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Field Emission Cathode Gating for RF Electron Guns (IN-04-039)

Argonne National Laboratory

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<p>
	Field emission (FE) gun&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</p>

Field emission (FE) gun          

<p>
	<span style="line-height: 115%; font-family: &quot;Calibri&quot;,&quot;sans-serif&quot;; font-size: 10pt; mso-fareast-font-family: Calibri; mso-bidi-font-family: &quot;Times New Roman&quot;; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA;"><font color="#000000">Simulation of FE gun emittance versus beam current density</font></span></p>

Simulation of FE gun emittance versus beam current density

Technology Marketing Summary

Scientists at Argonne National Laboratory have created an innovative way to enhance the performance of radio frequency (RF) electron guns: a method of gating electron emissions from field-emitter cathodes for these guns that also provides a highly focused electron beam without magnetic fields or a curved cathode surface. The innovations provide a high-brightness, very low emission beam with beam energies from 1 to 5 MeV. The gun also eliminates the need for lasers or hot filaments to produce electrons, making it simpler to use and able to operate in a superconducting environment.

Description

RF electron guns work by establishing an electromagnetic field inside a cavity. This field is used to accelerate electrons emitted from a cathode, down the bore of the gun and out an exit port in the form of an electron beam. The phase of the RF field at which a given electron is emitted from the cathode determines whether it can exit the cavity and, if so, at what energy. Electrons emitted when the electric field is positive accelerate in the cavity and exit the gun with good quality, i.e., well focused, low energy spread, and at a desired energy. Electron emission is suppressed when the RF field is negative. The worst condition is when the RF field accelerates the electrons, but the RF field changes sign and the electrons are accelerated back to the cathode and cause heating and damage.

 

Argonne’s invention uses two RF sources to generate high-brightness electron beams: one at the fundamental resonance of the cavity and another at a harmonic frequency, such as the third harmonic. The phase and strength of the harmonic field is adjusted relative to the fundamental field so that a field emission cathode can emit electrons. The electrons are emitted in a short pulse. In addition, the cavity is shaped to focus the emitted electrons even as they are exiting the cathode. This self-focusing eliminates the need for external magnetic fields. The resulting electron beam has exceptional beam emittance and high average current.  A cross section of an RF cavity for an electron gun with a field emitter cathode shows the location of the cathode, the RF power feed, and a electric field map at maximum acceleration (figure 1).  Computer simulations of the beam emittance versus the electron current density at the cathode illustrates the low emittance that is possible with the two-frequency electron gun (figure 2).  Even at 20 mA total current, the emittance is less than 6 nanometer-radians.  At higher current, e.g., 50mA, space charge causes the electron beam to diverge at high current density.

Benefits

Argonne’s design is compatible with both superconducting and normal-conducting RF electron guns, since unlike conventional processes, the method does not rely on high temperatures to induce electron emission.

Applications and Industries
  • High-power accelerators, such as free-electron lasers and terahertz sources;
  • Development of electron microscopes that do not require chromatic correction;
  • Improvements in electron-beam welding, electron-beam lithography, and vapor deposition systems; and
  • Cancer treatment using electrons.
More Information

Proof of Concept. A fully operational prototype for a normal conducting electron gun is estimated to cost $250,000. Of this amount, $75,000 is for the RF cavity. A normal conducting gun includes cold cathode electron emission and high brightness, but limited electron current.  A prototype of a superconducting RF gun with a field emission cathode would require approximately $1.5M to develop.  An SRF gun will be capable of high average electron current in addition to cold cathode and high brightness.  It is estimated that production costs for a SRF field emission gun would be $500,000.

Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Date
Patent 7,394,201
Patent
7,394,201
Field emission cathode gating for RF electron guns and planar focusing cathodes
A novel method of gating electron emission from field-emitter cathodes for radio frequency (RF) electrode guns and a novel cathode that provides a focused electron beam without the need for magnetic fields or a curved cathode surface are provided. The phase and strength of a predefined harmonic field, such as the 3rd harmonic field, are adjusted relative to a fundamental field to cause a field emission cathode to emit electrons at predefined times for the generation of high-brightness electron beams. The emission time is gated responsive to the combined harmonic and fundamental fields and the response of the FE cathode to the combined fields. A planar focusing cathode includes a selected dielectric material, such as a ceramic material, to provide an electron beam emission surface. Metal surfaces are provided both radially around and behind the dielectric material to shape the electric fields that accelerate and guide the beam from the cathode surface.
Argonne National Laboratory 07/01/2008
Issued
Patent 6,987,361
Patent
6,987,361
Field emission cathode gating for RF electron guns and planar focusing cathodes
A novel method of gating electron emission from field-emitter cathodes for radio frequency (RF) electrode guns and a novel cathode that provides a focused electron beam without the need for magnetic fields or a curved cathode surface are provided. The phase and strength of a predefined harmonic field, such as the 3rd harmonic field, are adjusted relative to a fundamental field to cause a field emission cathode to emit electrons at predefined times for the generation of high-brightness electron beams. The emission time is gated responsive to the combined harmonic and fundamental fields and the response of the FE cathode to the combined fields. A planar focusing cathode includes a selected dielectric material, such as a ceramic material, to provide an electron beam emission surface. Metal surfaces are provided both radially around and behind the dielectric material to shape the electric fields that accelerate and guide the beam from the cathode surface.
Argonne National Laboratory 01/17/2006
Issued
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
ANL-IN-04-039PrototypeAvailable05/28/201305/28/2013

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To: Andrea Sagols<partners@anl.gov>