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Description  |
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This invention relates to improved method and apparatus for the selective
production of fluoroscopic or radiographic x-ray images for diagnostic
purposes in a readily portable, hand-held and battery powered x-ray
system.
Many devices employing x-rays or other types of radiation have been used
and/or proposed for use to produce fluoroscopic or transitory images and
radiographic images for diagnostic purposes. The majority of such devices
are of bulky and heavy character and are either fixed in location or
rendered mobile by using special carts to permit limited movement thereof.
Most of such units, by their nature, produce large dosage of x-rays and
consume large amounts of power necessitating specialized electrical power
sources and, for mobil units, heavy and bulky arrays of batteries.
Illustrative of such mobile units are the General Electric Polarix and
Fisher Omni 325 systems which weight in excess of 600 lbs. and require 220
volt power at up to 70 amperes or equivalent battery packs. Other
manufacturers supply generally similar units.
In recent years, various diagnostic systems have been advanced which offer
increased mobility and, in at least one case, portability, with the latter
being attended by sacrifice of performance capability and versatility.
These latter systems include the Healthmate Fluoroscan, the Lixiscope and
the Bowie portable unit, the latter being specifically intended for
veterinary application. The Healthmate Fluoroscan and the Lixiscope, which
both employ microchannel plate image intensifiers, are purportedly
licensed under U.S. Pat. No. 4,142,101 and function only as fluoroscopes.
The Healthmate, Bowie and Lixiscope weight respectively 200 pounds, 21
pounds and 5 to 8 pounds with the first two being operable from standard
115 VAC line power. Both the Fluoroscan and Bowie unit utilize x-rays
while the Lixiscope utilizes gamma-rays from a radioactive isotope source.
Such gamma ray usage requires special handling and the replacement of the
source at three to six month intervals as the isotope decays.
A matter of concern in any diagnostic process utilizing x-rays or gamma
rays is the potential for biological damage to the patient and the hazards
presented to the operator of the device. Most x-ray systems currently in
use for both fluoroscopy and radiography utilize high intensity
x-radiation, which high intensity is dictated, in large part, by the
relatively low gain or limited degree of light amplification provided by
conventional image intensification techniques and also by the relatively
long source to image distances employed in such systems. The high
radiation intensities employed in these systems also require the use of
x-ray tubes employing large area focal spots since otherwise the high beam
currents would generate too much heat and lead to rapid deterioration of
the tube anode. X-ray tubes employing large area focal spots necessitate
operation at long source to image distances in order to maintain
satisfactory image resolution or definition.
This invention may be briefly described, in its broad aspects, as improved
method and apparatus techniques for x-ray fluoroscopic and radiographic
imaging. Improved apparatus constructed in accord with the principles of
this invention broadly includes a small, portable, hand-held x-radiation
generating and imaging means suitable for both fluoroscopic and/or
radiographic operation, at the option of the user, powered by a small
battery and in which small focal spot x-ray tube areas are employed and
the source to image receptor distance is markedly reduced. In a narrower
aspect, such apparatus is desirably in the form of a C-shaped housing
containing a small focal spot x-ray tube and shielding assembly with beam
collimation and directing means situated at the end of one arm of the C
shaped housing. A small sized high voltage DC power supply is located
immediately adjacent the x-ray tube housing assembly. Low voltage power
control circuitry and monitoring devices are located within the center
portion of the C-arm assembly. The other arm of the C shaped housing
disposed opposite that containing the x-ray head assembly may be used as a
handle for the device and preferably contains switch mechanisms to control
the production of x-rays. Mounted at the end of the second arm portion of
the C shaped housing are interchangeable means for producing enhanced
fluoroscopic or radiographic images of objects disposed between the two
extremities of the C-arm assembly. In a still narrower aspect the
invention includes effecting radiographic imaging of an interposed object
by utilizing a cassette suitably retained in a tray mechanism rigidly
attached to the second or handle end of the C-arm assembly. Such cassettes
contain sensitive screens which emit visible light when exposed to
x-radiation. Such visible light emissions are utilized to produce a
photographic image on a conventional negative film or an instant Polaroid
positive film. Fluoroscopic imaging is conveniently effected using a
similar type of x-ray sensitive screen, and amplifying the brightness of
the emitted visible image by suitable high gain light intensification
means, preferably of microchannel plate configuration. Optical coupling
means which may contain magnification or minification components may be be
interposed between the x-ray image receptor screen and the image
intensifying means and between the output screen of the image intensifier
and the viewing screen or lens.
