Mean transverse energy of ultrananocrystalline diamond photocathode

Abstract

Nitrogen incorporated ultrananocrystalline diamond ((N)UNCD) could be an enabling material platform for photocathode applications due to its high emissivity. While the quantum efficiency (QE) of UNCD was reported by many groups, no experimental measurements of the intrinsic emittance/mean transverse energy (MTE) have been reported. Here, MTE measurement results for an (N)UNCD photocathode in the photon energy range of 4.41 to 5.26 eV are described. The MTE demonstrates no noticeable dependence on the photon energy, with an average value of 266 meV. This spectral behavior is shown to not to be dependent upon physical or chemical surface roughness and inconsistent with low electron effective mass emission from graphitic grain boundaries, but may be associated with emission from spatially-confined states in the graphite regions between the diamond grains. The combined effect of fast-growing QE and constant MTE with respect to the excess laser energy may pave the way to bright UNCD photocathodes.Photocathode-based RF and pulsed DC guns are bright electron injectors for free electron lasers and advanced time resolved microscopes 1 . Further progress of electron laser and microscopy facilities (improved sensitivity, spatiotemporal resolution, high throughput) largely depends on development and understanding of materials with the potential to be utilized as photocathodes. Photocathode development challenges include achieving simultaneously (i) high QE, (ii) high transverse coherence (meaning low intrinsic emittance/low MTE), (iii) rapid response time.The ratio of the charge to the MTE determines the photocathode brightness, which in many applications is the most critical figure of merit. For a classical metal photocathode such as copper, the Fowler-Dubridge law 2 predicts that the emitted charge is a fast-growing function of excess energy (a power law), where excess energy ∆E is the difference between the laser primary incident photon energy ω and the work function φ defined as ∆E = ω−φ. Dowell and Schmerge 3 have found that the transverse momentum for metals also grows with excess energy as ∼ √ ω − φ. For the latter reason, to attain the highest quality (low divergence) electron beam metal photocathodes are often operated in the near threshold region (having the smallest ∆E, with the primary photon energy nearly matching the work function), although brightness increases with excess energy.A great number of metal and thin film alkali antimonide photocathodes obey the Dowell-Schmerge (DS) model 3-5 . However, some semiconductor photocathodes, e.g. GaAs and PbTe, show various MTE versus excess energy trends that are different from those specific to metals. Negative electron affinity (NEA) GaAs photocathodes 6 , for instance, demonstrate ∼1,000-fold QE increase as the excess energy increases from 0 to about 1 eV while the MTE remains low and nearly constant with the same ∆E range (within measurement precision).(N)UNCD is another example of a NEA photocathode that has high electron conductivity through the bulk of a semi-me...

Confirmation of Transit Time-Limited Field Emission in Advanced Carbon Materials with a Fast Pattern Recognition Algorithm

“…At 1 keV energy, v/c and γ are taken as 0.063 and 1, respectively. MTE values are 100−200 meV 25,26 for the UNCD. For CNTs, its Fermi energy of 50−100 meV 27 can be used as MTE.…”

Confirmation of Transit-Time Limited Field Emission in Advanced Carbon Materials with Fast Pattern Recognition Algorithm

“…At 1 keV energy, v/c and γ are taken as 0.063 and 1, respectively. MTE values are 100-200 meV 23,24 for the UNCD; and for CNT its Fermi energy of 50-100 meV 25 can be used. Then, the estimated values of the angle is 0.9 • for the UNCD sample 26 and 0.4 • for the fiber sample 2 .…”

Ampere-class bright field emission cathode operated at 100 MV/m

“…[21] and then exported to GPT in order to further track the particles in three dimensions. The initial energy spread on the cathode surface was set to the intrinsic value of 0.1 eV, and the UNCD work function was assumed to be 4 eV from previous Kelvin probe measurements [24]. Fig.…”