Cathodoluminescent gas discharge image display panel

This disclosure depicts a high-voltage cathodoluminescent gas discharge image display panel having an ordered array of display elements. The panel includes envelope means containing an ionizable gas at a predetermined very low pressure. The envelope means includes a transparent faceplate on the inner surface of which are disposed cathodoluminescent target elements. Electron source means produces at a given time at least one high-density electron beam, and includes means to cause a plasma sac to generate and gather electrons, and accelerate them to form a concentrated electron beam. An ultor electrode receiving a predetermined relatively high ultor voltage establishes a high voltage gradient in a plasma-free acceleration section which is effective to straight-line accelerate said electron beam in a substantially collision-free path directly into high-energy bombardment of the cathodoluminescent target elements. The panel includes light-stopping means whereby the useful visible light is solely that produced by high-energy electron bombardment of the target elements. Other structures including means for electron beam modulation are disclosed.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
This application is related to but in no way dependent upon copending 
application of common ownership herewith assigned including Ser. No. 
588,737 filed June 20, 1975; now U.S. Pat. No. 3,992,644, issued July 4, 
1978; Ser. No. 730,114 filed Oct. 6, 1976; now U.S. Pat. No. 4,009,082, 
issued July 4, 1978; and Ser. No. 828,792 filed Aug. 29, 1977 now U.S. 
Pat. No. 4,009,032, issued Dec. 19, 1978. 
BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT 
This invention relates to image display panels. It is particularly directed 
to a highly efficient cathodoluminescent panel useful for image displays 
such as alphanumeric and computer graphics, and well suited to television 
displays. 
Although the field of the gas discharge display panel has been diligently 
explored, no device has yet been able to meet the standards of performance 
and cost as high as those established by the cathode ray television 
picture tube in its current state of development. 
Ideally, the gas discharge display panel offers many benefits. First of 
all, it is not size-limited as stringently as the picture tube, wherein 
any increase in picture area much greater than the twenty-five inch 
diagonal measure results in an inordinate increase in bulk and weight. For 
example, a picture tube with a twenty-five inch diagonal measure weighs 
about fifty pounds while a tube with a thirty inch diagonal measure may 
weigh more than a hundred pounds. To cite other advantages, flat panel 
displays, which are commonly built in a matrix of linear rows of columns 
of discrete picture elements, are inherently capable of producing pictures 
of near-perfect raster linearity, interlace and color field registration. 
But these theoretical benefits have been largely offset by undesirable 
performance characteristics such as inadequate brightness, low luminous 
efficiency, luminance non-uniformity, and lack of contrast. 
Of these problems, inadequate brightness and low luminous efficiency have 
proved to be among the most troublesome impediments to commercial 
viability. The maximum level of brightness produced by a discrete picture 
element in prior art gas discharge panels has been but a fraction of the 
brightness level of an equivalent picture element in a television picture 
tube. As a consequence, it has not been feasible to scan gas discharge 
picture elements point-at-a-time as is done in the picture tube and yet 
achieve an acceptable brightness level. However, a greater level of 
brightness can be obtained in gas discharge panels by operating a full 
line of display panel picture elements at a time. Even by this expedient, 
however, a brightness level adequate for comfortable television viewing, 
and competitive with the current television picture tubes, has not been 
shown. 
Luminance non-uniformities have also proved troublesome in display panels. 
This problem may manifest itself as spots, rings or striations of light 
brighter than surrounding areas of the image field. These manifestations 
may remain fixed, or move about. Since the human eye is particularly 
sensitive to even slight differences in luminance intensity, the effects 
can prove deleterious, especially in panels used for the reproduction of 
images having a full gray scale, such as the television picture. 
Operation of the gas-discharge display panel is based upon the principles 
of the widely known glow-discharge tube, an example of which is shown by 
FIG. 1. Enclosed within an evacuated envelope 12 is cathode 14 and an 
anode 16. Envelope 12 may contain one of the noble gases such as krypton 
or argon, or common gases such as nitrogen, hydrogen, mercury vapor, or a 
mixture thereof. A suitable potential applied between cathode 14 and anode 
16 results in a glow discharge within the envelope. The entity exhibits 
classic gas discharge phenomena including a cathode dark space 20, a 
negative glow 22, a Faraday dark space 24, and a positive column 26. 
FIG. 2 shows an element of a prior art gas discharge display panel for 
producing spots of light utilizing the medium of the gas discharge tube. 
In essence, an intermediate apertured insulator 30 is located in a 
positive column 32 of a gas discharge cell 33. A "plasma sac" 34 (also 
called an "electrostatic double layer" in the art) forms on the cathode 
side of the aperture 31. Primary electrons from the cathode 35 generate 
secondary electrons in the gas discharge and are gathered by the plasma 
sac 34 and channeled into aperture 31. Light visible to the viewer, 
indicated by 36, is produced within sac 34 due to the higher electron 
temperature within the sac as compared to the electron temperature outside 
the sac. The phenomenon is described in a journal article entitled "A 
Picture-Display Panel Using a Constricted Glow Discharge", by H. Hori et 
al, IEEE transactions on Electron Devices, Vol. ED-21, No. 6. June, 1974. 
A gas discharge display apparatus utilizing the plasma sac is disclosed by 
Miyashiro et al in U.S. Pat. No. 3,749,969. 
Further with regard to FIG. 2 and the concept it represents, it is said 
that a gas such as neon can be used at a nominal pressure of five torr. An 
intermediate electrode 38 plated inside aperture 31 is used for 
propagation of the plasma sac 34 to an adjacent aperture having a similar 
intermediate electrode (not shown). Propagation is due to a priming effect 
in that the presence of the discharge in one aperture lowers the breakdown 
voltage of a discharge in an adjacent aperture to encourage the formation 
of a plasma sac in that aperture. At the same time, the discharge in the 
first cell is switched off. Thus, by this scheme, point-by-point scanning 
can be obtained, and as luminance is a linear function of current, the 
intensity of the light 36 can be varied so intermediate values of gray of 
limited scope can be obtained. 
Another approach followed by the prior art is to utilize ultraviolet 
emissions emanating from a positive column to stimulate the emission of 
light in the visible spectrum. 
