Outline For Thesis

Abstract, Intro (obviously)

Current Experiments - Motivation for Research

• Current experiments requiring the use of GaAs photo-guns - polarized electron beams
• What is a photo-gun?
• CEBAF Injector - What is it? How do we (currently) model it?

Description of problem - Ionization

• What is ionization & why is it bad for electron beam production & electron guns?
  • Ion Back-Bombardment & QE degradation of the photocathode
    • How/where are ions formed in the photogun?
    • How many ions reach the photocathode and what are their energies?
    • How many of these ions contribute/correlate to QE damage?
  • Other ion effects
    • Fast-ion instability
    • Charge neutralization
    • Effect on emittance and tune
    • Recombination - can lead to unwanted light incident on the photocathode active area, which may lead to beam halo

Current Solutions

• Experimental ion mitigation techniques that are currently available:
  • Clearing Techniques:
    • Clearing electrodes - ion precipitator
    • Repelling electrodes - biased anode
    • Beam Gaps
    • Beam Shaking Techniques
  • Damage mitigation techniques:
    • Varying laser spot size
    • Photocathode elemental makeup (GaAs vs Mb)?
• Theoretical Techniques
  • Other ion tracking codes (IBSimu, SIMION, etc.)

The "Thesis" statement

• What do **I** bring to the table? Brief description/summary of
  • GPT ionization custom element
  • Biased anode as ion mitigation technique
  • Analysis of how the charge lifetime scales with laser spot size (2017 experiments)
  • Ion trapping experiments/simulations
• How all of these help to solve the problem described above?
  • Knowing how ions are formed and where they go through measurements and simulations, we can...
    • Predict the effectiveness of ion mitigation techniques (such as the biased anode)
    • Predict the QE degradation of the photocathode and its charge lifetime under various beam conditions
    • Identify the conditions under which ions can cause deleterious effects on the beam (in its creation and its stability) That is, we can identify the sources/causes of damaging ions.

Ionization Simulations with GPT Custom Element

GPT Description

• Purpose - to create particle simulations, often of electron beams, and to track their movement within electromagnetic fields in real-time (as opposed to just getting the trajectory info)
• How does it work? (perhaps with a flow chart?)
  • Description of how particle distributions are created and what "macro-particles" are
  • Built-in and custom elements with their respective locations (i.e. coordinate systems) are called by the GPT kernel. These elements include:
    • E-Field and B-Field maps, usually due to beam line components
    • Space charge routines
    • Custom elements (like the ionization custom element)
  • Equations of motion are derived by solving the Poisson equation using the 5th-order Runge-Kutta Method with an adaptive stepsize control

GPT Custom Element Algorithm

• Description of how the custom element works and is used to simulate ionization
  • Ionization theory - Can be pulled from PSTP proceedings and tech notes
  • Secondary Electron Differential Cross Section (SEDCS)
  • Ion Energy Distribution (Maxwellian)
  • Momentum/Energy Conservation?
• Benchmarking with theory and IBSimu (Can be pulled from future GPT/IBSimu paper)...maybe put in the LIfeSize Runs Section?

Biased Anode To Mitigate Ion Back-Bombardment

• Description of ion back-bombardment
  • CEBAF vacuum pressures
  • Description of mechanism - electron beam ionizes gas, accelerates towards photocathode, damages it, causing QE decay (describe how this occurs)
• Description of how the biased anode will mitigate it (Can be pulled from PSTP) - compare E-Fields
• Summer 2019 Biased Anode Experiments
  • Description of Experiment at CEBAF
  • QE Measurements & Charge Lifetime Analysis
  • QE Scan Analysis
  • GPT Simulations (w/custom element)
  • Results/Discussion
• Similar for Winter 2019-2020 Experiments?

Analyzing 2017 LifeSize Runs with GPT/IBSimu

• Description of Experiment - varying laser spot size and calculating photocathode charge lifetime
  • QE Scan analysis
  • Comparing analysis results with GPT simulations using the ionization custom element
• Comparison & benchmarking with IBSimu

Ion Trapping at the Gun Test Stand (GTS)

• Description of phenomenon & experiments
  • Anode bias/Gun solenoid systematic study
  • Experiments where ghost beam returns when gun solenoid current is lowered and then raised - perhaps ion trap filling or field emission?
• GPT Simulations (w/custom element)
• Ion Trapping Experiments - Recreating Ghost Beam using Steel Shield. These experiments will help confirm/benchmark the GPT simulations and vice-versa.
• Results/Discussion

Discussion/Conclusions

• What is the most important info that I've learned so that I can pass it on to the next grad student?
  • GPT is extensible!
    • GPT can accurately simulate and predict the effect of ions on the operation of the photogun, both in the electron beam production (QE decay) and its stability (ion trapping & charge neutralization)
  • A biased anode is effective at increasing the charge lifetime of the photocathode by repelling ions downstream of it.
  • Ghosts are real! (ion trapping experiments at GTS)

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