J. D. Jarvis, H. L. Andrews, C. A. Brau, J. Driscol, B. Ivanov, C. L. Stewart, and K. Varga

1. Field-Emission Cathodes for Free-Electron Lasers J. D. Jarvis, H. L. Andrews, C. A. Brau, J. Driscol, B. Ivanov, C. L. Stewart, and K. Varga Department of Physics and Astronomy Y. M. Wong, B. K. Choi, J. Davidson, & W. Kang Department of Electrical and Computer Engineering

2. Diamond Field-Emitter Arrays (DFEAs) • Fabrication • Conditioning and high current density operation • Emittance measurements • Electron energy spectrum measurements • Carbon-Nanotube Field-Emission Cathodes • Conclusions and outlook

3. FEAs are microfabricated arrays of individual field emitters: • Can be made with and without gate electrodes • Emission can be time gated using various techniques • Additional self-aligned electrodes may be included for beam collimation • Many varieties, including Spindt type, CNT, Diamond (DFEA) etc.. ungated DFEA Gated DFEA w/ collimation x-px phase space jonathan.d.jarvis@vanderbilt.edu

4. Diamond field-emitter arrays are a promising new beam source: • Developed at Vanderbilt by Davidson et al. • Fabricated with an inverse mold transfer technique • CVD diamond w/ boron and nitrogen, ~5 nm tip radius • Can be customized with multiple nanotips jonathan.d.jarvis@vanderbilt.edu

5. Two different types of gated DFEAs have been fabricated: • Typical turn-on voltages of ~ 40 V (gate to cathode) • Gated DFEAs can be produced using either SOI or volcano processes SOI process Volcano process jonathan.d.jarvis@vanderbilt.edu

6. DFEAs have several advantages over photocathodes: • Rugged: high thermal conductivity and chemically inert • Tolerate poor vacuum operation, >10-6 Torr, transport in air • Can be conditioned for highly uniform emission • No drive laser required • Compatible with NCRF/SRF technology • No heat generated • No laser window required • Photocathode survival in Ampere-class injectors is uncertain jonathan.d.jarvis@vanderbilt.edu

7. DFEAs can be conditioned for highly uniform and stable emission: • Fluctuations are observed in each beamlet due to transient adsorbates • Heating and operation at high fields - stable emission achieved • Sustained operation at high fields - uniform emission achieved by dulling sharpest tips (ms to DC) Quartz capillaries jonathan.d.jarvis@vanderbilt.edu

8. (a) (b) (c) (d) (e) High-current conditioning provides a high degree of spatial uniformity: • Evaporation of nanotips is self limiting, leading to highly uniform emission. • Similar to pulsed conditioning of Spindt cathodes. • DC Studies limited by anode destruction (maximum per-tip current thus far : 15 mA) (b) (d) (e) (a) (c) After 0.5hr at ~500nA/tip After 0.5hr at ~20nA/tip After 0.5hr at ~100nA/tip After 1hr at ~ 1.5mA/tip unconditioned jonathan.d.jarvis@vanderbilt.edu

9. Close-diode DC conditioning is only possible for large-pitch arrays: • Anode destruction prevents DC conditioning of dense arrays • For 4 mm pitch array, 15 mA/tip, and 3 kV beam energy, the power density at anode is ~300 kW/cm2 jonathan.d.jarvis@vanderbilt.edu

10. Pulsed operation at microsecond time scales allows conditioning of dense arrays: • Microsecond pulsing avoids anode sputtering • Successful pulsed conditioning of 224x224 (14-mm pitch) array • Marginal increase in turn-on field Before After (b) (d) (e) (a) (c) jonathan.d.jarvis@vanderbilt.edu

11. UngatedDFEAs are now being tested at high currents and high current density: • Recently, 0.6 A, achieved from both 224x224 (14-mm pitch) and 224x448 (20-mm pitch) (b) (d) (e) (a) (c) • Pulsed operation of a 4-mm pitch (20x250) array operated at ~30 A/cm2 (~5mA/tip), limited by power supply. jonathan.d.jarvis@vanderbilt.edu

12. Emittance for 20-mm pitch array has been successfully measured: • Fabricated PP by fs laser machining; 50 mm thick, 1-mm pitch, ~ 50-mm hole diameter • For a 1-mm cathode, normalized x emitance is: • Previous results (28-mm pitch): jonathan.d.jarvis@vanderbilt.edu Aperture size

13. DFEA spectrum is examined with a high-res. retardation energy analyzer: • Based on University of Maryland (UMER) design • Deconvolved resolution function from thermionic spectrum: 150 meV FWHM • Integrated into DC test stand capable of anode-cathode planarity and gap adjustment during high-voltage operation jonathan.d.jarvis@vanderbilt.edu

14. From previous experiments, adsorbates are expected to have significant effects: • Adsorbates modify the intensity and spectrum of field-emission • Resonant tunneling greatly enhances emission at certain energies • Typically, adsorbates remain stable for many seconds (long enough to acquire spectra) • Analyzer aperture examines current from a single emitter jonathan.d.jarvis@vanderbilt.edu

15. Adsorbates significantly modify the energy spectrum of a clean emitter: • Each spectrum taken for identical experimental conditions during various periods of stable emisison • Transitions between spectra are concurrent with emission current fluctuations jonathan.d.jarvis@vanderbilt.edu

16. Spectrum from a clean emitter is similar to that of metals: • Expect “clean” spectra to occur at low intensity and near the Fermi energy • FWHM of ~0.3 eV, narrowest spectrum observed • The effects of varying sp2, sp3, boron, and nitrogen unknown jonathan.d.jarvis@vanderbilt.edu

17. Observe order-of-magnitude current enhancements without spectral broadening: jonathan.d.jarvis@vanderbilt.edu

18. For a large 6-D brightness the anti-symmetry of electrons is important: • The uncertainty principle and Pauli exclusion set a maximum phase-space number density of 2/ h3 for fermions. • A quantum-degenerate beam would have special properties such as suppressed e-e scattering (similar to electrons in a metal) • Beams with high degeneracy exhibit antibunching jonathan.d.jarvis@vanderbilt.edu

19. A field-emission microscope was built for studying CNT field emisison: • Large fields at the emitter surface “freeze” in an image of the emission area • Pentagonal rings, indicative of a clean nanotube cap, are clearly visible • Adsorbate migration and large, localized current enhancements are seen jonathan.d.jarvis@vanderbilt.edu

20. An adsorbate on a carbon nanotube field emitter may produce a QDEB: • A single adsorbate emitted more current than the entire nanotube (+6 mA) • Transient event, only lasting a fraction of a second • If source ~0.1 nm, emittance is close to Heisenberg limit • If DE is ~0.3 eV, brightness is close to Pauli limit Pauli limit for DV = 0.3 eV: jonathan.d.jarvis@vanderbilt.edu

21. DFEAs can tolerate poor operating conditions, be conditioned for highly uniform emission, be produced in an arbitrary footprint Numbers to remember: for 1 mm cathode, ~1 A, ~1 mm-rad Production and testing is beginning on gated devices DFEAs are slated for testing in several RF guns (ANL, NPS, LANL) Adsorbed species are important Individual adsorbates may provide quantum-degenerate beams of ~10 mA CONCLUSIONS: jonathan.d.jarvis@vanderbilt.edu

22. THANKS TO ALL THE GROUP! jonathan.d.jarvis@vanderbilt.edu