A phosphor is disposed on the transparent walls of the cavity surrounding 
the positive column. One execution of this approach utilizes the plasma 
sac phenomenon as described in a journal article by H. Hori et al (Op. 
cit.), but alleges to be an improvement thereon in that it is said to 
utilize a more efficient ultraviolet excitation of phosphors from a 
positive column, rather than a negative glow luminance light production 
phenomenon taught by Hori. The method of ultraviolet excitation of 
phosphors is described in "Electron Accelerating Display Cell," Y. Okamoto 
et al, Preprint Number 464 of the 1975 national meeting of the Institute 
of Electrical Engineers of Japan, 1975. 
According to the referenced document, the device is said to be operable in 
three modes, as illustrated by FIGS. 2A, 2B, and 2C. It will be observed 
that the gas discharge cell structures in the figures are identical, in 
that each has a cathode 1 and associated negative glow, an anode 2 and 
intermediate electrode 3 having an aperture therein, and a phosphor 4 
disposed on an inner surface of evacuated, gas-charged envelope 5. The 
particular mode that develops in these common configurations depends upon 
the potential on anode 2; that is, a progressive increase in levels of 
potential on anode 2 results in modes I, II, and III as shown in FIGS. 2A, 
2B, and 2C respectively. Mode II is the favored mode and would seem to be 
the most feasible mode of operation, with modes I and II considered as not 
being viable for display applications. 
With regard to the operation of mode I illustrated by FIG. 2A, electrons 
generated in the interspace between cathode 1 and intermediate electrode 3 
diffuse into display cell area 6 and are accelerated by relatively low 
potential on anode 2, said to be on the order of 200 volts or less. An 
electron e may follow a typically random collison-determined path 7 to 
impinge upon low-voltage phosphor 4, causing emission of light 8. 
Alternatively, an electron e following path 9 may collide with an atom 11, 
resulting in the emission of ultraviolet light which, upon impact with 
phosphor 4, also results in the emission of visible light 8A. It appears 
that no positive column is generated in mode 1 operation. 
With regard to FIG. 2B and mode II operation, a relatively greater 
potential on anode 2, assumed to be more than 200 volts, results in the 
formation of a positive column 13 which emits abundant ultraviolet light 
for the excitation of phosphor 4. When a predetermined threshold level is 
reached in the discharge current, a plasma sac 15 forms. Plasma sac 15 
provides for amplification of the electron current drawn through the 
aperture in intermediate electrode 3, providing for greater phosphor 
excitation than mode I through enhanced ultraviolet emission from positive 
column 13. 
In mode III operation shown by FIG. 2C, a potential said to be even greater 
on anode 2 causes the intermediate electrode 3 to become a second cathode, 
as shown by the presence of a second negative glow 17. A positive column 
19 also appears, resulting in photoluminescence as in mode II. The 
presence of the negative glow 17, however, causes the discharge current to 
"run away," as a self-sustained discharge develops between intermediate 
electrode 3 and anode 2 as in an ordinary gas discharge. To prevent this 
run-away condition a resistive entity (not shown) must be placed in series 
with the intermediate electrode 3. This in turn reduces the panel speed 
response as compared with preferred mode II. (See "A New DC Gas Discharge 
Display With Internal Memory," Y. Okamoto et al. Japan J. Appl. Phys. Vol. 
15 (1976), No. 4; also, Patent Disclosure No. 26 01 925 (German). 
Schwartz, in U.S. Pat. No. 3,845,241 discloses a gas discharge structure as 
a source of free electrons which are accelerated in an adjoining, second 
section into impingement with an electron-excitable phosphor screen. 
Establishment of a gas discharge in the second section is precluded by 
appropriate selection of certain dimensions, gas pressure and accelerating 
voltage according to Paschen's law. Moderating means are provided in 
certain embodiments for causing the energy range of free electrons 
entering the second section to be narrow relative to the range of energies 
of free electrons generated in the gas discharge. A number of control grid 
arrangements are also disclosed. 
Displays in which a light-emissive material is directly excited by electron 
bombardment are known as cathodoluminescent displays. Obtaining an 
adequate number of electrons for adequate excitation of the light-emissive 
material, and hence adequate brightness, has been a problem in panel 
displays utilizing cathodoluminescence, as the standard planar cathode in 
its present state of development does not yield enough electrons at low 
gas pressures for an effective display. To remedy this deficiency, a 
structure known as a "hollow cathode" has been introduced into 
cathodoluminescent panel displays. The use of a hollow cathode in gas 
discharge displays and the advantages thereof, are disclosed in U.S. Pat. 
Nos. 3,992,644 3,938,135 and 3,999,094 assigned to the assignee of the 
present invention. 
OTHER RELATED PRIOR ART 
Luminance non-uniformity in the form of anode spots is the topic of an 
article in the Encyclopedia of Physics, "Gas Discharge II" Vol. XXII, S. 
Flugge, Ed. Springer-Verlag, Berlin. 1956. Pp. 151-152. 
Examples of the use of hollow cathodes in gas discharge displays and in 
other applications may be found in U.S. Pat. Nos. 3,662,214; 3,701,918; 
3,831,052; 3,875,442; 3,882,342. 
With regard to prior art disclosing a plasma sac, the following are cited: 
An article entitled "Theory of a Double Sheath Between Two Plasmas", by J. 
Andrews and J. Allen, Proc. of the Royal Society of London, A 320 Pp. 
459-472. 1971; an article entitled "A New Gas-Discharge Display Device 
Using Through-Hole Enhancement", H. Hori et al. Conference Record, 1970 
IEEE Conference on Display Devices, Pp. 140-143; and an article entitled 
"The Double Sheath at a Discharge Constriction", by F. Crawford and I. 
Freeston, Microwave Laboratory, Stanford University. 
Patents relating to plasma sac technology and display panels having 
relevance in general include: U.S. Pat. Nos. 3,622,829; 3,800,186; 
3,801,864; 3,956,667 and Patent Specification No. 1 433 256, Great 
Britain. 
The glow discharge tube shown by FIG. 1 is an excerpt from a section of the 
Encyclopedia of Physics titled "The Gas Discharge at Low Pressure", by 
Gordon Francis. (Op. cit.) P. 56. 
Priming and self-scanning, deionization and recovery time are discussed in 
a book entitled Cold Cathode Glow Discharge Tubes, by G. F. Weston. ILIFFE 
Books Limited. 1968. Pp. 63-65, and 281-287. 
Further relevant art includes: "Extract From `Anomalous Variations of the 
Sparking Potential as a Function of PD`," by F. Penning. Reproduced from 
Proc. of the Royal Academy of Sciences, Amsterdam 34, 1305 (1931), from 
Electrical Breakdown in Gases, J. Rees, Ed. John Wiley & Sons; and 
"Principles and Techniques in Multicolor DC Gas Discharge Displays." Z. 
