Screen Grid Research Articles

Ion extraction from a plasma

An experimental investigation of the physical processes governing ion extraction from a plasma is presented. The screen hole plasma sheath of a multiaperture ion accelerator system is defined by equipotential plots for a variety of accelerator system geometries and operating conditions. A sheath thickness of at least 15 Debye lengths is shown to be typical. The electron density variation within the sheath satisfies a Maxwell-Boltzmann density distribution at an effective electron temperature dependent on the discharge plasma primary-to-Maxwellian electron density ratio. Plasma ion flow up to and through the sheath is predominately one dimensional and the ions enter the sheath with a modified Bohm velocity. Low values of the screen grid thickness to screen hole diameter ratio give good ion focusing and high extracted ion currents because of the effect of screen webbing on ion focusing.

Influence of Fluorescent Lights on Hyperactivity and Learning Disabilities

During the first five months of 1973, a pilot project was conducted by the Environmental Health and Light Research Institute in four first-grade windowless classrooms of a school in Sarasota, Florida. In two of the rooms, the standard cool-white fluorescent tubes and fixtures with solid plastic diffusers remained unchanged. In the other two rooms, the cool-white tubes were replaced with full-spectrum fluorescent tubes that more closely duplicate natural daylight. Lead foil shields were wrapped around each end of the tubes where the cathodes are located. Aluminum “egg crate” diffusers with an additional grounded aluminum screen grid replaced the solid plastic diffusers in these latter rooms. A dramatic improvement in behavior was demonstrated in hyperactive children.

A modified ionization gauge for measurement of relatively high pressures

Distortions of pressure measurements with a Bayard-Alpert gauge in the range 10−4−10−3 Torr were investigated, and changes in gauge sensitivity were found to be correlated with the observation of different electric charges on the glass envelope. A modified Bayard-Alpert gauge was designed. The gauge incorporated a screen grid between the gauge elements and the glass envelope. The screen grid was connected to the gauge filament and was intended to reduce the impact energy of electrons on the glass surface and the interaction of electric charges on the envelope with charged particles in the gauge. Four modified gauges were built and tested, and the comparison of the gauges with a McLeod gauge showed that the ion-collector current was a linear function of gas pressure over the range from 5 × 10−5 to 7.2 × 10−4 Torr. For further improvements, a beam type ionization gauge is proposed.

A New Programmable Lamp Current Supply for Atomic Absorption Spectrophotometry

Abstract This paper describes a programmable high voltage power supply. It is capable of delivering triggered currents of up to 100 ma at 400 volts. The compelling reason for its design and construction was in order to reduce inherent functional complexity built into some existing commercial systems. This was accomplished by redesignings the power supply, trigger controls and metering components. Control of the lamp current was effected by changing the screen grid voltage, thereby leaving the control grid to be used exclusively for switching. This resulted in a calculated rise time of less than five micro-seconds. The device herein described was used as a direct replacement in the Techtron A.A. Spectrophotometer. When used at switching times of more than 400/sec it supplied accurate phase control with minimum background noise. The following four figures describe the apparatus in detail.

1556. Calculation of power incident on a screen grid of a beam tetrode due to radiation from cathode: M S Epshteyn, Elektron Tekh Gener Modul Rentgen Prib, 1, 1970, 43–47 ( in Russian)

1556. Calculation of power incident on a screen grid of a beam tetrode due to radiation from cathode: M S Epshteyn, Elektron Tekh Gener Modul Rentgen Prib, 1, 1970, 43–47 ( in Russian)

Measurement of the electrical properties of a flowing plasma, including a critical comparison of probe experiment and theory

The primary quantities characterizing the electricaJ carriers of a flowing plasma in a low density wind tunnel have been determined from measurements using electrostatic probes immersed in the plasma. With the exception of the ion temperature, the plasma parameters have been obtained from the current—voltage characteristics of two types of single electrode probe. The probes consist of a cylinder, the major axis of which is aligned parallel to the flow of the plasma, and a disk, the exposed surface of which is normal to the direction of flow. Experiments with a double electrode probe consisting of a disk-shaped collector electrode which is screened from direct exposure to the plasma by a fine wire mesh, grid electrode, made it possible to obtain current—voltage characteristics with the ion and electron components separated from one another. From the current—voltage characteristic corresponding to collection of ions, using the screen grid probe, values of the ion temperature and drift velocity have been obtained. The measurements have been made at various points along the centre line of flow, for one particular value of the flow rate using argon as the test gas. For a given position of the probes, one value of the ion temperature has been evaluated, together with two independent values of each of the other primary quantities characterizing the electrical carriers of a flowing plasma. Each pair of values agree satisfactorily amongst themselves, good agreement being generally obtained between probe theory and experiment.

