We report the application of evaporative cooling to clouds of trapped antiprotons, resulting in plasmas with measured temperature down to 9 K. We have modeled the evaporation process for charged particles using appropriate rate equations. Good agreement between experiment and theory is observed, permitting prediction of cooling efficiency in future experiments. The technique opens up new possibilities for cooling of trapped ions and is of particular interest in antiproton physics, where a precise CPT test on trapped antihydrogen is a long-standing goal.Historically, forced evaporative cooling has been successfully applied to trapped samples of neutral particles [1], and remains the only route to achieve Bose-Einstein condensation in such systems [2]. However, the technique has only found limited applications for trapped ions (at temperatures ∼ 100 eV [3]) and has never been realized in cold plasmas. Here we report the application of forced evaporative cooling to a dense (∼ 10 6 cm −3 ) cloud of trapped antiprotons, resulting in temperatures as low as 9 K; two orders of magnitude lower than any previously reported [4].The process of evaporation is driven by elastic collisions that scatter high energy particles out of the confining potential, thus decreasing the temperature of the remaining particles. For charged particles the process benefits from the long range nature of the Coulomb interaction, and compared to neutrals of similar density and temperature, the elastic collision rate is much higher, making cooling of much lower numbers and densities of particles feasible. In addition, intraspecies loss channels from inelastic collisions are non-existent. Strong coupling to the trapping fields makes precise control of the confining potential more critical for charged particles. Also, for plasmas, the self-fields can both reduce the collision rate through screening and change the effective depth of the confining potential.The ALPHA apparatus, which is designed with the intention of creating and trapping antihydrogen [5], is located at the Antiproton Decelerator (AD) at CERN [6]. It consists of a Penning-Malmberg trap for charged particles with an octupole-based magnetostatic trap for neutral atoms superimposed on the central region. For the work presented here, the magnetostatic trap was not energized and the evaporative cooling was performed in a homogeneous 1 T solenoidal field. Figure 1a shows a schematic diagram of the apparatus, with only a subset of the 20.05 mm long and 22.275 mm radius, hollow cylindrical electrodes shown. The vacuum wall is cooled using liquid helium, and the measured electrode temperature is about 7 K. The magnetic field, indicated by the arrow, is directed along the axis of cylindrical symmetry and confines the antiprotons radially: due to conservation of angular momentum, antiprotons do not readily escape in directions transverse to the magnetic field lines [7]. Parallel to the magnetic field, antiprotons are confined by electric fields generated by the electrodes.Also shown are the two ...

We present the results of an experiment to search for trapped antihydrogen atoms with the ALPHA antihydrogen trap at the CERN Antiproton Decelerator. Sensitive diagnostics of the temperatures, sizes, and densities of the trapped antiproton and positron plasmas have been developed, which in turn permitted development of techniques to precisely and reproducibly control the initial experimental parameters. The use of a position-sensitive annihilation vertex detector, together with the capability of controllably quenching the superconducting magnetic minimum trap, enabled us to carry out a high-sensitivity and low-background search for trapped synthesised antihydrogen atoms. We aim to identify the annihilations of antihydrogen atoms held for at least 130 ms in the trap before being released over ∼ 30 ms. After a three-week experimental run in 2009 involving mixing of 10 7 antiprotons with 1.3 × 10 9 positrons to produce 6 × 10 5 antihydrogen atoms, we have identified six antiproton annihilation events that are consistent with the release of trapped antihydrogen. The cosmic ray background, estimated to contribute 0.14 counts, is incompatible with this observation at a significance of 5.6 sigma. Extensive simulations predict that an alternative source of annihilations, the escape of mirror-trapped antiprotons, is highly unlikely, though this possibility has not yet been ruled out experimentally.

Control of the radial profile of trapped antiproton clouds is critical to trapping antihydrogen. We report the first detailed measurements of the radial manipulation of antiproton clouds, including areal density compressions by factors as large as ten, by manipulating spatially overlapped electron plasmas. We show detailed measurements of the near-axis antiproton radial profile and its relation to that of the electron plasma.

Antihydrogen production in a neutral atom trap formed by an octupole-based magnetic field minimum is demonstrated using field-ionization of weakly bound anti-atoms. Using our unique annihilation imaging detector, we correlate antihydrogen detection by imaging and by field-ionization for the first time. We further establish how field-ionization causes radial redistribution of the antiprotons during antihydrogen formation and use this effect for the first simultaneous measurements of strongly and weakly bound antihydrogen atoms. Distinguishing between these provides critical information needed in the process of optimizing for trappable antihydrogen. These observations are of crucial importance to the ultimate goal of performing CPT tests involving antihydrogen, which likely depends upon trapping the anti-atom

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