Transient Hollow Cathode Discharge Research Articles

Variable beam entrance Faraday cup system for pulsed electron beam current profile characterization.

A high accuracy variable beam entrance Faraday cup (VBEFC) system is designed in this work. The presented VBEFC system is designed for the beam current profile measurement of the transient hollow cathode discharge (THCD) generated pulsed electron beam, which is a new source of high energy flux for metallic material processing. By proper designs of the beam entrance array, fast response electron collector, grounding, and shielding, this VBEFC system is capable of determining the radial profile and its temporal evolution of the THCD generated electron beam. The results of the electron beam current and current density distributions collected at varying radial locations and times under multiple voltages are presented in this paper. The experimental results show that both the amplitude and the current density of the THCD electron beam at a given radius always increased with the increase in the accelerating voltage, which is coincident with the self-focused propagation of the electron beam promoted by the voltage.

Experimental investigations of nanosecond-pulse electron beam profile for metal surface treatments

In this paper, a transient hollow cathode discharge (THCD) generated pulsed electron beam is applied for the metal surface treatment. And the profile of this pulsed beam current density distribution is experimentally investigated via a high accuracy variable beam entrance Faraday Cup (VBEFC) system. The influences of the varying beam pulses on the post-treated surface microstructure and properties of 306 stainless steel and 6061 aluminum alloy are investigated in this paper. The experimental results demonstrate that the THCD generated pulsed electron beam profile has a characteristic pattern of concentrated energy flux within a 2 mm circular region where the high energy electrons are the most concentrated. And such a concentrated energy flux pattern can be further promoted by the increased accelerating voltage and the self-focusing space charge effect. The pulsed e-beam surface treatment results on 306 stainless steel show that the surface morphology variations are identical with the evolutions of the beam current profiles under varying accelerating voltages. Additionally, the surface treatment results on 6061 aluminum alloy show that the micro-hardness of the alloy can be improved from the initial 63.1 HV to 85.1 HV by increasing the beam accelerating voltage up to 25 kV, promoting the concentrated energy flux of the generated beam pulse.

Operation of an electron beam initiated metallic plasma capillary discharge

In this work, a simple and different technique of generation of a metallic capillary discharge plasma is being studied. The capillary is initially evacuated to a low pressure of below 10−2 Pa. The transient hollow cathode discharge (THCD) produces high energy electron beam that subsequently is employed to vaporize the anode material that will inject into the capillary and initiates the discharge through the capillary. The EUV energy emission produced is calculated to be an estimation in the order of tens of milijoules. At the peak of the current of around 9 kA, the electron temperature of the plasma achieved is estimated to be around 11 eV.

Experimental investigation of time-resolved electron beam energy distributions generated in a transient hollow cathode discharge

In this paper, a retarding potential energy analyzer (RPEA) specific for pulsed electron beams within the pressure range of tens of mTorr is developed and used to investigate the energy of transient hollow cathode discharge produced electron beams. This RPEA has been applied in a pseudospark-based electron beam source at applied potential up to 20 kV. Experimental investigations under applied potential of 5 kV, 10 kV, 15 kV, and 20 kV were carried out and the time-resolved electron energy distributions are constructed. The numbers of electrons within various energy groups are calculated from the time-resolved electron energy spectra. Results show that the maximum number of electrons is emitted within the energy range of 40%–60% of the full applied potential on the pseudospark device, and varies from 22.5 ± 2.0% to 38.9±2.0% of the total number of emitted electrons. Additionally, the energy transformation efficiency of stored electrical energy to electron beam energy is calculated from presented data. The energy transformation efficiency increases from 11.4 ± 0.5% at 5 kV breakdown voltage to 23.2 ± 3.5% at 20 kV breakdown voltage.

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.

