Commun. Comput. Phys., 9 (2011), pp. 231-239. Improved Unlike-Particle Collision Operator for delta-f Drift-Kinetic Particle Simulations R. A. Kolesnikov 1*, W. X. Wang 2, F. L. Hinton 31 Los Alamos National Laboratory, Los Alamos, NM 87544, USA. 2 Plasma Physics Laboratory, P.O. Box 451, Princeton, NJ 08543, USA. 3 Center for Astrophysics and Space Science, University of California, San Diego, La Jolla, CA 92093, USA. Received 9 April 2010; Accepted (in revised version) 26 April 2010 Available online 27 August 2010 doi:10.4208/cicp.090410.260410a Abstract Plasmas in modern tokamak experiments contain a significant fraction of impurity ion species in addition to main deuterium background. A new unlike-particle collision operator for $\delta f$ particle simulation has been developed to study the non-local effects of impurities due to finite ion orbits on neoclassical transport in toroidal plasmas. A new algorithm for simulation of cross-collisions between different ion species includes test-particle and conserving field-particle operators. An improved field-particle operator is designed to exactly enforce conservation of number, momentum and energy. Notice: Undefined variable: ams in /var/www/html/issue/abstract/readabs.php on line 163 PACS: 52.30.Gz, 52.25.Xz, 52.25.Dg Key words: Drift-kinetics, magnetized plasmas. *Corresponding author. Email: rkolesni@lanl.gov (R. A. Kolesnikov), wwang@pppl.gov (W. X. Wang), fhinton@ucsd.edu (F. L. Hinton)