Paschen Discharge Research Articles

Heaterless 300A lanthanum hexaboride hollow cathode.

Hollow cathodes used in electric propulsion typically have an external heater to raise the thermionic electron emitter to emission temperatures. Heaterless hollow cathodes that are heated by a Paschen discharge have been historically limited to low discharge currents (<5A) due to arcing and inefficient heating. A new heaterless technology was previously developed for cathodes up to 50A, utilizing a refractory metal tube to extend the gas feed line partway into the thermionic insert region. A high voltage (>700V) Paschen discharge is ignited between the keeper and the tube, which quickly transitions to a lower voltage (<80V) thermionic discharge from the inner tube surface and heats the thermionic insert by radiation. This "tube-radiator" configuration eliminates arcing and inhibits the long-path-length discharge between the keeper and gas feed tube upstream of the cathode insert that caused inefficient heating in prior designs. This paper describes extending this technology developed for a 50A cathode to one that is capable of 300A. The larger cathode uses a 5-mm diameter tantalum tube-radiator and a 6-A, 5-minignition sequence. Ignition was challenging because the high heating power required (≥300W) is difficult to maintain with the low voltage (<20V) keeper discharge that exists prior to igniting the thruster discharge. To achieve self-heating from the lower voltage keeper discharge, the keeper current is raised to 10A once the LaB6 insert starts emitting. This work shows that the novel tube-radiator heater is scalable to large cathodes capable of tens of thousands of ignitions.

Configuration of instrumentation wire extraction for ITER magnet feeder system

For the quench detection, the high voltage (HV) instrumentation wires should be led out from the electrical insulation layers of ITER magnet feeders. Based on the latest design, at least four HV instrumentation wires need to penetrate from the busbar joint, which break the integrity of the intact insulation layer. A perforated crack may generate along the instrumentation wire in the insulation layer. At some specific ambient pressure and operational voltage, the crack will cause Paschen discharge inevitably, which is the most serious risk threatening the safe and steady operation of the feeders. There are over 200 busbar joints with the instrumentation wires penetration need to be manufactured on the ITER site, and a reliable and durable insulation structure and technology need to be developed for the extracting HV instrumentation wires of ITER magnet feeder. In this paper, the HV instrumentation wire extraction and the specimen design are presented. The electrical properties of the specimen, including the DC hi-pot test, Paschen test and Partial Discharge (PD) test, are reported and discussed.

Development of a 50-A heaterless hollow cathode for electric thrusters.

Hollow cathodes in electric thrusters normally use an external heater to raise the thermionic electron emitter to emission temperatures. These heaters are a potential single-point failure in the thruster and add a separate power supply to the power processing unit. Heaterless hollow cathodes are attractive for their compact size and potential higher reliability but have only been reliably demonstrated to date in small hollow cathodes capable of discharge currents below around 5A. A new heaterless LaB6 hollow cathode has been developed that is capable of discharge currents from 5 to 50A. The cathode configuration extends the gas feed tube at cathode potential part way into the emitting insert region of the cathode. A high-voltage Paschen discharge is struck from the tube to the keeper that heats the tube tip, which then efficiently heats the insert by radiation. This configuration eliminates the arcing observed in prior large heaterless designs that coupled the high-voltage Paschen discharge to the orifice plate or the insert itself. Discharge current-voltage characteristics show that the presence of the tube does not significantly perturb the insert-region plasma. Startup uses a simple 3minignition procedure, and voltage traces of the keeper discharge reveal that much of the present tube-radiator's 100-to-150W heating power comes from an intermediate thermionic discharge sustained by the tube during the transition between the Paschen discharge and LaB6 insert thermionic regime. This novel heating mechanism enables an unprecedented class of higher-current heaterless hollow cathodes for the next generation of high-power electric propulsion systems.

Arcing on Solar Generators by Collection of Ionospheric Plasma Currents

There are several well-known possibilities to trigger a secondary arc on solar arrays by the creation of conductive plasma produced in the gap between two solar cell strings. It could be by electrostatic discharges due to an inverse potential gradient condition, micrometeoroid impacts, or laser impacts. The development of electrical propulsion for satellites requires higher voltages on solar arrays. These voltages strongly modify the plasma–satellite interactions, especially by increasing the collection of plasma currents. By studying this plasma collection current on solar cell samples, we have detected secondary arcs between two strings without any triggering event as previously described. Focusing on this event, we found that a series of physical phenomena may lead to the secondary arcing situation: heating by plasma current collection, outgassing, Paschen discharge conditions, and then secondary arcing. We have reproduced on solar cell-like samples what occurs when an arc starts and the conditions for its occurrence. Thereby, we have defined when this phenomenon is able to happen on real solar arrays.

