Outline For Thesis

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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


  • 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)

Go to To-Do List

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