Low-pressure Gas Discharge Research Articles

A study of indirect-heated cathode for low-pressure gas discharge lamp

A study of indirect-heated cathode for low-pressure gas discharge lamp

Pulsed electron beam generation in a simple discharge device

We report a phenomenon of pulsed electron beam generation in a low-pressure gas discharge device. The cathode of the device is of simple geometry without a hollow cavity that is employed in the transient hollow cathode discharge (THCD) devices. The discharge and beam characteristics are measured and found to be similar to that of THCDs such as pseudospark and channelspark.

Landau damping and anomalous skin effect in low-pressure gas discharges: Self-consistent treatment of collisionless heating

In low-pressure discharges, where the electron mean free path is larger or comparable with the discharge length, the electron dynamics is essentially nonlocal. Moreover, the electron energy distribution function (EEDF) deviates considerably from a Maxwellian. Therefore, an accurate kinetic description of the low-pressure discharges requires knowledge of the nonlocal conductivity operator and calculation of the non-Maxwellian EEDF. The previous treatments made use of simplifying assumptions: a uniform density profile and a Maxwellian EEDF. In the present study a self-consistent system of equations for the kinetic description of nonlocal, nonuniform, nearly collisionless plasmas of low-pressure discharges is reported. It consists of the nonlocal conductivity operator and the averaged kinetic equation for calculation of the non-Maxwellian EEDF. This system was applied to the calculation of collisionless heating in capacitively and inductively coupled plasmas. In particular, the importance of accounting for the nonuniform plasma density profile for computing the current density profile and the EEDF is demonstrated. The enhancement of collisionless heating due to the bounce resonance between the electron motion in the potential well and the external rf electric field is investigated. It is shown that a nonlinear and self-consistent treatment is necessary for the correct description of collisionless heating.

The hairpin resonator: A plasma density measuring technique revisited

A microwave resonator probe is a resonant structure from which the relative permittivity of the surrounding medium can be determined. Two types of microwave resonator probes (referred to here as hairpin probes) have been designed and built to determine the electron density in a low-pressure gas discharge. One type, a transmission probe, is a functional equivalent of the original microwave resonator probe introduced by R. L. Stenzel [Rev. Sci. Instrum. 47, 603 (1976)], modified to increase coupling to the hairpin structure and to minimize plasma perturbation. The second type, a reflection probe, differs from the transmission probe in that it requires only one coaxial feeder cable. A sheath correction, based on the fluid equations for collisionless ions in a cylindrical electron-free sheath, is presented here to account for the sheath that naturally forms about the hairpin structure immersed in plasma. The sheath correction extends the range of electron density that can be accurately measured with a particular wire separation of the hairpin structure. Experimental measurements using the hairpin probe appear to be highly reproducible. Comparisons with Langmuir probes show that the Langmuir probe determines an electron density that is 20–30% lower than the hairpin. Further comparisons, with both an interferometer and a Langmuir probe, show hairpin measurements to be in good agreement with the interferometer while Langmuir probe measurements again result in a lower electron density.

Influence of the longitudinal magnetic field on the length of the Faraday dark space region

Magnetized plasma is used in many technological applications. Investigation of the magnetic field influence is therefore of great interest. In present paper a low pressure gas discharge in neon in presence of the longitudinal magnetic field is investigated. Experiments were performed on the cylindrical discharges with three different value of tube radius. The length of the faraday dark space region was recorded as a function of magnetic field value and the discharge current. The length has shown a steep increase with the magnetic field. Under some conditions almost whole interval between the electrodes was dark. A simple hydrodynamic model of the faraday dark space is considered in order to explain observed phenomena. Intensive diffusion processes lead to the field reversal in this region. With increasing magnetic field the effective losses of the charged particle to the wall become smaller. This serves as a reason for the extension of the dark space. The applied description yields good qualitative agreement with experiment. The discrepancies in quantitative values are discussed.

On the formation of the EDF in the inelastic energy region in the ionization waves

Low pressure gas discharges in the inert gases frequently exist in striated mode. The occurrence of the striations at low pressures and small currents is a consequence of the resonant formation of the energy distribution function of electrons (EDF) in spatially periodic electric fields. At these conditions the energy relaxation length in elastic energy region is much bigger than the striation length and the relaxation length in the inelastic region is much smaller than that. This difference in the relaxation lengths in elastic and inelastic energy regions causes the different ways of the EDF formation in those regions. Before the excitation threshold the EDF is formed non-locally, what is connected with the big value of the energy relaxation length, while after the excitation threshold the small value of the relaxation length makes the EDF to form under the influence of the local value of the electric field with additional phase shift. It is shown in the presented paper, that in the limiting case of small values of relaxation length in the inelastic energy region the solution of the Boltzman equation can be obtained by sewing the non-local solution in the elastic energy region with the local solution in the inelastic region and also the appropriate phase shift can be determined.

