F. Belloni, G. Caretto, D. Doria, A. Lorusso, P. Miglietta, V. Nassisi, A. Perrone, M. V. Siciliano, G. Accoto Department of Physics University of Lecce Photoemission studies from metal by UV lasers

In this experimental work we study the possibility to increase the quantum efficiency of metallic photocathodes, in order to produce intense electron beams. Nowadays the use of electron beams is widespread among scientificic, industrial (particles accelerators, FEL, new polymers realization) and medical fields (radiotherapy e radiology). It is very important to have availability of good electron beams of high intensity, short temporal duration and low emittance.

In details we have studied the photoemission quantum efficiency for two different metals: zinc (φ = 4.3 eV) and yttrium (φ = 3.1 eV), utilizing two different excimer lasers: KrF (λ=248 nm, Δt=23 ns) and XeCl (λ=308 nm, Δt=10 ns). In the future work we will study the quantum efficiency for other metals: magnesium (φ = 3.7 eV), copper (φ = 4.7 eV), carbon (φ = 4.8 eV) and gold (φ = 5.1 eV). Two other laser sources with different wave-length will actived and utilized: KrCl (λ=222 nm, Δt=10 ns) and Nd- YAG (λ=532 nm, 2 a harmonic).

Photo of the experimental apparatus Sketch of experimental apparatus

KrF λ=248 nm Laser spot Excimer Lasers τ=23 ns (FWHM) 40 mm^2 XeCl λ=308 nm Laser spot 72 mm^2 τ=10 ns (FWHM) KrCl λ=222 nm 3 τ=10 ns (FWHM) Laser solid-state Nd-YAG λ=532 nm 2 a harmonic

Global Efficiency Global Efficiency Temporal Efficiency Temporal Efficiency

for semiconductor with cesium for semiconductor with cesium like K 2 CsSb and Cs 2 Te It is not convenient to use this photocathodes for such applications, because their realization and their utilization require high vacuum conditions. Metallic photocathodes, on the other hand, need acceptable vacuum conditions, they are very tough and they are not so expensive. Metallic photocathodes, on the other hand, need acceptable vacuum conditions, they are very tough and they are not so expensive.

The general expression of Richardson equation that governs photoemission process is: with: where is Fowler function of argument:

The thermionic component is: The n-photons process component is: The current component relative to the N+1-photon process, where N is the first integer number below the ratio  h :

Schottky Effect: L. Martina, V. Nassisi, G. Raganato and A. Pedone Nucl. Instr. Meth. B 188, 272 (2002) J.P. Girardeau-Monteaut and C. Girardeau-Monteaut, J. Appl. Phys. 65, 2889 (1989)

Waveform of laser pulse and photoemitted current Temporal quantum efficiency (TQE) calculation

Schottky Effect:

Temporal quantum efficiency (TQE) calculation Waveform of laser pulse and photoemitted current

Schottky Effect:

Time (s) Temporal Quantum Efficiency Normalized Peaks

Effetto Schottky:

Time (s) Normalized Peaks Temporal Quantum Efficiency

TEMPERATURE CALCULATION FOR METALLIC TARGET where I 0 g(t) is the temporal profile of laser intensity, C is a constant that depends on cathode material and T 0 is the initial temperature. where I 0 g(t) is the temporal profile of laser intensity, C is a constant that depends on cathode material and T 0 is the initial temperature. Convolution integral in this equation was calculated numerically, beacuse the temporal profile of laser pulse hasn’t an exact analytical espression. The equation that describes the temperature-time trend on the target surface was obtained solving the equation of heat-conduction by using Green formulas. The solution, for z=0, is:

Temporal evolution of the Y film irradiated by KrF laser at 24 mJ. The temperature trend is described by former equation; his maximum value was 1070 K, reached at 26 ns.

Temporal evolution of the Zn target irradiated by XeCl laser at 48 mJ. The temperature trend is described by former equation; his maximum value was 500 K, reached at 10 ns.

Temporal evolution of the Zn target irradiated by KrF laser at 16 mJ. The temperature trend is described by former equation; his maximum value was 440 K, reached at 23 ns.

SEM image (magnifying x2000) of Zn photocathode before laser irradiation

Electron-beam simulation by using OPERA 3-D for a system with rough cathode and grid anode: electron beam visualization

Electric field mapping (strength) on cathode surface

Comparison between Child-Langmuir curve (blue curve), I-V curve obtained from simulation with smooth anode and cathode (violet curve), I-V curve obtained from simulation with grid anode and smooth cathode (light-blue curve) and the one obtained with grid anode and rough cathode (yellow curve)

Photoemitted current from cathodes with different roughness Photoemitted current density versus accelerating voltage for cathodes with different roughness SEM images for copper targets with different roughness: a) Ra=0.05, b) Ra=0.12, c) Ra=0.17 R a = a/b roughness coefficient where a is the average deepness of the furrows and b is the average periodicity of the furrows