VORPAL for Simulating RF Breakdown

1. VORPAL for Simulating RF Breakdown Kevin Paul kpaul@txcorp.com VORPAL is a massively-parallel, fully electromagnetic particle-in-cell (PIC) code, originally developed for laser-plasma simulation. Since it's creation in 2004, VORPAL has expanded its capabilities to include electrostatics, cross-section-based particle-particle interactions, hybrid particle-fluid modeling, and a variety of numerical models for everything from field ionization, impact ionization, secondary electron emission, field emission, and particle-impact heating. Fermilab MuCool RF Workshop III – 7 July 2009

2. Tech-X Corporation Projects • Breakdown Phase II: • Seth Veitzer • July 2008 – July 2010 • Developing VORPAL to do 3D simulations of RF breakdown • Built off of a Phase I project using OOPIC (2D/r-z) • eSHIELD Phase I: • Me • July 2009 – March 2010 • More VORPAL development to test magnetic insulation • Will couple small-scale with large-scale simulations Fermilab MuCool RF Workshop III – 7 July 2009

3. VORPAL: Versatile Plasma Simulation Code • Technical Features: • Object-oriented C++ • 1D/2D/3D Massively Parallel Scaling to 10,000+ Processors • Compressed Binary Data Formatting (HDF5) • Mac OS X / Microsoft Windows / Linux • Multi-physics Capability: • Kinetic Plasma Model • Field & Impact Ionization • Field & Secondary Emission • Hybrid Particle-Fluid Modeling • Electrostatic & Electromagnetic • Uses: • Laser wake-field accelerators • Electron cooling • Photonic Band Gap Devices • RF Heating in Fusion Plasmas • Breakdown in Microwave Guides • Simulation of Ion Sources & Penning Sources • Modeling of Plasma Thrusters • Availability: • Consulting • Purchase • SBIR/STTR Collaboration • Web interface (In development!) Fermilab MuCool RF Workshop III – 7 July 2009

4. Electrostatic Particle-in-Cell Simulation:One Simulation Time Step Initialization Steps... Fields defined and initialized on a grid {Ei, Bi} Particle positions & velocities initialized {xα, vα} Particles accelerated by the fields {v'α} Particles moved based on new velocity {x'α} One Time Step New fields computed from charges {E’i} Charge “deposited” on the grid {ρi} Fermilab MuCool RF Workshop III – 7 July 2009

5. Electromagnetic Particle-in-Cell Simulation:One Simulation Time Step Initialization Steps... Fields defined and initialized on a grid {Ei, Bi} Particle positions & velocities initialized {xα, vα} Particles accelerated by the fields {v'α} Particles moved based on new velocity {x'α} One Time Step New fields computed from old fields {E'i, B'i} Currents “deposited” on the grid {Ji} Fermilab MuCool RF Workshop III – 7 July 2009

6. Electromagnetic Particle-in-Cell Simulation:One Simulation Time Step New particles added (lost removed) {xα, vα} Particles accelerated by the fields {v'α} Particles moved based on new velocity {x'α} One Time Step Collisions and interactions computed New fields computed from old fields {E'i, B'i} Currents “deposited” on the grid {Ji} Fermilab MuCool RF Workshop III – 7 July 2009

7. Electromagnetic Particle-in-Cell Simulation:One Simulation Time Step This is where all the interesting physics for RF breakdown takes place!!! New particles added (lost removed) {xα, vα} Particles accelerated by the fields {v'α} Particles moved based on new velocity {x'α} One Time Step Collisions and interactions computed New fields computed from old fields {E'i, B'i} Currents “deposited” on the grid {Ji} Fermilab MuCool RF Workshop III – 7 July 2009

8. RF Breakdown Physics: What must be modeled? • Field emission of electrons from conductor surfaces • Secondary emission of electrons from conductor surfaces • Sputtering • Neutral Desorption • Field-induced ionization (Tunneling ionization) • Impact ionization • X-ray production from electron impact on conductor surfaces • Surface heating due to particle impact • Surface deformation due to melting • Radiative cooling of ions Fermilab MuCool RF Workshop III – 7 July 2009

9. Physics Models in VORPAL/TxPhysics: What can VORPAL do now? • Fowler-Nordheim model for field emission from “assumed asperity” • Jensen model for field, thermal, and photo-induced electron emission • Rothard model for ion-induced secondary electron emission (depends strongly on nuclear stopping power of material) • Furman-Pivi (LBNL) model for electron-induced secondary electron emission • Yamamura model for sputtering (nuclear stopping dependent threshold model) • Molvik model for neutral desorption (akin to Rothard model) • Tunneling ionization rates for various materials from Keldysh • Parameterized impact ionization, excitation, and recombination cross sections for electrons and ions • Diagnostics for recording energy deposited in absorbing boundaries • Coronal model for computing radiated power by ions in a plasma (a diagnostic, no radiation transport) Fermilab MuCool RF Workshop III – 7 July 2009

10. VORPAL/TxPhysics Development: What will VORPAL be able to do? • X-ray emission model for various materials due to electron bombardment • Impurity radiation model for ion cooling • Simple radiation transport • Couple VORPAL simulations to molecular dynamics models for surface damage and deformation • Temperature and emission yield “diagnostic mapping” to more easily visualize the simulations • A web-based interface to VORPAL with the capability of providing computational resources to researchers anywhere • Surface damage and heating model due to bombardment • Multi-scale simulation capability, coupling “fine-grain” (surface asperity) simulations with “course-grain” (RF cavity) simulations …all are about 1 year away! Fermilab MuCool RF Workshop III – 7 July 2009

11. Example: Impact Ionization, Elastic Scattering & Excitation • A beam of 40 eV electrons is incident on a “droplet” of Xenon and Argon gas. • Impact ionization, elastic scattering, and neutral gas excitation are all computed. Fermilab MuCool RF Workshop III – 7 July 2009

12. Example: Impact Ionization, Elastic Scattering & Excitation Fermilab MuCool RF Workshop III – 7 July 2009

13. Example: Impact Ionization, Elastic Scattering & Excitation Fermilab MuCool RF Workshop III – 7 July 2009