Anode-cathode Spacing Research Articles

Light Ion Sources for ICF

PBFA II, Sandia National Laboratories' (SNLA) advanced light-ion accelerator [1] (now under construction), will deliver a 30-MV, 5 MA lithium ion beam to an Inertial Confinement Fusion (ICF) target. Although most ICF experiments have been performed with proton beams, lithium ions were selected for PBFA II for several reasons. The use of lithium enables a large anode-cathode spacing, resulting in less impedance change during the pulse. Lithium ions have less magnetic deflection in the diode, so they can be focused for a longer period of time. Also, since the ionization potential of Li+ is low, while that of L++ is very large, a Li+ source should be achievable without significant contamination from Li++ Ions heavier than Li+ would require operating voltages greater than 30 MV, or the ions would have to be accelerated in a multiple charged state. The selection of the lithium option for PBFA II has made the development of a suitable lithium source one of the highest priority areas in the light-ion ICF program. Following that decision in the fall of 1983, SNLA organized a comprehensive ion source program to develop an optimum lithium source for PBFA II and to understand near-term sources.

A 2D MWPC area detector for use with synchrotron X-radiation at the Daresbury laboratory for small angle diffraction and scattering

A 2D multiwire proportional chamber area detector is being developed to provide a real time data acquisition system for small angle scattering and diffraction experiments with synchrotron X-radiation at the Daresbury synchrotron radiation source (SRS). The chamber has a circular aperture, 200 mm diameter with an anode and cathode wire pitch of 1 mm; a front cathode-anode spacing of 6 mm and a 6 mm spacing between anode and rear cathode. A 1 mm thick front beryllium window and a rear aluminium cover plate with indium seals provide a gas-tight system. Previous experiments with a similar chamber design allowed continual use of the chamber for up to 2 years without refill. A digitising time of 2 μs is expected based on a 260 mm delay line and Lecroy TDC linked to a mass semiconductor memory of 512×256 elements. The experiment will be controlled by a PDP 11/04 computer with 28 K memory interfaced to a CAMAC create with 64 K fast access CAMAC memory. The system should be relatively easy to use with good order to order resolution and reasonable rate for small angle diffraction and scattering experiments on biological systems. Evaluation of the set-up for protein crystallography is planned though a TV based image intensifier (Enraf-Nonius) is preferred for this application at the SRS.

Synthesis of the Pierce gun

The design of a Pierce gun is uniquely defined by four parameters: beam voltage, current, waist radius, and cathode current density. The gun convergence angle, cathode spherical radius, anode-cathode spacing, and throw (distance to the waist) are calculated by an iterative procedure which typically converges in four cycles to 0.1°. The anode lens formula of Danielson, Rosenfeld, and Saloom is combined with inverted forms of the Langmuir-Blodgett solution for the spherical diode and the Universal Beam Spread equation for the tunnel region; the iterations match the trajectories at the anode plane. Correction for spherical aberration is made by the method of Frost and Purl as quantified by True. The inverted equations are simple enough to be solved in a few minutes with a programmable calculator. The experimental data of Frost and Purl is used to demonstrate the validity of the procedure for guns of medium perveance.

Flash x-ray tube for diffraction studies on a two-stage light-gas gun

A flash x-ray tube has been fabricated and has been driven by a Blumlein pulse generator to peak voltages of 60 to 80 kV and peak anode currents of 8 to 10 kA. The pulse width, dependent upon the anode-cathode spacing, can be made ∼80-ns wide, and a small spot or line focus is possible. These features make the device useful for flash x-ray diffraction studies. Current in the diode region exceeds the calculated space-charge limited values, probably because of neutralization of charge by plasma ejected from the cathode during the initial phase of the discharge.

Direct measurement of broad spectral electromagnetic wave energy generated by an intense relativstic electron beam

A direct determination is made of the total power radiated in a single pulse, ℱ P (ω) dω, by an intense relativistic electron beam cyclotron maser configuration as a function of applied magnetic field. The existence of other possible instabilities over a broad spectral range notwithstanding, the total output is dominated by the maser instability occurring at the relativistic cyclotron resonance magnetic field. The results for two different anode-cathode spacings, corresponding qualitatively to cold and warm beam conditions, are presented and compared. A possible explanation accounting for the existence of high power noise radiation at low magnetic fields for the warm beam case is discussed.

Generation and extraction of microsecond intense relativistic electron beams

The Physics International Pulserad 1140 has been modified so that a pulse length of 1.7 μsec can be obtained with a beam energy of 50 kJ. This modification involves substituting a pulse-forming network for the conventional Blumlein. This long-pulse generator has been used to study diode behavior on 1-μsec time scales. The dependence of impedance on voltage and anode–cathode spacing is discussed here. A semiempirical model has been developed. The diode time history and closure velocity are discussed in terms of this model.

