Jan Hasker

Properties and manufacture of top-layer scandate cathodes

Abstract The life and/or the staying power against ion bombardment of scandate cathodes can be improved by using a top layer of W + Sc2O3 or W + W/ScH2 on a tungsten matrix. The latter is impregnated with the usual 4-1-1 impregnant. Even at high voltage pulses the current densities are so high that the deviation from space charge limitation is small. The manufacture is discussed and the cathode life at the operating temperature of 1220 K is shown to be very long. Moreover, the relationship between processing parameters and emission recovery after ion bombardment is examined with the aid of combined sputter- and scanning-Auger measurements. It is shown that thin-layer coverage of tungsten by scandia is important to the high emission. This coverage is related to the impregnation process. After extended sputtering it cannot be completely recovered by reactivation. Consequently, the top layer cathodes cannot withstand sufficiently the usual processing and operation of television display tubes. On the other hand, they can improve the life and performance of electron devices with good vacuum and/or relatively low accelerating potentials. Moreover, activated top-layer scandate cathodes are relatively insensitive to exposure to (moist) air.

Scandium supply after ion bombardment on scandate cathodes

A scandium, barium, and oxygen coverage of monolayer order is essential to the high-emission capability of scandate cathodes. The relationship between surface coverage and emission is demonstrated by combined scanning Auger and emission measurements. A problem with the known cathodes is that the emission recovery after ion bombardment, as may occur during CRT processing, is not yet complete. The problem cannot be solved by simply constructing a mixed-matrix cathode of W and Sc grains because of huge evaporation of Sc during sintering and oxidation of the remainder during subsequent impregnation. How the scandium can be effectively surrounded by a protective layer resulting in considerably improved scandium supply, is discussed. Moreover, experimental results indicate the possibility for further simplification of the cathode manufacture. The latter is shown with the aid of cathodes containing compounds which exhibit Sc surface segregation on heating. >

Some experiments on the role of oxygen and surface reactions for tungsten and scandate thermionic emitters

It is shown that the state of oxidation of the tungsten before activation on an S-type cathode is irrelevant to the eventual emission level. The reason for this is that during activation the original surface oxygen is partly removed by desorption, while the remaining tungsten oxide is reduced by means of atomic barium produced by the reaction between impregnant and tungsten. This can also be demonstrated by exposing an unimpregnated heated tungsten matrix to an external atomic barium flux: the surface oxygen disappears. In addition, experiments showed the transport of oxygen to the surface. The possibility for the formation of stable complexes by means of surface reactions is shown by exposing a heated (1125 degrees C/sub b/) matrix with a W+Sc/sub 2/O/sub 3/ top layer to an external barium flow. This may be related to the surface compound formation on activation of a cathode made by pressing and sintering a mixture of W, Re/sub 24/Sc/sub 5/, and Ba-Ca-aluminate. >

Astigmatic electron gun for the beam-indexing color television display

Though the beam-indexing color television display has some obvious advantages when compared with other display systems, the electron-optical demands are very high. First, these demands and the imaging system to be used are discussed. It turns out that the beam must be astigmatic to obtain optimum performance. The beam shape is prescribed by the deflection-coil design. This beam shape, giving rise to a vertically elongated spot of sufficiently small dimensions, is obtained by using an astigmatic gun of special design in combination with a simple rotation-symmetrical focusing lens. After a general discussion of the focusing properties of this combination, the intensity distributions along both the minor and the major axis of the spot on the screen are calculated and compared with experiments. The gamma of the beam current characteristic is measured as well as calculated. Its dependence on the beam current is small up to beam currents of about 3 mA. Moreover, the cathode loading is calculated as a function of the beam current. Without special means of dynamic correction, satisfactory color reproduction in normal picture material is obtained up to peak currents of 1.5 mA in 110°-25 in tubes. Higher beam currents would be attainable in the absence of a residual deflection error which causes a tilt of the spot near the corners of the screen. The structure of the picture is quite acceptable at a triplet pitch of 1.2 mm without using a lenticular screen.

Imaging, beam acceptance, and beam-discharge lag in camera tubes

The effect of beam interception on the initial-energy distribution of the electrons contributing to the operating beam and on the imaging in camera tubes is examined. It is shown that the imaging properties of different imaging systems can be compared mutually by calculating their figure of merit, which is defined as the product of beam angle and spot diameter. The beam acceptance by the low-potential target depends on the imaging system used in the tube. This is clearly demonstrated by calculations on three different imaging systems, the imaging properties of which are compared with one another. These systems have been chosen such that: 1) the influence of the Coulomb interactions between individual electrons on the beam acceptance is of minor importance; and 2) the initial-energy distributions of the beam electrons are different particularly in the low-energy region. This gives rise to differences in the lag for peak white in the three systems which are calculated. The importance of these differences depends on operating conditions and layer capacitance. It is shown that a space-charge minimum between mesh and target, which could affect the beam acceptance, does not occur at normal values of the mesh potential and of the distance between mesh and target. The influence of increasing the beam angle on the beam acceptance is calculated and found, to give rise to a paralell shift of the acceptance curve which looks like the effect of an increasing contact potential difference.