Experimental Observations of the Virtual Anode Motion and Streamer Breakdown Mechanisms in a Pseudospark Discharge

A pseudospark (PS) discharge or transient hollow-cathode discharge is a high-voltage low-pressure discharge, which is characterized by the cathode being hollow and the cathode's interior connected to the anode-cathode (A-K) gap by a hole in the center of the cathode. Some initial electron emission from the cathode region is the starting point of the different ionization processes occurring in the subsequent phases until the final breakdown. These initial electrons create an avalanche of charged particles whose tail is constituted by ions, meanwhile the head is essentially formed by electrons. An avalanche moving from the cathode toward the anode establishes physical conditions in order to create a virtual anode into the A-K region. Successive avalanches produced by spontaneous electron emission at the cathode, secondary electron emission due to ion impact in the cathode, or impact of the ionized particles with the residual gas, give access to the virtual anode arrives to the cathode leading the final electrical breakdown. During breakdown, electron and ion beam activities are present as consequence of the precedent ionization processes. In this paper, we present experimental observations of the moving virtual anode as well as the ion and electron beams emitted from a PS discharge for seven different gas pressures. The experiments were performed in hydrogen at a pressure of between 10 and 70 Pa, with a cathode aperture of 5 mm. A pulsed-charged capacitor scheme is used to produce up to a 30 kV step across the electrodes.

Comparative studies on intense electron beams generated in transient hollow-cathode discharges

For a large range of pulsed power applications, intense pulsed electron beams with 0.1-2-mm diameter, power densities of about 10/sup 9/ W/cm/sup 2/, peak currents in the 50 to 1000 A range, and electron energies up to several tens of kiloelectronvolts are generated in high voltage (5-30 kV) transient hollow cathode discharges (THCD). The most efficient THCD configurations for generating such electron beams are the pseudospark, the channel spark, and preionization-controlled open-ended hollow-cathode configurations, which differ from each other by the tube geometry and the typical working gas pressure (0.5-500 Pa). The beam parameters, such as current, pulse length, and electron energy are compared for all three configurations using the same diagnostics. The dependencies of these beam parameters on the discharge geometry, voltage, external capacitance, and gas pressure is discussed in these three configurations.

Electron energy distribution function in a transient open-ended hollow cathode discharge

An energy characterization of the electron beam produced in a transient open-ended hollow cathode discharge is presented. The entire energy distribution function (EDF) of the beam ranging from 1 keV to tens of keV was measured for the first time using two methods: the bremsstrahlung emission from the interaction of the beam with a thick target and the magnetic deflection of the beam. It was experimentally proved that the EDF is composed of lower-energy electrons described by one or two exponential functions decreasing with energy and a quasi-Gaussian hump of high-energy electrons with a maximum at ~0.5-0.7 of the energy corresponding to the breakdown voltage. The EDF is specific for each discharge configuration and depends on the breakdown voltage.

Further statistical studies of ionization growth and breakdown formation mechanisms in the final breakdown phase of a transient hollow cathode discharge

A transient hollow cathode discharge (THCD) is a high-voltage low-pressure discharge, which is characterized by an axial hollow in the cathode electrode. The temporal sequence of the different parts of ionization growth in breakdown formation, which take place just before electric breakdown, are statistically studied. The von Lane formalism has been used to characterize in detail the statistical time distribution of the different processes required for electric breakdown to occur. The experiments have been performed in Hydrogen at pressure between 13.3 and 53.2 Pa, with different sizes of the cathode aperture. It has been found that the different processes of ionization growth and breakdown formation mechanisms involved in the sequence leading to breakdown are not associated with a single characteristic time. Time-shifted Gaussian distribution functions have been identified which, when added together, reproduce the cumulative time distribution for each observed event, especially at low pressure and smaller cathode aperture, where the hollow cathode effect is less effective.

Plasma development in the low-pressure channel spark for pulsed intense electron beam generation

Pulsed intense electron beams with remarkable parameters are generated in transient hollow-cathode discharges (THCD). The channel spark is different from the other THCD configurations with respect to the low gas pressure and the tube geometry, which consists of a dielectric capillary tube and an anode mounted on its external surface. In this paper, the channel-spark discharge is systematically studied by electrical and fast shutter camera measurements. The buildup of electrostatic field and the equipotential lines are calculated by the numerical code Electrostatic Field (ElFi) 3.0. The influence of the gas pressure, breakdown voltage, and external capacitance on the discharge current and the beam current are outlined. Depending on the gas pressure, two different discharge modes are observed, i.e., the high resistive and the regular modes, which can be distinguished by the shape of the electrical signals. The plasma development in these modes is studied by electrical and fast shutter camera measurements, performed spectrally integrated and with interference filters to separate the emission of the ions originating from the working gas and capillary material. Compared to the pseudospark, the plasma development in channel spark presents similar as well as specific features due to the low gas pressure and to the capacitive anode-plasma coupling.

