A high power radiation source in the THz range with long pulse and narrow line width is required for diagnosing fusion type plasmas by collective Thomson scattering. Gyrotrons currently meet the requirements concerning power, pulse length and line width when operating in the 50-150 GHz range. They may have the potential also to do this in the THz range. In this paper two methods based on harmonic operation are investigated. Both look promising, if high conversion efficiency can be attained. This, however, still has to be demonstrated. (C) 1999 Published by Elsevier Science B.V. All rights reserved.