U.S. patent number 3,783,333 [Application Number 05/229,136] was granted by the patent office on 1974-01-01 for x-ray tube with improved control electrode arrangement.
This patent grant is currently assigned to Picker Corporation. Invention is credited to Zed J. Atlee.
United States Patent |
3,783,333 |
Atlee |
January 1, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
X-RAY TUBE WITH IMPROVED CONTROL ELECTRODE ARRANGEMENT
Abstract
Electron emissive device such as an x-ray tube having a cathode
for generating electrons and an anode for receiving the same is
provided with an auxiliary electrode insulated from the focusing
cup electrode and positioned behind the cathode filament. A
positive potential may be applied to such auxiliary electrode
relative to the filament so that such auxiliary electrode attract
electrons and prevent their flow to the anode.
Inventors: |
Atlee; Zed J. (Oak Brook,
IL) |
Assignee: |
Picker Corporation (Cleveland,
OH)
|
Family
ID: |
22859964 |
Appl.
No.: |
05/229,136 |
Filed: |
February 24, 1972 |
Current U.S.
Class: |
315/30;
378/138 |
Current CPC
Class: |
H01J
35/066 (20190501); H01J 2235/068 (20130101) |
Current International
Class: |
H01J
35/00 (20060101); H01J 35/06 (20060101); H01j
029/52 () |
Field of
Search: |
;315/30
;313/55,56,57,59,60,308 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Quarforth; Carl D.
Assistant Examiner: Nelson; P. A.
Attorney, Agent or Firm: Stephen W. Blore et al.
Claims
I claim:
1. An electron emissive device comprising:
a cathode means including an electron emitter for generating
electrons;
an anode for receiving said electrons;
an auxiliary electrode positioned entirely behind said emitter of
said cathode means with respect to said anode;
means for applying a potential between said cathode means and said
anode to cause electrons to flow from said cathode means to said
anode;
and means for connecting said auxiliary electrode to a source of
positive electrical potential with respect to said emitter thereby
drawing electrons from said cathode means and decreasing the flow
of electrons from said cathode means to said anode.
2. An electron emissive device as set forth in claim 1, said
cathode means comprising a filament.
3. An electron emissive device as set forth in claim 1, said
cathode means comprising a metal body having an electron focusing
cup recess therein,
and a filament for thermionic generation of electrons mounted
within said recess.
4. An electron emissive device as set forth in claim 3, said metal
body having a front surface wherein said focussing cup recess is
provided and a rear surface,
said auxiliary electrode being positioned adjacent to but spaced
from said rear surface,
said body having an opening extending from said rear surface into
said recess,
whereby upon application of a positive potential to said auxiliary
electrode electrons will flow from said filament to said auxiliary
electrode through said opening.
5. An x-ray tube comprising:
a cathode means including an electron emitter and a focusing
electrode insulated from said emitter for generating electrons,
an anode for receiving said electrons;
an auxiliary electrode positioned behind said emitter with respect
to said anode and insulated from said focusing electrode;
means for applying a potential between said cathode and said anode
to cause electrons to flow from said cathode means to said
anode;
and means for connecting said auxiliary electrode to a source of
positive electrical potential with respect to said cathode means
thereby drawing electrons from said cathode means and decreasing
the flow of electrons from said cathode means to said anode.
6. An x-ray tube in which the improvement comprises:
a cathode means including a pair of filaments for emitting
electrons;
anode means for receiving said electrons;
a focusing cup structure for said filaments comprising a pair of
metal bodies each having a focusing cup recess formed therein, said
bodies being electrically insulated from each other;
said filaments being mounted one in each of said recesses and
electrically insulated from said bodies;
at least one auxiliary electrode insulated from said focusing cup
structure and positioned behind said filaments relative to said
anode;
means for connecting said filaments to a source of electrical
potential;
and means for connecting each of said bodies to a source of
negative electrical potential relative to the filament potential
independently of the other body whereby a cutoff bias can be
applied selectively to said bodies to restrict flow of electrons
from a selected one of said filaments to said anode.
7. An x-ray tube as set forth in claim 6 in which the auxiliary
electrode is positioned rearwardly of at least one of said metal
bodies,
and means for connecting said auxiliary electrode to a source of
positive potential with respect to the filament in the adjacent
body whereby upon application of said positive potential electrons
will be drawn from such filament to said auxiliary electrode.