Minimization of Impedance Fluctuations in Cold-Cathode Pseudospark Switches (PSS)

The pseudospark discharge is characterized as a low-pressure gas discharge located on the left branch of the Paschen curve. Based on this discharge, a family of fast gas discharge closing switches for pulsed-power applications have been developed at Erlangen for more than 10 years. Due to the similarity to thyratrons, however, without having a hot cathode, the pseudospark devices are often described in literature as cold-cathode thyratron. The main features of a pseudospark switch are a high current rise closed to 10/sup 12/ A/s, the ability to carry reverse currents up to 100% and a high lifetime. The cold cathode also reduces the required standby power one magnitude lower than in thyratrons. On the other hand, especially at low-peak currents, undesired phenomena appear by the physics of the cold cathode. Current quenching, chopping, and impedance fluctuations are problems which occur at currents in the range of a few kiloamperes. Especially, the impedance fluctuations cause pulse-to-pulse fluctuations of the energy transferred to the load which could influence, i.e., the output energy of a laser system. For reducing this described problem of impedance fluctuations the dependencies of the geometry and material are investigated. With use of tungsten compound electrodes, this behavior of a cold cathode could be reduced significantly. This material shows also an improved reignition behavior after current zero.

The Influence of an External Permanent Magnetic Field on the Dynamics of Plasma Channels in a Radial Pseudospark Switch

High-current gas discharges can reach current densities of 10/sup 8/ A/cm/sup 2/ in order of magnitude and, therefore, damage the electrodes of nearly all types of low-pressure gas discharge switches. In pseudospark switches with radial geometry, the total current is divided by the number of radial channels and so the current per channel is strongly reduced. However, because of electromagnetic forces the plasma channels attract themselves and pinch together already in the first maximum of the switching current. Therefore, the main advantage of the radial pseudospark switch, the division of the total current, is lost. In this paper, a basic idea of how this movement of the plasma channels can be suppressed, will be introduced. Strong permanent magnets of different types (polarization B/sub x/ of type A: 0.48 T; type B: 1.31 T; and type C: 1.88 T) are attached to the pseudospark switch near the bores of the hollow cathode. The most effective way to apply the magnets was to put them face to face with the bores of the hollow cathode with the center line of the bores and the cylindric magnets coinciding. In this case, the magnetic field looks like a magnetic bottle viewed along the charge carriers path. The magnetic fields capture the plasma channels for a short time and so the pinch effect is delayed until the first maximum of current is over. The investigated pseudospark switch was a radial switch with three channels. The charging voltage was 10 kV and the discharge capacity was 2.2 /spl mu/F, resulting in a total discharge current of 40 kA, respectively, 13.3 kA per channel. In the switch under consideration, the pinching is fulfilled after 800 ns when no magnets are applied. The smallest possible distance between the front of the magnet and the end of the discharge channel was 2 mm. The strength of the magnetic field in this distance with magnets of type A, B, and C was 390, 1065, and 1525 mT, respectively, and the pinching was delayed for 400, 2000, and 1600 ns, accordingly.

Particle-beam experiment to study heterogeneous surface reactions relevant to plasma-assisted thin film growth and etching

An ultrahigh-vacuum-based particle-beam experiment to study heterogeneous surface reactions relevant to plasma–surface interaction processes is presented. The experiment comprises two radical beam sources and a source for low energy ions. As diagnostic tools real-time in situ ellipsometry and infrared spectroscopy are implemented. The infrared sensitivity for thin films is enhanced through application of an optical cavity substrate. The fluxes of the radical beam sources are quantified absolutely for the production of hydrogen atoms and methyl radicals. The ion source is also quantified for a wide variety of ionic species, e.g., He+, Ar+, H+, H2+, H3+, and CH3+. Ion energies from above 1 keV down to 1 eV are achievable. The setup allows one to investigate heterogeneous surface processes of one single species or simultaneous interaction of up to three different, individually controllable species with a surface of interest. By running the radical sources to produce hydrogen and methyl radicals and the ion source with the ions mentioned, microscopic surface processes relevant to deposition and erosion of hydrocarbon layers in low-pressure gas discharges were studied in great detail. Sample results for the hydrogen CH3 growth synergism, ion-assisted film growth, and chemical sputtering are presented to demonstrate the capability of the setup.

