Electron multiplier with ion bombardment shields

A chain of planar dynodes is divided into two groups which are spaced from and substantially parallel to each other. A cathode which is capable of emitting electrons upon ion bombardment is at one end of the dynode chain. An envelope encloses the dynodes and the cathode. Also enclosed by the envelope are a plurality of shields. Each shield is located so as to prevent gas ions, present within the envelope, from striking the dynodes, while allowing the ions to strike the cathode.

BACKGROUND OF THE INVENTION 
The present invention relates to electron multipliers which employ ion 
feedback and more particularly to such devices having means for preventing 
the ions from striking the electron emissive regions of the dynodes of the 
multiplier. 
Various types of cathodoluminescent image display devices have been 
recently suggested employing ion feedback electron multipliers as electron 
sources. Such devices incorporate flat electron multipliers, each formed 
by a dynode chain having a cathode at one end and some form of 
cathodoluminescent screen at the other end. In these devices, the 
electrons in the first multiplier stages are amplified forming many 
electrons in the final stage, which in turn strike residual gas molecules 
in the atmosphere of the device converting the molecules to positive ions. 
These ions travel to the cathode which is coated with a secondary emissive 
material. The ions bombard the cathode emitting additional electrons which 
travel to the first stages of the electron multiplier completing a 
feedback loop. 
The device can achieve self-sustained electron emission if the gain of the 
feedback loop gain exceeds one. In this case, the device may be turned on 
and off by controlling the electrical potentials applied to the dynodes or 
the cathode so as to switch the feedback loop gain above or below one 
respectively. It has been found, however, that the ions can strike the 
dynodes in the electron multiplier, as well as the cathode. When the ions 
strike the dynodes, additional electrons are given off, creating problems 
with respect to the on-off control of the device. The control of the 
device may be lost if the ions strike and create secondary electrons on 
higher stage dynodes than the dynode or the cathode whose potential has 
been changed. These secondary electrons will not be suppressed by the 
switching of lower dynode or cathode potentials. Thus a sustained electron 
emissive feedback loop may exist in the final stages which can not be 
turned off by potential changes at the cathode or the early multiplier 
stage dynodes. This control problem is most serious in certain 
applications where one desires to control the device only through 
potential changes on the cathode. In this situation, ion bombardment of 
the first multiplier stages can cause loss of cathode control. Such 
control problems can be avoided if this ion bombardment of the dynodes can 
be eliminated. 
SUMMARY OF THE INVENTION 
An ion feedback electron multiplier comprises a chain of planar dynodes 
divided into two groups. The groups are spaced from and parallel to each 
other. A cathode capable of emitting electrons upon ion bombardment is at 
one end of the dynode chain. The cathode and the dynode chain are enclosed 
by an envelope. Also within the envelope is means for preventing ions from 
bombarding at least some of the dynodes. Such a means, however, do not 
significantly inhibit the electron flow through the dynode chain or the 
ion flow to the cathode.

DETAILED DESCRIPTION OF THE INVENTION 
With initial reference to FIG. 1, an electron multiplier, generally 
designated as 10, has a tubular envelope 12. Enclosed by the envelope 12 
is a cathode 14, a dynode chain formed by nine planar dynodes 21-29, a 
plurality of ion shields 31-37 and an electron collection means 16. Each 
of the dynodes 21-29 is planar and has its surface coated with a material 
which is secondary electron emissive, such as MgO. The dynodes 21-29 are 
divided into two groups which extend along the longitudinal axis of the 
envelope 12 and are spaced from and parallel to one another. The first, 
third, fifth, seventh and ninth dynodes 21, 23, 25, 27 and 29 
respectively, are in one group and the second, fourth, sixth and eighth 
dynodes 22, 24, 26 and 28 are in the other group. The dynodes 21-29 form 
an electron multiplier dynode chain. At one end of the dynode chain is the 
cathode 14 whose surface is coated with a material which emits electrons 
upon ion bombardment, such as MgO or BeO. At the opposite end of the 
dynode chain is a means for collecting electrons 16, such as a 
cathodoluminescent screen. Conventional terminology refers to each dynode 
as a stage of the multiplier, with the adjectives early or later referring 
to the stage's proximity to the cathode. 
