Self-amplification of coherent spontaneous emission in a Cherenkov free-electron maser

Ultrashort pulses of microwave radiation have been produced in a dielectric -lined Cherenkov free-electron maser (FEM) amplifier. An intense initial se ed pulse, due to coherent spontaneous emission (CSE), arises at the leading edge of the electron pulse. There is evidence to show that 3-4 cycle spike s are produced through the amplification of these seed pulses. A strong dep endence of the start-up power on the rise time of the electron pulse has be en found. The experimental results are verified by a theoretical analysis. Our study shows that amplification in a FEM amplifier is always initiated b y CSE arising from the edge of the electron pulse when the rise time is com parable to the electromagnetic wave period.