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 method involves transferring plasmas emanating from a region with a low magnetic field (0.14 T) and relatively high pressure (10^-9 mbar) into a 15 K Penning-Malmberg trap immersed in a 3 T magnetic field with a base pressure better than 10^-13 mbar. 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 compatible scheme. Subsequent stacking resulted in a plasma containing more than 1.2 10^9 positrons, which is a factor 4 higher than previously reported. Using a rotating wall electric field, plasmas containing about 20 10^6 positrons were compressed to a density of 2.6 10^10 cm-3. This is a factor of 6 improvement over earlier measurements.
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