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 Fermilab MuCool RF Workshop III – 7 July 2009 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

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 Fermilab MuCool RF Workshop III – 7 July 2009 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!)

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

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

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

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

8. RF Breakdown Physics: What must be modeled? Fermilab MuCool RF Workshop III – 7 July 2009 –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

9. Physics Models in VORPAL/TxPhysics: What can VORPAL do now? Fermilab MuCool RF Workshop III – 7 July 2009 –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)

10. VORPAL/TxPhysics Development: What will VORPAL be able to do? Fermilab MuCool RF Workshop III – 7 July 2009 –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!

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

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