Fine structure of carbon films deposited from a microwave low-pressure gas discharge

We have studied the conditions of synthesis and the fine structure of carbon films of various structural modifications possessing preset electrical properties. The phase diagram of polymorphous transformations is constructed for carbon films grown from the plasma of microwave discharge in low-pressure ethanol vapor. Technological factors influencing the efficiency of extracting diamondlike microcrystals from a hydrocarbon matrix have been studied. Depending on the extraction regime and initial layer thickness, the size of extracted microcrystals can vary from 10–12 to 100–120 nm, while their surface concentration may reach 1.4×107 cm−2.

Dust-acoustic wave instability at the diffuse edge of radio frequency inductive low-pressure gas discharge plasma

A spontaneous excitation of a grain density wave in a dusty cloud of monodisperse particles suspended at the diffuse edge of an rf inductive gas discharge has been discovered. The main physical parameters of this wave (phase velocity, wavelength, and growth rate) and of the background plasma (distributions of the electron density, electron temperature, and space potential) were measured. A theoretical model of the observable phenomenon based on the theory of dust acoustic waves in a collisional dusty plasma correlates well with the experimental data in a broad range of experimental conditions. The influence of a varying dust grain charge on the development of the observed dusty plasma instability has been analyzed. It is shown that the necessary condition for the instability excitation is the availability of a permanent electrical field (E0 ⩾3 V/cm) in the dusty cloud region.

Experimental investigation into low pressure gas discharges in microwave electric field optical sensor probes

This paper investigates the feasibility of using the emission intensity of low‐pressure argon and nitrogen gas discharges as the sensing mechanism for a microwave electric field optical sensor probe in microwave resonant cavities. The emission is coupled to a photodiode for detection through an optical fibre due to the difficulty in using conventional optoelectronic devices in close proximity to microwave cavities. The discharge emission intensity is monitored at a range of different input powers to the cavity. The proposed designs for the electric field sensing probe are also included.

低圧ガス放電管用新電極の検討

低圧ガス放電管用新電極の検討

Deposition of highly adhesive amorphous carbon films with the use of preliminary plasma-immersion ion implantation

The usability of the following two types of plasma generators for deposition of highly adhesive a-C:H films on the large area substrates has been studied: (1) a source of plasma generated by means of a non-self-sustained arc discharge in low-pressure gas; and (2) an ion-plasma source on the basis of a Hall current accelerator with closed electron drift. The distinctive features of both sources are: (a) the possibility of the generation of extended flows (up to 2 m) of relatively dense plasma (∼10 10 cm −3); and (b) control of the plasma ionization degree, allowing realization of both preliminary plasma-immersion ion implantation (PIII) of a substrate and subsequent plasma-immersion ion-assisted deposition (PIID) of a-C:H film. The results of experimental investigations into the characteristics of the sources in different operational regimes are presented. Taking into account the probe measurements of plasma parameters, both generators have been optimized to operate in the PIII and PIID regimes. Characteristics of the pulsed negative bias applied to the substrate in both regimes have also been determined. It was shown that both sources allowed deposition of a diamond-like film on large-area substrates with a growth rate of 100–300 nm h −1. A hard (20–30 GPa) a-C:H coating containing approximately 60% of carbon atoms with sp 3 hybridization and having satisfactory adhesion to the substrate can be obtained if short (∼60 μs) high-voltage (∼6 kV) bias pulses are applied to the substrate.

Device for calibration of low pressure gauges development

Vacuum system set-up is presented and used for calibration of pressure gauges in the region of rough vacuum, from 103 mbar to 10-1 mbar, with dry air. The capacitance manometer is used for the calibration of piezoresistant manometer, oil, mercury and mercury micrometers U manometers. The applicability of the experimental set-up and obtained results for the low pressure gauges calibration in the medium and high vacuum region is approved. It was shown how calibration of some cheaper pressure gauges might be performed and reliable measurements of the pressure may be made in the region that is of interest for atomic and molecular collision physics and low-pressure gas discharges. It was also shown that mentioned calibration procedure could be used for calibration of different U manometer types in order to renormalize older atomic and molecular collision data.

Prebreakdown Phenomena and Formation Processes of Low-Pressure Glow Discharges in Ar, Ar/N2 and Ar/O2 Mixtures

In this research, both the prebreakdown phenomena and the transient processes from the prebreakdown phenomena to the glow discharge in low-pressure Ar gas, Ar/N2 and Ar/O2 mixtures were investigated by electrical and optical methods. The prebreakdown phenomenon in the Ar/N2 mixture was similar to that of Ar gas, and a pulseless component has been observed. The prebreakdown phenomenon in the Ar/O2 mixture differed from those in the Ar gas and Ar/N2 mixture, and formed a pulse discharge. Thus, the prebreakdown phenomenon in the Ar gas has been changed to pulse discharge by mixing a small amount of O2 gas which played an important role in the formation of pulse discharge.

