Schottky Enabled Photoemission & Dark Current Measurements John Power, Eric Wisniewski, Wei Gai Argonne Wakefield Accelerator Group Argonne National Laboratory U.S. High Gradient Workshop SLAC, Feb 9, 2011 at the S-band RF Gun Facility at Tsinghua CERN, CLIC Studies Tsinghua EP department

John Power, SLAC 2011 Tsinghua U has an rf gun facility available to study copper surfaces under high fields Laser 400 nm, 1 mJ 0.1 – 3 ps S-band RF gun 57 – 73 MV/m cathode Laser alignment Faraday Cup ICT Dark Current Measurements photocathode Schottky Enabled Photoemission Measurements 2 configurations

John Power, SLAC 2011 Tsinghua S-band rf gun Facility Features Dark current measurement The cathode is a solid copper plate (no gap) Schottky photoemission measurement RF field level and laser parameters are suitable = 400 nm laser (h = 3.1 eV) E = MV/m The research facility is operational

John Power, SLAC 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, SLAC 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, SLAC 2011  Short Laser Pulse (~ 0.1ps)  E=55 injection phase=80  55sin(80)=54 First results from Tsinghua Data Q(pC) laser energy (mJ) photocathode input window Q I single photon emission

John Power, SLAC 2011  Short Laser Pulse (~ 0.1ps)  E=50 injection phase=30  50sin(30)=25 First results from Tsinghua Data Q(pC) laser energy (mJ) photocathode input window Q aI + bI 2 multiphoton emission

John Power, SLAC 2011 Dark Current Measurements Experimental parameters –work function of copper =  0 = 4.65 eV –Field (57– 73 MV/m) –Note: field could be lowered more but Faraday Cup signal was too weak to measure current First results from Tsinghua S-band RF gun 57 – 73 MV/m cathode Faraday Cup

John Power, SLAC 2011 Copper work function Φ 0 =4.65 eV Fit: β ~ 130 Fowler Nordheim plot of dark current data First results from Tsinghua Data Field (57– 73 MV/m)

John Power, SLAC 2011 Summary of the first measurements Schottky Enabled Photoemission Measurements Schottky enhanced emission observed at all the field levels measured. h  =  eV,  0 =4.6 eV   =1.5eV (Schottky effect required) The lowest field 25 MV/m  (Schottky effect)  implies  >=60 note: also observed emission at lower fields, but data was noisy. This implies even larger  exists. Dark current measurements  is 130. (this is consistent with typical SLAC data  E~10 GV/m)

What questions about the surface can be investigated with an s-band gun? Some possibilities/speculation … Alternative interpretation of Fowler-Nordhiem plots Measurement of the field enhancement and work function

John Power, SLAC 2011 Image potential e 2 /16  0 z Electrostatic potential -eEz Effective potential z 0  eff EFEF    eff =   -  The Schottky Effect: applied field lowers the effective potential metal e- field emission Field emission

John Power, SLAC 2011 Electron emission Copper surface typical picture  geometric perturbations (  ) Fowler Nordheim Law (RF fields): 1.High field enhancements (  ) can 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, SLAC 2011 Field emission enhancement factor  =130 seems unphysical –h/  ~ 100 –fresh surfaces machines to ~10nm roughness –h=10 nm,  =0.1 nm “a tower of single atoms”

John Power, SLAC 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, SLAC 2011 A e from Fowler-Nordheim plot Raw Data –Field emitted current –E-field on surface Fit –Typical fits give areas so small that they are difficult/impossible to measure. Does this give us a way to probe whether  or  0 dominates?

John Power, SLAC 2011 Image potential e 2 /16  0 z z 0  eff EFEF  The Schottky Effect: applied field lowers the effective potential metal Photoemission hh hh h   photoemission h   No photoemission hh

John Power, SLAC 2011 Image potential e 2 /16  0 z Electrostatic potential -eEz Effective potential z 0  eff EFEF    eff =   -  The Schottky Effect: applied field lowers the effective potential metal hh e- photoemission excess energy E excess, metal = ħ  -  eff Normal Photoemission in an rf gun 1,2 Photoemission 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)

John Power, SLAC 2011 Image potential e 2 /16  0 z Electrostatic potential -eEz Effective potential z 0  eff EFEF    eff =   -  The Schottky Effect: applied field lowers the effective potential metal hh e- photoemission excess energy Schottky Enabled Photoemission via (external field) Photoemission

John Power, SLAC 2011 Image potential e 2 /16  0 z z 0  eff EFEF  The Schottky Effect: applied field lowers the effective potential metal hh e- photoemission excess energy Schottky Enabled Photoemission via   (work function lowering) Photoemission e- photoemission

John Power, SLAC 2011 z 0  eff EFEF   The Schottky Effect: applied field lowers the effective potential I II III -eEz -e  Ez (bulk of cathode) (high  ) 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 hh  eff  Q h  eff Q (h  eff ) 2 Ideas to measure the effective work function?? –sweep the laser energy (OPO) –sweep the RF phase, which changes field (Schottky effect)

John Power, SLAC 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

John Power, SLAC 2011 Can we measure the relative strength of  and  0 Q h  eff Q (h  0 ) 2 E (MV/m)  eff (eV)

John Power, SLAC 2011 summary An S-band facility at Tsinghua University is available to study surface emission –Schottky Enabled Photoemission –Dark Current Emission Facility Parameters –laser: 400 nm laser (pulse length: 0.1 ps, 3 ps) –rf field: <73 MV/m First measurements have been made Alternative interpretation of FN plots being investigated –  and  o Developing techniques to measure the effects