Recent work in the design and development of high definition television receivers and high resolution cathode ray tube (CRT) monitors has been directed to reducing electron beam spot size and increasing electron beam intensity or the charge density in the beam. Reducing electron beam spot size improves picture resolution, while increasing beam current density permits increased display brightness. One approach to increasing beam current density is to raise the temperature of the electron gun's cathode which then emits a large number of electrons. A conventional oxide cathode is capable of producing an emission current density of only 0.5 A/cm.sup.2 over an extended operating lifetime. While electron emission density increases exponentially with increasing cathode temperature, cathode useful lifetime is correspondingly reduced exponentially with increasing operating temperatures. Therefore, in a conventional electron gun employing a typical oxide cathode, it is impossible to achieve a high resolution spot size without shortening cathode useful operating lifetime.
Electron beam optics dictates that at low current (i.ltoreq.500 .mu.A) the focused electron beam spot is roughly proportional to the aperture size of the CRT's G.sub.1 control grid and that the total maximum current drawn from the cathode is roughly proportional to the square of the G.sub.1 aperture (assuming that cathode emission density remains the same). Therefore, a high resolution electron beam requires a small G.sub.1 aperture in the beam forming region (BFR) of the electron gun. This, in turn, reduces beam current resulting in an undesired reduction in video display brightness. Attempts to resolve this dilemma generally involve replacing the conventional oxide cathode with one having a higher current density capability and a long operating lifetime. This combination in a cathode offers a small spot size with both acceptable display brightness and a reasonably long operating lifetime. In order to provide small beam spot size, high video display brightness levels, and acceptable cathode operating lifetimes, many CRT manufacturers have turned to using the dispenser cathode which can sustain many times the current density of a conventional oxide cathode while continuing to offer extended operating lifetimes. However, a dispenser cathode is on the order of 20-50 times more expensive than a conventional oxide cathode. Even when a dispenser cathode is employed, the power requirements of the CRT are usually higher.
Referring to FIG. 1, there is shown a simplified diagrammatic cross-sectional view of pertinent electrical portions of a prior art electron gun 10 such as used in a conventional CRT. Electron gun 10 includes an electron source 12, a low voltage beam forming region (BFR) 14, and a high voltage beam focusing region 16. Although only a single electron gun 10 is shown in the sectional view of FIG. 1, the typical color CRT employs three such electron guns, one for each of the primary colors of red, green and blue. The electron gun 10 has a longitudinal axis A-A' along which an electron beam is directed onto the phosphor coating 20 of a display screen 18 in a CRT. The electron beam is shown for simplicity as a series of closely spaced electron rays 22 extending between a cathode K and the display screen 18. A plurality of charged grids, or electrodes, are disposed along axis A-A' for forming and directing the electron beam onto the display screen 18 as described below.
The electron source 12 includes the heated cathode K and the combination of a G.sub.1 control grid and a G.sub.2 screen grid for directing energetic electrons from the cathode surface generally along the electron gun's axis A-A' toward the display screen 18. The G.sub.1 control grid is disposed adjacent cathode K, while the G.sub.2 screen grid is disposed intermediate the G.sub.1 control grid and a G.sub.3 grid. Each of the G.sub.1 control grid and the G.sub.2 screen grid includes a generally circular aperture having a diameter d.sub.G1 and d.sub.G2, respectively. Apertures d.sub.G1 and d.sub.G2 are typically of the same size, although d.sub.G2 may in some cases be larger than d.sub.G1 for manufacturing purposes. In addition, the G.sub.1 and G.sub.2 grids are generally in the form of thin plates having thickness t.sub.G1 and t.sub.G2, respectively. Although only one aperture is shown in the cross-sectional view of FIG. 1 for simplicity, each of the G.sub.1 control and G.sub.2 screen grids includes three spaced apertures, each adapted to receive and pass a respective electron beam in a color CRT. Cathode K, the G.sub.1 control grid, the G.sub.2 screen grid, and a portion of the G.sub.3 grid facing the G.sub.2 grid comprise the low voltage BFR 14 of the electron gun 10. The G.sub.3 grid also includes an aperture 33 through which the electrons are directed. The G.sub.3 grid is coupled to a focus voltage (V.sub.F) source 36 for focusing the electrons beam to a sharply defined spot on the display screen 18.
One or more beam focusing grids (G.sub.4, G.sub.5, etc.) can be disposed intermediate the G.sub.3 grid and the display screen 18 for focusing the electron beam to a spot on the display screen's phosphor coating 20. Usually the last grid has the anode voltage V.sub.A which combines with the adjacent focus voltage V.sub.F grids to form the main focusing lens. In our case (FIG. i), the main lens is formed of the G.sub.3 and G.sub.4 grids. The path of travel of the electrons between cathode K and the display screen 18 is shown as a plurality of the aforementioned closely spaced electron rays 22 in the figure. The electrons are drawn from the cathode K over a generally circular area having a diameter d.sub.K. With each of the grids charged to a predetermined potential, or voltage, a complex electrostatic field is established within the electron gun 10. The electrostatic field within a portion of the electron gun 10 is represented by a series of equipotential lines 24 shown in dotted-line form disposed about the longitudinal axis A-A' of the electron gun 10. The electrostatic field represented by the equipotential lines 24 causes the convergence of the electron rays 22 in the BFR 14 such that the electron rays typically form a crossover of axis A-A' intermediate the G.sub.2 screen grid and the G.sub.3 grid. The electron rays 22 are then permitted to diverge somewhat to a diameter of d.sub.s before being focused by one or more focusing grids represented by the G.sub.4 grid. The electron beam is focused to a small spot on the screen's phosphor coating 20.
In a conventional CRT electron gun design, the G.sub.1 and G.sub.2 aperture diameters are generally equal which facilitates assembly of the electron gun. There has thus been no incentive to make the G.sub.1 grid's aperture larger than that of the G.sub.2 grid. In addition, during operation the "hot" cathode-to-G.sub.1 grid spacing D.sub.G in a conventional CRT electron gun design is preferably on the order of 0.08 mm. However, due to manufacturing difficulty, the actual "hot" spacing can be controlled to only a limited degree. Increasing the cathode-to-G.sub.1 grid spacing gives rise to a "halo" about the focused electron beam spot on the CRT display screen caused by energetic electrons having a large thermal velocity component transverse to the axis of the electron beam. These high transverse thermal velocity electrons are incident upon the display screen about the center image of the electron beam spot giving rise to a halo, or haze, surrounding the individual electron beams pixel in the pattern array which significantly detracts from the quality of the video image.
The present invention addresses and overcomes the aforementioned limitations of the prior art by providing a beam forming arrangement in an electron gun capable of providing a high density electron beam having a small spot size using conventional cathode materials operating at normal temperatures.