External Cathode Research Articles

A numerical study of chaos in transferred electron oscillators and amplifiers with the use of an improved computer model

A model of a transferred electron device including reactive circuit elements and realistic boundary conditions is used to study chaos. We find that any simulation of chaos is suspect if the external circuit and cathode boundary conditions are ignored; including them reduces the range of parameters in which chaos occurs. © 1996 John Wiley & Sons, Inc.

Factors influencing electrochemical removal of chloride from concrete

Electrochemical chloride removal was studied using prisms made from concrete containing various levels of chlorides derived from sodium chloride added during mixing. The amount of chloride removed during the treatment was assessed by analysing the anolyte. Chloride removal increased with increasing applied potential, number of reinforcing bars at a particular depth and initial chloride content of the concrete. A greater percentage of chloride was removed from prisms where the thickness of the chloride bearing layer of concrete was less than the depth of cover to the reinforcement. Where the thickness of the chloride bearing layer exceeded the cover to the reinforcement, the use of an external cathode significantly increased the total amount of chloride removed. Chloride removal from a face remote from the source of the chloride contamination (soffit desalination) was shown to be feasible.

Determination of the fate of the current in the stress corrosion cracking of sensitized type 304SS in high temperature aqueous systems

The experimental work reported in this paper was designed to resolve the question concerning the fate of the current that is generated during crack growth in engineering alloys in power plant environments. Two limiting possibilities exist: the cathodic processes that consume the positive ion current generated at the crack tip reside within the confines of the crack; or they reside on the external surfaces. Experiments were conducted which made use of sensitized type 304 stainless steel (SS) compact specimens C(T) that had been modified to provide external surfaces that are isolated from the specimen, so that the current flowing between the crack and the external cathode could be readily monitored. This was done by coating the C(T) specimens with baked-on polytetrafluoroethylene so that only the crack (after fatigue precracking) was exposed to the environment. Cathodes were then mounted on the sides of the C(T) specimen and the current was monitored as the stress intensity and water chemistry were varied. The experiments reported here clearly demonstrate that current flows from the crack to the external cathode during crack propagation in sensitized type 304SS in pure water at the elevated temperatures typical of light water reactor heat transport systems. Thus, in spite of the high resistivity of the environment, the cathodic reaction is not restricted to the flanks of the crack or to the crack mouth, but occurs predominantly on the external surfaces, as postulated in the coupled environment fracture model. New insight has been gained into the physical mechanism of crack growth in high temperature aqueous environments.

Porous polycrystalline silicon: a new material for MEMS

A new technique for the fabrication of thin patterned layers of porous polycrystalline silicon (polysilicon) and surface micromachined structures is presented. First, a multilayer structure of polysilicon between two layers of low-stress silicon nitride is prepared on a wafer of silicon. Electrochemical anodization with an external cathode takes place in an RF solution. A window in the outer nitride layer provides contact between the polysilicon and the HF solution; the polysilicon layer contacts the substrate through openings in the lower silicon nitride layer (remote from the upper windows). Porous polysilicon growth in the lateral direction is found at rates as high as 15 /spl mu/m min/sup -1/ in 12M (25%, wgt) HF to be controlled by surface-reaction kinetics. A change in morphology occurs when either the anodic potential is raised or the HF concentration is decreased, causing the polysilicon to be electropolished. The etch front advances proportionally to the square root of time as expected for a mass-transport-controlled process. Similar behavior is observed in HF anodic reactions of single-crystal silicon. Dissolution of the polysilicon layer is confirmed using profilometry and scanning electron microscopy. Enclosed cavities (chambers surrounded by porous plugs) are formed by alternating between pore formation and uniform dissolution. Porous polysilicon also forms over a broad-area layer of polycrystalline silicon that has been deposited without overcoating the silicon wafer with a thin film of silicon nitride. The resulting porous layer may be useful for gas-absorption purposes in ultrasonic sensors. >

Optimum neutralization of negative space charge in cylindrical and spherical diodes