8. In an x-ray tube:
an anode;
a focusing electrode comprising a conductive metal body having at
least one focusing cup recess formed therein in the surface facing
said anode and at least one opening extending from said recess to
the opposite surface of said body;
a cathode filament mounted in said recess for generating electrons
for impingement upon said anode having a lead extending through
said opening;
an auxiliary electrode of an electrically conductive material
positioned behind said filament relative to said anode and adjacent
to said opposite surface but electrically insulated from said
focusing electrode;
means for connecting said filament to a source of electrical
energy;
means for connecting said focusing electrode and anode to a source
of electron accelerating electrical potential;
and means for connecting said auxiliary electrode to a source of
electrical potential positive with respect to said filament whereby
at least some of the electrons generated at said filament will be
drawn to said electrode thereby lessening the flow of electrons
from said cathode to said anode.
Description
BACKGROUND OF THE INVENTION
In electron emissive devices such as x-ray tubes, utilizing
tungsten filaments, it has been possible to maintain the electron
emissive level by simply changing the current level supplied to the
filament. The response of a pure tungsten filament is rapid enough
both in heating and cooling and thus in emission to give the
control which is desired. Where additional control has been
necessary, conventional wire grids positioned between the anode and
the cathode have been utilized in some circumstances. However, the
grid wires distort the electron beam which is undersirable in many
circumstances such as, for example, in the production of x-rays
where an anode focal spot of regular confirmation and even
distribution of electrons thereover is desired.
Thoriated tungsten filaments have many desirable features and many
efforts have been made to utilize them in x-ray tubes and other
electron emissive devices. In come circumstances the thoriated
tungsten filaments are adaptable, but in others, such as x-ray
tubes, they are generally undesirable under control arrangements
which have heretofore been available. Since thoriated tungsten
filaments are operated at a lower temperature than pure tungsten
filaments the cooling rate of a thoriated tungsten filament is
slower than that of a tungsten filament and thus control of
emission level by a decrease in filament current is frequently too
slow in the case of a thoriated tungsten filament to permit its
use. The use of a grid for control is undesirable for the reasons
outlined above.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with th present invention, an x-ray tube or similar
electron emissive device having an electron generating cathode
filament, a focusing cup electrode and an anode is provided with an
auxiliary electrode that is positioned behind the cathode filament
with respect to the anode. By applying a positive potential to such
auxiliary electrode, that is, positive with respect to the
filament, electrons can be drawn from the filament to the electrode
thus reducing the number of electrons which flow from the cathode
to the anode. Conversely, reducing the potential will permit more
electrons to flow to the anode. The auxiliary electrode thus
provides a means for controlling the level of electron flow to the
anode.
The auxiliary electrode arrangement can also be utilized to control
operation of x-ray tubes having dual filaments which are provided
in order to produce focal spots of different sizes on the anode. In
such tubes, it is frequently desirable to place a bias upon the
cathode itself in order to cut off flow of electrons from the
filament to the anode. X-ray tubes have been provided heretofore
with cathodes having two filaments, one being longer than the other
and arranged so as to form on the anode, depending upon the
filament which is operated, a large or a small focal spot. The
larger focal spot is utilized, for example, for spot film
radiography whereas the small focal spot is utilized for
fluoroscopic examination. In some instances it is desirable to be
able to switch rapidly from one filament to another. In accordance
with a modified embodiment of the present invention, such rapid
switching is enabled by constructing a cathode with two filaments
mounted in electrically insulated focusing cup eletrodes. This
enables both filaments to be operated continuously and the electron
flow from the respective filaments cut off by selectively applying
a negative bias to the respective focusing electrode, thereby
cutting off the flow by reason of the space charge effect accross
the mouth of the focusing cup recess. Each cathode portion may also
be provided with an auxiliary control electrode behind the filament
such as discussed above.
