Further laboratory measurements of the beam-plasma discharge (BPD) produced when an energetic electron beam traverses an initially neutral gas are presented. The experimental data indicate that the critical beam current Ic at energy V, magnetic field B, and system length L required for ignition obeys the empirical relationship Ic ∝ V3/2/B0.7 PL at pressures P < 2 × 10−5 torr. This relationship appears to primarily describe the accumulation of ambient plasma density, collisionally produced by the beam itself, to a critical value where ωpe² ≈ ωce² at which ignition occurs. Additional measurements of the narrow-band cyclotron-related waves (fce < f < 1.4fce) observed at beam currents below BPD threshold do not clearly establish whether this instability is a necessary precursor to the BPD. The relevance of these experiments to past and planned electron beam experiments in space is briefly discussed.

Further laboratory measurements of the beam-plasma discharge. [for aerospace technology application]

One-dimensional Vlasov simulations are used to study the propagation of electron beams injected from a spacecraft into an ambient plasma and the associated potential structures. It is shown that, for a given beam velocity, the propagation velocity and the potential structure depends on the beam density. In the case of moderate beams, a double layer forms near the beam head which propagates into the ambient plasma much more slowly than the initial beam velocity.

Electric potential structures and propagation of electron beams injected from a spacecraft into a plasma

Experiments done in space and laboratory on the interaction of an electron beam and a magnetized plasma exhibit the simultaneous excitation of waves near harmonics and half harmonics of the electron gyrofrequency. The dispersion equation for electrostatic modes has been solved for an anisotropic Maxwellian plasma and a suprathermal population (“beam” in the text) presenting anisotropy and regions of ∂ƒ0/∂v⊥>0 in its distribution function. For oblique propagation to the magnetic field, results similar to those observed in experiments are obtained. The hydrodynamic/kinetic nature of the instabilities is discussed.

Simultaneous emissions at nω c and (n+1/2)ω c in space and laboratory plasmas

The theory of electron-beam interaction with Bernstein plasma modes is developed for conditions of active space plasma experiments as well as experiments in large vacuum chambers. Translational movement of the beam along a magnetic field and Larmor rotation of the beam are considered as sources of free energy for wave build-up. Part of the energy of translational movement is lost to the excitation of Langmuir plasma oscillations. Bernstein modes then slowly grow owing to a 'fan'-type instability. In this case the emission spectrum consists of lines near the half-harmonics of the electron gyrofrequency, (n+1/2)ω c . Larmor gyration of the beam is not destroyed by the excitation of Langmuir turbulence, and thus there is strong hydrodynamic interaction of the rotated beam with the Bernstein modes. Integer harmonics of gyrofrequency nω c are generated in this case, with the spectral maximum near the upper-hybrid plasma frequency.

Generation of Bernstein modes in space and laboratory plasmas by injection of an electron beam

Phenomena Induced by Charged Particle Beams (PICPAB) are planned to be investigated during the first Spacelab flight (FSLP) using a European payload. The PICPAB experiment (Beghin et al., 1979) consists of two accelerators of electron and ion beams and associated diagnostic instruments including wave receivers, thermal plasma probes and return current particle energy-analysers. The functional tests using prototypes of those instruments have been carried out in the large vacuum chamber SIMLES (~ 300 m3) at CNES-S0PEMEA (Toulouse-France) in february 1980. The main purpose was to perform a simulation under conditions where the ambient neutral and ionized gas, magnetic field strength and lay-out of the different packages were as close as possible to those anticipated for FSLP mission. But such a simulation cannot be undertaken ignoring plasma physics processes which are associated with the neutralization of the gun body and of the beam itself, those problems which are precisely among scientific objectives of this experiment for FSLP mission

Electron beam injection and associated phenomena as observed in a large space simulation chamber

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Electron beam injection and associated phenomena as observed in a large space simulation chamber