Hollow cathode discharge as pulsed X-ray source

Pre-ionization controlled transient hollow cathode discharges operating at pulsed high voltages of some tens of kV and gas pressure around 0.1 mbar are used to generate electron beams of some tens of A during about 10 ns. These latter are used to produce bremsstrahlung X rays which are energy analyzed. X ray spectrum is used as a tool for beam diagnostic. Evidenced beam properties and discharge versatility allow to consider the use of these devices as miniature X ray source. Its feasibility is studied.

Ultrafast hollow cathode triggered capillary discharge device as a strong XUV source

Several modifications of ultrafast capillary discharge devices have been operated in 3 gases (Xenon, Argon, Methane) at low pressure to obtain XUV radiation. Combining the features of a transient hollow cathode discharge with the inherent characteristics of the capillary discharge leads to a new starting regime different from the standard one. An electron beam being supported by the Hollow Cathode Effect makes the discharge initiating on-axis without any pollution by wall material. The capillary acts only as a limiter. After initiation, the heating process finishes within one ns time scale. Using a stored energy of less than 0.5 J, we achieve plasma temperatures higher than 80eV. With respect to future applications, our table top device gives the edge on other systems like laser produced plasmas or synchrotons through compacity, simplicity and cost effectiveness. With respect to basic research, we are close to the lasing conditions of the recombination scheme. We report on the electric parameters, the spatial and temporal development of the emitting source and the time-resolved XUV-spectroscopy including radiance analysis.

Time-resolved measurement of the energy distribution function of an electron beam created by a transient hollow cathode discharge

The energy distribution function of the electron beam produced in a transient hollow cathode discharge is measured using a method which is based on the collector self-biasing, allowing an access very close to the extraction hole. The distribution obtained is a mainly polyenergetic one extending over a range of up to some tenths of kiloelectronvolts. It is composed both of runaway electrons, whose energies we assume are equal to the instant discharge voltage, and a main part composed of a monotonically decreasing energy distribution. The ratio between these two populations is around 10% and strongly increases with distance from the extraction hole.

Time evolution of hollow cathode ionization processes in the final breakdown phase of a transient hollow cathode discharge

The enhanced ionization processes taking place inside the hollow cathode region (HCR) of a transient hollow cathode discharge (THCD) are essential events which lead to final electrical breakdown. This ionization growth is permanently assisted by a virtual anode moving in the anode-cathode gap (A-K gap), which extends the anode potential to within the hollow cathode region. In the paper, the ionization growth inside the HCR under the enhanced field due to the close proximity of the anode potential has been studied using a statistical technique in a range of pressures, with three different cathode apertures. Statistical time distributions of an extensive experimental data set are analyzed to understand the mechanisms involved in the final stages, just before electric breakdown.

Fast pulsed hollow cathode capillary discharge device

A fast pulsed capillary discharge device has been developed and implemented. The device combines the features of a transient hollow cathode discharge with the inherent characteristics of the capillary discharge, to obtain VUV to XUV radiation with ns rise time. The discharge operates in a 0.8 mm inner diameter alumina capillary, at 10–30 kV applied voltage. On axis discharge initiation is assisted by electron beams, which are characteristic of the hollow cathode effect. A short, 5 ns full width at half maximum XUV pulse is produced in association with a ∼1 kA, 5 ns current pulse.

Pulsed intense electron beams generated in transient hollow cathode discharges: fundamentals and applications

For the commercial application of pulsed power, material processing with intense pulsed particle beams is a very interesting subject. Recently, high-voltage (1-70 kV), low-pressure (1-100 Pa) transient hollow-cathode discharges turned out to be sources for pulsed intense electron beam generation suitable for this application. The remarkable parameters of these electron beams-beam currents of 50-1000 A (10-30% of the maximum discharge current) with a high energy component (mean energy of about 0.25-0.75 of maximum applied voltage) of 20-70% of the maximum beam current, power density up to 10 W/cm/sup 2/, beam diameters of 0.1-3 mm, beam charge efficiency of 3-5%-captured the attention not only of the scientific community in the last decade. The electron beam is emitted during the early phases of the discharge, and only weak dependence of the high energetic peak of the beam current was found on the external capacity, which determine the development of the later high-current phases. However, the beam parameters depend on the breakdown voltage, gas pressure, and discharge geometry (including self-capacity). In this paper, the characteristics of the pulsed intense electron beams generated in two configurations-multigap pseudosparks and preionization-controlled open-ended hollow-cathode transient discharges (PCOHC)-are described. Such electron beams already were used successfully in a variety of pulsed power applications in material processing, deposition of superconducting (YBaCuO) and diamond-like thin films, microlithography, electron sources for accelerators, and intense point-like X-ray sources, and some preliminary experiments revealed new potential applications such as pumping of short-wavelength laser active media. These pulsed electron beams could be used further in any kind of pulsed power applications that require high-power density, small or high electron energy, and small-beam diameters.

