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

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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
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• 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

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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
x-px phase space
w/ collimation
jonathan.d.jarvis_at_vanderbilt.edu
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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_at_vanderbilt.edu
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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_at_vanderbilt.edu
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DFEAs have several advantages over photocathodes

• Rugged high thermal conductivity and chemically
  inert
• Tolerate poor vacuum operation, gt10-6 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_at_vanderbilt.edu
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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

jonathan.d.jarvis_at_vanderbilt.edu
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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)

(a)
(b)
(c)
(d)
(e)
After 0.5hr at 500nA/tip
After 0.5hr at 100nA/tip
After 1hr at 1.5mA/tip
After 0.5hr at 20nA/tip
unconditioned
jonathan.d.jarvis_at_vanderbilt.edu
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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_at_vanderbilt.edu
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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

(a)
(b)
(c)
(d)
(e)
jonathan.d.jarvis_at_vanderbilt.edu
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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)

(a)
(b)
(c)
(d)
(e)

• Pulsed operation of a 4-mm pitch (20x250) array
  operated at 30 A/cm2 (5mA/tip), limited by
  power supply.

jonathan.d.jarvis_at_vanderbilt.edu
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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_at_vanderbilt.edu
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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_at_vanderbilt.edu
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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_at_vanderbilt.edu
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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_at_vanderbilt.edu
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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_at_vanderbilt.edu
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Observe order-of-magnitude current enhancements
without spectral broadening
jonathan.d.jarvis_at_vanderbilt.edu
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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_at_vanderbilt.edu
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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_at_vanderbilt.edu
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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_at_vanderbilt.edu
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CONCLUSIONS

• 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

jonathan.d.jarvis_at_vanderbilt.edu
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THANKS TO ALL THE GROUP!
jonathan.d.jarvis_at_vanderbilt.edu