Degradation in gain for a free electron laser amplifier due to electron momentum spread
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A finite spread in axial momentum for the electron beam in a free electron laser amplifier is shown to decrease the small-signal gain. For millimeter and sub-millimeter wave amplifiers, where exponential growth dominates the gain, it is shown that the gain is approximately 3 db below that for a cold beam if the relative momentum spread (Δu/u)1/2 = (Go/248)1/2 (λo/L), where Go≫1 is the gain in db for the cold-beam case, λo is the magnetic wiggler period, and L is the amplifier length. Exact numerical examples are given for representative FEL amplifiers at 35 and 550 GHz.
Key wordsfree electron laser amplifier electron momentum spread
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