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A microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor comprising: a plasma chamber;a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate on which the synthetic di

A microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor comprising: a plasma chamber;a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate on which the synthetic diamond material is to be deposited in use;a microwave coupling configuration for feeding microwaves from a microwave generator into the plasma chamber; anda gas flow system for feeding process gases into the plasma chamber and removing them therefrom;wherein the microwave plasma reactor further comprises an electrically conductive plasma stabilizing annulus disposed around the substrate holder within the plasma chamber.

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1. A microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor comprising: a plasma chamber;a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate on which the synthetic

1. A microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor comprising: a plasma chamber;a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate on which the synthetic diamond material is to be deposited in use;a microwave coupling configuration for feeding microwaves from a microwave generator into the plasma chamber; anda gas flow system for feeding process gases into the plasma chamber and removing them therefrom;wherein the microwave plasma reactor further comprises an electrically conductive plasma stabilizing annulus disposed around the substrate holder within the plasma chamber when viewed down a central axis of the plasma chamber,wherein the electrically conductive plasma stabilizing annulus is in the form of a projecting ring which protrudes into the plasma chamber from a side wall of the plasma chamber,wherein the electrically conductive plasma stabilising annulus has an upper surface, a lower surface, and an end portion which forms the projecting ring which protrudes into the plasma chamber from the side wall of the plasma chamber,wherein the upper surface of the electrically conductive plasma stabilising annulus is spaced apart from the a microwave coupling configuration, the electrically conductive plasma stabilizing annulus being positioned at a height within 50 mm, of a height of the supporting surface of the substrate holder, andwherein the electrically conductive plasma stabilizing annulus has an axial depth relative to a height of the plasma chamber in the range 1% to 30%. 2. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus has a radial width relative to a diameter of the plasma chamber in the range 1% to 30%, 3% to 20%, 5% to 15%, or 8% to 12%. 3. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus has a radial width in a range: 10 mm to 165 mm, 20 mm to 100 mm, or 40 mm to 80 mm for a microwave frequency f in the range 400 MHz to 500 MHz; 5 mm to 100 mm, 10 mm to 50 mm, or 20 mm to 40 mm for a microwave frequency f in the range 800 MHz to 1000 MHz; or 2 mm to 30 mm, 4 mm to 20 mm, or 6 mm to 15 mm for a microwave frequency f in the range 2300 MHz to 2600 MHz. 4. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus has an inner diameter relative to a diameter of the plasma chamber no less than 50%, 60%, 70%, 75%, or 80%. 5. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus has an inner diameter relative to a diameter of the plasma chamber no more than 95%, 90%, 85%, or 80%. 6. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus has an inner diameter no less than: 300 mm, 400 mm, 450 mm, or 500 mm for a microwave frequency f in the range 400 MHz to 500 MHz; 150 mm, 200 mm, 240 mm, or 280 mm for a microwave frequency f in the range 800 MHz to 1000 MHz; or 50 mm, 70 mm, 85 mm, or 95 mm for a microwave frequency f in the range 2300 MHz to 2600 MHz. 7. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus has an inner diameter no more than: 950 mm, 850 mm, 800 mm, 720 mm, or 680 mm for a microwave frequency f in the range 400 MHz to 500 MHz; 450 mm, 400 mm, 350 mm, or 330 mm for a microwave frequency f in the range 800 MHz to 1000 MHz; or 170 mm, 150 mm, 130 mm, or 120 mm for a microwave frequency f in the range 2300 MHz to 2600 MHz. 8. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus has an inner diameter wherein a ratio of the inner diameter / plasma chamber height is in the range 0.9 to 2, 0.95 to 1.8, 1.0 to 1.5, or 1.1 to 1.3. 9. A microwave plasma reactor according to claim 1, wherein the axial depth of the electrically conductive plasma stabilizing annulus relative to a height of the plasma chamber is in the range 2% to 20%, or 5% to 15%. 10. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus has an axial depth relative to a vertical distance between two electric field antinodes of the plasma chamber in the range 1% to 30%, 2% to 20%, or 5% to 15%. 11. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus has an axial depth in a range: 10 mm to 200 mm, 10 mm to 150 mm, 20 mm to 100 mm, 40 mm to 80 mm, or 40 mm to 60 mm for a microwave frequency f in the range 400 MHz to 500 MHz; 5 mm to 100 mm, 5 mm to 75 mm, 10 mm to 50 mm, 20 mm to 40 mm, or 20 mm to 30 mm for a microwave frequency f in the range 800 MHz to 1000 MHz; or 2 mm to 35 mm, 2 mm to 30 mm, 4 mm to 20 mm, 7 mm to 15 mm, or 7 mm to 10 mm for a microwave frequency f in the range 2300 MHz to 2600 MHz. 12. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus is positioned to have a lower surface at a height equal to or less than 50%, 40%, 30%, 20% or 10% of a height of the plasma chamber from an end of the plasma chamber on which the substrate holder is disposed. 13. A microwave plasma reactor according to claim 1, wherein the height of the electrically conductive plasma stabilizing annulus is within 30 mm, 20 mm, or 10 mm of the height of the supporting surface of the substrate holder. 14. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus is formed of a material having a melting point of at least 100° C., an electrical conductivity of at least 1×106 Sm−1, and/or a thermal conductivity of at least 10 WK−1m−1. 15. A microwave plasma reactor according to claim 14, wherein said material comprises at least 80%, 90%, 95%, or 98% of one or more of stainless steel, copper, aluminium, molybdenum, and tantalum. 16. A microwave plasma reactor according to claim 1, wherein the electrically conductive plasma stabilizing annulus comprises at least a portion having a curved cross-sectional shape with a radius of curvature in a range 1 mm to 40 mm, 2 mm to 30 mm, 3 mm to 20 mm, or 5 mm to 15 mm. 17. A microwave plasma reactor according to claim 16, where said portion has a varying radius of curvature including a lower portion which has a larger radius of curvature than an upper portion. 18. A microwave plasma reactor according to claim 17, wherein said lower portion has a radius of curvature in a range 10 mm to 40 mm, 10 mm to 30 mm, or 12 mm to 20 mm and said upper portion has a radius of curvature in a range 1 mm to 10 mm, 2 mm to 8 mm, or 3 mm to 7 mm. 19. A microwave plasma reactor according to claim 1, wherein the plasma chamber is configured such that the electrically conductive plasma stabilizing annulus is exposed to a plasma in use without any bell-jar between the plasma and the electrically conductive plasma stabilizing annulus.