Elevation of the space-charge-limited electron current in cylindrical and spherical diodes, brought about by the introduction of neutralizing positive ions between the electrodes, is considered. The ion distribution is chosen to belong to the particular mathematical class that enables one of the three successive integrals of Poisson's equation to be performed analytically. For the external cathode configuration, it is found that there is no upper limit to the electron current provided there is sufficient ion generation and provided the power supply can give sufficient current. These results are applied, for the enhancement of current, to the electron-beam-pumped excimer laser in cylindrical symmetry and a suitable means of generating the required ion production is suggested. In the internal cathode configuration the limited electron current can be increased severalfold by the introduction of ions. However, an upper limit is reached when the electric field is reduced to zero at some point between the electrodes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An optically triggered, glow switch Marx bank

Operation of high-powers lasers, electron and other accelerators, and other devices is often limited by power modulator performance. Results of an investigation of a concept for modulator switching that has the potential to significantly extend these limitations presented. The switching device is a three-stage Marx bank based on back-lighted thyratrons that are triggered using optical fibers. Potential advantages include optical isolation of triggering, operation of the switches in a glow mode, no external cathode heater, reduced volume and supporting systems, and high-precision (subnanosecond) triggering. A three-stage proof-of-principle Marx bank has been operated at 105 kV with a rise time of >

Space-charge-limited electron current flow between coaxial cylinders and concentric spheres with ion production at an intermediate radius

The steady-state Poisson equation is formulated in cylindrical and spherical geometries assuming the emission of electrons at zero energy from the cathode and the emission of both ions and electrons, again at zero energy, from an intermediate surface at an arbitrary radius between the electrodes. The equation is solved numerically with the boundary conditions of zero field at the charged particle emitting surfaces to give the maximum electron current. An optimum radius of the intermediate surface is then found corresponding to the overall maximum electron current. The calculations are performed for a ratio of electrode radii ranging from 1 to 400, and, in the case of the external cathode configuration, the increase in electron current afforded by bipolar flow in comparison to unipolar flow can be several orders of magnitude. Application is briefly considered to electron beam pumping of vacuum ultraviolet excimer lasers and to fusion pellet compression by electron beams.

He-Cd&amp;lt;sup&amp;gt;+&amp;lt;/sup&amp;gt;white light laser by a novel tube structure

A newly designed external cathode type He-Cd+ hollow-cathode white light laser has been realized by using a "twin cathode" and a cylindrical grid electrode which is distributed coaxially along the rod anode. The electric input power of this system is 300 V, 1 A, and 300 W, almost double that of the flute type, 250 V, 0.6 A, and 150 W, but the cathode temperature is almost the same in both systems. The output power of the blue laser line was increased over that of the flute and hybrid hole types, while that of the green and red laser lines could not exceed both types.

Discharge of ions and space charge formation in CdF2 crystals with different kinds of electrodes

CdF2 samples provided with different kinds of electrodes and insulations are investigated. It is found that, due to an electron injection which cannot be avoided, metal particles are formed below the surface which act as a kind of « reversible dendritic electrode »; that the cations are discharged by segregation there and/or at the external cathode; that the two known space-charge formation TSC-peaks are related to the blocking of the external electrode and to an electron injection resp. It is finally shown that there are indications of a similar situation in alkali halides.

A controllable ion gun with high brightness

A concentric discharge with radial carrier transport is used for obtaining centrally located, dense, non-equilibrium plasmas. The inwardly directed electron beam. from an external ring cathode is accelerated over a short distance and traverses the slit of a high-energy ring anode. The electron beam is then decelerated to produce the maximum ionisation cross section during irradiation inside the central plasma anode. This results in high drain current from the cathode, low ionisation outside the plasma and reflection of plasma ions. The plasma electrons are reflected near the cathode. Thus the plasmas produced are confined to the centre and the discharge is not self-maintained but is controllable up to an argon pressure of 10 Pa. Short-time pulsed probe plasma diagnostics have shown an additional contraction of the plasma, caused by unidirectional movement of electrons and positively charged ions which results in a channelling effect. The ion emissivity of the source has reached 6 kA m-2 for argon and hydrogen.

Injection and drift studies of high current relativistic electron beams in a toroidal-like system

The successful use of an external field shaping cathode to inject high current relativistic E beams (500 kV, 20 kA, 50 nsec) across a toroidal magnetic field (1–4 kG) into a metallic vessel is presented. Injection of up to 50% of the diode energy into trajectories far removed from the metal wall (≳5rL) was achieved. Drift properties if the E beam in the toroidal vessel were studied. Energy fluence down the torus was measured. Up to 60% of the injected beam was recovered after propagating a distance of 8 m. The drift time around the torus was found to be long enough to enable trapping the beam in the toroidal system.

