Computer experiments on electron guns

Computer techniques for analyzing an axially symmetric or two-dimensional electrode system with an emitting surface can be used to obtain cathode current density distribution, beam minimum radius and its location, as well as possible electrode current interception. Here, comparison is made of available experimental data from two axially symmetric Pierce-type guns. The choice of the two axially symmetric Pierce-type guns was made for their different value of PV/T ratio, an invariant for transverse scaling. One of them has a low ratio of PV/T(approx 0.1 times 10^{-6}), which signifies considerable beam spread due to the effects of the Maxwellian velocity distribution of the thermal electrons leaving the cathode. The other gun has a high perveance of2 times 10^{-6}, and thus relatively high PV/T ratio where beam spreading is mainly due to space-charge forces alone. Good agreement with experimental data is shown. Computer techniques of this type can be a valuable tool for diagnostic purposes of any type of space-charge-flow devices (e.g., electron guns, ion guns for space propulsion, linear accelerators, etc.) and enable the engineer-designer to arrive quickly and cheaply at an optimum configuration.     

Thermionic emission model of electron gate current in submicronNMOSFETs

A thermionic emission model based on a non-Maxwellian electron energy distribution function for the electron gate current in NMOSFET's is described. The model uses hydrodynamic equations to describe more correctly the electron transport and gate injection phenomena in submicron devices. A generalized analytical function is used to describe the high-energy tail of the electron energy distribution function. Coefficients of this generalized function are determined by comparing simulated gate currents with the experimental data. This model also includes the self-consistent calculation of the tunneling component of the gate current by using the WKB approximation, and by using a more accurate representation of the oxide barrier by including the image potential. Good agreement with gate currents over a wide range of bias conditions for three different technological sets of devices are demonstrated by using a single set of coefficients     

Focus reflex modulation of electron guns

High resolution and low drive features have been successfully combined in a new type of electron gun for cathode-ray tubes. The gun has a spot-defining aperture of 0.007 inch upon which emission from a large cathode area is concentrated by a retarding electron lens. This unit modulates the beam by electron reflection, while focusing it upon the aperture ("Focus Reflex Modulation"). Immediately ahead of the aperture, a modulated virtual cathode is formed with an emission capability of over four amperes per square centimeter. A1000-microa beam with a 6° divergence is controlled by a signal of 12 volts centered at ground potential. Highlight brigntness of 250 foot lamberts was read at 17,500 volts, while more than 500 lines were resolved on a television test pattern. In more recent forms of the FRM gun,1600 microa are measured in the screen return, out of1800-microa cathode current. This is 88 per cent over-all transmission, using the same defining aperture (0.007 inch). 18 volts of drive signal will completely modulate the above current.     

Use of the BFCPIC and BFCRAY codes to describe space charge limited emission in electron guns for gyrotrons

Numerical modelling of an electron gun in the space charge limited regime requires determining the current density distribution as well as the electric fields and electron trajectories. This is a rather complicated self-consistent problem, since the space charge influences the electric field, which in turn influences the electron trajectories. Previous simulations of magnetron electron guns using the BFCPIC and BFCRAY codes used a simple emission model (constant current density) that is approximately valid for thermionic emission. The code has been modified to include space charge limited emission. Several different ways of doing this are considered. One of the models considered uses Gauss’s law to force the electric field on the emitter to vanish; it was used in the original version of BFCPIC for the simulation of ion diodes. A second is based on the use of Child’s law (locally), which may be more appropriate for extension to fully electromagnetic particle-in-cell (PIC) codes. Calculations were performed with both models, and the results compared with each other and with experiments performed at FZK.     

On the performance of high perveance electron guns

A design of high perveance electron gun has recently been proposed by Müller. An experimental investigation has been made of the electron beams produced from: 1) a gun designed directly from Müller's charts, and 2) a gun whose design is a modification of the Müller design to make it more suitable for high-voltage operation. The perveance in each case was about 2 × 10^-6. The distribution of the current density and the profile of the beam were examined, either by allowing the beam to fall on a plate coated with carbon, or by measuring the current passing through a pinhole in a screen which could be moved across the beam at a number of axial positions. Electrostatic experiments showed that the beams were initially annular, but further along the axis the current density became highest at the center. This is attributed to crossing trajectories resulting from lens aberrations in an anode aperture whose diameter is comparable with the cathode-anode spacing. With magnetic focusing, the current density distribution across the beam varied periodically along the axis to an extent which depended critically on the magnetic field conditions in the accelerating region of the gun.     

