In future fusion-oriented plasma devices alpha-particle diagnostics will be of great importance. It is generally recognized that collective Thomson scattering techniques show great potential in this field. For ion temperature measurements as well as for alpha-particle diagnostics a radiation source emitting in the submillimetre range is required. However, powerful radiation sources which meet the requirement of pulse durations in the millisecond range are not easily available at these wavelengths. A high frequency gyrotron, combined with an efficient frequency tripler could be an interesting alternative to the free-electron laser, the only source which has been considered up till now. We report experiments on third harmonic generation of radiation in n-doped Si, using a source with a wavelength of 676 mu m. A power of up to 2 kW could be generated at 225 mu m with a conversion efficiency of 0.1% While the onset of saturation could be observed, the power limit was due to surface breakdown effects rather than saturation.