Development of Diamond Field-Emitter Arrays for Free-Electron Lasers

Jonathan D. Jarvis, Heather L. Andrews, Charles A. Brau, Chris L. Stewart
Department of Physics and Astronomy, Vanderbilt University
Bo Kyoung Choi, Jimmy Davidson, Weng Kang, Supil Raina, Yong Mui Wong
Department of Electrical and Computer Engineering, Vanderbilt University

FEAs are microfabricated arrays of field emitters:
• Can be made with and without gate
electrodes
• Additional self-aligned electrodes may be
included for beam collimation to minimize
emittance
• Many varieties, including Spindt type, CNT,
Diamond (DFEA) etc..
ungated DFEA
w/ collimation
x-p x phase space
Gated DFEA

DFEAs have several advantages over photocathodes:
• Rugged
• Tolerate poor vacuum operation, 10 -5 Torr, transport in air
• Can be conditioned for highly uniform emission
• Degrade gracefully (ungated)
• No drive laser required
• Compatible with NCRF/SRF technology
– No heat generated
– No laser window required

Two main RF gun integration strategies for DFEAs:
• Gated FEAs can be driven by a low-voltage
harmonic feed
• For ungated FEAs, emission at a desirable
RF phase may be accomplished by 3 rd
harmonic mixing or cathode extension into the
cavity
Emission Time Gating

DFEAs are fabricated by a mold-transfer technique:
• MPCVD diamond grown in an oxide sharpened Si mold
• Brazed to a Mo substrate
• Tip radii of < 10nm
Thermal oxidation of Si
Oxide patterning
Anisotropic etching
Tip mold sharpening oxidation
MPCVD diamond deposition
Metal deposition Ni/Ti
High temperature diamond brazing
Si mold & oxide removal
TiCuSil
Mo

DFEAs can be conditioned for highly-uniform
emission:
Investigated several conditioning methods:
1. Selective gas exposure
• no improvement thus far
2. Vacuum Thermal Electric Conditioning (VTEC)
• High field annealing of tightly adsorbed species
• Greatly improved temporal stability
3. Sustained moderate/high current operation
• Morphology changes by thermal-assisted field evaporation
CVD diamond cathode
Quartz capillary
Glass/Y 2 O 3 :Eu/Ni (5nm)
+ HV
A
Mo substrate

High current conditioning most successful thus far:
• Evaporation of nanotips is self limiting, leading to highly uniform
emission.
• Similar to pulsed conditioning of Spindt cathodes.
• During conditioning studies, microtips achieved per-tip currents exceeding
15µA. Scales to 100A/cm 2 DC for highest densities
• Studies limited by anode destruction
(a) (b) (c) (d) (e)
(a) (b) (c) (d) (e)
unconditioned
After 0.5hr
at ~20nA/tip
After 0.5hr
at ~100nA/tip
After 0.5hr at
~500nA/tip
After 1hr at
~ 1.5µΑ/tip
~200°C VTEC

Preliminary emittance measurements performed:
• Measurement made with a
pepperpot technique
• 3x24 DFEA, 28µm pitch
• Estimated rms divergence of
~38mrad
• Value will be refined using higher
density arrays
• For 2kV beam and a 1mm cathode,
normalized x-emittance is:
+ HV
A
Collector:
ZnO/Y 2 O 3 :Eu
Metal spacer
Pepperpot anode
Quartz capillary
Mo substrate
CVD diamond cathode

Measured emittance agrees well with simulations:
• Field solving in POISSON, trajectory
calculations in GPT
• Emitter is conical rather than pyramidal
• rms angular divergence at the pepperpot
position is ~40mrad

High-resolution energy analyzer has been built for
measuring DFEA energy spread:
• Based on UMER design [1]
• Focusing electrode improves resolution by several orders of magnitude
• Integrated into DC teststand capable of anode-cathode planarity and gap
adjustment at high voltage
A
Collector: ZnO/Y 2 O 3 :Eu
A
Retarding mesh
V bias
Focusing electrode
V focus
Front aperture plate
V beam
Mo substrate
CVD diamond cathode

Estimated resolution is better than kinetic-energy
error in simulations:
• Modeled in SIMION
• Resolution better than KE error of 10ppm
• Optimum focusing strength found to be:
FWHM vs. focusing
Collected fraction
vs. bias

Preliminary spectra taken with a 3x24 DFEA:
• Beamlets from multiple adsorbates on a single
tip within analyzer’s angular acceptance
• Adsorbate fluctuations likely change the energy
spectrum and can not be normalized out
• FWHM of averaged spectra is ~1.3eV, much
larger than ~0.3eV for a metallic emitter
• UHV experiments with optimized cathode
geometries will yield clean diamond spectra

Conclusions:
• DFEAs are very rugged and operate well in extreme environments
• Self-limiting conditioning procedures have been developed that achieve highly
uniform emission.
• Currents in excess of 15µA/tip DC have been observed, which scales to
~100A/cm 2 for achievable tip densities (4µm pitch)
• Much higher current densities (1kA/cm 2 ) will be possible for pulsed RF
operation
• Emittance measurements agree with simulated values, 1.4µm for a 1mm
cathode
• Preliminary measurements of averaged energy spread (1.3eV) are much higher
than those of metallic field emitters (0.3eV)
• All presented results will be refined in the coming months