Computer designs for beam pentodes

A computer program for the solution of Poisson's equation in two dimensions with mixed boundary conditions has been used to solve for electron trajectories in beam pentodes with circularly cylindrical cathode contours and round wire grids in the general configuration of many Pierce guns in parallel. Such designs can have nearly uniform cathode current density over the entire cathode surface and zero grid interception under all conditions of interest, including positive grid conditions. Operation with the plate voltage as low as 5 per cent of the screen grid voltage provides satisfactory secondary electron retention in a typical design for which complete transfer characteristics are computed.

Some Characteristics of a beta-Ray Argon Tetrode Detector.

A fourth electrode can be inserted into the argon triode gas chromatographic detector to decrease the noise level and increase its sensitivity. Some of the properties of the resulting tetrode were determined with butane as the test substance and Sr/sup 90/ as the electron source. Graphs of sensitivity vs screen grid voltage and of anode and collector grid currents vs anode voltage are given. The ionization efficiency of the tetrode at maximum sensitivity is ~20 coulomb/g, and the linear range is ~10/sup 3/. Another electrode configuration is described in which the screen electrode may be used to collect negative ions formed by electron capture. (D.L.C.)

The double-grid ionization chamber

Abstract A new type of gridded ionization chamber is described. The chamber is provided with two sheets of grid between the cathode and the collector. Signals are taken out not only from the collector, but also from the grid closer to the cathode simultaneously. While the collector pulses afford the information on the energy of particles, the grid pulses give the information on the direction of emission. The other grid facing the collector is grounded and serves as a screen grid. Owing to the efficient shielding action of this screen grid, it is possible to make full use of the grid pulses while keeping the energy resolution better than 1 ≈ 2%. Several applications of the chamber are shown, including the improved “electronic collimation”, low-background counting and the measurement of angular distribution of proton recoils.

The Double-Grid Ionization Chamber

A double-grid ionization chamber is designed such that the additional grid serves as a screen grid and eliminates a serious loss in the energy resolution. The energy spectrum of the alpha particles from a natural uranium source obtained with this chamber is shown. The grid pulseheight distributions due to U/sup 2//sup 3//sup 4/ and U/sup 2//sup 3//sup 8/ are graphicall y shown, and the separation of the grid pulses is satisfactory in spite of the rather small difference in energy of the alpha particles. (N.W.R.)

Flicker noise in pentodes flicker partition noise

The method of approach used in the study of flicker noise in positive grid triodes is applied to pentodes with some modifications. It is demonstrated experimentally that the flicker noise current generated at the cathode of a pentode distributes over the positive electrodes in the ratio of the ac currents. Tomlinson's discovery of flicker partition noise is verified and it is shown that his results are caused by a noise current generator connected between screen grid and anode that is indeed independent of the flicker noise generated at the cathode.

High transconductance wide-band cathode-ray gun

The “screen-grid amplifier gun” is a combination of a conventional tetrode-type gun with an amplifier constituted in large measure by the gun's existing electrodes. In its simplest version the gun has, in fact, only one electrode more than conventional guns. The new gun has three grids participating in the beam mod ulation process: an auxiliary input grid and the two grids of the immersion lens. These latter grids are coupled together and operated 180° out of phase with respect to the input grid. Transconductance and resolution have been divorced from each other to a large extent in the screen grid amplifier gun. The low shunt capacitance makes the gun particularly suitable for wideband operation. Favourable possibilities are offered by this development for building guns with smaller “gamma” than hitherto possible. Analytical expressions correlating the gun's beam current to various types of gun drive, namely, combined screen and control grid drive, control grid drive and cathode drive are derived for cathode-ray guns of conventional design. An analytical expression for the gun's modulation constant is also given. Television guns possessing excellent resolution characteristics and transconductance in excess of 500 μmhos may be obtained rather simply. High transconductance guns and structures are shown.

