Electronic accumulation of extremely cold positrons in ultrahigh vacuum

Many cold positrons in ultrahigh vacuum are required to produce cold antihydrogen, to cool highly stripped ions, and to study ultracold plasmas. Up to 3.5 x 10⁴ such positrons have now been accumulated into the ultra-high vacuum of a 4.2 K Penning trap, at a rate exceeding 10³/hr. 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 tem-peratures in high vacuum. Cooling of high energy positrons (from²²Na decay) in a tungsten crystal near the trap, together with purely electronic trapping and damping, are key to the efficient accumulation and to pro-jected improvements.