THE DECAY INSTABILITY OF LANGMUIR-WAVES IN THE NONNEUTRAL ELECTRON-PLASMA COLUMN

The behaviour of large amplitude Langmuir waves is experimentally obse rved in a non-neutral electron plasma column with a cylindrical bounda ry. It is found that a Langmuir wave whose amplitude is above a thresh old decays into lower frequency Langmuir waves. Since the dispersion r elation is different from that of the unbounded neutral plasma, which is theoretically considered, the three-wave decay process of Langmuir waves becomes possible. In these processes, sidebands due to a lar ge amplitude Langmuir wave satisfy the energy and momentum conservation r elation. The threshold of this decay instability depends on the plasma temperature and becomes lower as the temperature becomes higher.