Paper NS-TuA3
Single Walled Nanotube (SWNT) Fiber Field Emission Cathodes

Tuesday, October 19, 2010, 2:40 pm, Room La Cienega

High power THz sources and amplifiers hold the potential to revolutionize the remote sensing and communication industries. Traveling wave tubes (TWT) are the most promising technology for compact high power amplifiers at THz frequencies. High frequency applications necessitate the need for small diameter beam tunnels and thus small diameter high current electron beams. Small diameter beams are typically achieved by emitting electrons from a thermionic cathode surface much larger than the THz TWT beam tunnel and focusing the beam using either electrostatic or magnetic field electron beam optics. To avoid using focusing optics, a micron sized high aspect ratio field emission (FE) cathode can be immersed in the confining magnetic field and used to generate an electron beam with a smaller diameter than the THz TWT beam tunnel. The key technical challenge with using a micron sized FE cathode is achieving the high current densities, long life time, and uniform current densities within the electron beam. The Air Force Research Laboratory (AFRL) began research on small diameter (<100 micron) single fiber DC cathodes for this application. Two types of fibers were tested, 1) those made from carbon with graphitized outer shells, and 2) fibers made solely from single walled carbon nanotubes (SWNT) that are densely packed and highly aligned along the axial direction of the fiber. Carbon fiber cathodes suffered serious degradation due to joule heating after only 25 hours of operation while only producing up to 200 microamps of current. A single SWNT fiber cathode has emitted approximately 3 milliamps of current for hundreds of hours while suffering minimal damage, as well as an 8x reduction in turn-on voltage over carbon fiber cathodes. Efforts are currently underway to optimize these SWNT fiber FE cathodes by tip shaping and applying low work function coatings.