Van Gelder and M. Mattheij. Pp. 1019-1024; "Electron Accelerating Plasma 
Display Cell," a series: Preprint No. 463; Mizushima et al, IEEJ, 1975; 
Preprint No. 1064, Okamoto et al, IECEJ, 1975; Preprint No. 18-2, Okamoto 
et al; and "Plasma Display Panel of an Electron Accelerated Type," Okamoto 
et al. ED75-58. Hitachi Ltd. 
OBJECTS OF THE INVENTION 
It is a general object of this invention to provide an improved gas 
discharge display panel. 
It is another object of this invention to provide a gas discharge display 
panel having an image brightness, luminance uniformity, luminance 
efficiency, and contrast and color reproduction capability comparable to 
the present television cathode ray picture tube; 
It is a further object to provide a display panel having a structure that 
is relatively simple and easy to manufacture; 
It is yet another object of this invention to provide a display that is 
fully compatible with NTSC standards and that can utilize standard 
television broadcast signals.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
This invention is particularly related to gas discharge image display 
devices wherein a gas discharge is used as a source of electrons. To add 
clarity to the description that follows, certain illustrative dimensions 
and values are given in the course of the specification, but in no manner 
are they to be considered limiting. 
With reference to FIG. 3, there is shown figuratively a very-low-pressure, 
high-voltage cathodoluminescent display panel 40 having an aspect ratio of 
three to four consonant with the aspect ratio of a standard television 
picture. The invention lends itself to a broad range of display sizes; for 
example, from nine inches to fifty inches in diagonal measurement (A), 
representing a practical range of interest for television viewing. The 
basic construction of a panel envelope 40 is shown by FIG. 3 in simplest 
form. Transparent faceplate 42 and back wall 44 are separated by side 
plates 46. All abutting faces are sealed to make an envelope which can be 
air-evacuated. The air is replaced by a suitable gas, or mixture of gases, 
at a predetermined very low pressure according to this invention. The 
panel envelope representing a preferred embodiment of this invention as 
described in this disclosure may have a front-to-back dimension of from 
about one to three inches in the thirty-inch diagonal measure size. This 
relative thinness is made possible by the fact that the surfaces of 
faceplate 42 and back wall 44 can be made fully self-supporting because of 
the nature of the internal structure, as will be shown. Because of this 
self-supporting ability, which represents an aspect of the preferred 
embodiment, the faceplate 42, which may, for example, be made from 
tempered plate glass, can be one-eighth of an inch thick or less. 
Similarly, back wall 44, which is also supported internally, can be 
equally thin. The value of this self-support is shown by the fact that the 
pressure on the faceplate and the back panel of a television display panel 
having a fifty-inch diagonal measurement may approach nine tons. 
Section 48 of FIG. 3 represents a very small area of the faceplate 42 
greatly enlarged to show an ordered array of red-light-emitting, 
green-light-emitting and blue-light-emitting cathodoluminescent target 
elements 50. To enhance color purity and contrast, and to reduce front 
reflection, each target element 50 may be surrounded by a light-absorptive 
material 52. The ancillary electronic circuits, which in the case of the 
television display would include circuits for video processing, are shown 
schematically as being contained in electronic section 54. 
FIG. 4 illustrates a single gas discharge display element 56 comprising the 
preferred embodiment of this invention. As used herein, the term "display 
element" is intended to mean those structures and partial structures which 
cooperate to define a single picture element of the panel. Display element 
56 as shown and described is to be considered as a microcosm of each of 
the hundreds of thousands of identical elements which, for example, may be 
incorporated into the figurative television display panel 40 shown by FIG. 
3. To contribute to a thorough understanding, the inventive concepts of 
the preferred embodiment of the invention shown by FIG. 4 will be listed 
first in brief, followed by a detailed functional description. 
Display element 56 is shown as being enclosed in an evacuated envelope 
containing an ionizable gas at a predetermined very low pressure. Envelope 
58, indicated by a broken line in FIG. 4, symbolically represents the 
panel-form envelope 40 shown in FIG. 3. The components according to the 
preferred embodiment of this invention comprise a rearwardly disposed 
hollow cathode 60 for receiving a relatively low applied voltage. 
Electron-transmissive anode 74 is spaced a predetermined distance from 
cathode 60 and receives a relatively intermediate applied voltage. 
Constriction-forming means 66 located between cathode 60 and anode 74 
define a constriction 68. A performance enhancement electrode 72 is 
located between cathode 60 and anode 74. Between cathode 60 and 
performance enhancement electrode 72 lies the Faraday dark space 70. The 
intermediate applied voltage of anode 74, the predetermined distance 
distance between cathode 60 and anode 74, the very low gas pressure, and 
the individual width of constriction 68 have values effective to support a 
gas discharge between cathode 60 and anode 74, and to cause a plasma sac 
76 to form in the plasma on the cathode side of constriction-forming means 
66. The plasma sac 76, by its nature, generates and gathers electrons from 
cathode 60 and accelerates them into constriction 68 to form a 
concentrated electron beam therein. 
An ultor electrode 86 is disposed contiguous to cathodoluminescent target 
element 88 on transparent faceplate 90 for receiving a predetermined 
relatively high ultor voltage. Ultor electrode 86 is separated by 
predetermined spacing from grid 75 to define an acceleration section 84 
therebetween. This predetermined spacing is so small that at the 
predetermined very low pressure and at the ultor voltage cited, no gas 
discharge plasma can possibly occur in acceleration section 84. The 
operating point is to the left of the Paschen curve minimum (more later on 
this). The ultor voltage establishes a high-voltage gradient in the plasma 
free acceleration section 84 which is effective to straight-line 
accelerate the beam of electrons 82 formed in constriction 68 in a 
substantially collision-free path directly into high-energy bombardment of 
the cathodoluminescent target element 88 disposed on faceplate 90. 
Light-stopping means, here shown as including a continuous, 
light-reflecting layer 87, blocks from view the light produced by the 
plasma whereby useful visible light produced by the panel is solely that 
produced by the high-energy electron bombardment of the cathodoluminescent 
target element 88 disposed on faceplate 90. Ultor electrode 86 as shown is 
light-transmissive. The grid means comprises anode 74 and at least one 
electron-transmissive modulating grid means 75 located down-beam of anode 
74. 
