High efficiency ultrananocrystalline diamond electron emitters Chunguang Jing, Sergey V. Baryshev, and Sergey Antipov Euclid TechLabs, LLC Anirudha V. Sumant Argonne National Laboratory Photocathode Physics for Photoinjectors, Berkeley CA, Nov. 2014

Euclid Techlabs LLC Euclid TechLabs LLC, founded in 1999 is a company specializing in the development of new dielectric materials and advanced designs for high power/high frequency applications. Additional areas of expertise include high power rf components, Photoinjectors, dielectric structure based accelerators and "smart" materials technology and applications. Fermilab • 2014: 12 people research staff and 3 administrative, • 2 offices: Bolingbrook, IL (lab) and Gaithersburg MD (administrative). • Tight collaborations with National Labs: Fermi, Argonne, BNL, Berkely.

Euclid Techlabs

www.euclidtechlabs.com

Argonne

Products and Projects

L-band high peak current LINAC

UHV L-band RF window

BPM compact readout

Linear and non-linear ceramics low loss; various form factors

Compact, detachable coaxial coupler for SRF cavity (joint project with FNAL) Photoinjectors: • L-band high peak current AWA-style • S-band high REP

Introduction 1) High quantum efficiency/yield 2) Robustness/lifetime in low vacuum/air 3) Emittance

Negative electron affinity (NEA) photocathodes GaAs:Cs-O CBM

e– e–

χ≈–0.1 eV

ε

Cs

εF

VBM

e–

surface

εg≈1.4 eV

e–

e–

O Vacuum level

x Karkare et al. PRL 112, 097601 (2014)

Diamond is a NEA material Excellent solar blind photocathode QE>~1-10% Himpsel et al. PRB 20, 624 (1979)

Tremsin et al. SPIE Proc. 4139, 16 (2000)

Doping of diamond Effective work function is activation energy of a dopant! CBM S:εa≈0.4 eV

χ≈–1 eV H C

P:εa≈0.6 eV N:εa≈1.7 eV

x

VBM

surface

εg≈6 eV

ε

Vacuum level

Nitrogen-incorporated ultrananocrystalline diamond. Basic characterization ~100 nm highly conductive (N)UNCD on Moly

Electron microscopy

Raman spectroscopy

Samples transfer. Work function results

2h in air Light in

e– out

V

A

KP tip

Quantum efficiency results

Photocathode Breakdown Study Beamline @AWA

(N)UNCD as field emitter: test-stand and cathode plug

Electron emission seen by YAGs and Faraday cup At surface gradients 45-65 MV/m, peak currents of 1-80 mA (equivalent to 0.3-25 mA/cm2)

Emittance and longitudinal energy spread

longitudinal FWHM and rms were 0.7% and 11% @ 2 MeV

σ waist 2 2 ε  σ L  σ waist L beam emittance was 1.5 mm×mrad/mm-rms

Surface Examination Before and After the Test

Summary and Outlook Planar UNCD is a unique platform for high efficiency e-emitters 1) n-type (N)UNCD with H-passivation is air resistance high QE photocathode: ~0.1% @ 254 nm and ~10-5 % @ 405 nm; 2) n-type (N)UNCD is field emitter producing self-bunched electrons when directly subject to RF field: energy spread <~1 %, emittance 1.5 mm×mrad/mm-rms. Next steps 1) Photoemission in Photoinjector; Preserving air resistance , improve QE to >1% @ 254 nm and sensitize UNCD toward 532 nm with QE ≥10-5 %; 2) Demonstrate planar field emitter performance in SRF injector @ CW operation as 1st step toward fully cryogenic SRF linacs.