Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
1999-11-03
2002-04-09
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S409000, C313S412000, C313S446000, C313S447000, C313S451000, C313S461000
Reexamination Certificate
active
06369499
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electron guns and more specifically to an electron gun design that improves cathode venting thereby extending cathode life and improving performance.
2. Description of the Related Art
A CRT typically includes a neck glass that houses an electron gun, a funnel that is tapered to accommodate the deflection of the beam, and a target. The electron gun is comprised of two or more optical parts; the triode that forms the beam and one or more focusing lenses that focus the beam at the target. The funnel is coated with a reactive element, typically barium, to neutralize the poisonous byproducts that are out gassed by the triode's cathode element (and other parts of the CRT).
Electron guns are typically given a name that describes their focusing lenses. A standard bipotential gun has an anode voltage and a focus voltage that together define a single main focusing lens. As shown in
FIG. 1
, a standard Einzel gun
10
has a pre-focus lens
12
and two main lenses
14
a
and
14
b.
The triode
16
is made up of the Emitter (cathode)
18
, the Wehnelt suppressor electrode (biasing grid)
20
and the extractor electrode (first accelerator grid)
22
. The heating of the cathode during operation causes the electrons to be emitted at the cathode surface
24
. The electrons are then pushed back to the cathode surface by the suppressor electrode. But, the suppressor electrode has an optical aperture that allows an extraction voltage from the first accelerator to penetrate through the aperture
26
and strip electrons off of the cathode. This results in a converging electron beam that crosses over at an axial position somewhere between the biasing grid and the first accelerator, typically referred to as the “first crossover”.
The biasing grid effectively forms an iris, which the beam passes through. This iris can be opened or closed by varying the voltage on the biasing grid. If the biasing voltage is brought closer to the cathode voltage then the cathode's active emitting surface becomes larger in diameter. This active area serves as the object in the total optical system. While this voltage change allows more current to escape from the cathode it increases the object size for the optical system. A smaller active area corresponds to a smaller spot size, provided that the cathode is healthy enough to emit the required peak current densities.
Increasing the extraction voltage on the first accelerating grid increases the biasing voltage required to “cutoff” the beam. This causes the active cathode surface to decrease in size but reduces the slope of the current vs. biasing voltage curve. This increase of the extraction voltage also increases the beam angle, which could be desirable or undesirable depending on the size of the main focusing lens.
This beam
27
is then sent through pre-focus lens
12
(volume between first accelerator electrode
22
and a second accelerator electrode
28
), first main lens
14
a
(volume between second accelerator electrode
28
and a focus electrode
30
) and second main lens
14
b
(volume between focus electrode
30
and a final accelerator electrode
32
) that focus the beam at the target. The higher the potential difference between the electrodes the stronger the lensing effects. But, a stronger lens has more spherical aberration.
In an Einsel gun the second accelerator electrode and final accelerator electrode are both held at anode potential and the focus electrode is at a lower potential. Second accelerator electrode
28
is electrically connected to final accelerator electrode
32
via a jumper
34
. Final accelerator electrode
32
is connected to an internal conductive coating
36
on the inside of a neck glass
38
, which is held at anode potential, by a number of snubber springs
40
. The diameter of the main lenses is limited to the space between a pair of mounting beads
42
a
and
42
b
. The smaller the main lenses
14
a
and
14
b
the greater the spherical aberration for a given beam size.
A large beam is desirable because it has a steeper crossover angle at the first crossover, which reduces the spot size at the target. But, as the beam increases in size the spherical aberration affects increase the spotsize. Thus, the Triode must be optimized for the best possible spotsize for a given focusing lens system.
Spot size and life are directly tied to the health of cathode
18
and more specifically active area
24
. If the cathode becomes poisoned during activation or normal operation its ability to deliver high peak current densities, e.g. 5 microamps per square centimeter, will suffer and its lifetime will shorten. Thermal stimulation of the cathode produces free elements that emit the electrons and byproducts that, if not removed, will recombine with the free elements thereby poisoning the cathode.
In most electron guns, these byproducts are removed by venting them to a funnel
44
whose inner surface has a reactive coating
46
such as barium that neutralizes the byproducts. The vent is simply a path from the cathode's active area
24
to the reactive coating
46
. In most guns, this path is the line of sight through the aperture holes
26
in the triode and pre-focus elements into the funnel
44
. A straight or “line of sight” path is the most efficient. Each ninety-degree turn that must be traversed to reach the funnel reduces conduction by fifty percent.
Most guns use a fairly large aperture
26
, on the order of 400 microns in diameter. In most cases, this is adequate to vent the cathode during normal operation. However, during cathode activation when the emission of byproducts is greatest the cathode is in danger of being poisoned. Furthermore, any attempt to reduce the aperture will choke off the vent capacity and poison the cathode. This will reduce the peak current density, which already needs to be higher if a smaller aperture is used.
More specifically, cathode
18
comprises a metallic base coated with a mixture of barium carbonate, strontium carbonate, calcium carbonate, and a nitro-cellulose binder. The cathode must be activated before it can efficiently emit electrons. This activation process is comprised of three basic steps. First, the cathode is brought to a temperature that will break down its nitro-cellulose binder. This causes compounds that are poisonous to the activated cathode to out gas and linger near cathode surface
24
unless sufficient venting is provided. The second step is to break down the three carbonates into their respective oxides by increasing the cathode temperature. This process causes more poisons to out gas and linger near the cathode. The final step is to partially break down the three oxides into free barium, strontium, and calcium respectively by again increasing the temperature of the cathode. Once, this third step is performed the cathode is subject to poisons permanently combining with the free barium, strontium and calcium. This irreversibly reduces the total amount of free barium, strontium, and calcium available and raises the work function of the cathode surface.
The cathode's emission current is subject to two limitations. First, the cathode has a temperature limited emission current density, which varies widely from cathode to cathode. Differences in the cathode's activation and the tube's vacuum quality can change the temperature limit of the current density. A cathode that was subjected to poisons during activation will have a lower temperature limited emission current density. Secondly, the cathode emission is subject to a space charge limitation at the surface of the cathode, which is determined by the physical geometry of the triode. Whichever limit is smaller prevails. Typically, the triodes are designed to operate in space charge limited conditions.
During normal operation, the cathode is heated to a temperature that normally ensures space charge limited conditions. If the cathode was not properly vented during activation then the typical operating temperature will be insufficie
Blakely , Sokoloff, Taylor & Zafman LLP
Intel Corporation
Patel Nimeshkumar D.
Roy Sikha
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