Abstract:
An ionization gauge that eliminates a hot cathode or filament, but maintains a level of precision of gas density measurements approaching that of a hot cathode ionization gauge. The ionization gauge includes a collector electrode disposed in an ionization volume, an electron source without a heated cathode, and an electrostatic shutter that regulates the flow of electrons between the electron source and the ionization volume. The electrostatic shutter controls the flow of electrons based on feedback from an anode defining the ionization volume. The electron source can be a Penning or glow discharge ionization gauge.

Description:
BACKGROUND OF THE INVENTION 
   Hot cathode ionization gauges are the most common non-magnetic means of measuring very low pressures and the most widely used version worldwide was disclosed in U.S. Pat. No. 2,605,431 in 1952. A typical ionization gauge includes an electron source or a cathode. The electrons emitted by the electron source collide with gas atoms and molecules in an ionization volume and produce ions. The rate at which the ions are formed is directly proportional to the density of the gas (pressure at a constant temperature) in the gauge. 
   Two types of ionization gauges exist: hot cathode and cold cathode. The most common hot cathode ionization gauge is the Bayard-Alpert (B-A) gauge. The B-A gauge includes a heated filament (cathode) that emits electrons toward a cylindrical wire grid (anode) defining an ionization volume (anode volume). The temperature spread for most commonly used cathodes is about 1,500 degrees Celsius to about 2,200 degrees Celsius. An ion collector electrode is disposed within the ionization volume. Electrons travel from the electron source toward and through the anode, and are eventually collected by the anode. In their travel, the electrons impact molecules and atoms of gas and create ions. The ions are attracted to the ion collector electrode by the electric field within the anode volume. The pressure of the gas within the ionization volume can be calculated from ion current (I ion ) generated in the ion collector electrode and electron current (I electron ) generated in the anode by the formula P=(1/S)(I ion /I electron ), where S is a coefficient with the units of 1/torr and is characteristic of particular gauge geometry, electrical parameters and pressure range. 
   The operational lifetime of a typical B-A ionization gauge is approximately ten years when the gauge is operated in benign environments. However, these same gauges fail in hours or even minutes when operated at too high a pressure or in gas types that degrade the emission characteristics of the gauge&#39;s electron source (hot cathode). Examples of such hot cathode interactions leading to decreased operational lifetime range from degradation of the electron emission properties of the oxide coating on the hot cathode to exposure to water vapor. Degradation of the oxide coating dramatically reduces the number of electrons generated by the cathode, and exposure to water vapor results in the complete burnout of a tungsten cathode. 
   Cold cathode gauges come in many varieties. They include the Penning, the magnetron, the inverted magnetron, and the double inverted magnetron. The cold cathode inverted magnetron ionization gauge, sometimes referred to as a glow discharge gauge, also includes a cathode and an anode; however, the cathode is barely heated, and may heat to about a twenty degree Celsius rise over ambient temperature. The initial source of electrons is by a spontaneous emission event, or by a cosmic ray. As the electrons circle about the anode, the electrons ionize gas molecules and atoms through electron impact ionization, and other electrons are released by this event. As the cathode captures the ions, a current is generated in the cathode. This current is used as an indication of gas density and pressure. The capture of ions at the cathode also releases more electrons which are contained by the crossed electric and magnetic fields and sustains the discharge. In this way, a “cloud” of electrons and ions known as a plasma is formed in the ionization volume. However, cold cathode gauges suffer from relatively large inaccuracies due to uncontrolled discharge of electrons and surface phenomena. 
   SUMMARY OF THE INVENTION 
   An ionization gauge or corresponding method in accordance with an embodiment of the present invention eliminates the hot cathode or filament, but maintains the same level of precision of gas density measurements provided by a hot cathode ionization gauge. The ionization gauge includes a cold electron source, a collector electrode disposed in an ionization volume, and a regulated electrostatic shutter. The electrostatic shutter controls the flow of electrons between the electron source and an ionization volume based on the number of electrons in the ionization volume. The collector electrode then collects ions formed by the impact between the electrons and gas molecules in the ionization volume. In another embodiment of the present disclosure, the ionization gauge may also include multiple, or at least two collector electrodes. The collector electrodes may be positioned in the same location or in different locations relative to one another. 
   The ionization gauge can include an anode which defines the ionization volume and retains the electrons in a region of the anode. The collector electrode can be disposed within the ionization volume. The ionization gauge can include elements of a Bayard-Alpert type vacuum gauge. The electron source can be an inverted magnetron cold cathode gauge or glow discharge ionization gauge. 
   In yet another embodiment of the present disclosure, there is provided an ionization gauge. The ionization gauge has a source that generates electrons and a collector electrode. The collector electrode is disposed in an ionization volume and collects ions. The ionization gauge also has an electrostatic shutter that is configured to control the flow of electrons between the electron source, and the ionization volume. The ionization gauge also has an envelope that surrounds the source. The electrostatic shutter and the envelope permit electrons from a predetermined electric potential region to enter into the ionization volume. 
