H. Krompholz

Plasma development in the early phase of vacuum surface flashover

The primary physical mechanism responsible for charge-carrier amplification, in a developing surface discharge, has eluded conclusive identification for decades. This paper describes the results of experiments to directly detect charge-carriers, above the dielectric surface, within the developing discharge. Free electrons are detected by measuring the deflection of a laser beam, focused to a 20 /spl mu/m 1/e diameter, with an angular sensitivity of 0.18 mVspl mu/rad and a risetime of 6 ns. The estimated detection threshold for electrons in the developing discharge is 10/sup 16/ cm/sup -3/ to 10/sup 17/ cm/sup -3/. A streak camera is used to gather spatial information regarding luminous processes with a maximum resolution of 25 /spl mu/m and 0.6 ns. Current measurements have a sub-nanosecond response time and a detection threshold of 100 mA. Laser deflection measurements demonstrate the rapid development of a particle gradient, generally within 10 /spl mu/m of the surface near the cathode and in the range of 75 to 175 /spl mu/m from the surface near the anode, during the developing discharge. Streak camera measurements demonstrate the formation of an intense, visible emission, 25 to 50 /spl mu/m in diameter, located near the insulator surface, during the formation of the discharge. These results imply that charge-carrier amplification occurs above the surface of the insulator, in a region of neutral particles desorbed or otherwise ejected from the insulator surface. >

Magnetic Insulation For The Space Environment

The influence of magnetic fields on dielectric surface breakdown voltages for space conditions (pressure in the range of 10-7 to 10-2 Torr, background plasma, and UV illumination) is investigated, both for DC and pulsed voltages as well as DC and pulsed magnetic fields, including conditions for magnetic self insulation. DC conditions are susceptible to corona-type discharges with major influences of magnetic fields on discharge paths and geometry. For pulsed volt-ages, insulation effects dependent on the polarity of the magnetic field (with respect to the electric field and the surface) are starting at field amplitudes of 0.2 T.

Pulsed Dielectric Surface Flashover at Atmospheric Conditions

Dielectric flashover along insulators in vacuum has been sufficiently researched in the past. Less studied, but of similar importance, is surface flashover at atmospheric pressures and the impact of various electrode geometries, humidity, and type of gas present. Previous research has shown distinct arc behavior in air and nitrogen for an electrode geometry in which the electric field lines curve above the dielectric surface. Specifically, flashover experiments in nitrogen have shown that the arc path will follow the electric field lines, not the dielectric surface. As a result, it was concluded that the arc development path, whether along the electric field line or the surface of the dielectric, is related to the oxygen content in the atmospheric background. It is believed that this dependence is due to the arcs production of UV radiation in an oxygen rich environment. Further testing, in a pure nitrogen environment with UV illumination of the surface prior to the pulse application, has shown that UV plays a significant role in the arc development path. There is a near linear relationship between the percentage of liftoffs and the time delay between UV application and flashover. Additional studies have also shown a relationship between the UV intensity and the percentage of liftoffs. Based on these results we will discuss the physical mechanisms primarily involved in unipolar flashover at atmospheric pressure. Additional experimental results regarding the effects of humidity on the liftoff phenomenon will be presented as well.

