HIGH-GAIN FREE-ELECTRON-LASER AMPLIFIER WITH WARM PLASMA BACKGROUND -LINEAR-ANALYSIS

The FEL instability with a warm plasma background is performed for all range of values of plasma and beam densities with unspecified wiggler number of period and strength. The excited plasma mode is responsible for a dramatic increasing of the exponential growth rate of the insta bility, in comparison with a plasma-free FEL, when the plasma has a ve ry small thermal speed, in comparison with the fixed beam resonant vel ocity and for small group velocity of the plasma mode in comparison wi th the radiation group velocity.