4 kV class splicing device to connect high-voltage instrumentation wires and cable

In ITER machine, the splicing devices (SDs) are designed to connect the high voltage (HV) individual wires bringing the signal from the sensors installed in the superconducting magnet system to the HV cables connected with the feedthroughs installed in the cryostat wall. SDs shall be designed and manufactured to ensure its safety operation at 4.5 K and under the environment of ionizing radiation and high magnetic field. As a kind of insulating structural part, the structural design of prototype SD aimed at achieving enough safe margin of insulating performance under helium (He) gas condition. To evaluate the electrical properties, a prototype SD was fabricated and the HV-leakage current tests were performed, the testing results indicate that the leakage currents are less than 2 μA under 4 kV DC and 20 μA under 4 kV AC respectively, which are less than the allowable values 5 μA and 30 μA. In addition, the testing results also indicate that there was no breakdown or current impulse occurred during Paschen discharge tests. However, when the bonded interface is not He gas tightness, SD can be damaged by Paschen discharge.

Experimental results on Paschen tests on the electrical insulation of the terminal service Box of the correction coils of ITER

The insulation performance of wet wrapping of terminal service box pipework is investigated by the Paschen discharge test in this study. The test mock-up was designed with a U-shaped pipework as the fundamental structure, consisting of insulator breaks, voltage taps, and high voltage wires. In addition, the design of the test mock-up and insulation structure is studied, and the investigation method used in this study is described in detail. A high voltage test was conducted at room temperature for various degrees of vacuum. The test results demonstrated that the insulation (wet wrapping) remains undamaged, and leakage current is less than 10 μA. Further, the test results conformed to the requirements of the ITER Organization. Finally, the performance of the insulation was determined, and the feasibility of the wet wrapping process was verified indirectly.

15 kV class high-voltage electrical vacuum feedthrough

In the feeder system of the Experimental Advanced Superconducting Tokamak (EAST), the high-voltage (HV) electrical vacuum feedthrough plays an important role in transmitting the voltage signals from one vacuum environment to anther vacuum environment under cryogenic temperature. Due to no commercial solutions, the feedthroughs for EAST were developed in the Institute of Plasma Physics, Chinese Academy of Sciences. In this paper, the structural design, electrical analysis, structural analysis and experiments involving thermal cycle, helium gas tightness, HV-leakage current and Paschen discharge are presented. The experimental result indicates that the leak rate of the developed feedthroughs was less than 1 × 10−9 Pa·m3·s−1. After the 5 thermal cycles from room temperature (RT) to 77 K, the HV-leakage current experiment under 10−3 Pa vacuum environment and the Paschen discharge experiments of feedthroughs were performed, the leakage current under 15 kV DC is lower than the allowable value 5 µA, and the leakage currents in helium gas and nitrogen gas from 1 Pa to 50 kPa are lower than the allowable value 10 µA.

Experimental Study on Paschen Discharge in Helium for High Voltage Cryogenic Insulation Material

According to the requirements of large superconducting components, all the solid superconducting insulation of current leads, magnets, cooling and measurement pipeline at high potential should withstand 80–300 K thermal cycles. If helium leak or pipe leak taken place, and insulation has been formed some flaws, Paschen discharge would be occur. The experimental program aimed at analyzing the low-temperature electro-physical performance of solid superconducting insulation, which was designed with different configuration such as thickness, material structure and so on. The dielectric performance of both pre-preg cryogenic insulation and the wet wrap cryogenic insulation were studied. Both of the dielectric strength of the two type insulation is in the range from 11 to 23 kV/mm at 80 K helium condition. Besides, both of the two type insulation samples of 3 and 5 mm thickness can withstand 30 kV under various helium pressures at 80 K.