Determination of work functions near melting points of refractory metals by using a direct-current arc

A direct-current arc in argon at atmospheric pressure was used to determine effective work functions of refractory metals, including tantalum, tungsten, molybdenum, and niobium. The procedure is experimentally advantageous because surface cleanliness of the specimen is not critical, high vacuum is not required, and the anode-cathode spacing is not critical. The experimental procedure involves striking an arc to a metal wire cathode to form a melted ball having an emitting area defined by its diameter. The literature melting point of the metal is taken as the emitting temperature. By using these parameters and the known arc current, effective work functions were calculated from the Richardson-Dushman equation. The calculated work functions agree with recommended handbook values to within about 0.1 V and have typical repeatabilities of 0.02 V. By varying the arc current, Richardson plots can be made over a temperature range from a few hundred degrees below the melting point to about 50° over the melting point of the test metal. A Richardson plot over this temperature range is presented for tantalum.

Shape of cathode dark space of a sputtering plasma determined from probe measurements

This paper reports measurements of the shape of the cathode dark space (CDS) of a dc sputtering system. Special attention was given the CDS enveloping the cathode since this region influences the sputtering of the cathode periphery. A Langmuir probe was used to map the CDS negative glow boundary which was found to remain unchanged with cathode-anode spacing until the anode neared the CDS boundary. Glass substrates placed on the anode also influenced the CDS shape. The Langmuir probe current-voltage characteristic changed with time until the probe was completely coated with metal sputtered from the cathode. The coated probe gave reproducible results over a long period of time.

Possible advance in thermionic conversion

A theory is outlined by which a thermionic convertor might operate at an anode-cathode spacing of about 1 mm, thus removing a major drawback to the exploitation of this direct generator of electric power. Successful testing of the theory could lead to a new low-pollution multifuel power unit.

The situation in China and the position of the CCP at the present stage

In this paper, we demonstrate high-performance lateral AlGaN/GaN Schottky barrier diodes (SBD) on Si substrate with a recessed-anode structure. The optimized rapid etch process provides results in improving etching quality with a 0.26-nm roughness of the anode recessed surface. By using the high work function metal Pt as the Schottky electrode, a low turn-on voltage of 0.71 V is obtained with a high uniformity of ±0.023 V for 40 devices. Supported by the flat anode recess surface and related field plate design, the SBD device with the anode–cathode spacing of 15 μm show the specific on-resistance (Ron,sp) of 1.53 mΩ·cm2 only, the physical breakdown voltage can reach 1678 V with a high power figure-of-merit (P-FOM) of 1840 MW/cm2. For the SBD device with the anode–cathode spacing of 30 μm, the physical breakdown voltage can be as high as 2705 V and the power FOM is 2217 MW/cm2.

Emission of visible radiation from extended plasmas in silicon diodes during second breakdown

Thin film silicon-on-sapphire diodes have been reverse biased into second breakdown and the formation of a filament has been observed between the anode and cathode. This filamentary conduction was accompanied by emission of visible radiation, the light being much more intense than that associated with avalanche or microplasma emission. The filamentary emission appeared to be associated with both thermal heating of the diode to temperatures as hot as 600-800°C and the formation of a plasma. Quantum efficiencies as high as 0.04 percent have been observed and plasmas extending over anode-cathode spacings of up to 100 µm have been observed.

The effect of tolerances in conical flow Pierce guns II—The perturbed flow in the magnetic field

The first-order perturbations to a beam initially in nonrippling Brillouin flow, which results from several common manufacturing errors in conical flow Pierce guns, are calculated. The manufacturing errors are changes in cathode-anode spacing, changes in cathode curvature, tilt, and transverse displacement of the cathode-focus electrode assembly, and tilt and transverse misalignment of the whole gun to the magnetic field. Errors in cathode-to-focus electrode spacing are not discussed. The tolerances which destroy the rotational symmetry of the gun are shown to cause the smooth beam to perform a helical motion; the tolerances which preserve the symmetry are shown to produce an undulating beam. Using the results of Part I of this paper [1], the magnitudes of individual gun tolerances are related to the effects they produce in the magnetic field. A method of setting limits to the individual gun tolerances is presented and illustrated by calculating tolerances for a gun used in an X-band traveling-wave tube. The calculated tolerances are found to have values which may readily be achieved by careful engineering.

The effect of tolerances in conical flow Pierce guns I—The perturbed flow within the gun

A first-order perturbation analysis of the dc effects of several important tolerances in conical flow Pierce guns is presented for the case in which the gun is part of a solenoid focused system used to produce a smooth Brillouin beam. In this, the first of two parts, the flow up to the anode of the gun is analyzed. The data presented permit calculation of the magnitude of the perturbations at the anode due to various manufacturing errors. These errors are: changes in cathode-anode spacing, changes in cathode curvature, tilt, and transverse displacement of the cathode-focus electrode assembly. The theory does not permit the analysis of cathode to focus electrode tolerances. The perturbations at the anode serve as boundary conditions for the flow beyond the gun which is calculated in the companion paper. A comparison of the magnitude of effects at the anode calculated by the theory and obtained from an experimental study shows that reasonably good agreement exists between theory and experiment.