Translation and application of aerospace management technology to socio-economic problems.

formance of both plasma bridge neutralizers was excellent. The total neutralize! input powers were constant at 10 w. Upon removal of each system from its test chamber, detailed inspections of all components were performed as the systems were disassembled. All components of both systems were found to be in extremely good condition. The only noticeable wear was on the accelerators and the DG-2 cathode emitter coil. About a gram of material was found inside each engine cathode. Although the mechanisms producing these contaminants are not known, spectrographic analyses showed the major constituent to be cesium with about 3% Mo and Ta, which were used in the cathode bodies and emitters, respectively. After cleaning, the Ta emitter of the DG-2 cathode was found to have been pitted due to chemical attack. No weight loss could be detected for either cathode, and it is estimated that less than 2% of the emitter mass was lost due to the pitting in the DG-2 test. The sheath of the coaxial external heater of the DG-2 cathode was ground away and the insulation was removed, revealing a crack in the core conductor that caused the open circuit failure. This crack was apparently a random material failure. Clogging of the neutralizer orifice was the reason for the loss of neutralizer emission current on the DG-2 test after 8100 hr. The orifice cap was removed from the neutralizer and examined on the inside, where the plugging material was found as a buildup only in the vicinity of the orifice. The origin of the plugging material is not known exactly, but because of the highly localized buildup around the orifice, it is suspected that the source was outside the neutralizer. The attack on the DG-2 cathode emitter in the 8150-hr test was insignificant as a limitation on lifetime, since less than 2% of the emitter mass was lost. No such attack was observed on the cathode emitter used in the 3670-hr test. The open circuit of the external cathode heater on the DG-2 engine was caused by a random material failure and can be prevented by more careful inspection of raw material. The plugging of the neutralizer orifice appears to be due to the test environment, and should not occur in space. Although the exact mechanism causing the arc power increases is not known, it is felt that the contamination found in the cathodes was a large factor. The arc-power increase is not a problem affecting lifetime, since the rate of rise decreased with time on both tests. The only observed effect that could be considered a limitation on lifetime was the erosion of the accelerators. Direct interception of ions caused enlargement of the accelerator apertures on the downstream sides. The upstream surfaces were not damaged, so the electrode optics were not changed. Most of the erosion of the accelerators occurred in sharply defined pits on the downstream side. The sharp definition and location of the pits indicated that they were caused by slow ions formed in the neutralized beam plasma and focused

Nucleation and growth of magnetite films on Fe in high-temperature water

The nucleation and early stages of oxide growth on pure Fe in de-ionised water at 250 °C have been observed by electron microscopy. Two different types of growth, corresponding to the well-known Potter-Mann and Bloom forms, were observed depending on the presence or absence of an external cathode during oxidation. The difference between the two modes of growth is ascribed to a field-enhanced difference in the surface mobilities of the reaction products.

Glass BF3 proportional counters and their low temperature characteristics

Glass envelope BF 3 proportional neutron counters of simple construction have been made for balloon-borne cosmic ray equipment. Both internal and external cathode counters having soda-lime envelopes with a BF 3 filling pressure of 45 cm Hg are described. These counters have a resolution of 15% fwhm after standing for 2 1 2 years and there is no evidence for deterioration of their characteristics over this period. Some improvement in resolution is reported at reduced temperatures, at least down to −60°C, the lowest investigated. However, at low temperatures the wall resistance of external cathode counters must be considered, especially if high counting rates and high gas gains are to be used.

On the External Cathode Proportional Counter

On the External Cathode Proportional Counter

Construction of External Cathode Geiger Counters

First Page

An external cathode hard-glass Geiger-Müller tube.

