Shielded hollow cathode electrode for fluorescent lamp

A hollow cathode electrode, particularly useful in fluorescent lamps, comprises an outer metal sleeve, an inner metal sleeve disposed within the outer sleeve and an emissive mix disposed on the inner sleeve. In one embodiment, the inner sleeve is a folded cylinder having a square cross section disposed within a circular cylinder. The object of the present invention is to prevent heat loss from the inner sleeve and to minimize sputtering. The electrode of the present invention may also include a third exterior sleeve surrounding but not contacting the interior sleeve or sleeves.

BACKGROUND OF THE DISCLOSURE 
This invention relates to electrodes, and, more particularly, to hollow 
cathode electrodes for use in fluorescent lamps. 
Conventional fluorescent lamp electrodes contribute to darkening of the 
ends of the lamp. This phenomenon reduces the luminous efficacy of the 
lamp as a function of lamp running time. The material deposited on the 
walls of the lamp is typically a mixture of evaporated barium and other 
material sputtered from the electrode. Additionally, this phenomenon also 
limits the life of the lamp because of the eventual removal of emission 
mix from the electrodes so that starting becomes difficult for the lamp 
ballast. 
Hollow electrodes are generally operable in the so-called hollow cathode 
discharge mode of operation and such hollow electrodes offer several 
advantages. First, these electrodes generally produce less darkening of 
the lamp ends and a longer lamp life with better lumen maintenance over 
the life of the lamp, than do lamps employing heated filament cathodes. 
The reason for this advantage is the containment of sputtered and 
evaporated material within the hollow portion of the electrode. For the 
case of barium emissive mixes, this is especially useful since it provides 
a low work function and a correspondingly low electrode fall voltage, 
which, in turn, reduces sputtering. The work function is a measure of the 
energy required for removal of electrons from the surface of the 
electrode. 
Simple cylindrical sleeve electrodes having emission mixes disposed on 
their interior surfaces are known in the prior art. In particular, A. 
Bouwknegt and A.G. Vanderkooi have reported on such structures in 
Proceedings of the First International Conference on Gas Discharges, 1971, 
pg. 217. In this paper, the authors indicated that desirable goals would 
include 50,000 lamp starts with 12,000 hours of average lamp life. 
However, using only the simple hollow electrode, the above authors 
indicated that such lamps operate for only 12,000 hours without serious 
depreciation, the lamp having incured only 6,000 starts. In contrast, 
conventional presentday fluorescent lamps generally exhibit an average 
life in excess of approximately 20,000 hours. 
Many of the problems associated with poor starting and shortened lamp life 
are related to the simple cylindrical electrode employed. In particular, 
it is first noted that the opposite electrode surface from that which 
holds the emission mix, namely, the outside of the cylinder, is exposed 
for thermal radiation. Since emission requires a temperature of 
approximately 800.degree. C. to approximately 900.degree. C., this results 
in a considerable rate of energy loss. The cathode fall voltage must 
increase and/or current must increase to supply this energy at the 
electrode surface. Additionally, the starting of the discharge involves 
some deterioration of the end of the cylinder. Furthermore, lamp starting 
tends to destroy emission mix and the high operating temperature can lead 
to punch-through of the cylinder. 
It should also be noted that it is highly desirable that any electrode, 
particularly fluorescent lamp electrodes, be configured in a structural 
arrangement which promotes rapid, facile and economical assembly. 
Accordingly, improvements in the conventional hollow cathode electrode 
design should not generally preclude rapid and economical manufacture. 
SUMMARY OF THE INVENTION 
In accordance with a preferred embodiment of the present invention, a 
hollow cathode electrode, especially for use in fluorescent lamps, 
comprises an outer metal sleeve, an inner metal sleeve disposed within the 
outer sleeve and substantially coaxial therewith and an emissive mix 
disposed on the inner sleeve. In one embodiment of the present invention, 
the inner sleeve possesses a rectangular cross section and is preferably 
formed by folding a strip of metal into a parallelepiped shape. In this 
embodiment, the outer sleeve is preferably a circular cylinder into which 
the folded inner sleeve is inserted. In accordance with still another 
embodiment of the present invention, there is included a third cylindrical 
sleeve supported in either a contacting or non-contacting relationship and 
surrounding the innermost sleeve. This provides a doubly shielded design. 
