Waves generated during electron beam emissions from the space shuttle

Some results from the SEPAC (Space Experiments with Particle Accelerators) experiment flown on the Spacelab 1 shuttle mission are presented. The active part of the experiment included the injection of electron beams at currents up to 300 mA, and energies up to 5 keV. The main focus of the paper is on the wave activity observed during these beam emissions. It is found that the noise in the 0.75- to 10-kHz VLF band has a spectral shape that can be characterized by an f−n law, and that the VLF signal level depends on the beam angle to the magnetic field, the strongest emissions being observed for parallel beams. These features are found to be consistent with a drift wave instability. In the 0.1- to 8-MHz frequency range, very little activity is detected, probably due to a too low gain setting. Reference is made to results from two other electron beam experiments performed from the space shuttle, namely PICPAB (Phenomena Induced by Charged Particle Beams), also flown on Spacelab 1, and FPEG (Fast Pulse Electron Gun) flown on STS 3. Some comments on the space shuttle as a platform for investigation of beam-plasma interactions are provided.