[en] The application of an asymmetric dipolar electric field rotating at a frequency close to that of the axial bounce of a collection of trapped positrons has, in the presence of a low pressure molecular gas to provide cooling, been used to achieve compression of the cloud. A theory of this effect has been developed for a Penning trap potential, with the cooling modeled in the Stokes viscous drag approximation. Good agreement between the theory and measurements of the frequency dependence of the cloud compression rate has been found, establishing that the phenomenon is a new form of sideband cooling.

[en] The anharmonic component of the electric field of a Penning–Malmberg trap is exploited to manipulate a subset of the radial (r) distribution of trapped positrons, using a dipole field made to rotate about the long-axis (z) of the trap. This ‘rotating wall’ technique (RW) induces inward transport at frequencies associated with the motion of trapped particles, although similarly it causes heating. The motional frequencies vary spatially within a non-ideal trap, thus resonant interaction with the rotating field may be restricted to a region selected to lie away from the trap centre, thereby forming a pseudo-potential barrier and reducing losses due to both heating and expansion. We demonstrate this effect for improved accumulation of positrons and further outline a technique to achieve strong compression with low RW amplitudes by chirping the drive frequency. (paper)

[en] Positron clouds are compressed following accumulation in a Surko-type two-stage buffer gas trap using an asymmetric rotating wall electric field. An analytic theory used to describe measurements of the rate of compression is discussed. Furthermore, we describe measurements taken without the rotating wall applied and with the rotating wall compression present during accumulation of the positron cloud. This has enabled total loss rates for the positrons via annihilation and collisional-induced radial transport to be isolated, with the latter mechanism found to be dominant. We have shown that the application of the rotating wall at a resonant frequency virtually eliminates radial transport, such that the positron loss is caused by annihilation in the gas. (paper)

[en] Following bombardment of a mesoporous silica sample by positrons ejected from a two-stage buffer gas trap, ortho-positronium (o-Ps) was emitted into vacuum with an efficiency of around 27%–28%. This ensemble, with a density close to 10¹¹ m⁻³, was then irradiated by multiple solid state-based laser beams up to 50 ns after positron implantation. Tunable over a wide range (230–2200 nm), the lasers have enabled excitation of the o-Ps Lyman-α transition, followed by subsequent excitation and, if desired, ionisation to vacuum. Excitation from the 2P state to intermediate states in the principal quantum number, $n_{\mathrm{P}\mathrm{s}}$, range $3⩽<!--\; ???\; -->n_{\mathrm{P}\mathrm{s}}⩽<!--\; ???\; -->18$ has been achieved with efficiencies ${\mathrm{ϵ<!--\; ??\; -->}}_{\mathrm{e}\mathrm{x}}^{n_{\mathrm{P}\mathrm{s}}}\gtrsim <!--\; ???\; -->80\mathrm{\%<!--\; \%\; -->}$, whilst the excitation efficiency of ground state o-Ps to the 2P state of ${\mathrm{ϵ<!--\; ??\; -->}}_{\mathrm{e}\mathrm{x}}^2\sim <!--\; ???\; -->13\mathrm{\%<!--\; \%\; -->}$ is currently limited by a mismatch between the Doppler broadening of the 1S-2P transition and the 225 GHz laser bandwidth at 243 nm. (paper)

[en] A tunable, pulsed laser was used to excite the Lyman-α transition (1S–2P) of positronium (Ps). The laser system has a large bandwidth of $\mathrm{\Delta <!--\; ??\; -->}\mathrm{\nu <!--\; ??\; -->}=225$ GHz at $\mathrm{\lambda <!--\; ??\; -->}=243$ nm, providing significant coverage of the Doppler-broadened, single-photon transition. The infra-red fundamental of a Nd:YAG laser was converted to ultraviolet by a series of solid-state, nonlinear processes, centred about an unseeded optical parametric oscillator, from which the bulk of the ultimate bandwidth derives. The Ps atoms were created by bombarding mesoporous silica with positrons, and the Doppler-width of the 1S–2P transition of the resulting ensemble was measured to be $\mathrm{\Delta <!--\; ??\; -->}\mathrm{\nu <!--\; ??\; -->}=672\pm <!--\; ??\; -->43$ GHz (equivalent to $T\approx <!--\; ???\; -->300$ K). It is envisaged that the UV laser will be incorporated into a two-step process to efficiently form Rydberg states of Ps, with potential applications in synthesis of cold antihydrogen, gravity measurements with antimatter, or for injection of electrons and positrons into a stellarator. (paper)

[en] A study of positron capture in a two-(pressure) stage buffer gas accumulation apparatus is presented for a variety of species, including some molecules which are known to be either efficient for positron trapping, or are frequently used to cool the particles when held in these devices. Absolute accumulation efficiencies are reported for all species. A detailed optimisation procedure, which has identified the main processes responsible for positron capture and loss in the trap, has been deployed to explore accumulation efficiency as the gas pressure and the electrostatic well depth in the trap are systematically varied. Accumulation exploiting energy loss via molecular vibrational transitions has been observed for the first time for a number of gases, though at much lower efficiency than achieved using electronic excitation processes. (paper)

[en] Weakly bound positron-electron pairs have been created in vacuum following low energy positron bombardment of a surface held at a temperature close to 4 K. The pairs, which behave as magnetized positronium atoms in the strong (>1 T) magnetic fields used in this experiment, were detected following their field ionization using an arrangement of Penning traps. Yields, which at highest are around 5 x 10^-6 per incident positron, are presented and compared with previous work. Measurements of the behaviour of the yield as the distance from the production target to the ionization well was varied are presented and discussed, as are results taken for a fixed well at different magnetic fields. Both data sets were found to be consistent with a model in which the positronium moves across the magnetic field lines with a constant drift speed.

[en] Magnetised positronium is formed by impacting low energy positrons onto a gas covered target immersed in a magnetic field (B ≥ 1T). The resulting weakly bound positronium atoms subsequently travel some distance in an arrangement of Penning-type traps whereupon they can be field ionised. The remnant positrons are accumulated and then detected by forced annihilation on the target. The production efficiency of the magnetised atoms has been measured for different species of gases, gas layer thickness and the strength of the magnetic field. The positronium loss as a function of the distance travelled has been measured and is shown to be caused by the magnetron drift of the positronium atom.

[en] We describe a simple and versatile method to manipulate the amplitude of the magnetron orbit of ions stored in a Penning trap, applied here to a cloud of low energy positrons. By applying a pulsed voltage to a split electrode in the trap, which is normally used for rotating wall compression of the particles, the size of the magnetron orbit can be changed at will. The modified orbit has been shown to be stable for many magnetron periods. The technique could find use in applications which require off-axis ejection of particles, for instance in the filling of arrays of traps for multicell positron storage.

[en] Knowledge of the residual gas composition in the ALPHA experiment apparatus is important in our studies of antihydrogen and nonneutral plasmas. A technique based on autoresonant ion extraction from an electrostatic potential well has been developed that enables the study of the vacuum in our trap. Computer simulations allow an interpretation of our measurements and provide the residual gas composition under operating conditions typical of those used in experiments to produce, trap, and study antihydrogen. The methods developed may also be applicable in a range of atomic and molecular trap experiments where Penning-Malmberg traps are used and where access is limited.