Positioned between adjacent dynodes in each group is a shield 31-37. Each 
shield extends into the space between the two groups of dynodes. The 
distance with the shields extend between the two groups of dynodes is 
dependent upon a number of factors, such as the distance between the two 
groups of dynodes and the inter-dynode distance. However, the distance 
which the shields extend should be kept to a minimum in order to protect 
the adjacent dynode from ion bombardment without appreciably inhibiting 
either the flow of electrons from dynode to dynode or the flow of ions 
back to the cathode 14. Most of the ions striking a dynode have a 
trajectory which nearly grazes the dynode immediately preceding the struck 
dynode. Therefore, these ions can be easily intercepted by shields that 
extend from each plane of the dynode surfaces 18 approximately 1/6 the 
distance between the two groups of dynodes. 
The potential applied to each dynode in a conventional multiplier increases 
as the collection means 16 is approached. The shields may be maintained at 
various potentials in relation to the dynode potential. In the embodiment 
shown in FIG. 1, the shields 31-37 are maintained at the potential of the 
dynode directly opposite the shield as indicated by voltages V.sub.1 
-V.sub.9. Specifically, the first shield 31 is maintained at the potential 
of the second dynode 22 and the second shield 32 is maintained at the 
potential of the third dynode 23 and so on. In an alternative potential 
distribution, the shields 31-37 are maintained at the potential of the 
adjacent dynode which is farther from the cathode 14. In this second 
variation, the first shield 31 is maintained at the potential of the third 
dynode 23. The second shield 32 would be maintained at the potential of 
the fourth dynode 24 and so on through the dynode chain. 
FIG. 2 shows an alternate, preferred embodiment of the present invention. 
In this embodiment, an electron multiplier designated 40 has an envelope 
42 enclosing a cathode 44, a plurality of dynodes 51-59 and an electron 
collection means 46. The dynodes are divided into two spaced parallel 
groups. The electron multiplier 40 is similar to the multiplier 10 in FIG. 
1, except that the ion bombardment shields are incorporated onto the 
dynode structure. The first and second dynodes 51 and 52 can be planar and 
similar to the first and second dynodes 21 and 22 in the device 10 in FIG. 
1. The remainder of the dynodes 53-59 have an L-shaped structure. The 
short portion 48 of the L-shaped dynodes 53-59 form the ion bombardment 
shields and project into the space between the parallel groups of dynodes. 
The elongated portion of the L structure forms the emissive surface of the 
dynode. The dynodes 51-59 may be coated with a secondary emissive 
material, such as MgO. Since each ion bombardment shield is incorporated 
into the structure of a dynode, the shield is maintained at the potential 
of that dynode. This potential distribution is equivalent to the second 
variation described in reference to the multiplier 10 of FIG. 1. 
As noted before, the distance which the ion shields 48 extend into the 
region between the two groups of dynodes depends upon a number of factors. 
The following specific example is illustrative of the dimension 
proportionality between the various elements. With respect to the 
embodiment of FIG. 2, the distance between the two groups of dynodes may 
be about one millimeter. The shields may extend 0.17 millimeters from the 
dynode surface 50. Each dynode may have a width of about one millimeter 
and be spaced about 0.4 millimeters from the adjacent dynodes. The ratio 
of dynode width to the group spacing should be about 1:1 and the ratio of 
dynode spacing to group spacing should be about 0.4:1. 
In the present invention, the cathode emits electrons which travel to and 
strike the first dynode. The dynodes in the multiplier chain are spaced 
and biased so that the electrons will flow from one dynode to the next 
dynode and increase in number with each stage, as is well known in the 
art. For example, the electrons emitted by the first dynode will strike 
the second dynode which emits a greater number of electrons than the 
number which strike it. The electrons from the second dynode will strike 
the third dynode and so on through the dynode chain. In the particular 
embodiment shown in FIG. 2, each dynode has an active multiplying region 
which comprises approximately the half of the surface 50 which is closest 
to the collection means 46. The electrons that strike this latter half of 
the surface 50 have the highest probability of generating secondary 
electrons which will travel to the next dynode in the chain. The electrons 
emitted from the first half of the surface 50 have an extremely low 
probability of reaching the next dynode in the chain. The majority of the 
electrons emitted by the ninth dynode will strike the collecton means 46. 