The primary object of this invention is the provision of improved method
and apparatus for x-radiation fluoroscopic and radiographic imaging
employing markedly reduced radiation levels.
Another object of this invention is the provision of small sized, light
weight and readily portable fluoroscopic and/or radiographic x-ray imaging
apparatus particularly adapted for use as a diagnostic tool for the
viewing of non-torso extremities, such as hands, arms and legs.
Another object of this invention is the provision of improved fluoroscopic
and/or radiographic x-ray imaging techniques in which the source to image
receptor distance is markedly reduced and small x-ray tube focal spots are
employed.
A futher object of this invention is the provision of improved techniques
for fluoroscopic and/or radiographic x-ray imaging, that markedly reduce
the needed electrical power requirements for operation thereof.
Referring to the drawing:
FIG. 1 is a schematic side elevational view of a preferred configuration of
a low intensity x-ray system for operation in the fluoroscopic mode
incorporating the principles of this invention.
FIG. 2 is a schematic side elevational view of the apparatus of FIG. 1, as
adapted for operation in the radiographic mode.
FIG. 3 is a vertical sectional view of the low intensity x-ray system shown
in FIG. 1 showing the positioning of major components therein.
FIG. 4 is a vertical section of a preferred fluoroscopic imaging receptor
assembly incorporating the principles of this invention.
FIG. 5 is a plot of scatter radiation from a fluoroscopic low intensity
imaging system embodying the principles of this invention.
FIG. 6 is a schematic circuit diagram of a suitable diminutively sized high
voltage power supply employing high voltage transformer and long chain
series multiplying means.
Referring now to FIGS. 1, 2 and 3 there are illustrated exemplary
components of a low intensity hand portable embodiment of an x-ray imaging
device capable of both fluoroscopic and radiographic imaging at the option
of the user that incorporates the principles of this invention. As shown,
x-radiation is emitted through a collimating cone 1 located near the end
of one of the arms of the C-arm assembly 2. A control panel 3 containing
mode and level switches 4, 5, 6 and 7 permits x-radiation to be emitted at
predetermined selected levels of voltage and intensity for either mode of
operation upon actuation of one or both of the actuating buttons 8 and 9.
The pre-set levels of voltage and x-ray beam current together with various
information pertaining to exposure time are presented on a display panel
10 preferably of a liquid crystal character, which may be back illuminated
for improved visibility. Preferably a microprocessor control system is
included in the device and the display panel 10 may also be utilized to
direct other information and error conditions, such as low battery power,
to the attention of the operator.
In the fluoroscopic mode of operation as depicted in FIG. 1, the spread or
divergence of the emitted x-ray beam 11 is further reduced by the addition
of a beam limiter 12 to the collimator cone assembly 1. The spread of the
x-ray beam is controlled and limited so that the fluoroscopic image
receptor assembly 22 effectively intercepts all of the emitted x-ray beam
11 to thereby minimize, if not avoid, exposure of the operator to the
emitted radiation. An additional shield 13, preferably of leaded plastic
material, may also be mounted on the image receptor assembly 22
surrounding the aperture therein to further protect the operator from the
fringe portions of the emitted radiation and leakage and scatter
radiation. The shield 13 is selectively shaped to especially protect the
eyes, thyroid and hand of the user. The device is preferably constructed
so that the fluoroscopic image receptor assembly 22 may be easily and
readily removed by loosening a knurled screw 14 and replaced with a
radiographic cassette holder tray 15 as shown in FIG. 2, for operation in
the radiograpic mode to produce permanent film records at the option of
the user. Suitable protective interlock mechanisms are incorporated to
prevent operation in such manner as might be dangerous to patient or
operator. For example, one interlock renders the unit inoperable if the
beam limiter is not positioned in place when operating in the fluoroscopic
mode, or if the image receptor head is not properly positioned under
either mode of operation. Also included are means responsive to the
selected positioning of the first image producing means in predetermined
spatial relation with the radiation source for limiting the x-ray tube
electron beam current to a value intermediate 50 microamperes and 300
microamperes for fluoroscopic operation and means responsive to the
selected positioning of the second image producing means in predetermined
spatial relation with said radiation source for limiting the x-ray tube
electron beam current to a value intermediate 500 microamperes and 3
milliamperes for radiation responsive photographic operation. Such
protective interlocks are most readily and desireably accomplished by
means of a microprocessor control system. Still another interlock or
microprocessor control may operate to prevent system operation in the
radiographic mode when the beam limiter 12 is in place since, under such
condition, the resulting film record would show only the central portion
of the image, necessitating a retake without the beam limiter 12 with
consequent additional exposure of the patient to radiation.
