Very low frequency signals and whistler-mode amplification in the magnetosphere and limit-cycle behavior in the CEBAF infrared free electron laser
Caudle, Daryl L.
Colson, William B.
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Very low frequency (VLF) electromagnetic wave injections experiments were conducted on 23-24 January 1988 from a 42-km horizontal dipole antenna located at Siple Station, Antarctica. The experiment consisted of a diagnostic format transmitted for one minutes every five minutes for a 10-hour period between 1700 UT and 0300 UT. These signals were received and recorded at the conjugate magnetic field point at Lake Mistissini, Canada. A detailed analysis of this data clearly demonstrates hot plasma effects such as saturated power levels, exponential growth rates, sideband formation and triggered emissions due to wave-particle interactions. These hot plasma effects remain constant over a time scale of 30 seconds but show large variations over a time scale of 5 minutes. These VLF signals were used to simulate "whistler waves" which occur naturally and are amplified by energetic electrons spiraling around magnetic field lines near the geomagnetic equator. Navy VLF communications are strongly affected by the presence of whistler waves. The electron and whistler wave interaction can be described by a theoretical model which is very similar to that used for free electron lasers (FEL). Using computer simulation most of the hot plasma effects seen in the Siple Station data can be modeled and compared to free electron characteristics such as saturation, electron trapping, tapering, and sensitivity to energy distributions. Two dimensional computer simulations in coordinates z and t have predicted that the CEBAF Infrared (IR) FEL can observe limit-cycle behavior when operating within its design parameters. The IR FEL is driven by a high quality electron beam with a micropulse length comparable to the slippage distance. At moderate values of the desynchronism, the optical power will oscillate periodically over several hundred passes through the resonator. The limit-cycle power oscillations are caused by "marching subpulses" that grow at the trailing edge of the optical pulse through a super-radiant process, and pass through the main optical envelope.
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