Positron plasma control techniques for the production of cold antihydrogen

An observation of a clear dependence of antihydrogen production on positron plasma shapes is reported. For this purpose a plasma control method has been developed combining the plasma rotating-wall technique with a mode diagnostic system. With the help of real-time and nondestructive observations, the rotating-wall parameters have been optimized. The positron plasma can be manipulated into a wide range of shapes (aspect ratio $6.5⩽\alpha \lesssim 80$) and densities $(1.5\times {10}^8⩽n\lesssim 7\times {10}^9{\mathrm{cm}}^{-3})$ within a short duration $\left(25\mathrm{s}\right)$ compatible with the ATHENA antihydrogen production cycle.

Figure 1

The relationship between the mode frequencies (ratio of the quadrupole mode frequency to the dipole mode frequency) and the aspect ratio $\alpha$ $(>1)$.

Figure 2

Schematic diagram of the rotating-wall apparatus. The region from EL2 to ER2 is the central region of the nested Penning trap. The split electrode EL1 was connected to the rotating-wall modules and ER2 was connected to the plasma-mode system.

Figure 3

(a) Effect of the rotating-wall on the aspect ratio and density of the positron plasma for various amplitudes. The lines are used to guide the eye. (b) Comparison of the effects of a $25\mathrm{s}$ wall with an amplitude of $10\mathrm{V}$ and a $100\mathrm{s}$ wall with $1\mathrm{V}$. The shaded areas represent the duration of the application of the rotating-wall voltage. After the rotating-wall voltage has been switched off, the signals on the plasma-mode system cannot be used to determine plasma mode frequencies for several seconds (see text). The data points on the dashed horizontal lines, however, are reliable and can be used to derive plasma parameters.

Figure 4

Schematic illustration of the ATHENA $\overline{\mathrm{H}}$ production cycle. Positrons are accumulated and transferred to the mixing trap every $\sim 200\mathrm{s}$ and $\overline{p}$ are accumulated by stacking 2 AD shots and then mixed in the nested Penning trap for $60\mathrm{s}$. There is a $\sim 40\mathrm{s}$ pause between $\overline{p}$ transfer and the start of mixing.

Figure 5

Trigger signals on the imaging detector during mixing. From top to bottom: compressed plasma, standard plasma, and expanded plasma. Inset: the same for the first $500\mathrm{ms}$. All the data are normalized to a single standard mixing cycle with $7000\overline{p}$.

Figure 6

The integrated number of $\overline{\mathrm{H}}$ triggers, after subtraction of background, scaled to a single standard mixing cycle with $7000\overline{p}$. The samples are at times of 0.1, 0.2, 0.5, 1, 20, and $60\mathrm{s}$ after the start of mixing.

Figure 7

(a) Time spectrum of the $\overline{p}$ annihilation signals. From top to bottom, the graphs are for the compressed plasma, the standard plasma, and the expanded plasma. $\overline{p}$ annihilations are monitored by the external scintillators [21] in this case. (b) The three step potential dump procedure (see text). The region labels are equivalent in both figures and in Table 2.