A Time of Flight mass analyser comprising: an annular ion guide having a longitudinal axis and comprising a first annular ion guide section and a second annular ion guide section; a first device arranged and adapted to introduce ions into said first annular ion guide section so that said ions form substantially stable circular orbits within said first annular ion guide section about said longitudinal axis; an ion detector disposed within said annular ion guide; a second device arranged and adapted to orthogonally accelerate ions in a first axial direction from said first annular ion guide section into said second annular ion guide section; and a third device arranged and adapted to maintain an axial DC potential along at least a portion of said second annular ion guide section so that said ions are reflected in a second axial direction which is substantially opposed to said first axial direction and so that said ions undergo multiple axial passes through said second annular ion guide section before being detected by said ion detector, wherein said annular ion guide comprises an inner cylindrical electrode arrangement and an outer cylindrical electrode arrangement, and wherein an annular time of flight ion guiding region is formed between said inner cylindrical electrode arrangement and said outer cylindrical electrode arrangement, characterised in that the Time of Flight mass analyser further comprises a device arranged and adapted to apply DC potentials to said inner cylindrical electrode arrangement and/or said outer cylindrical electrode arrangement in order to maintain a radial DC potential which acts to confine ions radially within said annular ion guide. an annular ion guide having a longitudinal axis and comprising a first annular ion guide section and a second annular ion guide section; a first device arranged and adapted to introduce ions into said first annular ion guide section so that said ions form substantially stable circular orbits within said first annular ion guide section about said longitudinal axis; an ion detector disposed within said annular ion guide; a second device arranged and adapted to orthogonally accelerate ions in a first axial direction from said first annular ion guide section into said second annular ion guide section; and a third device arranged and adapted to maintain an axial DC potential along at least a portion of said second annular ion guide section so that said ions are reflected in a second axial direction which is substantially opposed to said first axial direction and so that said ions undergo multiple axial passes through said second annular ion guide section before being detected by said ion detector, wherein said annular ion guide comprises an inner cylindrical electrode arrangement and an outer cylindrical electrode arrangement, and wherein an annular time of flight ion guiding region is formed between said inner cylindrical electrode arrangement and said outer cylindrical electrode arrangement, characterised in that the Time of Flight mass analyser further comprises a device arranged and adapted to apply DC potentials to said inner cylindrical electrode arrangement and/or said outer cylindrical electrode arrangement in order to maintain a radial DC potential which acts to confine ions radially within said annular ion guide.