Aperture method to determine the density and geometry of antiparticle plasmas

The density and geometry of p-bar and e+ plasmas in realistic trapping potentials are required if the rate of antihydrogen formation from them is to be understood. A new measurement technique determines these properties of trapped positron (e+) and antiproton (p-bar) plasmas, the latter for the first time. The method does not require the common assumption of a spheroidal plasma geometry, which only pertains for a perfect electrostatic quadrupole trapping potential. Plasma densities, diameters, aspect ratios and angular momenta are deduced by comparing the number of particles that survive transmission through an aperture, to that obtained from self-consistent solutions of Poisson's equation. For p-bar the results differ substantially from the spheroid plasmas of an ideal Penning trap. The angular momentum of the plasma emerges as smooth function of the number of particles in the plasma, independent of the depth of the potential well that confines them