Onset of Breakdown at Voltages below Ionization Potential in High-Pressure Thermionic Diodes

Early breakdown effects have been measured in relatively high-pressure thermionic diodes under given conditions of pressure and anode spacing. Apparent breakdown conditions are obtained at anode voltages considerably lower than the ionization potential of the gas. Measurements on xenon- and mercury-filled thermionic diodes, under space-charge conditions of operation, are presented which show that this breakdown voltage seems to be associated with the first excited state of the gas. Experimentally, early ignition occurs in diodes with a fixed cathode-anode spacing in a limited pressure range only. A theoretical explanation for this behavior is based on the model that, in the limited pressure range, electrons traversing the cathode-anode space can pick up almost all the energy of the applied voltage. In addition, data are presented showing that early breakdown in an inert gas-filled diode can occur at voltages much lower than the first excited state. This has been accomplished in thermionic diodes exposed to the radiation of a nuclear reactor. A qualitative explanation for this behavior is proposed.

The preparation of very flat surfaces of silicon by electropolishing

Slices of silicon can be very successfully polished by anodic dissolution in a cell using a horizontal rotating disk as cathode, a viscous electrolyte, and a very small anode-cathode spacing (e.g. 150 μ). The surfaces of n-type material of resistivity > 0·05 Ω-cm must be illuminated during polishing, necessitating perforation of the disk to admit a beam of light to them. The optimum cell voltage is always above that for which the differential conductance of the cell is negative, and is usually about 10 V. The dissolution of 50 μ from a surface initially lapped or mechanically polished leaves it very flat and free of all work damage; electron micrographs show the residual local departures from flatness to be <0·01 μ. Material so prepared offers advantages in planar technology and as substrates for epitaxial growth. A full quantitative understanding of the mechanisms involved is still lacking however.

Avalanche Breakdown-Double Injection Induced Negative Resistance in Semiconductors

Current-controlled negative resistances have been observed in a variety of point-contact Ge diodes. Experiments were done at 300°K and 243°K, using both dc and pulsed currents. In these experiments both contacts are of the ohmic, non-injecting type. The effect has been seen in both n and p -type Ge single crystals having resistivities that range from 0.1 to 50 ohm-cm. The frequency of the relaxation oscillations resulting from the negative resistance has ranged from 1 KG to 10 MC depending on the cathode-anode spacing and the external capacitance used. Analysis shows that thermal effects can not explain these results. The negative resistance is believed to be due to the two carrier current flow (double injection) in the bulk material after avalanche injection has been produced at the point contact. This explanation for the negative resistance differs from those previously proposed and allows predictions of the I-V curve as well as frequency limitations. Experiments with different sized points, varying pul...

Oscillations and Saturation Current Measurements in Thermionic Conversion Cells

Radio-frequency oscillations observed in cesium-filled thermionic diodes are interpreted on the basis of a model which assumes that the cesium ions oscillate in an excess negative charge potential well outside the cathode. A simplified theoretical treatment shows that the period at the onset of oscillations is linear with cathode-anode spacing in agreement with experiment, and relates the oscillation period to the ratio of cell current to saturation emission current. It is found that the rapid decrease of oscillation amplitude as the cell current reaches a critical value can be used to measure saturation emission currents.

Negative Sections of Moving Striations†

ABSTRACT An experimental investigation of moving striations in neon discharges at pressures (p) from 2-50 mm Hg has been carried out by rotating mirror photography. Supplementary photoelectric measurements have also been made. The appearance of the discharges has been described in detail. Particular attention has been given to the negative end of the column, where an increase in striation frequency usually occurred at a distance from the anode determined by current and pressure. The behaviour of this‘ branched ’ section was found to be independent of the position of the anode, and its length decreased when either the current or pressure was increased. The velocity of the waves varied as p −½ in the unbranched part of the column and was only slightly dependent on anode-cathode spacing except when the latter was small. In the branched region the striations travelled more quickly and the velocity varied in a more complicated way with pressure. Moving striations disappeared from the column with large currents...

Theoretical Efficiency of the Thermionic Energy Converter

An analysis of the efficiency of the thermionic energy converter is given in terms of Vc and Va, which represent the potential differences between the top of the potential barrier between the electrodes and the Fermi levels of cathode and anode, respectively. The analysis applies to all vacuum converters, and to gas-filled converters when the gas pressure is low enough so that the electron mean free path is greater than the cathode-anode spacing. In special cases, the analysis may also apply at higher gas pressures. It is shown (with certain assumptions) that optimum values of Va and Vc exist. For Va, the optimum is determined by the onset of thermionic emission from the anode to the cathode. For Vc, the optimum is determined principally by radiation losses. It is concluded that no fundamental reason exists why efficiencies of 30% or more cannot be realized in converting heat to electricity by this method.

Electrode spacing in disc-seal triodes

Four different methods have been investigated for determining the working cathode-grid clearance in disc-seal triodes. The methods involved (a) direct observation with a microscope, (b) capacitance measurements, (c) measurement of current flow and (d) determination of magnetron cut-off conditions. Methods (a), (c) and (d) gave reasonably consistent results. The range of clearances involved was 0.1 to 0.5 mm. Methods (a) and (d) were also used to find the cathode-anode spacing in the same triodes. The capacitance method, though of little use for determining the clearances directly, has been applied to the extension and checking of the clearances determined by the other methods.