High voltage, sub nanosecond feedthrough design for liquid breakdown studies

Experiments in self-breakdown mode and pulsed breakdown at high over-voltages in standard electrode geometries are performed for liquids to gain a better understanding of their fundamental breakdown physics. Different liquids of interest include liquids such as super-cooled liquid nitrogen, oils, glycerols and water. A typical setup employs a discharge chamber with a cable discharge into a coaxial system with axial discharge, and a load line to simulate a matched terminating impedance, thus providing a sub-nanosecond response. This study is focused on the feed-through design of the coaxial cable into this type of discharge chamber, with the feed-through being the critical element with respect to maximum hold-off voltage. Diverse feedthroughs were designed and simulated using Maxwell 3-D Field Simulator Version 5. Several geometrically shaped feed-through transitions were simulated, including linearly and exponentially tapered, to minimize electrostatic fields, thus ensuring that the discharge occurs in the volume of interest and not between the inner and outer conductor at the transition from the insulation of the coaxial cable to the liquid. All feedthroughs are designed to match the incoming impedance of the coaxial cable. The size of the feedthroughs will vary from liquid to liquid in order to match the coaxial cable impedance of 50Ω. The discharge chamber has two main ports where the feed-through will enter the chamber. Each feed-through is built through a flange that covers the two main ports. This allows the use of the same discharge chamber for various liquids by changing the flanges on the main ports to match the particular liquid. The feedthroughs were designed and built to withstand voltages of up to 200 kV. The feedthroughs are also fitted with transmission line type current sensors and capacitive voltage dividers with fast amplifiers/attenuators in order to attain a complete range of information from amplitudes of 0.1mA to 1 kA with a temporal resolution of 300 ps.

Investigation of the transmission properties of High Power Microwave induced surface flashover plasma

When dealing with the propagation of High Power Microwaves (HPM), special precautions must be used to prevent the onset of plasma generation. In this paper, we investigate the plasma located on the high pressure side of the dielectric boundary separating the vacuum environment of the microwave source from the high pressure environment of the transmitting medium, e.g., atmosphere. Because the collisional ionization rates are a monotonously increasing function of Eeff/p in the range of interest, the effective electric field normalized with pressure, implementation of HPM in high altitude (low pressure) environments are subject to dielectric breakdown due to this generated plasma, more than at sea-level altitudes. Dielectric breakdown causes the interruption in transmission of electromagnetic radiation due to the reflection and absorption properties of the plasma generated on the dielectric surface. In this paper, transmission, reflection, and absorption data is presented for plasma generated under various pressures ranging from 5 to 155 torr in N2 and air environments. In addition, seed electrons from UV illumination of the dielectric surface and physical vapor deposited metallic points are implemented and their implications to the overall transmission properties are discussed.

EXPERIMENTAL INVESTIGATION OF THE EARLY PHASE OF DIELECTRIC SURFACE FLASHOVER IN A SIMULATED LOW EAR

An experimental apparatus has been designed and constructed to acquire information on the physical mechanisms involved in the early phase of dielectric surface flashover in a simulated Low Earth Orbit (LEO) environment. The setup consists of a cable discharge pulser connected to the dielectric test gap via a spark gap in selfbreakdown mode. The geometry of all interconnecting lines and of the discharge chamber is coaxial and the impedances are closely matched to the cable pulser. As a first stage of the simulated LEO environment a low energy argon plasma is used. A xenon UV source, a low energy electron gun, and a combination of UV radiation and plasma will be applied to the dielectric test sample in the near future. Preliminary results with plasma have shown a change in the duration and development of the breakdown process compared to the case with vacuum.

Increasing surface flashover potential using magnetic insulation

Magnetic fields can be used to increase the flash over potential across a dielectric surface when applied in the appropriate direction. The first experimental arrangement utilizes a DC magnetic field capable of producing up to 0.7 T. In a second experimental arrangement, the insulating effects of pulsed magnetic fields produced by currents of up to 175 kA are investigated. Experiments with inhomogeneous fields indicated that the condition for magnetic insulation has to be satisfied in the cathode region only to increase the surface flashover voltage. Permanent magnets can be used to accomplish this. Magnetic insulation effects for space conditions (low earth orbit) have been investigated.

