Fireballs are luminous regions produced by double layers in front of positively biased electrodes in plasmas. Although fireballs have been investigated previously there are a great variety of unexplained nonlinear phenomena, some of which are addressed in this work. First, it is shown that a fireball is not an isolated local phenomenon but an integral part of the entire discharge plasma. Current closure and limits are discussed. Fireballs with currents from milliamperes to tens of amperes are created depending on whether the electron source is temperature limited, space-charge limited or limited by ion currents in afterglow plasmas. Fireballs are created with highly transparent grids which allow electron transmission through the electrodes and optimize the ionization efficiency. The physics of pulsating fireballs is investigated. Fireballs disrupt when density outflow exceeds production, leading to density collapse and current disruption when the electron drift exceeds the Buneman limit. The current disruption causes a density decay in the entire discharge causing the electrode sheath to widen, starting sheath ionization and the formation of a new fireball. Finally, novel fireball properties have been observed in nonuniform magnetic fields of dipole, mirror and cusp topologies.
A new fireball configuration has been developed which produces vircator-like instabilities. Electrons are injected through a transparent anode into a spherical plasma volume. Strong high-frequency oscillations with period corresponding to the electron transit time through the sphere are observed. The frequency is below the electron plasma frequency, hence does not involve plasma eigenmodes. The sphere does not support electromagnetic eigenmodes at the instability frequency. However, the rf oscillations on the gridded anode create electron bunches which reinforce the grid oscillation after one transit time or rf period, which leads to an absolute instability. Various properties of the instability are demonstrated and differences to the sheath-plasma instability are pointed out, one of which is a relatively high conversion efficiency from dc to rf power. Nonlinear effects are described in a companion paper ͓R. L. Stenzel et al., Phys. Plasmas 18, 012105 ͑2011͔͒.
Instabilities in an electron-rich sheath on a plane electrode in a discharge plasma have been investigated experimentally. The high-frequency sheath-plasma instability near the electron plasma frequency is observed. With increasing dc voltage, the instability exhibits bursty amplitude and frequency jumps. The electrode current shows spikes and jumps, and the plasma potential near the electrode shows large fluctuations below the ion plasma frequency. Sheath-ionization has been identified as the cause for these low frequency instabilities. Electrons energized in the sheath produce ions which reduce the space charge in the sheath and the electric field and the ionization rate. Ions are ejected from the sheath which increases the charge density, electric field, and ionization rate. The positive feedback between these processes leads to a relaxation instability whose time scale is determined by ion inertia and ionization rates. The associated density and potential fluctuations affect the amplitude and frequency of the sheath-plasma instability. When the sheath ionization rate exceeds the ion losses, the sheath expands into an anode plasma or “fireball.” The potential drop across the sheath decreases and the sheath-plasma instability vanishes. The electrode current-voltage characteristics develop a region of negative conductance. For short grid voltage pulses, the ionization effects can be avoided.
High-frequency instabilities are observed in connection with unstable fireballs. Fireballs are discharge phenomena near positively biased electrodes in discharge plasmas. They are bounded by a double layer whose potential is of order of the ionization potential. Fireballs become unstable when plasma losses and plasma production are not in balance, resulting in periodic fireball pulses. High-frequency instabilities in the range of the electron plasma frequency have been observed. These occur between fireball pulses, hence are not due to electron beam-plasma instabilities since there are no beams without double layers. The instability has been identified as a sheath-plasma instability. Electron inertia creates a phase shift between high-frequency current and electric fields which destabilizes the sheath-plasma resonance. High-frequency signals are observed in the current to the electrode and on probes near the sheath of the electrode. Waveforms and spectra are presented, showing bursty emissions, phase shifts, frequency jumps, beat phenomena between two sheaths, and nonlinear effects such as amplitude clipping. These reveal many interesting properties of sheaths with periodic ionization phenomena.
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