A cold spray material deposition system comprising: a gas heater comprising: an outer housing defining an inlet and an outlet for a gas stream; a pressure vessel defining a gas path through the outer housing; a heating element supported within the pressure vessel and within the gas path for heating the gas stream; a layer of thermal insulation disposed between the heating element and an interior wall of the pressure vessel; and a diffuser arranged within the gas path between the inlet and the heating element for spreading the gas stream prior to entering the heating element; and a cold spray device configured to receive a heated gas stream from the gas heater, inject a material for deposition into the heated gas stream and propel the gas stream and material onto a substrate; characterised in that the inlet includes an inlet tube extending through the pressure vessel, wherein the diffuser comprises apertures in the inlet tube to facilitate diffusion of incoming gas flow; and the cold spray material deposition system further comprises thermocouples arranged at both the inlet tube to the outer housing and the outlet from the outer housing configured to provide information indicative of a temperature of the gas stream, a power supply for supplying power to the heating element and a temperature controller configured to control the power supply and the operation of the heating element to control the temperature of the gas heater based on information from the thermocouples. a gas heater comprising: an outer housing defining an inlet and an outlet for a gas stream; a pressure vessel defining a gas path through the outer housing; a heating element supported within the pressure vessel and within the gas path for heating the gas stream; a layer of thermal insulation disposed between the heating element and an interior wall of the pressure vessel; and a diffuser arranged within the gas path between the inlet and the heating element for spreading the gas stream prior to entering the heating element; and an outer housing defining an inlet and an outlet for a gas stream; a pressure vessel defining a gas path through the outer housing; a heating element supported within the pressure vessel and within the gas path for heating the gas stream; a layer of thermal insulation disposed between the heating element and an interior wall of the pressure vessel; and a diffuser arranged within the gas path between the inlet and the heating element for spreading the gas stream prior to entering the heating element; and a cold spray device configured to receive a heated gas stream from the gas heater, inject a material for deposition into the heated gas stream and propel the gas stream and material onto a substrate; characterised in that the inlet includes an inlet tube extending through the pressure vessel, wherein the diffuser comprises apertures in the inlet tube to facilitate diffusion of incoming gas flow; and the cold spray material deposition system further comprises thermocouples arranged at both the inlet tube to the outer housing and the outlet from the outer housing configured to provide information indicative of a temperature of the gas stream, a power supply for supplying power to the heating element and a temperature controller configured to control the power supply and the operation of the heating element to control the temperature of the gas heater based on information from the thermocouples.