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VORPAL for Simulating RF Breakdown. Kevin Paul kpaul@txcorp.com.

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slide1

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

slide2

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

slide3

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

slide4

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

slide5

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

slide6

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

slide7

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

slide8

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

slide9

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

slide10

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

slide11

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

slide12

Example: Impact Ionization, Elastic Scattering & Excitation

Fermilab MuCool RF Workshop III – 7 July 2009

slide13

Example: Impact Ionization, Elastic Scattering & Excitation

Fermilab MuCool RF Workshop III – 7 July 2009

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