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Physical Review Letters

Extremely Cold Positrons Accumulated Electronically in Ultrahigh Vacuum

L. Haarsma, K. Abdullah, G. Gabrielse

Published Jul 31, 1995• 44 citations
Publisher

Abstract

Many cold positrons in ultrahigh vacuum are required to produce cold antihydrogen, to cool highly stripped ions, and for ultracold plasma studies. Up to $3.5\ifmmode\times\else\texttimes\fi{}{10}^{4}$ 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.

DOI
10.1103/physrevlett.75.806

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Physical Review A•Jan 29, 2007•4 citations
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\ifmmode×\else\texttimes\fi1043.5\ifmmode\times\else\texttimes\fi{}{10}^{4}3.5\ifmmode×\else\texttimes\fi104 such positrons have now been accumulated into the ultrahigh vacuum of a 4.2 K Penning trap, at a rate exceeding 103/{10}^{3}/103/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 22Na{}^{22}\mathrm{Na}22Na 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.
The production and study of cold antihydrogen will require the manipulation of dense and cold, single component plasmas of antiprotons and positrons. The undertaking will build upon the experience of the nonneutral plasma physics community. Annihilations of the antimatter particles in the plasmas can be imaged, offering unique diagnostic opportunities not available to this community when electrons and protons are used. The techniques developed by our TRAP collaboration to capture and cool antiprotons will certainly be used by our expanded ATRAP collaboration, and by the competing ATHENA Collaboration, both working at the nearly completed AD facility of CERN. We recently demonstrated a new techniques for accumulating cold positrons directly into a cryogenic vacuum system. The closest we have come to low energy antihydrogen so far is to confine cold positrons and cold antiprotons within the same trap structure and vacuum container. Finally, we mention that stored electrons have been cooled to 70 mK, the first time that elementary particles have been cooled below 4 K. In such an apparatus it should be possible to study highly magnetized plasmas of electrons or positrons at this new low temperature.
Only our ATRAP Collaboration is yet able to accumulate and store 4.2 K antiprotons and positrons. The antiprotons come initially from the new Antiproton Decelerator facility at CERN. Good control of such cold antimatter plasmas is key to aspirations to produce and study antihydrogen atoms that are cold enough to confine by their magnetic moments. In the closest approach to cold antihydrogen realized to date, the cold positrons have been used to cool antiprotons, the first time that positron cooling has ever been observed. The Penning-loffe trap, one possibility for simultaneously confining antihydrogen and the cold ingredients from which it is formed, is introduced and discussed.
A new physical mechanism for positron accumulation is explained and demonstrated. Strongly magnetized Rydberg positronium is formed and then ionized, allowing us to trap equal numbers of either positrons or electrons over a wide range of conditions. Antiprotons are trapped, cooled, and stacked from the new Antiproton Decelerator facility for the first time. Combining positrons and antiprotons, we have demonstrated the first positron cooling of antiprotons. The cooling takes place in a 4.2 K, nested Penning trap where conditions are ideal for the eventual goal of the formation of antihydrogen.
The author's ATRAP team has managed to simultaneously accumulate cold antiprotons and cold positrons. Cold antihydrogen is produced when antiprotons are repeatedly driven into collisions with cold positrons within a nested Penning trap. Efficient antihydrogen production takes place during many cycles of positron cooling of antiprotons. A first measurement of a distribution of antihydrogen states is made using a preionizing electric field between separated production and detection regions. Surviving antihydrogen is stripped in an ionization well that captures and stores the free antiproton for background-free detection. A background-free observation of cold antihydrogen atoms was made using field ionization followed by antiproton storage, a detection method that provided the first experimental information about antihydrogen atomic states. More antihydrogen atoms can be field ionized in an hour than all the anti-matter atoms that have been previously reported, and the production rate per incident high energy antiproton is higher than ever observed. The high rate and the high Rydberg states suggest that the antihydrogen is formed via three-body recombination. This report presents bibliographic citations for 10 publications and 36 lectures that were produced during 2004 under this contract. The author also was the recipient of the George Ledlie Prize at Harvard University in 2004.
Antihydrogen, the bound state of an antiproton and a positron, is of interest for use in precision tests of nature's fundamental symmetries. Antihydrogen formed by carefully merging cold plasmas of positrons and antiprotons has recently been trapped in magnetic traps. The efficiency of trapping is strongly dependent on the temperature of the nascent antihydrogen, which, to be trapped, must have a kinetic energy less than the trap depth of . In the conditions in the ALPHA experiment, the antihydrogen temperature seems dominated by the temperature of the positron plasma used for the synthesis. Cold positrons are therefore of paramount interest in that experiment. In this paper, we propose an alternative route to make ultra-cold positrons for enhanced antihydrogen trapping. We investigate theoretically how to extend previously successful sympathetic cooling of positrons by laser-cooled positive ions to be used for antihydrogen trapping. Using simulations, we investigate the effectiveness of such cooling in conditions similar to those in ALPHA, and discuss how the formation process and the nascent antihydrogen may be influenced by the presence of positive ions. We argue that this technique is a viable alternative to methods such as evaporative and adiabatic cooling, and may overcome limitations faced by these. Ultra-cold positrons, once available, may also be of interest for a range of other applications.
Both experiments and simulations have shown that the temperature of antihydrogen atoms (H‾)(\overline{\mathrm{H}})(H) formed in a strongly magnetized positron-antiproton plasma is mainly determined by the heavy particles' temperature. A routine to keep cooling antiprotons (p‾)(\overline{p})(p​) by using ultracold (<1K)(<1\phantom{\rule{0.3em}{0ex}}\mathrm{K})(<1K) electrons is proposed under attainable experimental conditions. When cold positrons are loaded into such a low-temperature electron-antiproton mixture, ultracold antihydrogen atoms can be produced at a temperature below $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Our large-scale molecular-dynamics simulations, which involve 2000020\phantom{\rule{0.2em}{0ex}}00020000 particles (${e}^{\ensuremath{-}}$, e+{e}^{+}e+, and p‾\overline{p}p​) evolving up to $1\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{s}$, have confirmed the generation of trappable H‾\overline{\mathrm{H}}H atoms.