A detailed functional description of the preferred embodiment of the 
invention as illustrated by FIG. 4 now follows. The ionizable gas enclosed 
in evacuated envelope 58 may comprise a single gas, or a mixture of gases 
according to this invention. Typical gases that may be used in the panel 
are the noble gases such as krypton and argon, or the more common gases 
such as hydrogen, nitrogen, mercury vapor or mixtures thereof, such as a 
Penning mixture. A preferred gas according to this invention is pure 
nitrogen; a typical Paschen curve for nitrogen is shown by FIG. 7. The gas 
pressure is a fraction of a torr for electrode spacings of a few 
millimeters, and operation is well to the left of the Paschen minimum 57. 
A useful operating point, for example, occurs at 0.06 torr-centimeters. 
This corresponds, e.g., to a pressure of 0.2 torr with the electrodes 75 
and 86 separated by 0.3 centimeters in the plasma-free acceleration 
region. The benefits of panel operation at this very low pressure will be 
described in relation to component operation in the following description. 
The U-shaped configuration of rearwardly disposed hollow cathode 60 creates 
an efficient collecting cavity that traps metastable atoms, ions and 
ultraviolet photons on the enclosing walls, liberating as a result copious 
electrons. Also, electrons are reflected inside the cavity to provide a 
"circulating" electron current to greatly enhance the probability of 
ionizing gas atoms. This circulating current permits operation at lower 
gas pressures than with a planar cathode. The hollow cathode effect is 
evident at low gas pressure as the negative glow, which normally covers 
each metal surface in a sheath at high gas pressure, merges into one large 
negative glow which comprises a plasma in the center of the hollow cathode 
at low pressure. The effect is shown schematically in FIG. 4 as plasma 62 
is surrounded by a cathode dark space 64. The hollow cathode may, e.g. be 
at a potential of approximately minus 300 volts. 
In the FIG. 4 configuration, one hollow cathode 60 is shown as supplying 
one plasma sac 76 with a flow of electrons 63 within one display element 
56. In the preferred embodiment of this invention, however, a single 
hollow cathode is not so limited in area, but preferably spans a 
predetermined number of rows and columns of display elements. Within a 
large-area hollow cathode configuration, a single plasma sac, or group of 
sacs, may draw electrons from the large area within the hollow cathode. 
The use of the hollow cathode in the preferred embodiment of this invention 
provides many benefits. In addition to providing copious electrons, the 
hollow cathode offers the fast switching characteristics of the planar 
cathode and provides efficient operation at higher current levels. Another 
benefit lies in the fact that the hollow cathode functions efficiently at 
very low values of "Pd" (gas pressure times distance) between the anode 
and the cathode which as mentioned, and with reference to FIG. 7, is, for 
example, 0.06 torr-centimeters in pure nitrogen. This ability of the 
hollow cathode to function at very low Pd values provides, in turn, a 
singularly desirable characteristic; that is, the ability to produce at a 
given time one or more high-density electron beams for high-energy 
bombardment of cathodoluminescent target elements without forming a gas 
discharge plasma in the acceleration section. The accelerating section 84 
is the region between modulating grid 75 and the accelerating 
electrode--ultor electrode 86. 
The break-down voltage between the hollow cathode 60 and performance 
enhancement electrode 72 is of the order of a kilovolt. This relatively 
high breakdown voltage exceeds the capabilities of standard transistor 
circuits where it may be desirable to selectively pulse groups of hollow 
cathodes in a display. This voltage can be reduced, however, to a few 
hundred volts by first priming the hollow cathode with an auxiliary, or 
priming discharge. A feasible means for producing such a priming discharge 
is by the use of the ignitor wire 61. As shown by FIG. 4, ignitor wire 61 
extends into the approximate center of cathode 60. The ionization of the 
gas in the vicinity of ignitor wire 61 as electrons orbit the wire and are 
trapped in the wire's radial field, effectively lowers the breakdown 
voltage to a few hundred volts. During operation, there is only a small 
trickle of current through ignitor wire 61, providing a "keep alive" 
current so that hollow cathode 60 remains in a primed condition wherein 
its breakdown voltage may be less than 400 volts, for example. Ignitor 
wire 61 may be energized by a pulse or by a steady flow of current. The 
use of an ignitor wire as shown is not mandatory in the preferred 
embodiment of this invention; other hollow cathode priming means may be 
used such as, for example, a point electrode located near the side of 
hollow cathode 60. 
This invention is no way limited to the use of the hollow cathode as an 
electron source. A planar cathode, for example, especially designed to be 
highly efficient, could as well be used. Also, in the interest of energy 
conservation, it is well within the scope of this invention to utilize 
other sources of electrons such as provided by field emission. 
Conventional thermionic cathodes, while increasing power consumption, 
could as well be used; however, the large thermal time lag would restrict 
the ability to switch groups of such cathodes on and off where it is 
desired to cause the plasma sac to move to different locations while 
scanning a display panel. Whatever type of cathode is used, it should 
preferably meet the performance standards set by the hollow cathode as 
described in the foregoing. 
Further with regard to FIG. 4, electron-transmissive anode 74 is located 
forwardly of cathode 60 and is spaced a predetermined distance from 
cathode 60 for receiving a relatively intermediate applied voltage. 
Constriction-forming means 66 is disposed between anode 74 and cathode 60, 
and defines at least one constriction as will be described infra. 
Performance enhancement electrode 72 is shown as being a distance D.sub.1 
from cathode 60, with anode 74 being at a greater distance D.sub.2 from 
cathode 60. Performance enhancement electrode is located contiguous to and 
parallel with constriction-forming means 66, which is shown as being an 
insulator, and receives a voltage intermediate to the relatively 
intermediate voltage on anode 74 and the relatively low voltage on cathode 
60. Constriction-forming means 66 defines at least one constriction 68 
registered with a construction in performance enhancement electrode 72. 
These registered constrictions are respectively associated with one or 
more display elements 56, as will be shown. The intermediate voltages 
cited, the distance D.sub.2, the very low gas pressure and the width of 
the registered constrictions have values effective to support a gas 
discharge plasma between cathode 60 and anode 74 and to cause a plasma sac 
76 to form in the plasma about constriction 68 in performance enhancement 
electrode 72 on the cathode side of constriction-forming means 66. The 
plasma sac by its nature generates and gathers electrons from a large area 
of hollow cathode 60 and accelerates them into registered constriction 68 
to form a concentrated electron beam therein. 