Numerical computation of the phase velocity of current on an infinite helix

A numerical method is given for the determination of the propagation velocity of current on a thin-wire helix. The tangential electric field is set to zero at a point on the conductor surface and the propagation constant is found by solving the resultant integral equation by digital computer using a direct-search technique.     

Analysis of the role of high-brightness electron guns in lithography

The development of reliable, high-brightness, temperature-field (TF) emission electron guns promises greater flexibility in electron-beam lithography. Detailed pattern analysis of two IC mask sets was performed for the purpose of identifying those areas of electron-beam lithography where high-brightness guns could be applied to advantage. Potential exists for throughput improvement in both Variable Shaped Spot (VSS) Vector Scanning Systems and in ultra-high-speed Raster Scanning Systems.     

Digital computer simulation of crossed-field electron guns

A digital computer program for the simulation of crossed-field electron guns is described. The program simulates the electrodes and fields within the gun on a matrix of up to 10 000 points. The program, written in FORTRAN II for the IBM 7090, is generally executed in six minutes or less. It is shown to produce results within four percent of theory and experiment for two different cases.     

A design method for crossed-field electron guns

A design method for crossed-field guns based on a space-charge-flow solution in crossed fields is given. By using the method of analytic continuation in the complex plane, it is shown that it is possible to find the exact form of the electrodes required The design results in a gun similar to the French "short gun" with the great advantage that the current emitted from the gun and the current density at the cathode can be predicted. It is also shown that by making certain approximations to the exact space-charge-flow solution, a new type of gun can be designed, a "long gun" which can have extremely high convergence. The theory for this latter gun is extremely simple and the electrode shapes can be given entirely in analytic form.     

Numerical computation of the current distribution and far-field-radiation pattern of the axial-mode helical aerial

The accepted model of the endfire helical aerial is examined, and the current distribution and radiation pattern are derived numerically. It is shown that the model is not adequate, and alternatives are proposed and examined.     

Gridded electron guns for high average power

The design and performance of electron guns producing high-average-power beams which are controlled by an intercepting-type grid are described. The design consists of a modification of the design of Pierce-type electron guns to allow for the effect of grid insertion, and calculation of the amplification factors based on the potential distribution. The performance is evaluated in terms of the beam shapes and average beam powers obtainable. Measured amplification factors are compared with calculated values. Factors influencing the measured beam shapes are discussed. Trajectories of electrons emitted as secondaries from the grid and those of electrons deflected by the lens effect of the grid wires were plotted using an analog computer and electrolytic tank. The shapes and focusing properties of electron beams from similar gridded and nongridded guns are compared. Expressions relating the average beam power capability to the gun and grid dimensions are developed. The calculated average beam power capabilities are compared with measured values. Average beam power as high as 8 kw has been obtained with electron guns of a size suitable for use in X-band twt's.     

Digital computer analysis of axially symmetric electron guns

A digital computer program written in the IBM 7090 FORTRAN programming system is described and then employed in the analysis of several axially symmetric electron guns. The digital program employs relaxation techniques while alternately computing electric fields and trajectories. One cycle through the program consists of three parts. 1) Calculation of the voltages within the electron gun by solving Poisson's equation, in difference form, on a matrix containing a maximum of 10 000 points on which electrode potentials are laid out. 2) Calculations of new current densities along the cathode and the setting up of new trajectory starting points along the cathode to simulate these current densities. 3) Calculations of trajectories, including magnetic fields and relativistic effects if desired, and comparison of the new beam with the previous one. Thermal effects and direct particle-particle interaction effects are ignored although space-charge effects, introduced through the field calculations, tend to prevent excessive or usual trajectory crossings. Execution time for the entire program is between 4 and 12 minutes, although for most guns 5 to 7 minutes is usually sufficient. Results produced by the program are presented and compared with experimental and analytical results.     

Transport of noise fluctuations in convergent flow crossed-field electron guns

The transport of noise fluctuations in convergent flow or so-called Kino-type crossed-field electron guns is investigated in terms of a two-dimensional computer simulation of the gun by the well-known Monte Carlo method for studying stochastic processes. The exact electrode configurations are simulated in the computer memory. After sufficient time has elapsed for the electron flow to achieve a steady-state condition 2000 additional time intervals are computed and then a statistical analysis is made of the fluctuation quantities. Six emission spots are considered on a finite-width cathode. The analysis is facilitated by the development of a rapid method for the solution of Poisson's equation (two-dimensional). Of particular interest in the results is the improved laminarity of the flow in the KG-M gun and the fact that there is significant space-charge smoothing throughout the gun region in both Kino-type guns. Of greater significance and as yet not explainable is the fact that Ψ versus ωct has local peaks when the space-charge density, which is spatially varying, passes through the Brillouin value (ω_{p}-ω_{c}for this condition). This knowledge could be profitably used in deciding on the location of the gun exit plane. The fact that the laminarity of the flow is sensitive to slight changes in the electric field near the cathode indicates that an absolute evaluation of the noise performance of KG-A and KG-M guns is difficult and may only be obtained through appropriate experiments.     

