SIMULATION OF COLLISIONAL RELAXATION OF TRAPPED ION CLOUDS IN THE PRESENCE OF SPACE-CHARGE FIELDS

A statistical formulation of ion-neutral interactions in rf Paul traps is presented which provides a basis for calculating the collisional r elaxation of a trapped ion cloud. These calculations describe the ion cloud as a statistical distribution in space and energy and treat coll isions by Monte Carlo methods. Ion motion is calculated in the presenc e of space charge fields in a mean field approximation which allows ea ch ion to be treated independently. This formulation provides an adequ ate description of the ion cloud dynamics for ion densities less than or equal to 10(8) cm(-3). Frequency shifts induced in the ion cloud en semble by space charge are characterized. Energy relaxation of a C-60( +) ion cloud initially at a temperature of 5000 K by collisions with a background He gas at 300 K is calculated in the presence of space cha rge. (C) 1995 American Institute of Physics.