The main ALPHA trap and positron accumulator are Penning-Malmberg traps, where charged particles are confined radially by solenoidal magnetic fields, and longitudinally by electric fields produced with hollow cylindrical electrodes. Both traps are separated by a valve that is only opened for positron transfers. A linear actuator is also located between the traps that either allows positrons to pass through, or inserts an MCP for charge detection. (a) Configuration for positron transfers. (b) Configuration for charged particle manipulations. (c) Detection of autoresonantly extracted ions by charge amplification with the MCP.
Electrostatic potential manipulations during a typical run. (a) Positrons are brought into the main trap from the right, at a time when there is no potential wall at the axial position ∼400 mm (dashed line). As soon as they enter, the potential wall is erected (solid line) and positrons are confined and forced to bounce inside the experiment. Ions are formed through collisions of the positrons with the residual gas. (b) Ions and positrons cool down and accumulate at the bottom of the potential well, which is meanwhile modified to a more localized shape. (c) Positrons are then separated and discarded, and (d) ions are placed back in a deep well. (e) Finally, ions are placed at the bottom of an anharmonic well (solid line). A chirp is then applied that autoresonantly forces the ions to escape onto the MCP. The maximum electrostatic potential variations due to an applied 5 V chirp are shown as dashed lines. (f) Potential variations due to the chirp near the well minimum.
Calculated bounce frequencies in the anharmonic potential well used for the experiments, of particles with mass-to-charge ratios 1 (green-squares), 2 (blue-circles), 4 (red-triangles), and 14 (black-diamonds), in units of proton mass/fundamental charge (m p /e). Here, and in what follows, the energy is chosen to be zero at the potential well minimum.
Simulation histograms obtained for a 100 ms, 1 MHz–10 kHz, 4 V linear chirp applied to particles with m/q = 1, 2, 4, 14, 16, 28, and 32 and initial energies distributed uniformly in 0–49.1 eV. This energy range includes all possible initial energies and shows the full complexity of the particle dynamics in this nonlinear system. The peaks marked with an “F” correspond to particles driven out by the fundamental of the chirp, while “F1/2” and “F1/3” mark the first and second subharmonics.
(a) Experimental trace obtained with a 5 V, 100 ms, 1 MHz–10 kHz linear chirp (top), and simulation histograms made with a 4 V, but otherwise similar, chirp (bottom). The ∼20% difference in drive amplitudes gives a better fit of the data, and is consistent with the uncertainty in our knowledge of the coupling between the drive electronics and the electrode. Unlike Fig. 4 , this plot only includes initial energies in the 0–1.5 eV range. (b) Experimental data for a 50 ms, – linear chirp compared to the simulations. (c) Data and simulations for a linear 25 ms, 250 kHz–10 kHz chirp. See text for explanation of labels A, B, C, D.
(a) Experimental traces for different accumulator gas pressures when a 5 V linear, 1 MHz–10 kHz, 100 ms chirp is used (1 × 10−7 mbar is the default setting). (b) Simulations for ions with initial energies uniformly distributed between 0 and 49.1 eV, and a chirp amplitude of 4 V.
Experimental traces obtained at different times during a cool down of the apparatus. Data were taken sequentially from start to end in the order (a) through (g). A linear 5 V, 1 MHz–10 kHz, 100 ms chirp was used for all measurements. See text for details of labeling.
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