A modular microwave plasma-assisted deposition reactor for manufacturing synthetic diamond, said reactor comprising: - a microwave generator configured to generate microwaves having a frequency comprised between 300 MHz and 3000 MHz, - a resonant cavity formed, at least partly, by the inner cylindrical walls of an enclosure of the reactor, - a gas inlet system able to supply gases within the resonant cavity, - a gas outlet module able to remove said gases from the resonant cavity, - a wave-coupling module able to transfer the microwaves from the microwave generator up to the resonant cavity so as to allow the formation of a plasma, and - a growth support present in the resonant cavity, said modular reactor comprising at least three modulation elements, said modulation elements being selected among: - a ring able to be positioned between a first enclosure part and a second enclosure part so as to modify the shape and/or volume of the resonant cavity, and a seal system, allowing the sealing in terms of vacuum and the electrical continuity of the walls of the enclosure, being disposed between the ring and respectively, the first part and the second part of the enclosure; - a substrate-holder module and a quarter wave, said substrate-holder module being movable in vertical translation and in rotation, being in contact with the quarter wave and including at least one fluid cooling system; - a platen movable in vertical translation so as to modify the shape and volume of the resonant cavity and including through-apertures for the passage of the gases; - a gas-distribution module, including: ∘ a removable gas-distribution plate comprising an inner surface, an outer surface and a plurality of gas-distribution nozzles forming channels between said surfaces (111, 112) able to conduct a gas flow, and ∘ a support device connected to a cooling system and able to accommodate the removable gas-distribution plate; and - a substrate-cooling control module, including a removable device for injecting thermal resistance gas, said removable device for injecting thermal resistance gas comprising one or several thermal resistance gas inlet(s) and one or several thermal resistance gas outlet(s). - a microwave generator configured to generate microwaves having a frequency comprised between 300 MHz and 3000 MHz, - a resonant cavity formed, at least partly, by the inner cylindrical walls of an enclosure of the reactor, - a gas inlet system able to supply gases within the resonant cavity, - a gas outlet module able to remove said gases from the resonant cavity, - a wave-coupling module able to transfer the microwaves from the microwave generator up to the resonant cavity so as to allow the formation of a plasma, and - a growth support present in the resonant cavity, - a ring able to be positioned between a first enclosure part and a second enclosure part so as to modify the shape and/or volume of the resonant cavity, and a seal system, allowing the sealing in terms of vacuum and the electrical continuity of the walls of the enclosure, being disposed between the ring and respectively, the first part and the second part of the enclosure; - a substrate-holder module and a quarter wave, said substrate-holder module being movable in vertical translation and in rotation, being in contact with the quarter wave and including at least one fluid cooling system; - a platen movable in vertical translation so as to modify the shape and volume of the resonant cavity and including through-apertures for the passage of the gases; - a gas-distribution module, including: ∘ a removable gas-distribution plate comprising an inner surface, an outer surface and a plurality of gas-distribution nozzles forming channels between said surfaces (111, 112) able to conduct a gas flow, and ∘ a support device connected to a cooling system and able to accommodate the removable gas-distribution plate; and ∘ a removable gas-distribution plate comprising an inner surface, an outer surface and a plurality of gas-distribution nozzles forming channels between said surfaces (111, 112) able to conduct a gas flow, and ∘ a support device connected to a cooling system and able to accommodate the removable gas-distribution plate; and - a substrate-cooling control module, including a removable device for injecting thermal resistance gas, said removable device for injecting thermal resistance gas comprising one or several thermal resistance gas inlet(s) and one or several thermal resistance gas outlet(s).