Accordingly, it is an object of the present invention to provide a long 
life, low sputtering fluorescent lamp electrode. 
It is also an object of the present invention to improve fluorescent lamp 
efficacy and to reduce lamp end darkening. 
It is a still further object of the present invention to minimize radiated 
heat loss from hollow cathode electrodes.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
Before a discussion of the operation of the present invention is 
undertaken, it is desirable to discuss its essential construction 
features, particularly as illustrated in FIGS. 1-3. In FIG. 1, there is 
shown an outer cylindrical sleeve 10 having inner sleeve 12 disposed 
inside it in a substantially coaxial arrangement. Both sleeves are 
preferably metallic and inner sleeve 12 is preferably coated on the inside 
thereof with emissive mix 16, more particularly described below. Inner 
sleeve 12 preferably comprises nickel and is in the form of a folded metal 
sheet having a thickness of approximately 2 mils. FIG. 1 also more 
particularly illustrates the presence of plasma discharge 26 which is 
particularly illustrative of the hollow cathode discharge mode of 
electrode operation. Furthermore, sleeve 10 may also include end wall 24 
which acts to partially close sleeve 10 to further promote its function as 
a heat shield. Lastly, outer sleeve 10 is supported on 
electrically-conductive rod 18 and is fastened thereon by spot welding, 
for example, this being the preferred mode of fastening herein. However, 
any convenient means of support may be employed. FIG. 2 illustrates an end 
view of the electrode embodiment shown in FIG. 1 and particularly 
illustrates the ease of assembly of this electrode structure and the fact 
that inner sleeve 12 is easily formed from a folded strip of material and 
is readily insertable into outer sleeve 10 in a fashion which tends to 
hold sleeve 12 in a fixed position therein by a friction fit. 
For purposes of description of the present invention, and for 
interpretation of the claims herein, it should be particularly noted that 
the term "cylinder" as employed herein refers to the general mathematical 
meaning of this term, and is not limited to the special case of right 
circular cylinders. The cylinders themselves might be right-elliptical 
cylinders, or the right-square cylinder illustrated in FIG. 3. More 
particularly, with reference to FIG. 2, it is noted that there is shown a 
square cylinder disposed within a circular cylinder. However, without 
departing from the essence of the present invention, it is also possible 
to dispose a right circular cylinder within a right-square cylinder or 
even an elliptical cylinder within a square or circular cylinder or 
vice-versa. 
The emitting material 16 may be any conventionally-employed material 
include lanthanum hexaboride or barium calcium aluminate but more 
preferably comprises the triple oxide of barium, calcium and strontium, 
because this mix is inexpensive and provides a low work function. A 
suitable inner emitting surface is that prepared for radio vacuum tubes 
where the triple carbonate of barium, calcium and strontium is rolled onto 
a nickel base, preferably active nickel. It is also possible to add 
zirconium metal powder or zirconium hydride to the mix. Nickel powder may 
also be added to improve conductivity through the oxide layer. 
Tests employing the electrode structure exhibited in FIGS. 1-3 have been 
conducted. The electrode comprises a square inner cylinder of thin nickel 
having rolled emission mix on its interior surface. This inner cylinder 
was slipped into a second thin nickel sleeve which acted as an outer 
radiation shield. In particular, while the inner cathode surface operated 
at a temperature of approximately 800.degree. C., the outer sleeve 
operated at a temperature of only approximately 500.degree. C. These 
electrodes have operated for over 500 hours, supporting a discharge in a 
2.5 torr mixture of argon and mercury with the mercury being at a pressure 
corresponding to room temperature. The overall discharge voltage increased 
by about 0.5 volts from an initial voltage of 10.5 volts. Even though 500 
hours appears to be a short time relative to a desired lifetime of at 
least 20,000 hours, experience indicates that the negligible depreciation 
of the electrodes predicts a long life for this electrode design. It was 
also observed that the walls of the discharge envelope were not darkened 
with deposits as would be the case if conventional "stick" electrodes were 
operated for this length of time under the same conditions. 
