Radial compression of protons and $\mathrm{H}_3{}^{+}$ ions in a multiring trap for the production of ultralow energy antiproton beams

Radial compression of a proton cloud was performed in a multiring trap which was designed to trap and cool a large number of antiprotons for the production of low-energy $\left(10–1000\mathrm{eV}\right)$ antiproton beams. The resonance frequency for the radial compression was almost constant from $3\times {10}^5$ to $3\times {10}^6$ protons. The collision process of the trapped protons was also investigated to estimate the energy of the protons inside the trap. This technique will be applied to the ASACUSA experiment at the antiproton decelerator, CERN, to extract ultraslow antiprotons with good emittance.

Figure 1

A schematic of the experimental setup. The external rf electric field for the radial compression of trapped particles is applied through segmented electrodes. The external rf electric field to select the ion species is applied through another ring electrode.

Figure 2

Time-of-flight spectra of extracted ions with extraction energy of $\approx 800\mathrm{eV}$. (a) Two peaks corresponding to protons and $\mathrm{H}_3{}^{+}$ ions are clearly observed. (b) rf field with a frequency of $142\mathrm{kHz}$ (axial harmonic oscillation for $\mathrm{H}_3{}^{+}$ ions) was applied to kick out $\mathrm{H}_3{}^{+}$ ions. (c) rf field with a frequency of $248\mathrm{kHz}$ (axial harmonic oscillation for protons) was applied to kick out protons.

Figure 3

The ratio between protons (solid circles) and $\mathrm{H}_3{}^{+}$ (solid triangles) as a function of time. The axial resonance frequency of protons (open squares) is also plotted.

Figure 4

The resonance frequency for the radial compression of protons (solid circles) and $\mathrm{H}_3{}^{+}$ ions (solid triangles) as a function of the particle number. Open circles and open triangles correspond to the second harmonic frequencies for the radial compression of protons and $\mathrm{H}_3{}^{+}$ ions, respectively. Solid circles are almost independent of the number of protons.

Figure 5

A tank circuit signal with rf electric field applied for $3\mathrm{s}$. The frequency of the externally applied rf field was swept from 150 to $250\mathrm{kHz}$.