CORRELATOR MEASUREMENTS OF MEGAHERTZ WAVE-PARTICLE INTERACTIONS DURING ELECTRON-BEAM OPERATIONS ON STS

We report on the analysis of megahertz modulation of electrons as meas ured by the Shuttle Potential and Return Electron Experiment (SPREE) d uring dc firing of the shuttle electrodynamic tether system (SETS) fas t pulsed electron generator (FPEG). The SPREE and FPEG were flown aboa rd the space shuttle Atlantis flight STS 46 as part of the Tethered Sa tellite System (TSS 1) mission. The principal data reported here are f rom the SPREE multiangular electrostatic analyzers (ESAs) and Space Pa rticle Correlator Experiment (SPACE). The ESAs, mounted on rotary tabl es, measured electrons and ions in the energy range from 10 eV to 10 k eV over a solid angle of 2 pi sr. The SPACE is a signal processing sys tem that analyzes the pulse stream from the SPREE ESAs to identify bun ching of the electrons and ions produced by coherent wave-particle int eractions (WPIs). The SPACE detects modulations in the electron fluxes in frequency range 0- to 10-MHz. This paper concerns 2- to 4-MHz modu lations of the electron flux detected by the SPACE when the FPEG was f iring in a de mode at pitch angles close to 90 degrees. During such op erations, FPEG emitted a current of 100 mA at an energy of 1 keV. For these times, electrons with energies from 10 to 1850 eV were measured by the SPREE. For energies between similar to 10 and 100 eV the electr on flux is basically isotropic. At higher energies the flux increases for pitch angles near 90 degrees. The electron distribution functions generally decrease monotonically with increasing energy up to 100 eV. At energies > 100 eV the distributions either monotonically decrease o r exhibit a peak or plateau at energies near the beam emission energy. Megahertz modulations were observed for electrons with energies from 10 to 1180 eV, on both positive and negative slopes in the distributio n function and throughout the 2 pi ST sampled by the ESAs. The occurre nce and strength of the modulations exhibit no clear dependence on the pitch angle at which the electrons are measured. However, they appear to be limited to low parallel velocities (< 3 x 10(6) m s(-1)) where beam-generated waves are in resonance with suprathermal electrons.