With respect to lamp operation, it is to be noted that, when starting, arc 
current concentrates on the outer cylinder and after this cylinder is 
heated to a dull red color, arc current moves inside the rectangular 
cylinder to initiate operation in the diffuse-emitting hollow cathode 
mode. This process takes approximately 3 seconds. During this time, and 
after a large number of starts, there is some sputtering away of material 
from this outermost surface. However, this is not normally the emitting 
surface and long cathode life is still exhibited. Moreover, during 
continuous running, there is some diffusion of barium outside of the inner 
cylinder and onto the outer cylindrical surface. In fact, this is helpful 
for subsequent lamp starting and, accordingly, is not an undesirable 
feature. 
The above tests were conducted at a discharge current of approximately 1 
ampere since lower currents promote increased cathode fall voltage. This 
is due to heat loss from the electrode. While 1 ampere is required for 
some fluorescent lamps, a more typical current is approximately 430 
milliamperes. Thus, to get this lower current with a low cathode fall 
voltage, additional energy conservation is desirable. Accordingly, for 
such fluorescent lamps and lamp applications, the structure illustrated in 
FIGS. 4-6 is preferred. In FIGS. 4-6, inner sleeve 12 and sleeve 10 are 
preferably as previously described. Additionally, sleeve 10 is held within 
third sleeve 14 which acts as an additional radiation shield. In this 
embodiment, sleeve 10, which was previously described as an outer sleeve, 
is actually an intermediate sleeve with sleeve 14 being the outermost 
sleeve. Sleeve 10 is held in a fixed position within outer sleeve 14 by 
any convenient means. For example, indentations 22 in outer sleeve 14 may 
be provided to achieve this purpose. In any event, whatever means of 
support for sleeve 10 is provided, it is desirable that sleeves 10 and 14 
exhibit minimal physical contact, so as to reduce thermal conductive 
losses. In the embodiment shown, sleeves 10 and 12 project slightly from 
outermost sleeve 14. The entire assembly comprising sleeves 10, 12 and 14, 
and emissive mix 16 may be supported on conductive rod 18' such as that 
shown in FIG. 4. FIG. 5 provides an isometric view of the structure shown 
in FIG. 4 and FIG. 6 provides an end view of the same structure more 
particularly illustrative of indentations 22 for support of sleeve 10. 
Additionally, any of the sleeves illustrated in FIGS. 4-6 may also include 
a rear wall portion for further containment of thermal wavelength 
radiation. A similar structure 24 is shown in FIG. 1. 
Certain features of the structures illustrated in FIGS. 4-6 are worthy of 
note. In particular, it is to be noted that each sleeve is readily 
manufacturable and the entire assembly is readily put together simply by 
sliding the sleeves into one another. Furthermore, it is preferable that 
the inner structure protrude somewhat from the outer sleeve 14 in order 
that the inner cylinders be the starting surface and also to collect anode 
current for additional heating. Inner rectangular sleeve 12 also 
preferably contacts intermediate sleeve 10 at least at the tip of the 
intermediate sleeve so that the arc can transfer to the inner emitting 
surface. However, at other points this contact is minimized to reduce heat 
conduction. Furthermore, heat conducting along support rod 18' is 
minimized by using thin wires, preferably comprising nickel, or other 
weldable metal. Additional shields may also be provided particularly if 
the increasing cost and complexity is justified by the intended use. 
The electrodes illustrated so far in FIGS. 1-6 are primarily applicable to 
so-called instant start fluorescent lamps where application of auxiliary 
heating is not possible. However, with only minor modifications, the 
electrode structure of the present invention may be employed in so-called 
rapid start fluorescent lamps in which auxiliary heating is heating is 
employed. If such preheating or auxiliary heating is required, there are 
at least two ways to provide this function. In a first approach, the 
emitting surface or intermediate sleeve is made from a long thin ribbon 
which serves as a conductor to provide ohmic heating of the cathode 
surface. It is formed into a sleeve and ends attached to the lead-in wires 
for current continuity. In a second approach, the structure is heated with 
an indirect heater such as is done with the cathodes of radio tubes. In 
this case, heating wires are inserted between the two inner cylindrical 
sleeves and connected to lead-in/support wires 18 and 19 as shown in FIG. 