Referring now to FIG. 3, there is illustrated a preferred arrangement of
the major system components within the C-arm housing 2. As shown, the
x-ray tube housing assembly 16 is located within one extremity of the
C-arm assembly 2. Positioned in fixed spatial relation thereto by locating
pins or other appropriate means, not shown, is a collimating cone 1. The
beam limiter 12 is complimentally shaped and sized to accomodate insertion
thereof within the cone 1 in such manner that the axis of the narrowed
conical beam passing therethrough is coaxical with the central axis of the
generally rectangular collimating cone 1. As discussed above, when the
beam limiter 12 is properly positioned within the cone 1, a detector
mechanism, preferably in the nature of a small microswitch, or a magnetic
position detecting means, is activated to provide a positive signal
indicative of the beam limiters 12 presence to a microprocessor or other
centralized control system. Disposed within the housing 16 is a small
focal spot x-ray tube 40, suitably a Eureka EXR-80-20D. The x-ray tube
housing assembly 16 is oil filled and preferably employs other high
dielectric strength solid insulating materials, such as Kapton or Stycast
2850 FT, for electrical insulation purposes. Such tube housing assembly
includes means to accurately position the focal spot on the x-ray tube
target anode on the axis of the collimating cone 1.
High voltage power is fed to the x-ray tube anode 42 which is preferably
disposed at the lower end of the tube housing assembly 16, through a high
voltage connector assembly 17. The tube housing assembly 16 and the
adjacent portion of the high voltage connector assembly 17 are surrounded
by a suitable thickness of lead shielding, typically about 1 millimeter in
thickness in the vicinity of the anode of the x-ray tube and with a
reduced thickness to as little as 0.15 mm around the connector assembly
17. An x-ray window of suitable size is provided in the lead shield. The
material constituting the cylindrical x-ray tube housing assembly 16 is
preferably aluminum, typically about 0.5 mm in thickness which, together
with the oil and other solid insulating materials contained therein and
the glass of the x-ray tube provides sufficient filtration of low voltage
or low energy x-radiation to maintain good beam quality. Such low energy
radiation emission within the primary beam not only has insufficient
penetrating power for good diagnostic purposes, but also may cause harmful
effects to the patient.
The high voltage power supply 18 is desirably located immediately adjacent
the high voltage connector assembly 17 in order to minimize high voltage
leakage and transmission problems, and also to minimize possible
interference caused by exposure of high voltages on nearby low voltage
electronic components in the system. In order to minimize static build up
and high voltage noise, all high voltage systems are encased within a
grounded conducting shield which may suitably be a conducting paint
similar to that used on the inside of computer cabinets and the like.
The low voltage electronic power amplifying system 19 and associated
control system 20 are conveniently located in the elongated central
portion of the C-arm housing 2 as shown in FIG. 3. The low voltage power
amplifier system 19 is preferably disposed between the high voltage power
system 18 and the microprocessor control system 20 since, especially when
operated in radiographic mode, the high power level signals emanating from
the power amplifier system 19 are transmitted directly to the adjacent
high voltage power supply 18 and thereby minimize interference with the
sensitive microprocessor system 20.
The end portion 44 of the second arm of the C-arm housing 2 is conveniently
utilized as a handle for operation in the fluoroscopic mode and for
containing the manual actuation switches 8 and 9 and audible warning
transducers if such are desired. Power from an external battery pack, not
shown, is introduced via the multiwire cable assembly 21.
Referring now to FIG. 4, there are illustrated exemplary components
constituting a fluoroscopic imaging receptor assembly 22. As there shown,
incident x-radiation after emission from the x-ray source and passage
through an interposed examination subject, impinges upon and, passes
through an optically opaque but x-ray transparent window 23 at the front
of the receptor assembly 22. The window 23 may be fabricated from black
plastic material such as Delrin and the portion thereof in the path of
incident x-radiation is of small thickness, typically less than 1 mm.
