Title: Fabrication and Measurement of Regenerable Low Workfunction Photocathodes
Laser-switched photoemitters are a source of electrons for high current applications such as free electron lasers. Laser-modulated photoemission permits rapid switching of the electron beam, far surpassing what can be achieved using electric-field gated emission. Photoinjector systems consist of a drive laser producing…
Contents
1. Introduction
1.1 Overview
1.2 Photocathode Characteristics
1.2.1 Spectral Response
1.2.2 Lifetime
1.2.3 Response time
1.2.4 Damage Threshold
1.2.5 Transverse Energy Spread
1.3 Electron Beam Characteristics
1.3.1 Emittance
1.3.2 Brightness
1.4 Basic Photoemission Model
1.5 Types of Photocathodes
1.5.1 Metallic Cathodes
1.5.2 Effect of Surface Films on Metallic Cathodes
1.5.3 Semiconductor Photocathodes
1.5.4 Negative Electron Affinity
1.6 The Dispenser Cathode
1.7 Project Summary
2. Historical Landscape
2.1 History of Photocathodes
2.2 History of Photoinjection
2.2.1 Types of Injectors
2.2.2 Recent Developments
3. Experimental Setup
3.1 Introduction
3.1.1 Phase I: Cesiated Metal Substrates
3.1.2 Phase II: Cesiated Sintered Substrates
3.1.3 Phase III: Controlled Porosity Dispenser Photocathode
3.2 Vacuum Chamber
3.2.1 Chamber Geometry and Layout
3.2.2 Vacuum Characteristics
3.2.3 UHV Pumping Systems
3.2.4 Pressure at the Cathode
3.2.5 Source of Background Gas
3.2.6 Gas Valve Configuration
3.2.7 Gauges
3.3 Lasers
3.3.1 Diode Laser Basics
3.3.2 Diode Laser vs. Diode Pumped Solid State
3.3.3 Laser Specifications
3.3.4 Laser Safety
3.3.5 Robotic Control
4. Experimental Techniques
4.1 Data Acquisition and Control
4.1.1 Instrumentation
4.1.1.1 Cathode and Chamber Temperature
4.1.1.2 Coating Coverage
4.1.1.3 Photocurrent
4.1.1.4 Laser Power
4.1.1.5 Residual Gas Analyzer
4.1.2 Labview Introduction
4.2 Experimental Procedures
4.2.1 Chamber Pumpdown
4.2.2 Cathode Preparation
4.2.3 Cesium and Multi-Alkali Deposition
4.2.4 QE Measurements
4.2.5 Labview Automation
4.2.6 Ion Beam Treatments
4.3 Surface Characterization Techniques
4.3.1 Optical Microscopy
4.3.2 Profilometry
4.3.3 Scanning Electron Microscopy
4.3.4 Ion Induced Secondary Electron Emission
4.3.5 Focused Ion Beam Milling
5. Theory Overview
5.1 Introduction
5.2 Coating Dependent Work function
5.2.1 Gyftopolous-Levine Theory
5.2.2 Comparison to Literature Data
5.3 Photoemission Model
5.3.1 Modified Fowler-Dubridge Model
5.3.2 Evaluating Quantum Efficiency
5.4 Scattering Mechanisms
5.4.1 Temperature Dependent Scattering Rates
5.4.2 Post Absorption Scattering Factor
5.5 Application to Experiment
5.5.1 Bare Metals
5.5.2 Coated Surface
6. Experimental Results
6.1 Preliminary Observations
6.1.1 Background Noise
6.1.2 Monolayer vs. Multiple-Monolayer Cesium Evaporation
6.1.3 Space Charge
6.1.4 Calculating Error and Uncertainty
6.2 Solid vs. Sintered Tungsten
6.3 Surface Metrology of Sintered Tungsten
6.3.1 General Observations
6.3.2 Surface Pores
6.3.3 Crystallographic Orientation
6.4 Cs-on-Sintered Tungsten: Anneal Cleaned Only
6.4.1 QE vs. Coverage
6.4.2 Approximating Work function
6.4.3 Cathode Lifetime
6.4.4 Surface Diffusion of Cesium
6.5 Cs-on-Sintered Tungsten: Argon Ion Beam Treated
6.5.1 QE vs. Coverage
6.5.2 Approximating Work function
6.5.3 Cathode Lifetime
6.5.4 Surface Diffusion of Cesium
6.6 Cesium-on-Tungsten: Theory vs. Experiment
6.6.1 Predicted vs. Measured QE
6.6.2 Crystallographic Complications
6.7 Cesium-on-Silver Photocathode
6.7.1 Theory Validation: QE vs. Coverage
6.7.2 Cathode Lifetime
6.8 Discussion of Ion Beam Cleaning
6.8.1 Summary of Surface Characteristics
6.8.2 Surface Morphology
6.8.3 Hydrogen Ion Beam Results
7. Dispenser Cathode
7.1 Design and Fabrication
7.1.1 Cesium Chromate Pellets
7.1.2 Dispenser Cel
7.1.3 Heater Mount
7.2 Vacuum Insertion
7.2.1 Modified Anode-Cathode Configuration
7.2.2 Pump-down Procedure
7.2.3 Argon-Ion Cleaning
7.3 Dispenser Activation
7.3.1 Activation Temperature
7.3.2 Chamber Conditions during Activation
7.3.3 Post-Activation Performance
7.4 Rejuvenation Process
7.4.1 QE Recovery
7.4.2 Indirect Coverage Measurement
7.4.3 Chamber Conditions
7.5 Modes of Operation
7.5.1 Periodic Rejuvenation
7.5.2 Continuous Rejuvenation
8. Conclusion and Discussion
8.1 Precursor Studies
8.1.2 Coated Metals
8.1.2 Coated Metals
8.1.3 Surface Metrology
8.1.4 Cleaning Techniques
8.1.5 Theory Validation
8.2 Dispenser Studies
8.2.1 General Remarks
8.2.2 Operating Parameters
8.2.3 Literature Representation
8.3 Future Studies
8.3.1 Application to High QE Photocathodes
8.3.2 Conventional vs. Controlled Porosity
8.3.3 Alternative Forms of Cesium
8.3.4 Related Studies
8.4 Closing Remarks
Bibliography
Author: Moody, Nathan Andrew
Source: University of Maryland
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