A superconducting magnet feeder system that avoids the risk of Paschen discharge

Current is traditionally fed to magnets in fusion devices via a system of superconducting busbars in a thermally insulating vacuum enclosure. Some of the magnets are pulsed, taking the busbars to a potential of several kilovolts, so these must be well insulated electrically. There is a risk of helium leaking into the insulation vacuum, so the electrical insulation of the busbars must be permanently hermetic in order to avoid the risk of Paschen discharge, subsequent arcing and concomitant heavy damage. Such so-called Paschen-tight insulation is difficult to achieve, especially considering that the system is pulsed and the required lifetime may be of the order of 20 years. This report describes a conceptual study of such a feeder system that avoids the risk of Paschen discharge by routing the superconducting busbars via a pressurized helium environment, thus eliminating the necessity of thorough hermetic sealing of the insulating envelope. In the concept that is presented several other features address design issues that have occurred in recent feeder systems for fusion magnets.

Experimental Study on Paschen Tests of ITER Current Lead Insulation

An experimental Paschen test setup has been established to analyze the quality of ITER current lead (CL) insulation and extend the research on Paschen's law under various conditions. Insulation problems can destroy a machine if a Paschen discharge is triggered by an insulation defect that is caused by faulty manufacturing, electromagnetic force, and thermal stress load with a certain degree of vacuum helium or pipe leakage. The results show that the CL insulation mock-up worked well under normal temperature and pressure. Besides, the mock-up also worked well in helium conditions and at 80 K temperature at different pressures. One area of CL insulation was severely destroyed when the 80 K test was conducted after 5 thermal cycles, resulting in Paschen discharge phenomenon. The breakdown voltage is maintained at a relatively low level under different pressure conditions; the change of breakdown voltage was mainly due to the change of pressure, and such change was in line with the Paschen law.

Interactions between organic photoconductor and plasma discharge within an electrophotographic environment

Structural and chemical compositional changes occurring within the surface region of an organic photoconductor electrically charged with Paschen discharge over an extended period of time have been investigated with the help of attenuated total reflectance Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy. Experimental conditions were selected to simulate interactions between the photoconductor and the plasma discharge occurring in a typical electrophotographic printing environment. A long time exposure of the photoconductor to energetic charged and neutral species, and UV photons impinging upon the photoconductor caused oxidation of the surface, substantial chemical bond breakage, and the reformation of bonding configurations within the region below the surface. These resulted in the formation of a thin surface region with properties different from those of the original photoconductor. Since this study closely follows a modern electrophotographic process, the obtained conclusions are applicable to any electrophotographic printing engine.

Dependence of electrostatic field on properties of paper in a two-dimensional toner image transfer nip

Electrostatic elds in the toner image transfer nip of laser printers were calculated by exact solution of the Poisson equations for a two-dimensional (2D) model and by applying for the paper the approximation of dielectric thickness. Both the distribution and location of the electric eld strength component parallel to the paper surface were investigated and eld values were calculated for different paper and toner parameters. The conditions of the occurrence of Paschen discharge in the toner image transfer nip were analysed.

Seeking radio emissions from hypervelocity micrometeoroid impacts: Early experimental results from the ground

High-velocity impact experiments have been conducted to look for radio frequency (RF) emissions from impact-produced plasmas that could be used to identify micrometeoroid impacts to spacecraft in orbit. Launched by a three-stage light gas gun, 17 mm diameter by 0.9 mm thick Ti6Al-4V flyer plates impacted 0.75 mm thick indium (In) foil at more than 10 km s −1. The resulting collision presumably ionized some fraction of the vaporized In cloud, which was accelerated to about 12 km s −1. This weak In plasma then passed through a wide-band detection system that looked for RF emissions. Over the course of five shots during the experiment, no conclusive plasma emissions from the In were detected. However, strong evidence indicates that significant charge is accumulated on the flyer plate during acceleration and flight, possibly producing Paschen discharge to the chamber walls. Finally, plasma may be produced by the launcher secondary to launching the plate, leading to further contamination of the results. These effects have significant consequences for RF experiments attempted in launching systems of this type.