Disposed immediately behind the window 23 is a high resolution x-ray
sensitive screen 24 of Kodak Lanex or similar material which converts the
image defined by invisible incident x-radiation to an optically visible
image, albeit of very low light intensity. The visible light producing
screen 24 is disposed in immediate interfacial optical contact with the
front face of a fiber optic cone assembly 25. Such interfacial optical
contact may be enhanced by appropriate optical bonding materials or by
depositing the active screen ingredients directly on the face of the cone
25. The fiber optic cone assembly 25 operates to efficiently transmit the
low intensity visible image produced on the screen 24 to the input window
of an image intensifier assembly 26. Desirably the output face of the
fiber optic cone 25 is disposed in good optical contact with the image
intensifier input window to minimize transmission losses therebetween. If
desired, the visible image producing screen and the input window of the
image intensifier may be coupled directly, or other optical transmission
systems employing lenses may be interposed therebetween. The use of fiber
optics or lenses permits controlled magnification or minification of the
image thus permitting utilization of a larger or smaller field of view
than the diameter of the image intensifier tube assembly. Care should be
taken to maintain the entire screen 24 and the optical input assembly to
the image intensifier light tight to prevent undesired degradation of the
faint image produced by action of incident x-rays on the screen 24.
The image intensifier assembly 26 is preferably of microchannel plate
construction which provides high light amplification, small size and has
low power requirements. The image intensifier assembly 26 produces a
bright visible image, conforming to the incident x-ray image, on the
output screen 27. The image on the output screen 27 may be viewed directly
or through suitable magnifying optical means 28 or, alternatively, as the
output side of a second fiber optic cone assembly.
Lead shielding is disposed inside the image receptor housing 22 surrounding
the optical image path to prevent unnecessary incident radiation from
penetrating the image receptor assembly and to minimize operator exposure
to radiation. As before noted, a shield 13, preferably transparent lead
plastic, may be added to reduce to a minimum any radiation by-passing the
image receptor and reaching the operator.
FIG. 5 shows typical low radiation level contours produced by scatter from
the examination subject and leakage from various components of the system,
when operated in the fluoroscopic mode. Operation of small-sized portable
x-ray diagnostic devices of the type described above antithetically
requires the maintainance of low levels of radiation, both in the primary
beam and also in the area of leakage and scatter, together with the
provision of images of sufficient clarity to permit utilization of the
unit as a safe diagnostic tool. Within the latter area a critical
performance requirement is the resolution or the ability of the system to
distinguish detail. To be an acceptable and practical diagnostic tool, the
resolution of the system, in both fluoroscopic and radiographic modes,
should be at least 3.5 line pairs per mm and preferably 5 line pairs per
millimeter. Since the above described optical and fiber optical components
have a resolving power considerably in excess of this level, it is
necessary to ensure that the image defining detail of the emitted x-ray
beam and the resolving capacity of the screen produce an initial visible
image that is of high resolution and is above the threshold of intensity
level that the image intensifier requires to maintain resolution to the
required levels in the optically enhanced image.
As stated previously, we have found it desirable to use a microchannel
plate image intensification system, not only because of its high gain,
small size and low power characteristics, but also because such image
intensification is capable, when operated with suitable input light
levels, of resolution in the order of 30 line pairs per millimeter which
permits, for example, magnification and/or minification by a factor of
three while still providing inherent resolution level of 10 line pairs per
mm. Such resolution capabilities permits the use of a relatively small and
inexpensive image intensifier suitably having a 25 mm screen and viewing
area, adopted to be used with fiber optic cones or lenses to provide a 75
mm viewing field in fluoroscopic mode. Such a viewing field accomodates
the non-torso extremities such as feet and hands., which is a primary area
of intended usage for the above described device.
A prime operating requirement is to ensure that the inherent resolution of
the x-ray generation system is capable of an ultimate system resolution in
excess of 5 line pairs per millimeter. As hereinbefore stated, this
invention is directed to a hand-held portable x-ray generating system. As
such, the attendant physical constraints in size and weight dictate that
the x-ray source to image receptor distance is markedly less than that
employed in conventional diagnostic x-ray apparatus and practically should
not exceed about 50 centimeter. Such small x-radiation source to image
distances in conjunction with an x-ray source focal spot of conventional
size inherently leads to loss of image definition. Such small distance
also inherently requires that the subject being examined is disposed
undesirably close to the x-radiation source, where the radiation intensity
is a maximum. To accomodate these divergent requirements the subject
apparatus employs a minimum source to image receptor distance of 25
centimeters and preferably uses a source to image receptor spacing in the
range of 30 to 35 centimeters. Such reduced source to image receptor
distances require selective utilization of a markedly reduced size of
focal spot in the x-ray tube to maintain the desired degree of resolution.
