The Role of Antihydrogen Formation in the Radial Transport of Antiprotons in Positron Plasmas
Résumé
Simulations have been performed of the radial transport of antiprotons in positron plasmas under ambient conditions typical of those used in antihydrogen formation experiments in Penning traps. The simulations were performed using classical trajectories of both antiprotons and surrounding positrons with randomized initial conditions. In addition, friction and fluctuation forces on the antiproton due to interaction with the positron plasma were included. This gives rise to both axial and radial diffusion of the antiprotons at a rate given by the magnetic field, the positron temperature and the plasma density, and the parameter range explored includes at least two values of each. As the antiprotons travel through the positron plasma they undergo repeated cycles of antihydrogen formation and re-ionization. We find that when these effects are added to the simulations the properties of the radial diffusion alter dramatically, even changing from normal to anomalous diffusion. We attribute this to the azimuthal drift of the antiprotons in the crossed magnetic (from the trap) and electric fields (from the space charge of the plasma). When the antiprotons are neutralised through antihydrogen formation this drift is temporarily interrupted, and the antiproton (carrying a positron) continues in the tangential direction. On average this motion will give an increase of the radial position of the antiproton, relative the trap axis. At low positron plasma temperatures, repeated cycles of antihydrogen formation and destruction are the dominant source of radial (cross magnetic field) transport. On time scales of a few milliseconds (depending on plasma parameters) the antiprotons will reach the edge of the cylindrical plasma, where the radial transport will cease, and thus the antiprotons will accumulate there.