Transit-time effects on noise in planar crossed-field electron guns

A theory is developed for the noise in an unbounded parallel-plate crossed-field diode. The high-frequency regime is studied, where transit-time effects must be included. The diode is assumed to be operating in the space-charge-limited regime. Maxwell's equations and the Vlasov equation are expanded to first order. A key consideration in this expansion is the fluctuation in the boundaries of velocity space appearing in the integrals for the charge and current density. The first-order terms describe the linear response of the electric field and transmitted current resulting from the injection of a small additional cathode emission composed of electrons of a specified velocity. Using a statistical approach, these perturbations are combined to determine the noise factor. The special case of a parabolic average-potential distribution in space is considered.     

Beam transmission in electron guns having a single-apertured control grid

The ratio of target current to cathode current (beam transmission) has been studied for a number of vidicons and scan converters. These are high-resolution electron tubes that characteristically have a control grid with a single aperture. The results, as anticipated, show that beam transmission is affected much more than total cathode emission by tube aging processes. The transmission was determined by direct beam current measurement and the cathode condition recorded by the cathode imaging technique. Because of the centralized area in which the loss of emission occurred, beam transmission was reduced much more than total cathode current. In vidicons, the decreasing beam transmission was correlated with increasing residual signal after erasure and with saturation of the gamma transfer characteristics at lower light levels. In practice, beam current rather than cathode current, should be monitored.     

Computer-aided design of electron guns for injected-beam crossed-field amplifiers

A two-dimensional self-consistent computer-aided CFA gun analysis program is discussed which can be used to solve for the characteristics of "long" crossed-field guns with arbitrary electrode shapes and including the effects of space charge. The results of analyzing a "long" Kino gun show excellent agreement between the theoretical gun parameters and those obtained by the computer analysis. The model shows promise of also being useful in the analysis of noise in CFA guns.     

Effect of surface roughness on the velocity spread in electron guns for gyrotrons

The effects of the roughness of the cathode surface on the emittance of an electron beam in a gyrotron gun are investigated. Parameter studies using a very simple model are performed in order to determine a reasonable parametrization of the effect for use in the BFCPIC and BFCRAY codes. Calculations are then performed for a gun in use at FZK.     

Sources of velocity spread in electron beams from magnetron injection guns

Calculated trajectories for the electron beam of a known gyrotron have been analysed to determine the relative importance of some of the contributions to velocity spread. For the geometry analysed, space-charge and thermal velocities at the cathode produced only a small part of the calculated velocity spread; much more was generated by the variation of electric field over the length of the cathode surface. The total range of transverse velocity was substantially reduced by adjustment of the shape of one electrode.     

Experiments on high-perveance electron guns with a subfocusing electrode

High-perveance electron guns with a subfocusing electrode were designed analytically, produced, and tested. The experiments are carried out to evaluate the design method. Perveance of guns are 1.7 to 1.3 × 10^-6A/V^3/2, convergence angles being 30°. The results confirmed that it was possible to design electron guns of perveance up to 3.0 × 10^-6A/V^3/2.     

The design and performance of grid-controlled, high-perveance electron guns

The focusing electrode and a probe projecting through the cathode serve as control electrodes for the current from a convergent-beam electron gun. The principal advantage of this type of "grid" is that there is no interception of the high-current-density beam by the probe-grid. This paper presents the design procedure and experimental results for typical probe-gridded guns. The design procedure is used to obtain the desired perveance, beam diameter, and approximate laminar electron flow. The probe geometry that results in a minimum beam distortion is discussed. The range of values of amplification factor obtainable and the influence of probe geometry on this factor are discussed. The magnetic field required for focusing the beam from a probe-gridded gun is compared with that required for perfect laminar flow and for focusing the beam from a nongridded gun of similar design. An electrolytic tank in conjunction with an analog computer was used to plot electron trajectories, with the effect of space charge included, for the probe-gridded gun and a similar nongridded gun. A comparison of the electron optics of the gridded and nongridded gun is made. Electrical breakdown and beam current during the interpulse time are problems considered. Methods used to minimize electrical breakdown and interpulse beam current are presented. Several models of probe-gridded guns were constructed. The measured characteristics of these guns demonstrate that the advantages of grid control can be obtained with only a minor effect on gun perveance and beam focusing.