   In another embodiment, the anode of the electron source is connected to ground, and the envelope is connected to a negatively charged voltage potential. This permits electrons that are located near the anode to enter the ionization volume. 
   The ionization gauge may also be configured to have an annular ring of an electrostatic shutter (or shutters), which is located near the periphery of the envelope, or end. The electron source can be a cold cathode ionization source, which has an anode connected to a voltage source. The envelope can be grounded. This allows electrons that are located near the envelope to enter the ionization volume. 
   In yet another embodiment, there is provided a method of measuring gas pressure. The method generates electrons, and regulates the flow of electrons from a source to the ionization volume. The method also controls the energy of the electrons from the source, and then collects ions. The electron energy can be controlled by operating the anode at a ground potential and operating an envelope that surrounds the electron source at a predetermined negative potential. The electron energy can be controlled by operating the anode at a predetermined voltage, and operating an envelope that surrounds the anode at a ground potential. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
       FIG. 1  is a schematic view of an ionization gauge according to an embodiment of the present invention; 
       FIG. 2  is a flow diagram according to an embodiment of the present invention; 
       FIG. 3  is a schematic view of an ionization gauge according to a different embodiment of the present invention having a second collector electrode; and 
       FIGS. 4 and 5  show a schematic view of other embodiments of the ionization gauge for controlling electrons to allow electrons from a low potential region to enter the ionization volume. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A description of preferred embodiments of the invention follows. 
   As shown in  FIG. 1 , an ionization gauge  100  according to an embodiment includes a typical B-A ionization gauge  112  without a hot cathode but with a cold electron source  122 . The B-A ionization gauge  112  may be a nude or non-nude type ionization gauge. The cold electron source  122  may be a cold cathode ionization gauge such as a glow discharge cell, of which one version is the inverted magnetron. The glow discharge cell generates an electron cloud using crossed electrostatic and magnetic fields. Other cold cathode ionization gauges that may be used as the cold electron source  122  include the Penning gauge, magnetron and the double inverted magnetron. 
   Generally, the inverted magnetron type gauge has two electrodes with an electric field between them caused by an anode being positive with respect to a cathode. Likewise, the cathode is negative with respect to the anode. The inverted magnetron type gauge is surrounded by a magnet (not shown) which has lines of force going lengthwise through the volume perpendicular to the electric field. Generally, the anode attracts electrons which cannot go directly to the anode due to the crossed magnetic fields. The cathode attracts positively ionized atoms and molecules. The cathode generates electrons when ions impact it, and the cathode is generally sufficiently large so the ions do not miss the cathode during travel. The cold electron source  122  may also be a field emission electron source that includes a cathode or an array of cathodes with a gradient or a sharp point at the emitting end of the cathodes. 
   The cold electron source  122  includes an anode  125  that receives power from an anode voltage source  130 . The cold electron source  122  opens into a measurement chamber  119  of the B-A ionization gauge  112  through an electrostatic shutter  120 . The B-A ionization gauge  112  includes a collector electrode  105  and an anode or grid  110 . The grid  110  defines an anode or ionization volume. The grid  110  can take the form of a helical coil grid or a cylindrical mesh grid or any other shape that allows electrons to enter an ionization volume. A grid bias power supply  136  provides a constant positive voltage with reference to ground to the grid  110 . An ammeter  140  connects to the grid  110  and provides an output signal to an electron source control  150 . The electron source control  150 , in turn, provides an output signal to the electrostatic shutter  120 . Finally, the collector electrode  105  connects through an amplifier  160  to an electrometer  175 . 
   In operation, molecules and atoms of gas enter the measurement chamber  119  through a vacuum port  117 . The cold electron source  122  generates an electron cloud or plasma of copious amounts of energetic electrons. The electrostatic shutter  120  allows a regulated or controlled quantity of these electrons to exit from the cold electron source  122  into the B-A ionization gauge&#39;s measurement chamber  119  by, for example, providing a well-regulated, modulated high voltage power supply pulse at the exit to the cold electron source  122 . Alternatively, instead of a pulse as mentioned above, other configurations may also be possible to allow a controlled or regulated quantity of electrons to exit from the cold electron source  122  to the chamber  119 . 
   In another embodiment, a control voltage may vary continuously and the pulse may vary in height, width or shape in order to allow a controlled or regulated quantity of electrons to exit from the cold electron source  122  to the chamber  119 . Various configurations are possible and within the scope of the present disclosure. Most electrons do not strike the grid  110  immediately but pass through the grid  110  and into the ionization volume defined by the grid  110  where they create positive ions through electron impact ionization. 
   The ions, once created by electron impact ionization, tend to stay within the grid  110 . The ions formed within the grid  110  are directed to the collector electrode  105  by the electric field produced by a difference in potential between (a) the anode grid  110  at a potential that is positive with respect to ground and (b) the collector electrode  105  which is at a potential which is near ground potential (i.e., negative relative to the anode grid potential). The ions are collected by the collector electrode  105  to provide an ion current in the collector electrode  105 . The collector current is then amplified by the amplifier  160  and provided to an electrometer  175 . The electrometer  175  provides an indication of the magnitude of the collector current that is calibrated in units of pressure. 