In order to realize the desired objectives we have determined that the
focal spot should not desirably exceed 0.5 mm by 0.5 mm and certainly
should be no larger than 1 mm by 1 mm.
As will be now apparent from the foregoing, a paramount operating
requirement for hand-held portable x-ray systems as described herein is
the preservation and maintanence of high resolution for both the
fluoroscopic and radiographic mode of operation. Such high resolution can
only be maintained, by selective utilization of a high resolution screen,
such as Kodak Lanex, for the initial conversion of image defining
x-radiation to visible light. Such type of high resolution screen requires
a relatively high level of radiation to produce an image of acceptable
brightness. In accord therewith a certain minimum level of transmitted
x-radiation must be received at the screen to produce a visible image of
acceptable resolution. For operation in fluoroscopic mode using a high
resolution screen, such as Kodak Lanex, we have determined that subject
free mininum radiation levels of 0.15 Roentgen per min (R/min) at the
screen surface are necessary to provide required image quality, and
preferably a radiation level that is in excess of 0.4 R/min. No
substantial advantage is observed by further increasing screen radiation
levels and, to minimize the potential for biological damage, an operating
level of 2.0 R/min at the screen, without attenuation by passage of the
radiation through the subject being examined, should not be exceeded for
operation of a device constructed according to the principles of this
invention. In the radiographic mode of operation, the radiation levels at
the film cassette, without attenuation by objects interposed between the
source and screen, are desirably between 2.5 R/min and 15 R/min, with a
preferred value of 5 R/minute. In this latter case of radiographic
operation, it will be apparent to those skilled in the art that such
radiation levels depend substantially on exposure times and, accordingly,
the above recommendations are based on maximum exposure times consistent
with producing a distinct image of an extremity of a conscious human
subject with said extremity supported by the film cassette. We have found
that exposure times between 50 milliseconds and 3 seconds, but preferably
in the order of from 50 millisecond to 1 second, generally provide
acceptable images. The utilization of shorter exposure times and higher
radiation levels, while not exposing the patient to more total radiation,
has been found impractical for usage in a portable battery operated system
of the type being described because of the size of the electronic power
systems required.
It is well known to those practiced in the diagnostic arts, that the
examination of human extremities and other objects of similar density
require utilization of x-radiation of a certain energy content or
penetrating power. It is well understood in the field that x-rays produced
by application of peak voltages of between 35 kilovolts and 80 kilovolts
and preferably in the range of from 40 to 75 kilovolts are suitable for
this purpose when using fluoroscopy and somewhat lower levels are
necessary when using radiography, where compensation to some extent may be
made by variation in exposure time of the film cassette. Emitted
x-radiation intensities are dependent upon both the peak kilovolts applied
to the anode of the x-ray tube and also upon the level of electron beam
current flowing from the x-ray tube filament to the anode thereof. Based
upon the herein specified operating radiation levels and the applied peak
kilovolts necessary for the contemplated diagnostic examination of various
extremites, we have determined that a necessary range of electron beam
current in the x-ray tube of from 50 to 300 microamps in the fluoroscopic
mode will provide, in a device of the type described herein, a practical
range for optimum operation. A preferred range of operation for maximum
performance and safety is with tube currents of between 100 to 200
microamps. When operating in the radiographic mode a minimum beam current
of 500 microamps and maximum of 3 milliamps is required, though the
preferred range for optimum operating characteristics consistent with the
portable nature of the system is between 750 microamps and 1.5 milliamps.
In order to minimize the level of radiation exposure to which the patient
and operator may be exposed it is normally necessary to employ x-radiation
impervious mechanical barrier means to prevent the close approach of body
parts to the x-radiation source where said radiation intensities, due to
the governing inverse square laws, become very high. In addition, the
Federal Food and Drug Administration requires various forms of warnings,
including audible alarms, when a fluoroscope is operated in any mode where
possible skin exposure level exceeds 5 R/min. The provision of limiting
barrier means of excessive length clearly intrudes on the physical space
available to interpose bulkier body parts such as knees and shoulders.
Means are included in the disclosed device to prevent source to skin
distances of less than 6.5 cm and to normally operate at a minimum source
to skin distance of 10 cm in a fluoroscopic mode. Such level of source to
skin distances typically results in limiting skin exposure to less than 20
R/min. under normal operating conditions as heretofor described.
