Extremely Cold Positrons Accumulated Electronically in Ultrahigh Vacuum

Many cold positrons in ultrahigh vacuum are required to produce cold antihydrogen, to cool highly stripped ions, and for ultracold plasma studies. Up to $3.5\times {10}^4$ 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 energy positrons (from ${}^{22}\mathrm{Na}$ decay) in a tungsten crystal near the trap, together with purely electronic trapping and damping, is the key to the efficient accumulation and to projected improvements.