Effect of Space Charge in Cold-Cathode Gas Discharges

Ward, A. L.

doi:10.1103/physrev.112.1852

Cited by 137 publications

(34 citation statements)

References 6 publications

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“…Likewise, any decrease of L F below L F0 will increase the wave front E field strength, resulting in a runaway electron avalanche (α>β, case 2) which is bound to spread the potential gradient, thus increasing L F back toward L F0 . The condition setting the value of E F0 is derived using formula (25) and the expression of α from Ward [50], most suitable for noble gases, leading to the following relation:…”

Section: Pre-breakdown Wave Of Negative Polaritymentioning

confidence: 99%

Optical and electrostatic potential investigations of electrical breakdown phenomena in a low-pressure gas discharge lamp

Gendre¹,

Haverlag²,

Kroesen³

2010

J. Phys. D: Appl. Phys.

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The ignition phase is a critical stage in the operation of gas discharge lamps where the neutral gas enclosed between the electrodes undergoes a transformation from the dielectric state to a conducting phase, eventually enabling the production of light. The phenomena occurring during this phase transition are not fully understood and the related experimental studies are often limited to local optical measurements in environments prone to influencing these transient phenomena. In this work unipolar ignition phenomena at sub-kilovolt levels are investigated in a 3 Torr argon discharge tube. The lamp is placed in a highly controlled environment so as to prevent any bias on the measurements. A fast intensified CCD camera and a specially-designed novel electrostatic probe are used simultaneously so as to provide with a broad array of measured and computed parameters which are displayed in space-time diagrams for cross comparisons. Experiments show that three distinct phases exist during successful ignitions: Upon the application of voltage a first ionization wave starts from the active electrode and propagates in the neutral gas toward the opposite electrode. A local front of high axial E field strength is associated with this process and causes a local ionisation to occur, leading to the electrostatic charging of the lamp. Next, a second wave propagates from the ground electrode back toward the active electrode with a higher velocity, and in this process leads to a partial discharging of the lamp. This return stroke draws a homogeneous plasma column which eventually bridges both electrodes at the end of the wave propagation. At this point both electrode sheaths are formed and the common features of a glow discharge are observed. The third phase is an increase of the light intensity of the plasma column until the lamp reaches a steady state operation. Failed ignitions present only the first phase where the first wave starts its propagation but extinguishes in the lamp, leading to a charge memory effect. It is found that the full propagation of this first wave is a requirement for a successful lamp ignition. Differences in the properties of the waves were observed depending on the voltage polarity, and it was estimated that a photoelectric effect at the wall is the most likely source of electron for the ionisation wave of positive polarity. Finally a simple model of the first ionization wave is developed and used to analyse the fundamental differences between processes occurring at negative and at positive polarity. From this study three conditions are developed for the successful unipolar ignition of lamps and the relations between them are derived.

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Section: Pre-breakdown Wave Of Negative Polaritymentioning

confidence: 99%

Optical and electrostatic potential investigations of electrical breakdown phenomena in a low-pressure gas discharge lamp

Gendre¹,

Haverlag²,

Kroesen³

2010

J. Phys. D: Appl. Phys.

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“…When the electric field amplitude and space charge density at the boundary are assumed zero, the solution is known as the Child-Langmuir law [19]. Because these three conditions are not, in fact, satisfied at the same location, the assumptions create problems in matching of the solutions of the sheath and the bulk plasma models at the boundary [8], [10], [12]. For a physically accurate solution, not only must the charge densities, plasma potential, and electric field match at the boundary, but their slopes should also be continuous.…”

mentioning

confidence: 99%

“…To understand the electrodynamic properties and a variety of chemical and physical phenomena in processing discharges, many computational plasma models have been presented in the literature [8]- [13]. Because the sheath thicknesses are only a few times and the ion mean free path in presheath region could be up to hundreds of in low pressure, high density plasmas, computation of an exact solution is very time consuming.…”

mentioning

confidence: 99%

Sheath thickness evaluation for collisionless or weakly collisional bounded plasmas

Wang

Wendt

1999

IEEE Trans. Plasma Sci.

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Abstract-The widely used Child-Langmuir law for sheath thickness evaluation in semi-infinite collisionless plasmas makes the assumptions of quasi-neutrality (n e = n i ) and zero electric field intensity E = 0 at the sheath edge, as well as applying the Bohm criterion for ions entering the sheath. However, through a whole region fluid model, Poisson's equation has been solved numerically for the steady-state solution through the sheath and presheath without these assumptions. With the sheath edge defined, as in the Child-Langmuir law, at the place where the ion velocity is equal to the Bohm velocity, the sheath thickness of a bounded collisionless or weakly collisional plasma has been found with this model in some cases to be much larger than that obtained with the Child-Langmuir Law. The sheath thickness discrepancy is significant under conditions found in low pressure high density plasma (HDP) tools for plasma processing. Results presented indicate that the sheath thickness is very sensitive to the electric field and space charge density at the sheath edge. The electric field and space charge density can be successfully estimated by an intermediate scale matching method [1]-[5], and are used to derive a modified expression for the potential in the sheath that can be solved numerically for sheath thickness. With these results, the matching problem, arising when sheath and plasma are modeled separately, can be overcome.

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“…This changes the electric field, which in turn can increase µ to unity and above [7,14]. Previous attempts to model the effect of a steady-state constant photocurrent include numerical efforts by Crowe et al [4] and Ward [5]. These researchers have predicted a substantial decrease in the breakdown voltage induced by photocurrent.…”

Section: Theoretical Modelmentioning

confidence: 99%

“…Previous analyses of the problem of photoelectric induced breakdown have concentrated on steadystate photocurrents lowering the applied breakdown voltage of a gap [2,3,4,5]. Using a photocurrent pulse as an initiation method from an undervoltage is less well documented.…”

Section: Introductionmentioning

confidence: 99%

Laser discharge initiation for gas-fed pulsed plasma thrusters

Berkery

Choueiri

2001

37th Joint Propulsion Conference and Exhibit

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A method of discharge initiation at an undervoltage by a laser-induced pulse of electrons from a photocathode is presented. The intended application is to produce spatially uniform current sheet initiation in gas-fed pulsed plasma thrusters. The effect is explained by a space charge buildup which can increase the electric field of the gap. A theoretical model was developed and showed that an increase of the charge multiplication coefficient to a level above unity will produce a current rise to breakdown. Attempts to use an ultraviolet light pulse to create a photocurrent spike were unsuccessful probably due to inadequate surface preparation of the photocathode. Infared laser pulses, however, did lead to current production which lead to breakdown from a 15% undervoltage in argon.

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Effect of Space Charge in Cold-Cathode Gas Discharges

Cited by 137 publications

References 6 publications

Optical and electrostatic potential investigations of electrical breakdown phenomena in a low-pressure gas discharge lamp

Optical and electrostatic potential investigations of electrical breakdown phenomena in a low-pressure gas discharge lamp

Sheath thickness evaluation for collisionless or weakly collisional bounded plasmas

Laser discharge initiation for gas-fed pulsed plasma thrusters

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