A primary practical concern in the efficient operation of portable battery
operated x-ray imaging systems of character herein described is the
efficiency of conversion of source battery power to operating high voltage
power. In typical fluoroscopic operation the required high voltage power
levels are in the range of 10 to 30 watts and in the radiographic mode,
required high voltage power levels are about five times higher, although
in this latter case such high power levels are required for periods of
very short duration. The efficiency of power conversion in such operation
impacts not only upon battery life but also upon the amount of heat that
is dissipated in the electronic components. At a 15 watt power level
output an amplifier system operating at 20% efficiency must dissipate 60
watts as heat whereas an efficient system operating at 80% efficiency
dissipates less than 4 watts in the form of heat. Inefficient modes of
operation therefore generally require special and bulky heat dissipation
adjuncts, as well as adversely affecting the basic reliability of the
electronic systems. Conventional readily available electronic power
amplifier systems operate at a theoretical maximum efficiency of about
75%. However under realistic operating conditions they generally operate
at about 50% efficiency which, when coupled to inherent efficiencies of
less than 60% in an associated high voltage power supply, result in
overall efficiency levels of less than 30%. In order to overcome the
foregoing, the practice of this invention preferrably utilizes a Class D
switching power amplifier which has inherent efficiency in excess of 90%
to provide an overall efficiency in excess of 50% when operating in the
fluoroscopic mode and with substantially higher efficiencies when
operating in the radiographic mode.
As hereinbefore pointed out the hand portable x-ray system described
described is characterized by the utilization of a high voltage power
supply of diminutive size and unique design that is disposed within the
hand-held device and preferably located immediately adjacent the x-ray
tube housing assembly. As heretofor discussed relative to the low voltage
power conversion and amplification equipment, the efficiency of electrical
energy conversion is a prime concern in order to minimize heat generation
and the problems attendant thereto, as well as to maximize the useful life
of the battery power source. The high voltage power supply operates to
convert the output of the low voltage power amplifier, typically 20
kilohertz AC at up to 30 volts RMS into a DC voltage of up to 80 kilovolts
and typically delivering a current of 1 milliamp. The means to accomplish
this broadly comprise a high voltage transformer adapted to convert the 20
KHz low voltage signal to a considerably higher voltage level and an
associated long chain series multiplying means to multiply and rectify the
transformer amplified high voltage AC signal to the desired high DC
voltage. As recognized by those skilled in the high voltage art, the
efficiency of long chain series multiplying means degrades rapidly as the
number of stages in the multiplier increases and also as the DC current
level increases, unless the value of the capacitors included in the chain
increases accordingly.
Referring now to FIG. 6, there is provided a circuit diagram of a preferred
high voltage multiplier circuit of dimunitive physical size. The input
transformer 29 includes a low loss ferrite EE core, suitably of Magnetics
Inc. P material, with a center leg cross-section of 0.90 square
centimeters. The primary 30 of the transformer 29 contains 9 turns and the
secondary thereof 31, contains 3200 turns in 5 isolated segments. Such a
transformer, suitably impregnated and encapsulated, can produce a peak
voltage in excess of 10 kilovolts. The high turns ratio employed therein
results in a large capacitance reflected to the primary 30 of the
transformer necessitating a substantial center leg gap to prevent drawing
high quadrature currents from the primary source. We have found that a
four stage multiplier, as shown, is optimum for this application, with
each stage comprising a pair of high voltage diodes 32 and a pair of high
voltage capacitors 33. Each capacitor and diode is subject to a voltage of
twice the peak transformer voltage and accordingly in this application
must be constucted to withstand 20 kilovolts. With presently available
state of the art components, the utilization of peak to peak AC voltages
in excess of 20 kilovolts in order to reduce the number of stages will
result in a significant increase in the size of the capacitors and,
accordingly, in the size and weight of the power supply. Conversely,
reducing the voltage per stage requires an increase in the number of
stages and concomittantly results in significant loss of efficiency and
regulation unless the value of the capacitors is again increased
substantially with an attendant increase in size. We have found it
basically impractical to operate with more than six stages of voltage
multiplication and highly desirable to utilize no more than four stages
thereof. The output of the illustrated power supply is connected through a
limiting resistor 34 of high value, suitable in the order of 2 to 10
megohms, which serves to protect the components therein from high surge
currents in the event of external arc occurrence. In the operating
environment, the entire power supply is suitably wrapped and encapsulated
using materials of high dielectric strength to withstand voltage breakdown
and is further coated with a suitable conducting | | |
