We have developed a new method, based on the ballistic transfer of preaccumulated plasmas, to obtain large and dense positron plasmas in a cryogenic environment. The achieved positron accumulation rate in the high field cryogenic trap is more than one and a half orders of magnitude higher than the previous most efficient UHV coimpatible scheme. Subsequent stacking resulted in a plasma containing more tha 1.2x10^9 positrons, which is a factor 4 higher than previously reported. Using rotating wall electric field, plasmas containing about 20x10^6 positrons were compressed to a density of 2.6 10^10 cm^(-3). This is a factor 6 improvement over earlier measurements.