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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 × 104 such positrons have now been accumulated into the ultrahigh 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 temperatures in high vacuum. Cooling of high energy positrons (from 22Na decay) in a tungsten crystal near the trap, together with purely electronic trapping and damping, are key to the efficient accumulation and to projected improvements.

Positrons from a 12 mCi22Na source are slowed by a W(110) reflection moderator and then captured in a Penning trap, by damping their motion with a tuned circuit. Because of the stability of the Penning trap and the cryogenic ultra-high vacuum environment, we anticipate that positrons can be accumulated and stored indefinitely. A continuous loading rate of 0.14 e+/s is observed for 32 h in this initial demonstration. More than 1.6×104 positrons have thus been trapped and stored at 4 K, with improvements expected. The extremely high vacuum is required for compatibility with an existing antiproton trap, which has already held more than 105 antiprotons at 4 K, for producing antihydrogen at low temperatures. The extremely cold positrons in high vacuum may also prove to be useful for cooling highly stripped ions.

The storage of extremely cold (4 K) antiprotons in a Penning trap is an important step toward the creation and study of cold antihydrogen. The other required ingredient, the largest possible number of comparably cold positrons, is still lacking. These would be recombined in a high vacuum with the trapped antiprotons, already stored at a pressure below 5×10−17 Torr, thereby avoiding annihilation of the antihydrogen atoms before they can be used in high accuracy measurements or in controlled collision experiments. In an exploratory experiment, positrons from a 18 mCi22Na source follow fringing field lines of a 6 T superconducting solenoid through tiny apertures in the electrodes of a Penning trap to strike a tungsten (reflection) moderator. The positron beam is chopped mechanically and a lock-in directly detects a positron current of 2.5×106e+/s on the moderator. The use of a moderator, unlike an earlier experiment in which < 100 positrons were confined in vacuum, should greatly increase the number of positrons trapped in high vacuum.

The possibility to produce, trap and study antihydrogen atoms rests upon the recent availability of extremely cold antiprotons in a Penning trap. Over the last five years, our TRAP Collaboration has slowed, cooled and stored antiprotons at energies 1010 lower than was previously possible. The storage time exceeds 3.4 months despite the extremely low energy, which corresponds to 4.2 K in temperature units. The first example of measurements which become possible with extremely cold antiprotons is a comparison of the antiproton inertial masses which shows they are the same to a fractional accuracy of 4×10−8. (This is 1000 times more accurate than previous comparisons and large additional increases in accuracy are anticipated.) To increase the number of trapped antiprotons available for antihydrogen production, we have demonstrated that we can accumulate or “stack” antiprotons cooled from successive pulsed injections into our trap.

During the last several years, our TRAP collaboration has pioneered techniques for slowing, trapping, cooling and indefinitely storing antiprotons to energies more than 1010 times lower than previously possible. The radio signal from a single trapped antiproton is now being used for precision measurements. Many cold antiprotons are “stacked” as another important step toward the eventual production of antihydrogen, and positrons have been trapped in vacuum.

Over the last five years, a series of experiments by our TRAP Collaboration has made it possible to slow, cool and store antiprotons at energies 1010 lower than was previously possible. Despite the extremely low energy, the storage time exceeds 3.4 months. The first example of measurements which become possible with extremely cold antiprotons is a comparison of the antiproton and proton inertial masses which shows they are the same to a fractional accuracy of 4×10−8. This is 1000 times more accurate than previous comparisons and large additional increases in accuracy are anticipated soon. The availability of low energy antiprotons opens other experimental possibilities as well, such as the production and study of trapped antihydrogen atoms. To increase the number of trapped antiprotons we have demonstrated that we can accumulate antiprotons from successive pulsed injections of antiprotons into the trap.

Since antiprotons have been captured in an ion trap, we consider the possibility of producing antihydrogen by merging cold trapped plasmas of antiprotons and positrons. The calculated, instantaneous rate for antihydrogen production by the 3-body recombination $$p^ - + e^ + + e^ + \to \bar H + e^ + $$ is much higher than for other proposed techniques, opening up intriguing experimental possibilities.

Antiprotons from the Low Energy Antiproton Ring of CERN are slowed from 21 MeV to below 3 keV by being passed through 3 mm of material, mostly Be. While still in flight, the kilo-electron volt antiprotons are captured in a Penning trap created by the sudden application of a 3-kV potential. Antiprotons are held for 100 s and more. Prospects are now excellent for much longer trapping times under better vacuum conditions. This demonstrates the feasibility of a greatly improved measurement of the inertial mass of the antiproton and opens the way to other intriguing experiments. The possibility of producing antihydrogen by merging cold, trapped plasmas of positrons and antiprotons is discussed.

Cylindrical Penning traps with open-endcap electrodes are compared with the hyperbolic traps which are currently used for high precision experiments. Cylindrical traps are easier to construct and allow free access to the center of the trap for particle loading, laser beams, or microwaves. The trapping potential can be tuned while particles are inside the trap to improve harmonicity. Special geometries are noted which allow anharmonicities to be tuned out without affecting the trap well depth, and the effects of trap imperfections and gaps between the electrodes are discussed. Such traps will be used for measuring the antiproton mass, for colling of trapped antiprotons, and possibly for producing antihydrogen.

Now that antiprotons have been captured in an ion trap, we investigate the possibility of producing antihydrogen by merging cold trapped plasmas of antiprotons and positrons. The calculated rate for antihydrogen production by the three-body recombination p − + e + + e + → H + e + is many orders of magnitude higher than for any other proposed technique, opening up intriguing experimental possibilities.