Vanderbilt University, Nashville
We present new results in the measured electron energy spectrum from diamond field emitters. The energy spectrum from a clean diamond surface has been measured and is comparable in shape and width to that of metal emitters. The results suggest that the emitted spectrum is sensitive to the presence of adsorbed species on the emitter surface. Electrons significantly below the cathode’s Fermi level are emitted by resonant tunneling. Furthermore, these resonant surface states can increase the total emitted current by more than an order of magnitude while maintaining a narrow spectral width (~0.5 eV). Experiments are also being performed with individual multiwall carbon nanotubes (MWCNTs). We have observed beams emitted from individual residual gas molecules that approach the quantum-degenerate limit of electron-beam brightness. This limit has profound consequences for the behavior of an electron. Tightly bound designer adsorbates may greatly enhance the emission properties and improve performance in electron injector systems.
Vanderbilt University, Nashville
Diamond field-emitter arrays (DFEAs) possess several advantages over photocathodes: high brightness, ruggedness, no drive laser requirement, and minimal heating. A gated DFEA with micron-scale cathode-gate spacing has the added benefits of direct e-beam modulation and low operating voltages < 100 V. A second gate can be integrated, creating built-in focusing capability. We have developed two types of self-aligning gate fabrication methods. First, pyramidal molds are formed on a SOI (silicon on insulator) substrate then coated with CVD nanodiamond. The bulk layer of silicon is thinned, followed by oxide etching and opening the diamond tip isolating the gate electrode and insulating layer from the cathode. The second method uses additive physical evaporation depositions of insulating and gate electrode layers on top of the DFEAs. Chemical etching of the insulating layer separates and opens cathode tip due to ‘lift off’ type step coverage of the evaporation technique. A 2-mask fabrication process has been used to pattern the gate to optimize active gate area and increase yield. Fabrication techniques and electrical behavior of the gated DFEAs will be discussed.
Vanderbilt University, Nashville
We present recent advances in the uniformity conditioning of diamond field-emitter arrays (DFEAs), and new results from emittance measurements of their emitted electron beams. DFEAs have shown considerable promise as potential cathodes for free-electron lasers. They have demonstrated their rugged nature by providing high per-tip currents, excellent temporal stability, and significant resistance to back-bombardment damage during poor vacuum, close-diode DC operation. Until now, the successful conditioning of high-density arrays has been precluded by thermal damage to the anode. We report successful uniformity conditioning of densely packed DFEAs using microsecond-pulsed high-current conditioning (HCC). A high degree of spatial uniformity was confirmed in low-current DC testing following these HCC procedures. The conditioned arrays will be used to refine previous measurements of the normalized transverse emittance of the emitted electron beams.
Vanderbilt University, Nashville
High-quantum-efficiency photocathodes used for free-electron lasers tend to be fragile and demand complex drive lasers. Field-emitter arrays eliminate both these problems, but introduce other problems along with interesting new physics. Diamond field-emitter arrays are rugged and forgiving of poor vacuum. They are easily conditioned to give uniform emission, current density on the order of 100 A/cm2 before phase compression, and emittance smaller than 10 μm-radians. In gated versions the emission can be phased to the rf drive and the emittance can be reduced by the focusing effect of the gate. Experimental evidence from diamond pyramids and carbon nanotubes suggests that field emission is enhanced by resonant tunneling through molecules adsorbed on the surface. The emission from individual molecules appears to reach the fundamental limits imposed by the Heisenberg uncertainty principle and by the Pauli exclusion principle.