The grid emitting properties of titanium

Experiments are described in which the emission from thin films of barium and barium oxide deposited on titanium is compared with that from similar deposits on tungsten. With deposits of both barium and barium oxide the emission from titanium is very much less than that from tungsten throughout the temperature range investigated (700-1000? C). Above about 900? C the evaporation of titanium will poison the emission of an adjacent oxide cathode, but it is concluded that from 700? C up to this temperature titanium would possess good grid emission suppression properties. This could enable titanium to be used as a screen grid winding wire in the place of carbonized molybdenum wire in a number of receiving valves, and as an electrode material in other electronic devices where hot electrodes become contaminated with cathode material.

The suppression of screen grid emission by carbon

The screen grid emission level which obtains in a receiving valve in which there is no suppression is shown to be of the same magnitude as that of a thin activated layer of barium oxide. It is further shown that the mechanism by which carbon suppresses screen grid emission is by the chemical reduction of deposited barium oxide to barium which evaporates. Below 900? C this reduction proceeds at a negligible rate and is only sufficient to activate the grid film. At 950? C the emission decays to a negligibly small value after a few minutes.

High sensitivity and accuracy pulse trigger circuit

A high sensitivity pulse height selector is described obtained using a retarding field diode and a differential negative resistance of high stability obtained by means of a multi-electrode electronic tube using the current division between screen grid and anode. The threshold of the discriminator can be set in the range of 1 to 30 mV its stability being in this range better than 1%.

Partition components of flicker noise

The flicker noise fluctuations in the space current of a thermionic valve are reduced by operating the valve under space charge limited conditions. In a pentode, this ?smoothing? is less effective due to the division of the current between the anode and the screen grid; consequently, a partition component of flicker noise may be said to occur. Experiments are described which confirm the presence of this phenomenon and which give useful evidence on the origin of the flicker noise. When making measurements on standard valves, there were encountered a number of defects which lead to the generation of excess noise at low frequencies.

Space-Charge and Ion-Trapping Effects in Tetrodes

Under the assumption of absolute vacuum in a tetrode, the space-charge depression in the screen-grid anode region will at sufficiently high current prevent secondary electrons released at the anode from reaching the grid, thus making the anode-current, anode-voltage characteristic similar to that of a pentode. Under pressure conditions present in ordinary high-vacuum tubes, positive ions formed by collisions between electrons and residual gas molecules will be trapped in the potential trough between the screen grid and the anode. Provided there exists no transverse ion-sweeping fields from beam-forming plates (as in the beam-power tetrode) or from charged end insulators, ion trapping proceeds automatically until the electron charge is neutralized. The effects of ion-trapping are discussed and a pulse method is described for measuring the conditions with and without presence of ions and for studying the ion-buildup process. Plasma ion oscillations have been observed.

A rectractable screen grid for jet air intakes

A rectractable screen grid for jet air intakes

Transit-Time Effects in Ultra-High-Frequency Class-C Operation

Transit-time effects in ultra-high-frequency triodes and tetrodes used in class-C operation are analyzed. Effects discussed are electron-transit reactance; electron-transit phase-delay angle; cathode back-heating; use of a screen grid to improve efficiency; changes in optimum shunt impedance; secondary emission; and anode back-heating by secondary electrons. It is pointed out that by increasing voltage and current density simultaneously the frequency and power can be raised without sacrificing efficiency or bandwidth. An equivalent circuit is described which takes account of certain important transit-time effects.

The alignment of grids in thermionic valves

The system formed by the cathode, control grid and screen grid of a tetrode valve is considered from an electron-optical point of view. It is shown that the ratio of the screen current to the anode current, and its variation with grid bias, can be accounted for over the whole of the characteristic. The formulae developed yield the data required to obtain optimum performance under any given conditions.Attention is also given to the application of electron-optical considerations to the problem of noise in the so-called silent screen-grid valves, and also to an interesting filamentary-beam tetrode in which the screen current is kept low by the application of electron-optical principles different from those used in other aligned valves.