7. The embodiment illustrated in FIG. 7 is similar to that shown in FIGS. 
4-6 except that heater wires 20 for cathode heating are shown disposed 
between the two innermost sleeves. This indirect heating has the advantage 
that higher voltage (approximately 2-3 volts) and a lower current (less 
than 1 ampere) can be chosen for the heater. 
The entire electrode structure described herein may be automatically 
assembled and employs inexpensive, already developed emissive mix 
materials. The activation of these materials during lamp manufacture is 
most easily done by induction heating. However, if residual gases are 
sufficiently high enough in pressure to cause breakdown and it is not 
desired to spend assembly time to remove them by pumping, activation of 
these materials is accomplished through use of the auxiliary heater 
illustrated in FIG. 7, or by focussed laser radiation. 
For the embodiments illustrated in FIGS. 4-6, it has also been particularly 
noted that, for low current lamps, there is a certain amount of barium 
diffusion, either over surfaces or through the discharge gas to the outer 
and inner surfaces of the outer shield and that the small electron 
emission from these surfaces can reduce the current in the hollow cathode 
itself. Current reduction in turn tends to raise the electrode drop which 
increases current flow to the shield resulting in a pinkish glow due to 
the excitation of argon, which excitation is made possible by the higher 
energy electrons accelerated by the cathode fall voltage. Accordingly, for 
lamps operated at such current levels, it is not desirable to operate the 
electrodes of the present invention with all electrode surfaces at the 
same electrical potential even though a partially insulated structure 
tends to reduce manufacturing ease as well as the number of separate parts 
employed. However, for such lamps electrical insulation of the inner and 
outer sleeves is readily accomplished. The embodiments illustrated in 
FIGS. 8 and 9 more particularly illustrate such an embodiment for which 
insulation is provided. In particular, in FIG. 8 support 21 is disposed 
through insulating glass rod 28 so as to hold innermost sleeves 10 and 12 
in a non-conducting, substantially coaxial relationship with outer sleeve 
11. This structure is shown in more detail in FIG. 9 which particularly 
illustrates the presence of folded triangular flaps 23. Supporting lead 21 
may be disposed through aperture 27 to preserve its insulating function. 
This structure prevents, or at least renders negligible, any discharge 
current to the shield and forces essentially all the current into the 
cathode interior. Shield 11 assumes a sufficiently negative potential to 
repel electrons in excess of the positive ion current which flows through 
it. If these insulated and shielded electrodes are found to be detrimental 
to the starting of a lamp in a particular lamp design, due to increased 
electrical potentials they may acquire during starting, the shields may be 
grounded to the cathode proper through a sufficient resistance to impede 
current flow but yet to control their potential. Sleeves employed in the 
cathode structure of the present invention may be insulated from one 
another in any convenient manner. However, it is to be noted that the use 
of wire suspensions from a glass wire press are particularly useful and 
this technology has been proven successful in the manufacture of cathode 
ray electron guns and for dynodes in photomultiplier tubes. 
It is also to be noted that outer shield 11 in FIGS. 8 and 9 may be 
provided with slit 29. Slit 29 is provided in the event of activation of 
emissive material 16 by induction heating. Slit 29 in the outer shield 
prevents eddy current heating there by interrupting the current path but 
instead forces induced currents to flow in inner structure 10. 
From the above, it may be appreciated that the present invention provides 
an electrode structure for fluorescent lamps which does, in fact, reduce 
sputtering, increase lamp efficacy and life, and yet is, nonetheless, 
easily manufactured and assembled from materials well known in the 
electrode arts. Moreover, it is seen, particularly with the insulated 
shield embodiments, that the present invention is employable not only with 
fluorescent lamps operating at high current levels, but also in lamps 
operating at low current levels. 
While the invention has been described in detail herein, in accord with 
certain preferred embodiments thereof, many modifications and changes 
therein may be effected by those skilled in the arts. Accordingly, it is 
intended by the appended claims to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.