The performance enhancement electrode provides several functions. For 
example, it serves to stabilize plasma sac 76 in registered constriction 
68 by conducting electrons from a surrounding area to plasma sac 76, and 
thus discourages the formation of a sac in non-energized neighboring 
constrictions. Performance enhancement electrode also serves to prime the 
contained gas in the region of said constriction, thereby permitting a 
plasma sac 76 to be established in constriction 68 by application of a 
lower voltage on anode 74 than otherwise possible, and is believed to 
supply electrons to sac 76, as shown by arrows 77. The performance 
enhancement electrode thus appears to act as both an anode and a 
cathode--an anode which assists in establishing a gas discharge between 
cathode 60 and anode 74, and a cathode by supplying electrons to plasma 
sac 76. 
The performance enhancement electrode also contributes to the luminance 
uniformity of the display panel. Non-uniformities may appear in certain 
types of prior art displays as steady-state or moving spots, rings or 
striations of light. These undesired phenomena are attributed to the 
concentration of current on the surface of a small area electrode facing 
the cathode. Such concentrations are thought to be the result of very 
slight physical irregularities and/or discontinuities in the planar 
surface of the facing electrode. The performance enhancement electrode, 
according to the preferred embodiment of this invention, comprises a 
conductor having positive potential thereon relative to cathode 60, as 
noted. It may accomplish the alleviation of luminance non-uniformities by 
functioning as an equalizer of the electron current in the array of 
display elements for like element excitation. 
As noted, ignitor wire 61 initiates a gas discharge inside the hollow 
cathode. The performance enhancement electrode 72, functioning as an 
anode, initiates the discharge outside the hollow cathode 60 in order to 
prime the plasma sac. Thereafter, a plasma sac forms on the cathode side 
of constriction 68 when a positive potential of, for example, 150 volts is 
applied to anode 74. A plasma sac forms when the current demand through 
constriction 68 exceeds the current that can normally be conducted by the 
low-temperature plasma near constriction 68. As the voltage is raised on 
anode 74, a threshold current and voltage is reached wherein plasma sac 76 
suddenly forms. The threshold voltage will vary depending on gas pressure, 
gas constitution, the size of construction 68 and cell-wall geometry. Due 
to this threshold phenomenon, the plasma sac acts as a "switch" that can 
be scanned point-to-point in a display. The scanning means and method for 
the display panel that is the subject of this invention does not represent 
per se an aspect of this invention, but is described and claimed in U.S. 
Pat. No. 4,130,777. 
Primary electrons from cathode 60 ionize gas atoms and produce secondary 
electrons. These secondary electrons produce a plasma or "sea" of 
electrons that then act as the source of electrons from the plasma sac. 
Plasma sac 76, by its nature, gathers electrons emitted by hollow cathode 
60 and accelerates them into constriction 68 to form a concentrated 
electron beam 82 therein. Referring additionally to FIG. 5, plasma sac 76 
is comprised of an outer sheath 78 which comprises a negative space charge 
layer, and an inner sheath 80 which comprises a positive space charge 
layer. A potential of about 150 volts (in this example) exists between 
these two layers as shown by the associated 
relative-voltage-versus-distance curve of FIG. 5. Electrons are collected 
and accelerated from the outer sheath 78 into the sac by the 150-volt 
increase in potential. The 150-volt increase between the two sheaths 78 
and 80 provides an impedance-matching function necessary to increase the 
conductivity of the plasma within constriction 68, and thus allows a 
higher current to pass through the constriction. The conductivity of the 
plasma in the area outside the sac is lower than the conductivity of the 
plasma in the area inside the sac. Low conductivity corresponds to low 
plasma electron temperature while high conductivity corresponds to high 
plasma electron temperature, in this case. After electrons are accelerated 
from the outer sheath 78 into the sac, they may produce additional 
ionization within the sac itself. This also contributes to the higher 
current passing within constriction 68. 
In addition to being able to gather electrons from cathode 60 and 
accelerate them into a constriction to form a concentrated electron beam, 
plasma sac 76 offers another benefit in its ability to move from one 
constriction to the nearest energized neighboring constriction (not shown) 
very rapidly; e.g., in a period of less than 200 nanoseconds. This 
mobility is believed to be adequate for scanning a spot at conventional TV 
scan rates, which is 125 nanoseconds. 
An electron "drift space" can be of value in moderating the relatively high 
energy of several hundred volts of the electrons emitted by hollow cathode 
60. Electron energy can be lowered an order of magnitude to tens of volts, 
by means of the drift space 67. Drift space 67 of FIG. 4 represents the 
distance between hollow cathode 60 and constriction-forming means 66, 
which may, e.g., be about 0.75 inch. In the preferred embodiment of the 
invention, the drift space comprises the Faraday dark space. The provision 
of a drift space in display panels is described and claimed in U.S. Pat. 
No. 3,999,094 to Chodil, assigned to the assignee of this invention. 
The concentrated electron beam 82 emerging from constriction 68 passes 
through electron-transmissive anode 74 and electron-transmissive 
modulation grid 75, which is disposed between anode 74 and ultor anode 86. 
The beam is modulated by grid 75 which has thereon a time-varying signal 
which may range from zero volts through one hundred and fifty volts for 
example. The time-varying signal may represent television picture 
information. 
The concentrated electron beam 82 now enters acceleration section 84. The 
ultor voltage of ultor electrode 86 is a voltage in the range of many 
hundreds to tens of thousands of volts, establishing a high-voltage 
gradient in the plasma-free acceleration section 84. This relatively high 
voltage is, in any case, a voltage greater than any one of the discrete 
voltages or voltage differences existing in the plasma of display element 
56, such as the anode fall, cathode fall, positive column, negative glow 
column, or the voltage differential in the plasma sac. The ultor voltage 
is effective to straight-line accelerate the beam 82 of electrons 
(indicated by the symbol e) in a substantially collision-free path 
directly into high energy bombardment of cathodoluminescent target element 
88 disposed on transparent faceplate 90. 
Light-stopping means is provided for blocking from view light produced by 
the plasma, whereby the useful visible light produced by the panel is 
solely that produced by the high-energy electron bombardment of 
cathodoluminescent target element 88. The light-stopping means is here 
shown as including a light-reflective, electrically conductive film 87 (an 
aluminum layer, e.g.) disposed on cathodoluminescent target element 88. 
The film 87 may also comprise the ultor electrode. 
