Johannes Gruenwald

Transit time instabilities in an inverted fireball. I. Basic properties

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 (2011)].

Transit time instabilities in an inverted fireball. II. Mode jumping and nonlinearities

A fireball is formed inside a highly transparent spherical grid immersed in a dc discharge plasma. The ambient plasma acts as a cathode and the positively biased grid as an anode. A strong nearly current-free double layer separates the two plasmas. Electrons are accelerated into the fireball, ionize, and establish a discharge plasma with plasma potential near the grid potential. Ions are ejected from the fireball. Since electrons are lost at the same rate as ions, most electrons accelerated into the fireball just pass through it. Thus, the electron distribution contains radially counterstreaming electrons. High-frequency oscillations are excited with rf period given by the electron transit time through the fireball. Since the frequency is well below the electron plasma frequency, no eigenmodes other than a beam space-charge wave exists. The instability is an inertial transit-time instability similar to the sheath-plasma instability or the reflex vircator instability. In contrast to vircators, there is no ...

Electron-rich sheath dynamics. II. Sheath ionization and relaxation instabilities

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 frequenc...

Pulsed, unstable and magnetized fireballs

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.

Electron-rich sheath dynamics. I. Transient currents and sheath-plasma instabilities

The evolution of an electron-rich sheath on a plane electrode has been investigated experimentally. A rapidly rising voltage is applied to a plane gridded electrode in a weakly ionized, low temperature, and field-free discharge plasma. Transient currents during the transition from ion-rich to electron-rich sheath are explained including the current closure. Time-resolved current-voltage characteristics of the electrode are presented. The time scale for the formation of an electron-rich sheath is determined by the ion dynamics and takes about an ion plasma period. When the ions have been expelled from the sheath a high-frequency sheath-plasma instability grows. The electric field contracts into the electron-rich sheath which implies that the potential outside the sheath drops. It occurs abruptly and creates a large current pulse on the electrode which is not a conduction but a displacement current. The expulsion of ions from the vicinity of the electrode lowers the electron density, electrode current, and ...

Pulsating fireballs with high-frequency sheath?plasma instabilities

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. (Some figures in this article are in colour only in the electronic version)

Comparison of measured and simulated electron energy distribution functions in low-pressure helium plasmas

Knowledge of the electron energy distribution function (EEDF) is of great interest in different branches of plasma physics ranging from laboratory to fusion plasmas. In the frame of this work systematic measurements of the EEDF in low temperature helium plasmas (Te ≈ 2 eV) at different working gas pressures and discharge currents (Idis between 1 and 2 A) will be presented and compared with numerical particle-in-cell (PIC) code simulations. The experiments were conducted in the Innsbruck double plasma machine and in the Ljubljana linear magnetic plasma device with helium as the working gas. The EEDF was obtained by the second derivative of the characteristic of a Langmuir probe. The PIC code was used to simulate the EEDF by taking into account most of the physical parameters in the plasma vessel.

Multiple Fireballs in a Reactive

Inverted fireballs (FBs) are plasma phenomena, which develop inside a highly transparent, positively biased grid electrode through electron impact ionization. The ionizing electrons are accelerated into the grid by the high potential on the wires. This leads to an enhanced plasma density inside the hollow anode as well as a strong light emission. FBs show a wide variety of properties. This behavior may manifest itself in different shapes of FBs, which are very sensitive to the physical parameters of the discharge.

{\rm H}_{2}/{\rm CH}_{4}

Energy deposition into a plasma for an inverted fireball geometry is studied using a self-consistent two-dimensional Particle-in-Cell Monte Carlo collision model. In this model, the cathode is a pin which injects the fixed electron current and the anode is a hollow metal tube covered with the metal grid. We obtain an almost constant ratio between the densities of plasmas generated in the cathode-grid gap and inside the hollow anode. The results of the simulations show that there is no energy exchange between the beam and plasma electrons at low emission currents. For increasing current, however, we observe the increasing coupling between the electron beam and the thermal plasma electrons. This leads to the heating of plasma electrons and the generation of the so-called supra-thermal electrons.

Plasma

Electron inertia causes a delay between high-frequency currents and electric fields. When the transit time of a bunch of streaming electrons equals one RF period, the electrons can enhance the field and produce oscillations. Here, electron transit-time instabilities have been observed in magnetized sheaths and fireballs. Novel fireballs trapped in a gridded hollow anode have been developed. These produce large-amplitude RF oscillations.