Validation of current density measurements with a B-dot probe

The time varying magnetic flux density in an electrodeless high frequency discharge was measured with a bare wire B-dot probe immersed in plasma. Based on B-dot measurements, the spatial distribution of the current density was determined and integrated over the cross section of the discharge to determine the total discharge current. The total current determined from B-dot measurements was compared with total current measured externally with a current transformer. Agreement between the two methods was very good over a wide range of gas pressures and discharge currents. This work validates that a small, immersed B-dot probe does not significantly perturb the discharge current and can accurately determine current density in a low-pressure gas discharge.

Hybrid model for the calculation of ion distribution functions behind a direct current or radio frequency driven plasma boundary sheath

A hybrid fluid dynamic/kinetic model is presented which describes the sheath and the presheath regions of dc or rf driven low pressure gas discharges in a realistic and self-consistent way. The model assumes an infinite extended sheath parallel to the electrode, allowing a one-dimensional spatial description. It provides for the presence of multiple positive ion species and their collisional interactions with the neutral background, and takes into account the possibility of a nonharmonic modulation of the sheath potential and the application of an external dc bias; in this work, the model is applied to a two-species capacitively coupled argon and oxygen plasma. The input required by the model consists of the fluxes of the incoming ions, of the modulating current, and of the pressure, the composition, and the temperature of the background gas. On output, the model provides the values of the electric field and of the particle densities within the sheath and the presheath, the total voltage drop across the sheath, and also the energetically and angularly resolved distributions of the positive ions and the energetic neutrals which impinge the material substrate at the boundary. In general, the model is able to treat dc discharges as well as capacitively and/or inductively coupled rf discharges, it thus covers most of the plasmas used in very large scale integration microelectronics manufacturing and other surface modification techniques. Using the model, studies of the energy distributions of the incoming ions have been performed for a wide range of parameters, and the effects of varying process conditions have been investigated. At low and intermediate pressures (p<50 mTorr), the distribution functions of rf driven discharges exhibit a characteristic bimodal structure; this structure disappears with increasing pressure as ion-neutral collisions become significant. A comparison of calculated ion energy distributions with experimental measurements on capacitively coupled argon and oxygen discharges shows excellent quantitative agreement. In addition to the ion energy distribution, the angular distributions of the incident ions at various energies are also discussed as a function of the neutral gas pressure. It turns out that the details of the angular distribution not only depend on the field structure of the sheath itself but also on that of the presheath. The results of the presented model are therefore more reliable than those of previous models which restricted themselves to the sheath region. This high physical accuracy of the presented model, together with its flexibility and its high execution speed, allows its use as a tool for technology-oriented computer-aided design in the microelectronics industry.

Prebreakdown phenomena and formation process of the glow discharge in low-pressure Ar gas

The prebreakdown phenomena and the formation process of the glow discharge in a low-pressure Ar gas were investigated under a uniform field gap. Prebreakdown phenomena were observed for 0.5 Torr cm⩽pd⩽2 Torr cm (where p is pressure, d the gap distance) in Ar gas under conditions of a slowly increasing voltage. It was observed that the prebreakdown phenomena formed pulse discharges up to the transition to the glow discharge. The amplitudes of the photon and current pulses due to the pulse discharge increased with time, and then decreased as soon as the transition to a steady glow discharge occurred. When the overvoltage or external series resistance was increased, the pulse amplitudes increased with the applied voltage and decreased with the resistance. The characteristics of the prebreakdown phenomena were changed by the shape of the electrodes. The formation mechanism of the glow discharge can be qualitatively explained by that of the streamer in a high-pressure discharge. The transient glow discharge was observed, and its duration increased with an increase in resistance. The instability of the glow discharge was controlled by three factors, namely, Kaufmann’s criterion, the Child–Langmuir law, and the density balance between the production and removal rates of electrons.

Silicon carbide as electrode material of a pseudospark switch

Through the last years, the pseudospark switch, a low-pressure gas discharge switch with hollow cathode geometry, became established as a promising element of pulsed power technology and a serious alternative to other high-power switches. The use of a novel electrode material silicon carbide yields performance improvements in two main areas. Quenching phenomena, a long-standing problem for several applications, are suppressed completely and the switch lifetime can be distinctly increased, approaching that of thyratrons for operation with high repetition rate. As a crow-bar switch, the lifetime is nearby unlimited due to cold electrode usage. Spatial and temporal resolved spectroscopy revealed new insight into the extraordinary discharge behavior of silicon carbide electrodes. The radial plasma expansion from the central bore hole to the outer electrode regions, forming vesicular shells of different ionization stages of Si and C, are described in detail. The remaining problem, a significant loss of deuterium gas during discharge, has been long-term tested and is assumed to be the outcome of absorption in the silicon carbide electrodes. An envisaged promising remedy is presented.