Pre-Breakdown Processes in the Hollow Cathode Region of a Transient Hollow Cathode Discharge

Discharge formation in the A–K space of transient hollow cathode discharge (THCD) is causally linked with the emission of high energy electron beams originating in the hollow cathode region (HCR). Ionization in the A–K gap proceeds through the formation of a moving virtual anode, whose time evolution is strongly correlated with different periods in the electron beam activity. Here, we report on time and space resolved observations of different ionization events inside the HCR, which are time correlated with ionization processes inside the A–K gap. The experiments have been performed in Hydrogen, at pressures between 50 and 400 mTorr. A statistical study of the characteristic times associated with the different ionization events, based on von Laue plots, shows that the time distribution of events is well described by a single Gaussian distribution.

Hollow cathode effects in charge development processes in transient hollow cathode discharges

A detailed experimental study of space charge formation and ionization growth in transient hollow cathode discharges (THCD) is presented. The experiment was performed with an applied step voltage up to 30 kV, with rise time less than 50 ns. The discharge was operated in different gases, at pressures in the range 50-750 mTorr, with cathode apertures ranging from 1 to 5 mm diameter and 5 to 20 mm long, with 10 cm electrode separation. Spatial charge formation, both in the hollow cathode region (HCR) and inter electrode space, has been studied with a capacitive probe array. Properties of high energy electron beams have been measured with a beam-target scintillator-photomultiplier arrangement. Detailed correlations of the electron beam evolution with the charge probe signals inside and outside the HCR clearly demonstrate the role of the electron beam in the initial formation and late evolution of a virtual anode and, in turn, the field enhanced ionization when the anode potential is brought close to the HCR. These results clearly identify the different regimes in which the Hollow Cathode plays a significant role in ionization growth in the inter electrode space and in the processes which eventually lead to electric breakdown.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Characteristics of a vacuum spark triggered by the transient hollow cathode discharge electron beam

The discharge characteristics of a vacuum spark triggered by the transient hollow cathode discharge (THCD) electron beam is investigated over a wide variety of discharge conditions. Two systems of the vacuum spark device have been considered-the first system powered by eight 2700-pF doorknob capacitors charged to a voltage of 40 kV (input energy of 17.6 J); while the second system employs a single 1.85-/spl mu/F Maxwell capacitor discharged at a voltage of 20 kV (input energy of 370 J). The operating pressure of these systems has been varied over the range of 10/sup -2/ to 10/sup -5/ mbar in order to examine the effect of the operating pressure on the plasma formation of the vacuum spark discharge. The effectiveness of plasma heating has been found to be significantly enhanced in the two vacuum spark systems studied here. In particular, the plasma of the 17.6 J system has been observed to be heated to a condition hot enough to emit in the X-ray region when the operating pressure is reduced from 10/sup -2/ to 10/sup -5/ mbar. Similarly, in the case of the 370 J system, hot spot formation is also observed to occur only at a low operating pressure of 10/sup -4/ mbar. >

Space and time dependence of the electric field and plasma induced emission in transient and steady-state hollow cathode discharges.

We have performed time-resolved observations of the current, optical emission, and electric field profiles in Ar and Ar+1% K hollow cathode discharges. The geometry and pressure are similar to those used in pseudosparks. The discharge was operated with a repetition rate of the order of kHz, an applied voltage in the range 300--600 V and with discharge current up to 1.5 A. Calculations were performed using a 2D, hybrid fluid-particle model developed previously [J. P. Boeuf and L. C. Pitchford, IEEE Trans. Plasma Sci. PS-19, 286 (1991)]. Good agreement is found between the predictions of the model and the experimental results, and, in particular, both show an initial sharp peak (less than 100 ns in width) in the discharge current a short time after application of the voltage which is related to the expansion of the plasma into the hollow cathode. Further, the model shows that the sequence of events leading to the formation of the hollow cathode discharge is the same in the present experiments and in pseudospark switches operating at much higher voltages.