Teflon electret radiation dosimeter

Electrets are insulating materials with a quasi-permanent electric polarization. Since charge compensation occurs when subjected to ionizing radiation, electrets may be used for α, β, γ, X, e − and neutron radiation dosimetry. The compensating charge may either be produced in the electret material itself or by interaction by the radiation field with surrounding insulating material. We report the results of investigations of electrets produced from Teflon ® polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexa-fluoropropylene (FEP) and tetrafluoroethylene-per-fluoromethoxyethylene (PFA) films. The electret state was produced uniformly on one surface of the films by a corona method with a negative 15 kV potential applied to multiple pointed electrodes facing the film on a grounded plate. After polarization, the films were exposed to a known X radiation and the uncompensated charge was nondestructively measured with capacitive probes. The area of the probe was designed in accordance with the spatial resolution desired. The resolution and sensitivity of such probe is ultimately limited by the Paschen discharge between the electret and the probe. Response curves compared the ratio of uncompensated charge density after and before exposures to the radiation as a function of exposure. The linearity of these curves shows that the films may be used as a radiation dosimeter. The slope of the response curves indicates the sensitivity to the ionizing radiation. The PFA film displays two linear regions which correspond to two electron trap levels. We demonstrate that the image forming nature of these planar electret dosimeters has a millimeter of spatial resolution. We also report the development of innovative electret geometry for measurements of the directional dependence of the radiation and by choice of the surrounding insulating materials, an almost complete selectivity in mixed radiation fields.

Space and Time Dependence of Electrostatic Transfer Process in Electrophotography

Electrostatic transfer process of four layers structure (photoreceptor, toner, paper, and transfer belt) which is specified by four R&#x2013;C parallel circuits combined in series and expressed by a set of 3 differential equations, is investigated with coefficients of time constant and dielectric thickness instead of R and C. The analytic closed form solutions are obtained with a method of eigenvalue and eigenvectors. Initial conditions of solutions are determined by static voltage difference of each layer, which are given by electrostatic Poisson's equations, and then space dependence and time one of solutions are unified by this analysis. Time evolution of each voltage, transfer efficiency, and Paschen's discharge are discussed. It is shown that Paschen's discharge occurs in non-image white area, dominantly.

Space and Time Dependence on Transfer Process of Electrophotography

Electrostatic transfer process of 4-layers structure (Photoreceptor, toner, paper, and transfer belt) which is specified by multiple layers of R-C parallel circuit and expressed by a set of 3 differential equations, is investigated rigorously with a method of eigen value and eigen vectors. Initial conditions of solutions are determined by static voltage difference of each layer, which are given by electrostatic Poisson's equations, and then space dependence and time one of solutions are unified by this analysis. Time evolution of each voltage, transfer efficiency and Paschen's discharge are discussed.

Evidence for Martian electrostatic charging and abrasive wheel wear from the Wheel Abrasion Experiment on the Pathfinder Sojourner rover

The Wheel Abrasion Experiment (WAE) on the Mars Pathfinder rover was designed to find out how abrasive the Martian dust would be on strips of pure metals attached to one of the wheels. A specially modified wheel, with 15 thin film samples (five each of three different metals), specularly reflected sunlight to a photovoltaic sensor. When the wheel was rotated to present the different sample surfaces to the sensor, the resulting signal was interpreted in terms of dust adhesion and abrasive wear. Many data sequences were obtained. Ground tests of similar wheels in a simulated Martian environment showed that static charging levels of 100–300 V could be expected. To prevent the possibility of Paschen discharge in the low‐pressure Martian atmosphere, charge dissipation points were added to the Sojourner rover and were shown in ground tests to keep charging levels at 80 V or less. Nevertheless, significant dust accumulations on Sojourner's wheels may be interpreted as evidence for electrostatic charging. Simple considerations of the expected maximum level of charging and electrostatic dust adhesion lead to an estimate for the size of the adhering dust grains. From the WAE data, it is hypothesized that the photoelectric effect is the most important mechanism for slow discharge in Martian daylight. Sensor signals obtained late in the Pathfinder mission show that significant wheel wear was seen on the metal wheel strips, with the most wear on the thinnest aluminum samples and the least on the thickest nickel and platinum samples. An estimate is made of the reflectance of the adhering Martian dust. The depth of dig of the WAE wheel shows that the dust is in some places very loose and in others tightly packed. Finally, comparison of the WAE results with ground test results makes possible a comparison of the Martian soil with mineral grain types and sizes found on Earth and show that the Martian dust is fine‐grained and of limited hardness.

Measurements of electrical breakdown in the vicinity of a sounding rocket differentially charged to 10 kV

During the initial period of high voltage biasing of the SPEAR-3 sounding rocket payload, it was observed that electrical breakdown occurred in the gas surrounding the rocket. The breakdown occurred almost all the way to apogee of 289 km on the upleg, but did not recur on the downleg until the payload reached an altitude of ∼ 100 km. It is suggested that this behavior can be attributed to payload outgassing on the upleg leading to abnormally high gas pressure near the payload skin. Consideration of a modified Paschen discharge process with varying pressure along the discharge path was found to be consistent with the results.