   The ammeter  140  measures an electron current generated in the grid  110  from electrons that arrive at the grid  110 . This measured current represents the number of electrons being provided to the ionization volume from the cold electron source  122 . The measured current information from the ammeter  140  is provided to an electron source control unit  150  which uses the current information as feedback to control the electrostatic shutter  120 . 
   The electrostatic shutter  120  may act as a controlling grid (that is insulated from a mounting) at the port of the attachment of the cold electron source  122  to the ionization envelope  115 . A value of the grid  110  current measured by the ammeter  140  dictates the voltage on the controlling grid, which then controls the quantity of electrons flowing from the cold electron source  122  to the ionization gauge  100  when the controlling pulse occurs. It is envisioned that, in one embodiment, the ammeter  140  provides a signal and then, subsequently, the controlling voltage pulse occurs. The electron source control unit  150  regulates the quantity of electrons supplied to the ionization volume from the cold electron source  122  to ensure optimum ionization. 
     FIG. 2  is a flow diagram of a process of measuring a gas pressure  200  according to an embodiment of the present invention. After the process starts ( 205 ), an electron source generates electrons ( 210 ). Then, the flow of electrons between the electron source and an ionization volume is regulated ( 220 ) based on the number of electrons in the ionization volume. Finally, ions formed by impact between the electrons and the gas molecules and atoms in the ionization volume are collected ( 230 ). The process  200  then repeats ( 235 ). 
   In another embodiment of the present disclosure, the method  200  may further include filtering the flow of electrons to limit the flow to a predetermined energy range. Other embodiments of the electrostatic shutter are shown in  FIGS. 4 and 5 . The filtering can be electrostatic filtering, and a geometry of the gauge can be changed or modulated in order to further assist with filtering. The method  200  may also further include modulating a voltage of the electron source in response to pressure. In yet another embodiment, the method  200  may further include that a gauge geometry may also be modified to produce an electron current in response to a pressure. 
   Turning now to  FIG. 3 , in a further embodiment of the present disclosure. Here, the Bayard-Alpert ionization gauge  100  includes a second ion collector electrode  105 ′ in addition to the ion collector electrode  105 . Here, the second ion collector electrode  105 ′ is positioned inside the anode grid  110  to assist in better ion collection. The ions, once created by electron impact ionization, tend to stay within the anode grid  110 , while at higher pressures the ions also tend to stay outside the grid  100 . The collector current is then amplified by the amplifier  160  and provided to an electrometer  175 . The electrometer  175  provides an indication of the strength of the collector current that is calibrated in units of pressure. 
   Turning now to  FIG. 4 , there is shown an alternative embodiment of an ionization gauge  100  that has a cold electron source  122 . The electrostatic shutter  120  of  FIG. 1  is replaced with an annular electrostatic shutter generally shown as  120   a  and  120   b  which is located at the periphery of the opening of the envelope  125   a  or end of cold electron source  122 . One electrostatic shutter  120  is also envisioned with portions  120   a  and  120   b , and the present disclosure is not limited to any specific number of shutters. The ionization gauge  112  also includes a cold electron source  122  that releases electrons into the ionization envelope  115 . Again, the shutter  120   a ,  120   b  acts as a controlling grid to envelope  115 ; however, in this embodiment, electrons escape from the cold electron source  122  from a relatively low electrical potential region. This allows electron control in the envelope  115 .  FIG. 4  shows a configuration where electrons, from a low potential region, escape from the cold electron source  122  to the ionization volume  119 . Preferably, the cold cathode/glow discharge gauge has the anode  125  at a high voltage which is connected to anode voltage source  130 . Preferably, the anode  125  is housed in a cold cathode envelope  125   a  which is connected to the ground. Cold cathode envelope  125   a  preferably is a cylindrical shaped member with a circular cross section; however, the cold cathode envelope  125   a  is not limited to this shape, and may have a different shape. Thus, electrons near the cathode envelope  125   a  escape and are released into the ionization envelope  115 . This allows only electrons, which are located near the cold cathode envelope  125   a  to escape. This allows the energy spread of electrons to be controlled, and the ionization gauge  100  releases electrons at a relatively low potential into the ionization envelope  115 . 
   Turning to  FIG. 5 , there is shown yet another embodiment of the present ionization gauge  100 . In this embodiment, electrons near the anode  125  escape and enter the ionization volume  119 . In this embodiment, the center anode  125  of the cold electron source  122  is connected to ground, while the cold cathode envelope  125   a  is operated at a negative, high voltage value, and is connected to anode voltage source  130 . This permits electrons to escape from the cold electron source  122  at a low energy to control the energy spread of electrons by the value of the anode voltage source  130 , and by using an electrostatic shutter  120 . 
   While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.