Other objects and advantages of the invention will become more
apparent hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevation of an x-ray tube constructed in
accordance with the invention;
FIG. 2 is a fragmentary view of the anode target looking in the
direction of the arrows 2--2 of FIG. 1;
FIG. 3 is an enlarged end view of the cathode head taken generally
along the lines 3--3 of FIG. 1;
FIG. 4 is a sectional view taken substantially along the lines 4--4
of FIG. 3;
FIG. 5 is a view similar to FIG. 3 of further embodiment of the
invention; and
FIg. 6 is a sectional view taken substantially along lines 6--6 of
FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is illustrated in connection with an x-ray
generator comprising an envelope 10 of glass or other suitable
material, having a cathode unit 12 suitably mounted in one of the
envelope and a rotary anode unit 14 suitably mounted in the other
end of the envelope.
The anode unit 14 comprises a rotating target element 16 preferably
formed of a metal of tungsten or other refractory metal. The anode
16 is provided with a frusto-conical target surface 18 defining a
target area for the reception of electrons upon superimposed focal
spots 22, 24 respectively.
Turning now more particularly to FIGS. 3 and 4, the cathode 12
therein illustrated comprises a metal body 30 which is supported
within a tubular nonconductive sleeve 32 supported from the
envelope 10. The body 30 is formed within an indented face adjacent
the end facing the anode unit 14 defined by a pair of planar face
portions 34-36. The face portion 34 has an elongated focusing cup
recess 38 formed therein and the face portion 36 has a shorter
focusing cup recess 40 formed therein. Mounted within the recess 38
is a filament 42 for generating electrons which are focused upon
the anode target portion 18 so as to form the focal spot 24
thereon. Mounted within the smaller recess 40 is a shorter filament
44 for generating electrons which are focused upon the target
portion 18 to form the focal spot 22 thereon. The focusing cup
recesses 38-44 are each formed with a relatively narrow bottom slot
or gap 46, 48 respectively, extending through the back face of the
body 30 which in this instance is formed in a cup shape with a
hallowed out back portion 50. Leads for connecting the filaments to
a source of electrical energy are provided including a lead 52
connected to one end of the filament 42 and a lead 54 connected to
the corresponding end of the filament 44. The leads extend through
circular openings formed at the opposite ends of the slots 46, 48
from the cups to the back face of the body 30. The opposite ends of
the filament 42,44 are interconnected to a common lead 56. Thus the
filaments 42, 44 can be selectively energized to generate electrons
for focusing on the respective portions of the anode target.
Mounted within the hollowed out base back portion 50 is an
auxiliary electrode 60 comprising a body made of conductive
material such as metal, the body being mounted in closely spaced
but electrically insulated relation with respect to the cathode
body 30. The body 60 is cylindrical and is secured to the body 30
by suitable means such as a screw 62 threaded into the body 30 and
extending through an axial opening 64 in the body 60, but insulated
therefrom by washers 66. The screw 62 provides a means for
connecting the body 30 to a source of electrical potential through
a lead 68 which is secured to the screw by suitable means. Means
are provided for connecting the electrode 60 to a source of
electrical potential comprising a lead 70 secured in position by a
setscrew 72. It is to be understood that all such leads extend
outwardly from the envelope 10 through a suitable conventional
arrangement for connection to sources of electrical energy and
potential such as shown in U. S. Pat. No. 2,862,107 of H. R.
Cummings granted Nov. 25, 1958.
As will be understood by those skilled in the art, upon
energization of a filament 42 or 44 and the application of
potential between the cathode body 30 and the anode 14, electrons
will be accelerated from the energized filament to the anode. In
accordance with the invention the flow of electrons from an
energized filament to the anode can be controlled by changing a
positive voltage (with respect to the energized filament) applied
to the auxiliary electrode 60. The application of such a potential
to the electrode will cause some of the electrons which are being
boiled off the filament to be attracted to the auxiliary electrode
thus diminishing the flow of the electrons to the anode. By
increasing or decreasing the potential of the electrode 60, the
number of electrons diverted from the flow to the anode can be
changed. Since the electrode 60 is out of the high voltage field of
the anode, the potential applied to the electrode will not affect
the focal spot size or electron distribution thereover.
This invention has particular advantage in the operation of x-ray
tubes in which the filaments are thoriated tungsten. Such filaments
are operated at such a low temperature that the usual control by
decrease of the current applied to the filament can not be utilized
since the cooling rate of the filament is too slow. Control by
means of the application of potential to the auxiliary electrode 60
on the other hand is instantaneous and thus the emission level of
the filament to the anode can be maintained substantially
uniform.