Statistical modeling of high power microwave surface flashover delay times

Summary form only given. The development of high power microwave (HPM) systems and technologies has been inhibited by breakdown phenomena which limit their transmitting capabilities. For a system in which there is no obvious source of breakdown initiating electrons, HPM breakdown can be even less predictable. A system in which microwaves are generated in a vacuum environment for the purpose of radiating into atmosphere typically uses a dielectric window to separate the vacuum and atmospheric sides of the system. At sufficient field levels, surface flashover can occur across this dielectric window resulting in a severe drop in transmitted power. The time between the application of the HPM and the sharp drop in transmitted power is described as the delay time, which consists of a statistical waiting time for initiatory electrons combined with an electron amplification time, or formative time. The experimental setup for this project consists of a 4 MW HPM source operating a 2.85 GHz attached to a traveling wave structure and a dielectric window mounted on the output side of the system. Dielectric surface flashover has been observed in air and nitrogen with pressures ranging from 60 to 155 torr. To provide a constant source of seed electrons, a UV lamp is used to illuminate the window resulting in photo-emitted electrons appearing at the surface. Another way to provide seed electrons is the inclusion of metallic points on the window which provide a source of field emitted electrons, and also results in a field enhancement at the dielectric surface. In the absence of a constant source of seed electrons, it is expected that field detachment from ion clusters is the primary mechanism for providing the high field region with flashover initiating electrons. A statistical model has been developed for predicting surface flashover that takes into account relevant parameters such as field level, ionization rate, gas type, and pressure. This model has shown good agreement with experimental data in nitrogen with UV illumination providing a constant electron seed rate. Presented here is an adaptation of this statistical model to an environment consisting of field enhancing metallic points as well as a comparison of results for UV illumination and stochastic seeding through field detachment from ion clusters.

A Finite-Difference time-domain simulation of formative delay times of plasma at high RF electric fields in gases

A Finite Difference (FD) algorithm was developed to calculate the formative delay time between the application of an RF field to a dielectric surface and the formation of a field-induced plasma interrupting the RF power flow. The analysis is focused on the surface being exposed to a background gas pressure above 50 torr. The FD-algorithm is chosen over particle-in-cell methods due to its higher computational speed and its ease of being ported to commercial electromagnetics solvers. The dynamic frequency-dependent permittivity of the plasma is mapped to the time domain of the FD algorithm using the Z transform. Therefore, together with the electron density, the effect of the developing plasma on the instantaneous microwave field is calculated. The high observed value of absorption, up to 60 %, is a result of the momentum transfer collision frequencies in the developing plasma being much larger than the microwave frequency. As a result, the electron density increases to values well beyond the density calculated from setting a plasma frequency equal to the microwave frequency. In the experiment, flashover is induced across a Lucite window by a 4 MW S-band magnetron operating at 2.85 GHz with ∼ 50 ns rise time. The results of the FD simulation are compared with experimental data obtained from flashover with background gases such as nitrogen, air, and argon all at pressures exceeding 50 Torr.

Delay time reduction of high power microwave surface flashover using metallic initiators

High power microwave (HPM) surface flashover can be rapidly induced by introducing metallic points on to the dielectric surface with negligible effect on the transmission properties. An experimental setup comprised of a magnetron operating at 2.85 GHz to produce a 4.5 MW, 3 μs pulse is used for observing surface flashover in various atmospheric conditions. An active pulse sharpening mechanism is used to reduce the pulse rise time in order to apply the electric field in tens of nanoseconds. For a system in which HPM transmission must be quickly suppressed, field enhancing geometries can provide a way for flashover to develop rapidly while keeping insertion loss at a minimum (<0.01 dB). Initial experiments utilizing 0.2 mm2 aluminum points with a spatial density of 25/cm2 have increased the global effective electric field by a factor of ∼1.5. This increase in electric field has sharply reduced delay times for surface flashover (i.e. the time between the application of the HPM pulse and a sharp drop in transmitted power). For an environment consisting of air at 155 torr, for instance, the delay time is reduced from 455 ns to 101 ns. Presented in this paper is a comparison of various field-enhancing geometries and how they relate to flashover development. Also, an analysis of time resolved images will be given along with an estimation of field enhancement factors.