A centrifugal compressor comprising: an impeller having a high pressure outlet; a scroll; and a variable diffuser between the impeller and the scroll, wherein high pressure gas flows in operation from the high pressure outlet through the variable diffuser to the scroll, the variable diffuser comprising: a passage defined between a front disc face and a back disc face; the front and back disc faces extending radially from respective inner edges to respective outer edges; an opening defined between the respective inner edges coupled to the high pressure outlet and another opening defined between the respective outer edges coupled to the scroll; a plurality of vanes within the passage, each vane spanning at least between the front disc face and the back disc face and having a fixed angle with respect to an axis of rotation of the centrifugal compressor; each vane rigidly fixed to front disc face and extend through the passage into the back disc face; the back disc face having a first and second axial position with respect to the front disc face; in the first position, each of the vanes extends into the back disc face and in the second position, each of the vanes extends through and beyond the back disc face; and an actuator operably connected to the back disc face and translating the back disc face between the first and second positions, characterised in that the centrifugal compressor comprises a plurality of pocketed vane slots extending axially from the back disc face on the side opposite the front disc face, each pocketed vane slot being formed as a closed pocket, and in that the actuator is an actuator assembly comprising a unison ring, at least three crank arms, at least three cam mechanisms and at least three cam shafts distributed circumferentially around the back disc face, wherein each cam mechanism comprises a translationally fixed outer sleeve and a rotationally fixed inner sleeve, each outer sleeve is rotationally driven by the respective crank arm, each crank arm is coupled to the unison ring, and each cam shaft is translationally driven by the respective inner sleeve. an impeller having a high pressure outlet; a scroll; and a variable diffuser between the impeller and the scroll, wherein high pressure gas flows in operation from the high pressure outlet through the variable diffuser to the scroll, the variable diffuser comprising: a passage defined between a front disc face and a back disc face; the front and back disc faces extending radially from respective inner edges to respective outer edges; an opening defined between the respective inner edges coupled to the high pressure outlet and another opening defined between the respective outer edges coupled to the scroll; a plurality of vanes within the passage, each vane spanning at least between the front disc face and the back disc face and having a fixed angle with respect to an axis of rotation of the centrifugal compressor; each vane rigidly fixed to front disc face and extend through the passage into the back disc face; the back disc face having a first and second axial position with respect to the front disc face; in the first position, each of the vanes extends into the back disc face and in the second position, each of the vanes extends through and beyond the back disc face; and an actuator operably connected to the back disc face and translating the back disc face between the first and second positions, a passage defined between a front disc face and a back disc face; the front and back disc faces extending radially from respective inner edges to respective outer edges; an opening defined between the respective inner edges coupled to the high pressure outlet and another opening defined between the respective outer edges coupled to the scroll; a plurality of vanes within the passage, each vane spanning at least between the front disc face and the back disc face and having a fixed angle with respect to an axis of rotation of the centrifugal compressor; each vane rigidly fixed to front disc face and extend through the passage into the back disc face; the back disc face having a first and second axial position with respect to the front disc face; in the first position, each of the vanes extends into the back disc face and in the second position, each of the vanes extends through and beyond the back disc face; and an actuator operably connected to the back disc face and translating the back disc face between the first and second positions,