Method and apparatus for controlling electron beam current

Abstract

An x-ray generating device includes a field emission cathode formed at least partially from a nanostructure-containing material having an emitted electron current density of at least 4 A/cm 2 . High energy conversion efficiency and compact design are achieved due to easy focusing of cold cathode emitted electrons and dramatic reduction of heating at the anode. In addition, by pulsing the field between the cathode and the gate or anode and focusing the electron beams at different anode materials, pulsed x-ray radiation with varying energy can be generated from a single device. Methods and apparatus for independent control of electron emission current and x-ray energy in x-ray tubes are also provided. The independent control can be accomplished by adjusting the distance between the cathode and anode. The independent control can also be accomplished by adjusting the temperature of the cathode. The independent control can also be accomplished by optical excitation of the cathode. The cathode can include field emissive materials such as carbon nanotubes.

1 . An x-ray generating device comprising: 
 a. a chamber;    b. a field emission cathode, comprising a film comprising carbon nanotube-containing materials on a conducting substrate;    c. an anode target;    d. a gate electrode comprising a metal grid, wherein the grid is placed between the cathode and the anode; and    e. a power supply structure to place the gate electrode at a higher electrical potential than the cathode for extracting the electrons from cathode, and to establish a large electrical potential difference between the cathode and the anode for generating the x-ray radiations.    
     
     
         2 . The device of  claim 1 , further comprising a focusing structure to focus the field emitted electron beam to a narrow area on the target, wherein the focusing structure is placed between the cathode and anode.  
     
     
         3 . The device of  claim 2 , wherein the focusing structure comprises one or more electrodes placed at a different electrical voltage from that of the gate electrode.  
     
     
         4 . The device of  claim 1 , further comprising a feedback circuit constructed to vary the applied electrical potential difference between the gate electrode and the electrical potential for controlling the generated x-ray radiation.  
     
     
         5 . The device of  claim 1 , wherein a current density of the field emitted electrons is more than 1 mA/cm 2  from the field emitting cathode area.  
     
     
         6 . The device of  claim 1 , wherein the current density of the field emitted electrons from the cathode is more than 10 mA/cm 2 .  
     
     
         7 . The device of  claim 1 , wherein electrodes are emitted from the cathode when the electrical field between the gate electrode and the cathode is large than 2 V/micron.  
     
     
         8 . The device of  claim 1 , wherein the cathode is a film comprising randomly oriented carbon nanotubes deposited on a conducting substrate by one of the following processes: solution casting, sprauing, spin-coating, sputtering, screen-printing, or electrophoretic deposition.  
     
     
         9 . The device of  claim 1 , wherein the cathode comprises a carbon nanotube film grown directly on a conducting substrate by a chemical vapor deposition method.  
     
     
         10 . The device of  claim 1 , wherein the carbon nanotube containing material comprises a patterned film defined by electron emitting carbon nanotubes aligned with openings disposed in the gate electrode.  
     
     
         11 . An x-ray generating device comprising: 
 a. a chamber;    b. a group of field emission cathodes, wherein each cathode comprises a carbon nanotube-containing film on a conducting substrate;    c. an anode target;    d. a gate electrode placed between the cathode and the anode; and    e. a power supple structure to place the gate electrode at a higher electrical potential than the cathode for extracting the electrons from cathode; and to establish a large electrical potential difference between the cathode and the anode for generating the x-ray radiations.    
     
     
         12 . The device of  claim 11 , wherein the group of cathodes emits electrons at a given time when an electrical field is applied over the group of cathodes.  
     
     
         13 . The device of  claim 11 , wherein each cathode or group of cathodes emits electrons in a predetermined sequence and a predetermined and variable electron current.  
     
     
         14 . The device of  claim 11 , wherein the x-ray radiation is a scanning x-ray beam originating from different points of the anode target, the scanning x-ray beam produced by switching on and off each cathode or a group of cathodes in a predetermined sequence, wherein the electrode beams are directed to predetermined locations on the target surface.  
     
