Visible Signatures of the Multi-Step Transition to a Beam-Plasma-Discharge
The transition of an electron beam to a beam-plasma-discharge (BPD) is characterized in part by substantial changes in both the distribution and the magnitude of the visible emissions from the ambient gas. At low ambient pressure (<4 × 10-6 Torr) there are abrupt transitions between four visually recognizable states. These include the basic beam with its noded configuration (A1), the noded beam surrounded by a weak halo (A2), and two states (B and C) of the BPD characterized by very bright halos and significant alteration of the beam itself. The latter states are differentiated by the differing intensities and diameters of their associated halos. When the beam current is reduced, these transitions occur in reverse and with substantial hysteresis.
Analysis of the optical intensities (total light and λ3914Å (N 2 + ) suggests that, at the critical current, the BPD may be twenty to one hundred times as effective as direct collisions in ionizing the ambient gas. Even the weak halo of state A2 (tentatively associated with emissions near the electron cyclotron frequency and its harmonics) may indicate an ionization rate several times that due to direct collisions of the beam electrons.
At higher pressures, the transition to BPD is less abrupt and may not include these separate intermediate states. There is little or no observable hysteresis; nor are the cyclotron emissions observed. The ratio of the ion production rate in BPD to that of the beam alone is lower than it is at low ambient pressure.
KeywordsBeam Current Ionization Rate Abrupt Transition Direct Collision Electron Cyclotron Frequency
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