Plasma sheath effects in the sampler and skimmer cones of the ICP-MS

POSTER

Abstract

In the ICP-MS, plasma neutrality and the associated issue of the plasma potential are governed by what happens in the plasma sheath. Plasma sheaths can generally be described by two model types: collisional, where the Debye length is long compared to the mean free path; and collisionless, where the mean free path is long compared to the Debye length. In the sampler cone, the Debye length is .3 micrometers, while the ion mean free path is 5 micrometers, nearly in the collisionless regime. In the skimmer cone, the Debye length is 2 micrometers, while the ion mean free path is 400 micrometers, well into the collisionless regime. Doing a full calculation with the Direct Simulation Monte Carlo algorithm, FENIX, would involve simulating electron physics, performing electrostatic field calculations, and resolving the small Debye length, all of which are computationally expensive. To approximate sheath formation in the sampler and skimmer, a forced ion flux model is made by first estimating the number of ions per second that should recombine at the wall using a simple, planar, collisionless sheath model, and then forcing the ions near the wall to have that flux by modifying their velocities each time step. The ion loss through the sheath results in a steep drop in the ion density at the nozzle wall which both diffuses and is sheared by the nozzle flow. Another plasma effect is that the sheath inhibits electron flow to the wall, greatly reducing thermal conduction to the wall. This means that the electron temperature of the plasma in the nozzle is hardly affected by the presence of the metal wall. In particular, setting the electron temperature equal to the wall temperature at the wall is inappropriate.

Authors

  • Matthew Zachreson

    Brigham Young University

  • Benjamin Bloom

    Department of Physics, University of Arizona, Tucson, AZ, National Institute for Materials Science, Tsukuba, Japan, The University of Electro-Communications, Tokyo, Langmuir Laboratory, New Mexico Tech, The University of Arizona, Brigham Young University, Department of Physics Colorado State University, Colorado School of Mines, National Renewable Energy Laboratory, University of Colorado Boulder, Principal Investigator, Graduate Student, Colorado State University, SSRL, SLAC, Department of Chemistry and Biochemistry, Brigham Young University, Department of Physics and Astronomy, Brigham Young University, National Tsing Hua University, Hsinchu, Taiwan, Colorado State Univ, JILA, University of Colorado at Boulder, NIST, JILA, University of Colorado at Boulder, Heinrich-Heine-Universitat, Department of Physics, University of Colorado Denver, Denver, CO 80217, Biomedical Engineering, University of Texas at Austin, Austin, TX, The University Centre in Svalbard, Utah State University, Utah Valley University, New Mexico State University, The George Washington University Nuclear Physics Research Group, Institute for Nuclear Physics at the Johannes Gutenberg University of Mainz, None, Colorado State Engineering Research Center, St. Petersburg Electrotechnical University in Saint Petersburg, Russia, University of California San Diego, Argonne National Laboratory, Los Alamos National Laboratory, Imperial College London, Space Dynamics Lab, Utah State University, Physics and CASS, Utah State University, Department of Chemistry, Colorado State University, Fort Collins, CO 80523, Department of Physics, Colorado State University, Fort Collins, CO 80523, Dept. of Electrical, Computer, and Energy Engineering, University of Colorado at Boulder, Dept. of Physics and Astronomy, University of Denver, CU Boulder, RASEI, NREL, University of Colorado, Rutgers, UTK, Joint Institute for Heavy Ion Research \& ORNL, University of Guelph, Insitituto de Estructura de la Materia, University of Toronto, INFN Laboratori Nazionali del Sud, University of York, University of Surrey, TRIUMF, Simon Frasier University, Universdad de Sevilla, Simon Fraser University, Univ of Utah, Univ of Wyoming, New Mexico Tech, GLOBALFOUNDRIES, IBM Systems and Technology Group, IBM Research Division, Irvine Valley College, University of Colorado - Boulder, Department of Physics, Arizona State University, Tempe, AZ, Department of Physics, New Mexico State University, Las Cruces, NM, Department of Physics, University of Michigan, Flint, MI, High Altitude Observatory, JILA, University of Colorado