     
         15 . An x-ray generating device comprising: 
 a. a chamber;    b. a field emission cathode, comprising a carbon nanotube-containing film on a conducting substrate;    c. an anode target;    d. a gate electrode comprising a metal grid, wherein the grid is placed between the cathode and the anode;    e. an electron beam focusing structure placed between the gate electrode and the anode; and    f. a power supply structure to place the gate electrode at a higher electrical potential then the cathode for extracting the electrons from cathode, and to place the focusing structure at a lower electrical potential than the gate electrode for focusing the electron beam, and to establish a large electrical potential difference between the cathode and anode target for generating the x-ray radiations.    
     
     
         16 . The device of  claim 15 , wherein the focusing structure comprises one or more electrodes with a cylindrical geometry placed at a different electrical potential from that of the gate electrode.  
     
     
         17 . The device of  claim 15 , further comprising a feed back circuit constructed to vary the applied electrical potential difference between the gate electrode and the electrical potential for controlling the generated x-ray radiation.  
     
     
         18 . The device of  claim 15 , further comprises a circuit constructed to vary the applied electrical potential difference between the gate electrode and the focusing electrode for controlling the electron focusing spot at the target.  
     
     
         19 . The device of  claim 15 , wherein a current density of the filed emitted electrons is more than 1 mA/cm 2 .  
     
     
         20 . The device of  claim 15 , wherein a current density of the field emitted electrons from the cathode is more than 10 mA/cm 2 .  
     
     
         21 . The device of  claim 15 , wherein electrons are emitted from the cathode when the electrical field between the gate electrode and the cathode is large than 2 V/micron.  
     
     
         22 . The device of  claim 15 , wherein the emitted electron current density is at least 1 mA/cm 2  when the cathode is subjected to an applied electrical field of more than 2 V/micron.  
     
     
         23 . The device of  claim 15 , wherein the cathode is a film comprising randomly oriented carbon nanotubes deposited on a conducting substrate by one of the following processes: solution casting, spraying, spin-coating, sputtering, screen-printing, or electrophoretic deposition.  
     
     
         24 . An x-ray generating device comprising: 
 a. a chamber;    b. a field emission cathode, the cathode comprising a carbon nanotube-containing film having an emitted electron current density of at least 1 mA/cm 2  when subjected to an applied electrical field of more than 2 V/micron;    c. an anode target;    d. a gate electrode; and    e. a power supply structure to place the gate electrode at a higher electrical potential than the cathode for extracting the electrons from cathode, and to establish a large electrical potential difference between the cathode and the anode for generating the x-ray radiations.    
     
     
         25 . The device of  claim 24 , further comprises a focusing structure to focus the field emitted electron beam to a narrow area on the target, wherein the focusing structure is placed in between the cathode and anode.  
     
     
         26 . The device of  claim 25 , wherein the cathode is a film comprising randomly oriented carbon nanotubes deposited on a conducting substrate by one of the following processes: solution casting, spraying, spin-coating, sputtering, screen-printing, or electrophoretic deposition.  
     
     
         27 . An x-ray generating device comprising: 
 a. a chamber;    b. a field emission cathode, comprising a carbon nanotube-containing film electrophoretically deposited on a conducting substrate;    c. an anode target;    d. a gate electrode comprising a metal grid;    e. an insulating spacer between the gate electrode and the cathode;    and    f. a power supply that enables automatic adjustment of the electrical field between the gate electrode and cathode to maintain a constant x-ray intensity.    
     
     
         28 . A self-focusing x-ray source comprising: 
 a. a chamber;    b. a field emission cathode, comprising a carbon nanotube-containing film deposited on a conducting substrate with a concave surface;    c. a gate electrode with the same curvature as the cathode surface;    d. an anode target;    e. an insulating spacer between the gate electrode and the cathode;    and    f. a power supply that enables automatic adjustment of the electrical field between the gate electrode and the cathode such that a constant x-ray intensity is maintained.    
     
     
         29 . An x-ray generating device comprising: 
 a. a chamber;    b. a field emission cathode, comprising a film comprising carbon nanotube-containing materials on a conducting substrate;    c. an anode target;    d. a gate electrode comprising a metal grid, wherein the grid is placed between the cathode and the anode;    e. a power supply structure to place the gate electrode at a higher electrical potential than the cathode for extracting the electrons from cathode, and to establish a large electrical potential difference between the cathode and the anode for generating the x-ray radiations;    f. wherein the field emission cathode comprises carbon nanotubes; and    g. wherein the field emitted electrons are self focused to a small area on the anode.