Many cold positrons in ultrahigh vacuum are required to produce cold a
ntihydrogen, to cool highly stripped ions, and for ultracold plasma st
udies. Up to 3.5 X 10(4) such positrons have now been accumulated into
the ultrahigh vacuum of a 4.2 K Penning trap, at a rate exceeding 10(
3)/h. Both the accumulation rate (per high energy positron incident at
the trap) and the number accumulated are much larger than ever before
realized at low temperatures in high vacuum. The cooling of high ener
gy positrons (from Na-22 decay) in a tungsten crystal near the trap, t
ogether with purely electronic trapping and damping, is the key to the
efficient accumulation and to projected improvements.