In accordance with this invention, anode 74 in cooperation with modulating 
grid 75 located down-beam of anode 74, provides for modulating the 
concentrated electron beam with a time-varying voltage to provide in 
cooperation with anode 74 full control of the beam wherein a range of 
differences in potentials between anode 74 and modulating grid 75 provides 
a related range of differences in electron current, and thus a related 
range of differences in luminous output from cathodoluminescent target 
elment 88. Tests have shown that a gray scale of 1000:1 or more is 
possible. 
When the potential on anode 74 is raised from zero to 150 volts, a plasma 
sac 76 forms. If the potential on modulating grid 75 is zero, however, 
there will be no current flow from constriction 68, and display element 56 
will effectively be biased to cut-off, and the associated 
cathodoluminescent target element 88 will not be activated. As the 
potential on modulating grid 75 is raised, increasing beam current will 
flow, the level of which is proportional to the voltage level on 
modulation grid 75 to the point of maximum current flow for maximum 
luminous output. Thus a full range of grays is provided by the two 
cooperating grids. In actuality, a lower limit to the gray scale exists 
because some residual ionization may produce a small amount of background 
light. 
For a display requiring little or no gray scale such as the alphanumeric, 
display element 56 may comprise only anode 74, without the presence of 
modulating grid 75. The use of a single electron-transmissive anode 74, 
which represents an aspect of the preferred embodiment, provides a 
monochrome image display relatively devoid of intermediate gray tones. By 
itself, the single anode cannot fully control beam current flow, so there 
is an abrupt threshold at which the plasma sac 76 forms, and substantial 
current is initiated. As a result, when only a single grid is used, a very 
limited range of grays can be obtained due to the high threshold level; 
that is, nominally a gray scale ratio of about 10 to 1. 
In the FIG. 4 embodiment, the components that form the plasma sac; i.e., 
constriction-forming means 66 and associated parts may lie near the 
positive column. Constriction-forming means 66 and associated 
plasma-sac-forming components could as well be located within the positive 
column, or, in the negative glow region of the gas discharge. 
Referring now to FIG. 6, a section of a full display panel structure 
according to this invention is shown, comprising a very low pressure, 
high-voltage gas discharge image display panel 92 having a rod-and-column 
array of display elements. Column 1 of the array comprises the left-most 
column of the display from the viewer's aspect. In TV applications, for 
example, the array may comprise five hundred columns across the width of 
the panel, with the columns extending from top to bottom of the display 
area. There also may be five hundred rows of display elements of the array 
extending the entire depth of the panel. In addition, there may be about 
fifty row-wise extending hollow cathodes, each providing for ten rows of 
display elements, as in this example. 
The primary components of the preferred embodiment 92 are listed from back 
wall 98 to front of the panel which comprises transparent faceplate 100. 
Located toward the back of the panel are the electron source means for 
producing at a given time at least one high-density electron beam, the 
means comprising the following components. Contiguous to back wall 98 is a 
rearwardly disposed array of large-area hollow cathodes 102, each spanning 
a predetermined plural number of rows (here ten) and columns (here all), 
and capable of supplying copious electrons at the aforedescribed 
predetermined very low gas pressure. Each hollow cathode is electrically 
discrete and receives a relatively low voltage; for example, minus 300 
volts. Hollow cathode 102 is comprised of top plate 128 and bottom plate 
130 which are electrically isolated from forwardly located adjacent 
structures by insulators 103. Each hollow cathode 102 is electrically 
isolated from adjacent cathodes by insulators 131 located therebetween. An 
ignitor wire 132 extends row-wise in the center area of each hollow 
cathode 102 for priming the associated cathode. 
The constriction-forming means comprises a barrier 104 located between 
anodes 106 and cathodes 102 and defines a plurality of narrow openings 
(constrictions) 105 each associated with one or more cathodoluminescent 
target elements 124 or "display elements. In the preferred embodiment of 
this invention, barrier 104 comprises a planar-form insulative means 
having at least one constriction therein for each display cell, and about 
which is selectively formed a plasma sac. (Plasma sacs are not shown by 
FIG. 6.) As described in the foregoing in relation to FIG. 4, the plasma 
sac, by its nature, gathers electrons from a large surrounding area of the 
associated hollow cathode 102 and accelerates them into the associated 
constriction to form a concentrated electron beam therein. In the 
preferred embodiment there are preferably about 250,000 constrictions 105 
in barrier 104. 
Performance enhancement electrode 108 is shown as being located contiguous 
to and parallel with barrier 104 and on the cathode side of the barrier 
and co-extensive without interruption across the width and height of panel 
92. Electrode 109 receives through a single input terminal 111 a voltage 
intermediate to the relatively intermediate voltage on anodes 106 and the 
relatively low voltage on hollow cathodes 102. In the preferred embodiment 
of this invention, performance enhancement electrode may have an opening 
in alignment with each of the constrictions 105 in barrier 104, with both 
in registration. About each of said registered constrictions, and on the 
cathode side of barrier 104, a plasma sac may form as described in the 
foregoing. 
Located forwardly of cathode 102 and performance enhancement electrode 109 
are column-wise oriented electron-transmissive anodes 106 arranged in 
columns as shown. Each anode 106 covers a column of constrictions 105 in 
barrier 104, also as shown. Anodes 106 are electrically discrete and 
receive a relatively intermediate applied voltage. The combination of 
cathode 102, anodes 106, and barrier 104, together with said intermediate 
voltage, the predetermined distance between cathode 102 and anode 106, the 
very low gas pressure, and the individual width of the registered 
constrictions having values effective to support a gas discharge plasma 
between cathode 102 and anodes 106 to cause a plasma sac to form in the 
plasma on the cathode side of barrier 104 about the constriction of any 
selected anode 106. 
A single column of row-select grids 108 lie in the same plane as anodes 106 
and provide for the selection of the row to be scanned in the panel. 
Barrier 104 defines a plurality of narrow constrictions 107 associated 
with row-select electrodes 108. It will be noted that each row-select grid 
extends row-wise only far enough to cover only one column of constrictions 
107 in barrier 104. A plasma sac for initiating row-wise scanning is 
started at the beginning of any row by the energizing of the associated 
row-select grid 108. 
Spacer 110 may be a planar-form insulator having a plurality of openings 
105A in registration with openings 105 of barrier 104. It will be noted 
that there are no constrictions in spacer 110 in registration with 
constrictions 107 in the column of row-select grids 108 as this column is 
not a light-emissive display element. 
