In-situ Materials Characterization by Alexander Ziegler Heinz Graafsma Xiao Feng Zhang & Joost W. M. Frenken

Author:Alexander Ziegler, Heinz Graafsma, Xiao Feng Zhang & Joost W. M. Frenken
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg

Coating

Carrier

WA [eV]

Coating

Carrier

WA [eV]

Cs

Pt

1.39

Na

Pt

2.1

Cs

W-oxide

1.44

BaO

W

2.1

Rb

Pt

1.57

La

W

2.13

K

Pt

1.62

Ce

W

2.71

Cs

W

1.64

U

W

2.95

B

W

1.75

Zr

W

3.14

CaO

Ni

1.77

Th

W

3.2

Ba

PtIr

1.77

W-oxide

W

6.24

Ba

W-oxide

1.8

Ni-oxide

Ni

6.34

Na

W

2

Pt-oxide

Pt

6.55

The coating tends to be either molten or sintered to bond to the hair pin tip. Molten coatings exhibit often very smooth surfaces, occasionally with small elevations and bumps. The latter, however, instead of becoming preferred electron emission locations due to high electric field concentrations, they are most often preferred sites for electric discharge generation at high extraction and acceleration fields. Moreover, the molten material relocates away from the actual tip of the hair pin due to capillary forces driving the liquid up the hair pin shaft, and once in the liquid state, phase separation and/or alloy formation with the tungsten wire can take place. The aim is to not have the tungsten come through the molten layer, as its high work function is not desirable and it should thus not be exposed to the laser irradiation. Alternatively, the sintered material at the hair pin tip tends to exhibit rather rough surfaces, but bonds well to the tungsten wire and remains in place at the tip, and does not undergo alloy formation. The chances of electric discharges are given, but because the surface roughness covers the entire tip and is not just sparsely distributed over the entire surface area, electric discharge occurs less often than with the molten counterparts. In fact, the surface roughness creates a much larger surface area than the smooth molten kind of tip. This proves to be of great advantage when irradiated by the laser, because many more electrons can be emitted simultaneously. In projection, the irradiated area corresponds to the laser spot on a smooth surface, yet the true surface area that is excited and where photons deposit their energy can be much larger on a rough surface. The laser photons penetrate the rough surface at all locations within the laser spot. However, some locations are reached and excited earlier than others and, unlike on a smooth surface, many locations are not perpendicular to the incident irradiation. The consequence is that electron emission is most irregular and not perfectly in phase with each other. Moreover, the electrons tend to be emitted in all possible directions and are not necessarily pointed toward the anode or in the direction of the laser polarization. Hence, the number of emitted electrons is large, but their orientation and energy distribution is large too.

These sintered cathode tips are most promising in terms of high electron yield per laser pulse. A few carefully selected and manufactured material combinations have shown electron emission currents up to 1.5 milliamps. Especially ZrC coated tips fall into this category, and they have also proven to have acceptable lifetimes (weeks to months). Their emission and operation characteristics (work function, resistance to laser induced damage, temperature stability, low reactivity, good bonding to carrier wire) are satisfactory.

There are a few other cathode designs that are being used in UED and UEM setups. One of them is to back-illuminate a thin metal foil with the pulsed laser to emit electrons out from the front. A development relying on the same principle is using negative electron affinity photo cathodes, Fig.

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