A few of the electrons emitted by the latter dynode stages will strike gas 
molecules in the envelope changing the molecules to positive ions. The 
positive ions travel at high velocities toward the cathode. These ions 
strike the cathode emitting additional electrons completing a feedback 
loop. Some of the generated ions do not reach the cathode but strike other 
parts of the device. The shields prevent the ions from striking the 
dynodes and generating electrons which will travel to the next dynode. In 
particular, an ion traveling from the output region 60 of the multiplier 
can strike the surface of the ion shield on the sixth dynode 56, as 
indicated by the dashed line 62. This ion can create an ion induced 
secondary electron which may strike the first half of the surface 50 of 
the sixth dynode. However, these electrons will strike the sixth dynode's 
surface at very low secondary emission energies, producing few, if any, 
secondary electrons. If the shield 48 was not present on the sixth dynode 
56, the ion could reach the latter half of the fourth dynode 54. Any 
secondary electron emitted by this ion would have a high probability of 
reaching the fifth dynode 55 resulting in electron multiplication through 
the dynode stages. 
Without the ion shields, it would become difficult to turn off a high gain 
feedback electron multiplier without controlling the potential applied to 
the latter stage dynodes. In this case, even though the cathode or early 
dynodes were turned off, the ion feedback could continue since ions could 
strike the mid or latter stage dynodes generating electrons. These 
electrons then would be multiplied through the electron multiplier and in 
turn generate more ions which would strike these same dynodes. As a 
result, feedback and electron emission could be sustained on the mid and 
latter stage dynodes. The ion shields prevent the ions from striking the 
dynodes and thereby prevent the ion feedback from continuing due to ion 
bombardment of the dynodes when the cathode or early stage dynodes are 
turned off. The multiplier can then be turned on and off by regulating 
only the voltage applied to the cathode or early stage dynodes which 
simplifies control circuitry and structure of the device. 
It may not be necessary to place shields between all of the adjacent 
dynodes, particularly if the feedback multiplier is operated at a 
sufficiently low loop gain such that sustained feedback can only be 
attained through ion bombardment of the cathode or early stages. Under 
this condition, shields are only necessary in the region near the early 
stages of the electron multiplier. It is the ion bombardment of the early 
stages of the electron multiplier which has the greatest effect on device 
control, since secondary electrons generated in the early stages by ion 
bombardment of the dynodes, will be multiplied by each of the latter 
stages of the electron multiplier. The multiplication of these electrons 
gives rise to the formation of a considerable number of electrons. 
Electrons generated by ion bombardment of the mid or latter stages will 
not be multiplied sufficiently to generate a significant number of ions. 
Therefore, this mode of feedback can be largely ignored when considering 
control of devices having low loop gain. 
These novel electron multipliers may be used singly or in arrays to form 
alphanumeric or image displays. When the multiplier is used in an array, 
the electron output of the multiplier may be used to illuminate one 
element of the display. The present invention has particular application 
in multi-cathode multiplier displays, such as the one shown in FIG. 3. In 
this embodiment, a matrix display device 70 has an envelope 72 comprising 
a cathodoluminescent screen 74 and a back panel 76 sealed together by 
walls 78. The interior surface of the screen 74 may be coated with a 
plurality of phosphor stripes (not visible). A plurality of parallel 
cathode stripes 80 are on the back panel 76. The cathode stripes 80 are 
composed of a material which will emit secondary electrons, such as MgO. A 
plurality of equally spaced parallel vanes 82 extend between the screen 74 
and the back panel 76 orthogonal to the cathode stripes 80. The vanes are 
formed of an insulating material, such as glass and have a plurality of 
parallel dynode stripes 91-98 on their surfaces forming an electron 
multiplier (similar to the one in FIG. 2) between adjacent vanes. With the 
exception of the first two dynodes 91 and 92, each of the dynodes 93-98 
have an L shape with the short portion of the L forming an ion shield 
projection 84 extending toward the adjacent vane. 
The basic device without the ion shields and its operation are described in 
the copending application of John Endriz, et al. entitled, ALLEL VANE 
STRUCTURE FOR FLAT IMAGE DISPLAY DEVICE, Ser. No. 672,122, filed on Mar. 
31, 1976. In order to activate a particular element of the display, a 
single multiplier is activated by adjusting the dynode potentials so that 
the gain of the multiplier is sufficient to sustain feedback. A single 
display element along the full length of the multiplier is selected by 
adjusting the potential along the cathode stripe 80 opposite the display 
element, so that the cathode will emit electrons to the selected 
multiplier. The electrons are then multiplied and illuminate a portion of 
the screen 74 opposite the intersection of the activated cathode stripe 80 
and the selected multiplier. Since the selection of one of the two matrix 
dimensions is accomplished through cathode switching, the ion shielded 
multiplier design of the present invention is desirable in achieving 
adequate control of the display.