Adjacent to spacer 110 are located modulation grids 112 arranged in columns 
extending vertically the full height of the panel and substantially 
parallel to anodes 106. The configuration of modulation grids 112 
comprises a trio of grids numbered 112R, 112G, 112B for modulation of 
triads of cathodoluminescent target elements respectively associated with 
red, green and blue picture information of a color television display 
panel. The high-density electron beam which is co-extensive with the 
predetermined group of cathodoluminescent target elements 124R, 124G, and 
124B is similarly divided into a plurality of beamlets 118, one for each 
element in said group. 
Ultor anode 120 is disposed contiguous to a layer of cathodoluminescent 
material which defines the target elements 124 on transparent faceplate 
100. Ultor electrode 120 receives a predetermined relatively high ultor 
voltage; that is a voltage in the range of many hundreds to tens of 
thousands of volts; preferably four to twenty kilovolts. Ultor anode 120 
is separated by a predetermined spacing from modulation grid 112 to define 
an acceleration section 115 therebetween. The spacing is so small that at 
the predetermined very low pressure and at the cited ultor voltage, no gas 
discharge plasma can possibly occur in acceleration section 115. The ultor 
voltage establishes a high-voltage gradient in the plasma-free 
acceleration section 115 which is effective to straight-line accelerate 
beamlets 118 in substantially collision-free paths directly into 
high-energy bombardment of target elements 124 disposed on the inner 
surface of transparent faceplate 100. 
The plurality of electron-transmissive modulating grids 112R, 112G, and 
112B are located down-beam of anode 106 and are respectively associated 
with a group of target elements 124R, 124G, and 124B for effectively 
dividing the beam into a like plurality of beamlets 118. Beamlets 118 are 
individually modulated with a like plurality of time-varying voltages to 
provide in cooperation with anode 106 full control of beamlets 118, 
wherein a range of differences in potential between anode 106 and 
modulating grids 112R, 112G, and 112B provide a related range of 
differences in electron current in each of beamlets 118. Thus, a related 
range of differences in luminous output from the cathodoluminescent target 
elements 124R, 124G, and 124B respectively associated with a plurality of 
display elements is achieved. 
The triad of target elements 124R, 124G, 124B represents one group of such 
elements in a color display. In a panel comprising 500 rows and 500 
columns of such elements, each of which comprises three discrete colors, 
there would be a total of 750,000 target elements. In a simple monochrome 
image display capable of intermediate gray tones, modulation grids 112 
would be one continuous row-wise and column-wise extending grid. For a 
simpler monochrome display of limited intermediate gray tones (as 
previously described), modulation grids 112 would not be used in the panel 
and the sole grid means would comprise the electron transmissive anodes 
106, one for each column, for providing a monochrome image display 
relatively devoid of intermediate gray tones. (The row-select grids would 
of course be utilized in a monochrome display.) Similarly, the triads of 
target elements 124R, 124B, and 124G respectively associated with red, 
blue, and green picture information would instead each comprise a 
monochrome light-emitting phosphor disposed on an inner surface of 
transparent faceplate 100. 
Spacer 110 is shown as being spaced from faceplate 100 by a plurality of 
panel support members 114. There may be one panel support member 114 
disposed between each of the columnns, or, the support members may be 
dispersed, with many columns between each support member. These support 
members may be row-wise extending, or, a combination of row- and 
columnwise-extending members. These support members, together with the top 
and bottom plates exemplified by 128 and 130 of the row-wise extending 
hollow cathodes and the insulators 103 against which they abut, provide 
the back-to-front internal bridging support which makes the display panel 
self-supporting against atmospheric pressure. The material comprising 
panel support members 114 may, for example, be a high-strength ceramic. To 
prevent build-up of electrostatic charges, panel support members 114 may, 
for example, be coated with a conductive material 116 having a very high 
electrical resistance. 
In the preferred embodiment implementing the applicant's invention, panel 
support members 114 provide a spacing of 0.125 inch between spacer 110 and 
faceplate 100. The spacing dimension is dependent primarily upon the 
potential on ultor anode 120 which may be in the range of many hundreds to 
tens of thousands of volts, for example, and is a function of the gas 
pressure within display panel 92. The gas pressure-distance cited as an 
example in the foregoing, that is, 0.06 torr-centimeters for nitrogen and 
a spacing of 0.125 inch, provides a high-voltage breakdown resistance of 
the interspace in the range of four to twenty kilovolts depending on gas 
mixture, field emission points, and low work function surfaces that may 
liberate electrons and initiate a gas breakdown. Of all of these factors, 
a low value of Pd is of primary importance. Any value of Pd selected must 
be such as to prevent the propagation of a gas discharge forward of the 
preferred discharge area as too high a pressure could result in an 
undesired secondary discharge between ultor anode 120 and modulation grids 
112. As a result, it could not be possible to maintain a high enough ultor 
anode voltage for adequate excitation of the cathodoluminescent target 
elements. 
With regard to general structure, back wall 98 may comprise a material such 
as glass or other insulative material that can lend strength and rigidity 
to the panel 92. Back wall 98 serves both as a component of the outer 
envelope 92, and as a support member for the plurality of row-wise 
extending hollow cathodes 102. 
The material from which the plates of hollow cathode 102 are preferably 
made comprises thin metal strips having a thickness of some two to five 
mils, or alternately, thick film or thin films disposed on insulative 
walls. If metal strips are used, metals having an expansion co-efficient 
substantially the same as that of glass should be used (assuming that the 
panel enclosure is made of glass); also, the metal may be hermetically 
sealable with glass, it must have a low work function, and be resistant to 
sputtering. Good results have been obtained with plates made with metal 
designated as Carpenter 42-6, available from Carpenter Technology, Inc., 
of Reading, Pa. 
Dimensions of significance to the proper operation of hollow cathode 102 
include the depth of top plate 128 and bottom plate 130 as shown by the 
distance "D" in FIG. 6. Generally, the spacing "H" between top plate 128 
and bottom plate 130 is selected to be between 0.1 and 3.0 times the 
length of the cathode fall of a planar (as opposed to hollow) cathode made 
of the same metal as plates 128 and 130 and operating in an atmosphere of 
the same gas and at the same pressure at which the cited plates are 
operated. In the FIG. 6 embodiment, the spacing "H" between top plate 128 
and bottom plate 130 is approximately equal to the height of ten rows of 
picture elements in a fifty-inch diagonal measurement display. The depth 
"D" of hollow cathode 102 is preferably approximately 11/2 inches and the 
distance "H" between top plate 128 and bottom plate 130 is preferably 
approximately 3/4 inch, in this embodiment. 
