Oscillating plasma bubbles. IV. Grids, geometry, and gradients

Plasma bubbles are created in an ambient discharge plasma. A bubble is a plasma volume of typically spherical shape, which is separated from the ambient plasma by a negatively biased grid of high transparency. Ions and electrons from the ambient plasma flow into the bubble volume. In steady state the flow of particles and currents is divergence-free, which is established by the plasma potential inside the bubble. The grid has two sheaths, one facing the ambient plasma, the other the bubble plasma. The inner sheath is observed to become unstable, causing the plasma potential in the bubble to oscillate. The instability arises from an excess of ions and a deficiency of electrons. Its frequency is in the range of the ion plasma frequency but depends on all parameters which influence the charge density in the sheath. When the grid voltage is very negative, electrons cannot enter the outer sheath, and the inner sheath becomes a virtual anode which reflects ions such that the bubble interior is empty. When an electron source is placed into the bubble it can neutralize the ions and the bubble refills. Without plasma sources or sinks the bubble plasma is extremely sensitive to perturbations by probes. Modified current-voltage characteristics of Langmuir and emissive probes are demonstrated. A sequence of papers first describes the basic steady-state properties, then the time evolution of bubbles, the effects of electron sources in bubbles, and the role of the grid and bubble geometry. The physics of plasma bubbles is important to several fields of basic plasma physics such as sheaths, sheath instabilities, diagnostic probes, electrostatic confinement, and current and space charge neutralization of beams. V C 2012 American Institute of Physics. [http://dx.

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“…The growth and decay of plasma bubbles and other properties will be shown in three companion papers. [12][13][14] …”

Section: Discussionmentioning

confidence: 98%

“…[12][13][14] The present paper describes the basic bubble properties and is organized as follows: After describing the experimental setup in Sec. II, the observations of basic probe characteristics and instability properties will be presented in Sec.…”

Section: Introductionmentioning

confidence: 99%

Oscillating plasma bubbles. I. Basic properties and instabilities

Stenzel

Urrutia

2012

Physics of Plasmas

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“…The luminous core is surrounded by the cathode region where a significant potential drop between the grid and the plasma exists. Such structures are known in literature under the name of plasma bubbles (PB) [20][21][22][23]. Charge sheaths form on both inner and outer sides of the grid to shield the plasma from the negatively biased wires [21].…”

Section: Resultsmentioning

confidence: 99%

Concentric double hollow grid cathode discharges. Spectral investigations and phenomenological approach

Konrad-Soare

Enescu

Dimitriu

et al. 2021

Plasma Sources Sci. Technol.

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A discharge plasma is created by simultaneously biasing two concentric spherical grids with axisymmetric orifices. In this geometry, space charge structures in the form of multiple quasi-spherical luminous plasma bodies appear simultaneously inside and around the cathodes. The plasma formations are highly interdependent supplying each other with the particle flow and current closure necessary for the maintenance of the discharge. To diagnose these structures, space-resolved cold Langmuir probe measurements and optical emission spectroscopy investigations were performed in the axial direction allowing for the mapping of the axial profiles of plasma potential, electron temperature and density, ion density and optical emission. The existence of an accelerating double layer in the vicinity of the holes has been confirmed here, and in previous research (Teodorescu-Soare C T et al 2016 Phys. Scr. 91 034002; Schrittwieser R W et al 2017 Phys. Scr. 92 044001; Teodorescu-Soare C T et al 2019 Int. J. Mass Spectrom. 436 83). Besides the assessment of the relationship between discharge conditions and plasma parameters in the novel cathode system, the importance of a multiple concentric cathode discharge configuration is revealed for deposition applications.

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“…An ion sheath instability has been observed in double plasma devices 16,19,21,27 and plasma bubbles formed by spherical grids with negative bias in an ambient plasma. [22][23][24][25] The sheath oscillates near the ion plasma frequency.…”

Section: A Ion Sheath Instabilitymentioning

confidence: 99%

“…Potential oscillations also modulate the discharge current and produce magnetic oscillations as well as oscillating inductive electric fields. The ion sheath instability has been studied in double plasma devices [16][17][18][19][20][21] and gridded plasma bubbles [22][23][24][25] but without electromagnetic effects. Observationally, ion-rich sheaths with a small population of electrons create copious harmonics.…”

Section: Introductionmentioning

confidence: 99%

Magnetic dipole discharges. III. Instabilities

et al. 2013

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Instabilities in a cross-field discharge around a permanent magnet have been investigated. The permanent magnet serves as a cold cathode and the chamber wall as an anode. The magnet is biased strongly negative and emits secondary electrons due to impact of energetic ions. The electrons outside the sheath are confined by the strong dipolar magnetic field and by the ion-rich sheath surrounding the magnet. The electron energy peaks in the equatorial plane where most ionization occurs and the ions are trapped in a negative potential well. The discharge mechanism is the same as that of cylindrical and planar magnetrons, but here extended to a 3-D cathode geometry using a single dipole magnet. While the basic properties of the discharge are presented in a companion paper, the present focus is on various observed instabilities. The first is an ion sheath instability which oscillates the plasma potential outside the sheath below the ion plasma frequency. It arises in ion-rich sheaths with low electron supply, which is the case for low secondary emission yields. Sheath oscillations modulate the discharge current creating oscillating magnetic fields. The second instability is current-driven ion sound turbulence due to counter-streaming electrons and ions. The fluctuations have a broad spectrum and short correlation lengths in all directions. The third type of fluctuations is spiky potential and current oscillations in high density discharges. These appear to be due to unstable emission properties of the magnetron cathode. V

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Oscillating plasma bubbles. IV. Grids, geometry, and gradients

Cited by 12 publications

References 23 publications

Oscillating plasma bubbles. I. Basic properties and instabilities

Oscillating plasma bubbles. I. Basic properties and instabilities

Concentric double hollow grid cathode discharges. Spectral investigations and phenomenological approach

Magnetic dipole discharges. III. Instabilities

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