A microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor comprising: a plasma chamber comprising a base, a top plate, and a side wall extending from said base to said top plate defining a resonance cavity for supporting a microwave resonance mode, wherein the resonance cavity has a central rotational axis of symmetry extending from the base to the top plate, and wherein the top plate is mounted across said central rotational axis of symmetry; a microwave coupling configuration for feeding microwaves from a microwave generator into the plasma chamber; a gas flow system (12, 14) for feeding process gases into the plasma chamber and removing them therefrom; and 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; wherein the resonance cavity is configured to have a height (h), as measured from the base to the top plate of the plasma chamber, which supports a TM 011 resonant mode between the base and the top plate at a frequency in the range 400 to 500 MHz, 800 to 1000 MHz, or 2300 MHz to 2600Mz, wherein the resonance cavity is further configured to have a diameter (d), as measured at a height less than 50% of the height of the resonance cavity as measured from the base, which satisfies the condition that a ratio of the resonance cavity height / the resonance cavity diameter is in the range 0.3 to 1.0, characterised in that the resonance cavity comprises internal walls configured to be exposed to a plasma formed within the resonance cavity in use, said internal walls comprising metallic surfaces forming at least 75% of a total surface area of said internal walls within the resonance cavity, and wherein a portion of said internal walls is formed by an annular dielectric window, formed in one or several sections, said annular dielectric window forming no more than 25% of the total surface area of the internal walls within the resonance cavity. a plasma chamber comprising a base, a top plate, and a side wall extending from said base to said top plate defining a resonance cavity for supporting a microwave resonance mode, wherein the resonance cavity has a central rotational axis of symmetry extending from the base to the top plate, and wherein the top plate is mounted across said central rotational axis of symmetry; a microwave coupling configuration for feeding microwaves from a microwave generator into the plasma chamber; a gas flow system (12, 14) for feeding process gases into the plasma chamber and removing them therefrom; and 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; wherein the resonance cavity is configured to have a height (h), as measured from the base to the top plate of the plasma chamber, which supports a TM 011 resonant mode between the base and the top plate at a frequency in the range 400 to 500 MHz, 800 to 1000 MHz, or 2300 MHz to 2600Mz, wherein the resonance cavity is further configured to have a diameter (d), as measured at a height less than 50% of the height of the resonance cavity as measured from the base, which satisfies the condition that a ratio of the resonance cavity height / the resonance cavity diameter is in the range 0.3 to 1.0, characterised in that the resonance cavity comprises internal walls configured to be exposed to a plasma formed within the resonance cavity in use, said internal walls comprising metallic surfaces forming at least 75% of a total surface area of said internal walls within the resonance cavity, and wherein a portion of said internal walls is formed by an annular dielectric window, formed in one or several sections, said annular dielectric window forming no more than 25% of the total surface area of the internal walls within the resonance cavity.