Light-stopping means 121 may comprise a film of aluminum evaporated on the 
inner surface of faceplate 100. Since such a film is metallic and hence 
electrically conductive, it could also comprise the ultor anode. 
With regard to display panel fabrication, techniques well-known to those 
skilled in the art may be used. For example, barrier 104 and spacer 110 
and the openings therein may be fabricated by means such as photo-forming 
or thick-film screening. Also, other well-known techniques such as glass 
molding, etching, shaping and perforating may be utilized. 
With regard to the composition of the grids, insulators and spacers 
illustrated by the several figures, anodes 106 and modulating grids 112 
may be comprised of an electrically conductive electron-transmissive mesh 
or grid fabricated from a material such as a stainless steel alloy. 
Barrier 104 and spacer 110 may be comprised of a dielectric material such 
as a ceramic with a thickness range of, for example, two to twenty mils. 
Barrier 104 and spacer 110 serve to define the geometry of the electron 
beam, separate the grids, and impart structural strength to the panel. 
Openings or constructions such as those shown by 105, 105A and 107 of FIG. 
6, may as well be in the form of circles, ovals, slots, or, rectangles as 
shown, and be either horizontally or vertically oriented. The rectangular 
configuration of the openings or constructions as shown, represents the 
preferred embodiment of the invention, as this configuration is deemed to 
be one most suitable for the activation of target elements comprising 
color. 
A construction technique that provides for great accuracy of registration 
and inherent supportive strength of the insulative structure, coupled with 
ease and economy of manufacture, involves building up insulative sections 
from glass filaments. This technique does not constitute an aspect of this 
invention, but is described and claimed in U.S. Pat. No. 4,099,082. The 
resulting structure resembles a log cabin. The glass filaments are strung 
on an external harp-like jig that keeps the filaments taut and in place 
until they have been rigidified, as by spraying with a suspension of 
low-melting point solder glass (frit). The resulting structure is fired to 
a temperature wherein the frit reaches its melting point and flows between 
the filaments, which have a higher melting point, to fasten them together 
permanently. A structure has been fabricated which tests show is able to 
withstand a pressure of over 2,000 pounds per square inch with dimensions 
maintained to within 0.002 inch during the firing cycle. 
Concerning the selection of material for the panel generally, it is 
preferable that all materials have relatively the same coefficient of 
expansion. This coefficient is in turn based upon the flow temperature of 
the glass frit used to solder the panel sections together during the 
fritting cycle. All parts must expand and contract in concert to prevent 
cracking of glass and ceramic members, and to prevent component separation 
or changes in spacings. Expansion-compatible material suitable for panel 
construction are well known. Two widely used and representative materials 
that would lend themselves to construction of a display panel according to 
the preferred embodiment are glass of the soda-lime type, and metal 
sections of the aforementioned Carpenter 42-6. 
With regard to the composition of the cathodoluminescent material, the 
following commercially available phosphors are representative of those 
suitable for the electron-acceleration voltage values of the preferred 
embodiment of this invention: 
EQU RED Y.sub.2 O.sub.2 S:Eu.sup.+3 
EQU GREEN La.sub.2 O.sub.2 S:Tb.sup.+3 
EQU BLUE Sr.sub.5 Cl(PO.sub.4):Eu.sup.+2 
The weight of a self-supporting fifty-inch diagonal measure image display 
panel according to this invention has been determined to be between fifty 
and fifty-five pounds, a weight which compares most favorably with a fifty 
pound weight of the conventional non-self-supporting twenty-five inch 
color television picture tube which, it will be noted, has only 
one-quarter the image display area. 
The structure of the preferred embodiment of the invention illustrated in 
FIG. 6 and described in the foregoing may be embodied in a 
non-self-supporting display panel as shown by FIG. 8. In this embodiment, 
the display panel 92 structure shown by FIG. 6 is disposed within two 
facing standard picture tube faceplates 219 whose mating edges 220 are 
sealed to form an air-tight envelope. A row of hollow cathodes 221 are 
shown along with a faceplate 223 and associated grids, insulators and 
spacers as have been described. 
FIG. 9 shows another embodiment of the invention, indicated in highly 
schematic form by 220, enclosed within a single television picture tube 
faceplate 222. In this aspect of the preferred embodiment, electrons are 
shown as being supplied by a planar cathode 224 such as a gas discharge 
planar cathode, field emission cathode, or thermionic cathode. Back wall 
228, which may be of honeycomb structure for strength and rigidity, 
hermetically seals the back section of faceplate 222 to complete the 
structure. 
A brightness-optimized cathodoluminescent gas discharge image display panel 
suitable for the display of television picture images has been described 
and illustrated. The preferred embodiment of the invention as disclosed is 
an example of only one of many possible applications. It can be 
effectively utilized for the display of visual information of several 
kinds. Because of its space-saving, flat configuration and light weight, 
it can replace the bulky, heavy cathode ray picture tube display in 
locations where space is at a premium, such as in an aircraft or 
spacecraft cockpit, as well as in the homes of consumers. The invention 
brings to such applications the same benefits as high brightness, 
luminance efficiency, high resolution, and excellent contrast of the 
standard television picture tube, as well as a range of display area 
dimensions much greater than that supplied by the picture tube. 
Other changes may be made in the above-described apparatus without 
departing from the true spirit and scope of the invention herein involved. 
For example, in the FIG. 5 embodiment, the constriction means 66 is 
illustrated as taking the form of an insulator having one or more openings 
therein--i.e., physical constrictions. It is contemplated that the 
constriction means could as well be means which define an electrically 
constricting field, rather than an opening or other physical constriction 
in a structural member. 
FIG. 10 is analogous to FIG. 5 and illustrates such an alternative 
embodiment. It will be noted that the constriction 68 of FIG. 5 has been 
replaced by a field-forming electrode 81, here shown in cross-section as a 
pair of wires. By the application of an appropriate voltage to electrode 
81, which, for example, may be in the order of one hundred volts or less, 
a constricting field is formed which contributes to the formation of 
plasma sac 76. The effect of the constricting field in constricting the 
gas discharge plasma is analogous to the effect of the insulator shown by 
FIG. 5 with its narrow constriction. The field-forming electrode 81 could 
as well be in the form of bars, rods, plated-on sections, metallized 
glass-filaments, or any other configuration suitable for the purpose of 
electrically imposing a constricting field upon the plasma established 
forwardly of the cathode. 
It is intended therefore that the subject matter in the foregoing depiction 
shall be interpreted as illustrative and not in a limiting sense.