As will be apparent, in the embodiment of FIG. 3 and FIG. 4 the
filaments 42-44 can be operated independently of each other only by
selectively applying power to the filaments. Thus rapid switching
from one focal spot to another is difficult, particularly with
thoriated tungsten filaments because of the slow response time of
the filaments. Moreover, control of emission by applying a bias to
the cup 30 is difficult in the configuration of FIGS. 3 and 4
inasmuch as the application of a bias sufficient to cut off
emission from the large filament 42 will place stress across the
small gap 48 in the smaller cup 40. These problems are alleviated
by the cathode structure shown in FIGS. 5 and 6. The cathode
structure therein shown comprises a cup comprising two separate
semi-cylindrical sections 80, 82 suitably supported from the sleeve
32 and insulated from one another by a diametrically extending
insulator 84. The cup section 80 has an elongated focusing cup 86
formed therein having a focusing slot 88 in the bottom thereof with
a pair of openings 90, 92 for the passage of lead wires 94, 96
connected to the opposite ends of a filament 98. The section 82 is
provided with a shorter focusing cup 100 in the surface thereof
facing the anode 14 in the bottom of which cup is provided a
focusing slot 102 and suitable openings for passage of leads 104,
106 connected to the opposite ends of a filament 108. The rear
surfaces, that is the surfaces of the cup sections 80, 82 remote
from the anode 14, are co-planar. Mounted closely adjacent to such
surfaces, but spaced therefrom, are a pair of auxiliary electrodes
110, 112, each of which is semi-circular in configuration and which
are separated by a diametric insulator 114. The electrodes 110, 112
are secured to the corresponding electrodes 80, 82, respectively,
by means of bolts 116, 118, respectively, which are in electrical
contact with the respective cup section but are insulated from the
respective electrode by means of insulators 120. An electrical lead
122 is connected to the bolt 116 to provide means for connecting
the cup section 80 to a source of electrical potential and lead 124
is connected to the bolt 118 to provide means for connecting the
cup section 82 to a source of electrical potential. The various
filament leads 94, 96, 104, 106 extend outwardly through the
electrodes 110, 112, each filament lead being encased within a
tubular insulator 126, which in turn is surrounded by a metal
sleeve 128. Means are provided for connecting the electrodes 110,
112 to source of electrical potential, the plate 110 being
connected to a lead 130, extending into the plate and held in place
therein by a setscrew 132. The plate 112 is connected to a lead
134, held in place by a setscrew 136.
In the operation of the present device the filaments 98, 108 can be
simultaneously energized. If it is desired to secure a focal spot
from the emission from the filament 98 sufficient bias is applied
to the cup section 82 to suppress the emission from the filament of
108. If it is then desired to secure emission from the focal spot
generated by emission from the smaller filament 108, the bias can
be instantaneously switched from the cup section 82 to cup section
80 thus suppressing emission from the filament 98 while permitting
emission from the filament 108 to pass to the anode.
The auxiliary electrodes 110, 112, function to control the amount
of available electrons from each of the filaments in a manner
similar to the control electrode 60. Thus, if the filament 98 is
operating to generate an electron beam focused upon the anode 14,
when the desired emission level from the filament is reached a
suitable power supply circuit will be energized to apply a positive
potential to the auxiliary electrode 110. This will cause electrons
to be drawn from the filament 98 through the openings 90 to the
plate 110, thereby maintaining the electron flow to the anode at
the desired level. In similar fashion when the cup 80 is biased to
cut off the emission from the filament 98 and the cup 82 is neutral
thereby permitting emission of electrons 108 to the anode 14, the
auxiliary electrode 112 can be utilized to control the flow of
electrons to the anode and cut off excessive flow when the desired
level is reached. That is, when a desired level of current flow
through the tube is sensed in the suitable control circuit, a
voltage will be applied to the auxiliary electrode 112 to attract
electrons from the filament 108 through the openings for the lead
wires 104, 106.
Having illustrated and described preferred embodiments of the
invention, it will be apparaent it permits of modification in
arrangement and detail.
* * * * *