IN the course of work on the development of Geiger–Muller counters in this laboratory, some interesting properties of a hard-glass external cathode (‘Aquadag’) counter have been observed. Argon–alcohol counters filled to a pressure of 10 cm. of mercury, having internal diameters of one inch, and using a 7-mil central tungsten wire, were found to have plateaux extending from 1,200 volts to more than 3,000 volts. The slope of these plateaux was generally found to be less than 0.4 per cent per 100 volts. It was noted that the size of the pulses obtained from these counters is reduced with enhanced rate of counting. This phenomenon results in an apparent upward shift of the threshold voltage with increased rate of counting. The slope and length of the plateau remain unaffected by incidence of strong radiation, and the shift in the threshold voltage can be nullified by using a preamplifier at the input of the scaler. The decrease in pulse size and the consequent increase of the threshold voltage are shown in Fig. 1.

On the Temperature Dependence of Counter Characteristics in Self-quenching Geiger-M ller Counters

The effect of temperature on the counting rate-voltage characteristics of self-quenching Geiger-Muller counters with internal and external cathodes (in this paper denoted as counters Nos. 1 and 2) has been investigated within the temperatures ranging from 8° to 60° C. It is found that the average counting rate remains independent of temperature (within the statistical error limits), and this constancy is better in the case of the external cathode counter than with the other. As counter No 1 shows a greater increase in slope at higher temperatures, the rate appears to be greater at higher applied potentials. Further, it is noticed that the plateau decreases and disappears at lower temperatures in the case of counter No 1, while counter No 2 does not show this effect. This is partly explained by the formation of semiconducting paths between the central wire and the cathode, the discharges along which give rise to spurious counts in the case of counter No 1, and the absence of these in the case of counter No 2. It is also observed that the slope of the plateau increases and the width decreases with rise of temperature in both cases, though the increase in slope is more marked with counter No 1 than with counter No 2. The increase in slope is probably due to the presence of a greater number of multiple discharges or spurious counts at higher-temperatures, an important point which is under study. The investigation also brings about the advantage of the external cathode in the construction and design of G-M counters, especially when reliable observations are desired with a counter under widely changing temperature conditions.

Plasmoidal High-Frequency Oscillatory Discharges in "Non-Conducting" Vacua

Plasmoidal discharges. A new type of vacuum tube discharge is described, excited by comparatively low voltage high-frequency oscillations (1.7 meters up to 30 meters wave-length) in which remarkable luminous balls, spindles and pear-shaped bodies are formed. These have been reported in earlier papers in Nature and the Phil. Mag. A study of the conditions under which these bodies are formed and their behavior in magnetic fields has lead to the conclusion that surface charges play a part in their formation. They have been shown to be built of excited molecules, as their spectra are molecular band spectra (${\mathrm{O}}_{2}$ or CO) and since Langmuir has seen in their formation the possibility of plasma oscillations, such as he observed in the mercury arc; they have been named "plasmoids" provisionally.Red phosphorescence of the glass wall. This occurs in all of the tubes containing oxygen at very low pressure. It appears to be due to low velocity secondary electrons in combination with excited oxygen. Its behavior in magnetic fields has been studied, and it appears when a plasmoid is deflected against the wall by a magnet. The red phosphorescence does not appear until after the high frequency discharge has operated on the glass for some minutes, becoming finally a bright ruby red. The same glass surface phosphoresces green under the impact of high velocity electrons given off from the inner surface of the glass when the external cathode is excited by higher voltages. It may happen that superposed streaks of red and green phosphorescence may occur simultaneously and they can be separated by a magnetic field.Clean-up effects. Remarkable clean-up effects have been observed, it being possible to have a tube sealed off from the pump which can be made to show either a pure Balmer spectrum of atomic hydrogen (purple discharge) or the green discharge of molecular oxygen. The pressure changes occurring in the sealed off tubes as the type of spectrum changes have been followed with a vibrating quartz fiber manometer.Spectroscopic investigation. The spectra of the plasmoids, their dark sheaths, and the less luminous gas in which they are imbedded have been investigated by projecting their images on the slit of a spectroscope. The band spectrum of molecular oxygen is seen in the whole tube, but is concentrated in the plasmoids and diluted in their dark sheaths; atomic lines appear as well, the local concentration being different for singly and doubly ionized oxygen.General nature of discharge. There are two types of discharge, one at very low potentials, suggesting the Townsend discharge (unmodified by space or surface charges) and a more brilliant one which comes in suddenly as the potential is gradually raised. Plasmoids, however, appear to form with both types, though their shape and distribution in the tube changes as we pass from one type to the other. External electrodes are used in all cases.