wall (228, 230) has a radial length in a range between 2.5 to 10 x 10 metres and an axial width in a range of 0.5 to 2 x 10 metres, and in that said diffuser comprises a first plasma actuator (258, 302) disposed on the curved section of the second wall, the first plasma actuator (258, 302) being adapted to generate a first electric field to ionize a first portion of air flowing along the flow path to thereby form a plurality of ionized species capable of inducing a second portion of air to flow in a desired direction. a first wall of the diffuser having a section, extending radially with respect to a central axis of the diffuser, and a curved section; a second wall having a section, extending radially with respect to said central axis, and a curved section, wherein the radially-extending section and the curved section of the second wall are spaced apart from the radially-extending section and the curved section of the first wall such that the curved section of the second wall is disposed radially inwardly from the curved section of the first wall and a flow path is defined therebetween, and wherein said diffuser is so designed that in use the air leaving the compressor flows in a radial direction between the radially-extending sections (232, 236), and an axial velocity is imparted to the air as it flows between the curved sections (234, 238); said diffuser being characterised in that the curvature radius of said curved section of said second wall is between 0.38 and 1.52 x 10 -2 metres, and in that the radial section of said flow path between said radially extending sections (232, 236) of -2 said first and second wall (228, 230) has a radial length in a range between 2.5 to 10 x 10 metres and an axial width in a range of 0.5 to 2 x 10 metres, and in that said diffuser comprises a first plasma actuator (258, 302) disposed on the curved section of the second wall, the first plasma actuator (258, 302) being adapted to generate a first electric field to ionize a first portion of air flowing along the flow path to thereby form a plurality of ionized species capable of inducing a second portion of air to flow in a desired direction.