Measurements which show that a locally lower work function contributes to field emission enhancement, along with the geometrical field enhancement factor.

Schottky Enabled Photoemission & Dark Current Measurements John Power, Eric Wisniewski, Wei Gai Argonne Wakefield Accelerator Group Argonne National Laboratory MeVArc Workshop Helsinki, Finland June 29, 2011 at the S-band RF Gun Facility at Tsinghua CERN, CLIC Studies Tsinghua EP department

John Power, Helsinki 2011 Schottky Enabled Photoemission Measurements Experimental parameters –work function of copper =  0 = 4.65 eV –energy of =400nm photon = h = 3.1 eV –Laser pulse length Long = 3 ps Short = 0.1 ps –Laser energy ~1 mJ (measured before laser input window) –Field (50 – 73 MV/m) ICT  e- First results from Tsinghua Data Should not get photoemission

John Power, Helsinki 2011  Long Laser Pulse (~ 3ps)  E=55 injection phase=80  55sin(80)=54 Q(pC) laser energy (mJ) photocathode input window First results from Tsinghua Data Q I single photon emission

John Power, Helsinki 2011 Electron emission Copper surface typical picture  geometric perturbations (  ) Fowler Nordheim Law (RF fields): 1.High field enhancements (  ) can cause field emission. peaks grain boundaries cracks (suggested by Wuensch and colleagues) (  , A e, E 0 ) I FN oxides inclusions alternate picture  material perturbations (   ) 2.Low work function (   ) in small areas can cause field emission. E0E0 E0E0

John Power, Helsinki 2011 β from Fowler-Nordheim plot Raw Data –Field emitted current –E-field on surface Fit –Different combinations of  and   can fit the same raw data –Can we find a way to measure what role each effect plays? (β=5, Φ 0 =0.5 eV) (β=130, Φ 0 =4.66 eV)

John Power, Helsinki 2011 z 0  eff The Schottky Effect: applied field lowers the effective potential hh I II -eEz 1 D.H.Dowell,J.F.Schmerge,Phys.Rev.Spec.Top.Accel.Beams (2009) 2 K.L. Jensen et al., J. Appl. Phys. 104, (2008) Photoemission    eff III -eEz Different work functions