Radial Compression of an Antiproton Cloud for Production of Intense Antiproton Beams

We report here the radial compression of a large number of antiprotons ($\sim 5\times {10}^5$) in a strong magnetic field under ultrahigh vacuum conditions by applying a rotating electric field. Compression without any resonant structures was demonstrated for a range of frequencies from the sideband frequency of 200 kHz to more than 1000 kHz. The radial compression achieved is a key technique for synthesizing and manipulating antihydrogen atoms and antiprotonic atoms.

Figure 1

A schematic view of the main part of the experimental setup (not to scale) including the multiring trap with a cross section of the segmented electrode (right lower figure) and the transport beam line. Antiprotons ($\overline{p}s$) come from the left side. The left lower figure shows the potential distribution along the axis ($z$). The dotted line shows the potential distribution during the antiproton extraction. The trap potential depth was 50 V.

Figure 2

The number of trapped (solid circles) and transported (solid triangles) antiprotons as a function of the electron plasma radius, $r_e$. A 2-MHz field was applied with its amplitude $V_r=1.1\mathrm{V}$, to expand the plasma to a radial size ranging from $r_e\sim 0.8$ to 5 mm. The corresponding volume density of the plasma of $3\times {10}^8$ electrons varied from $2.7\times {10}^9/{\mathrm{cm}}^3$ to $6.5\times {10}^7/{\mathrm{cm}}^3$, and the length from 62 to 45 mm. The solid line is the best fit to the solid circles, assuming that the radial distribution of the incoming antiproton beam can be expressed by a Gaussian function $\rho (r_{\overline{p}})=\exp \{-(r_{\overline{p}}/a_{\overline{p}}{)}^2/2\}$, for $a_{\overline{p}}=1.95\mathrm{mm}$. The uncertainty of antiproton number primarily comes from the fluctuation of the antiproton beam intensity from the AD. The uncertainties of $r_e$ mainly comes from the uncertainty of the plasma oscillation frequencies [3].

Figure 3

(a)–(d) The annihilation position of extracted antiprotons along the transport beam line for the rotation time of $t_r=0$, 60, 120, and 200 s. (e)–(h) The PSD images of extracted antiprotons for $t_r=0$, 60, 120, and 200 s. The rotation frequency was $f=247\mathrm{kHz}$ with its peak-to-peak amplitude $V_r=0.56\mathrm{V}$. The electron plasma used had a radius of 3.4 mm and was removed from the MRT before application of the rotating field and the extraction.

Figure 4

The transport efficiency ${\epsilon }_{\exp }$ of antiprotons as a function of the rotation time $t_r$ for $f=247\mathrm{kHz}$ and $V_r=0.56\mathrm{V}$. The error bars mainly come from the fluctuation of the antiproton beam intensity from the AD.

Figure 5

Simulated antiproton annihilation position distributions along the transport beam line [(a)–(c)] and antiproton beam profiles at the PSD [(d)–(f)]. (a) and (d) $a_{\overline{p}}=3.4\mathrm{mm}$, (b) and (e) $a_{\overline{p}}=0.25\mathrm{mm}$, (c) and (f) a superposition of two Gaussian distributions, 45% of $a_{\overline{p}}=0.25\mathrm{mm}$ and 55% of $a_{\overline{p}}=4.0\mathrm{mm}$.

Figure 6

The transport efficiency ${\epsilon }_{\exp }$ of antiprotons as (a) a function of the rotating field frequency $f$, and (b) in expansion around $f=247\mathrm{kHz}$ and $t_r=120\mathrm{s}$. The negative frequency refers to the counter-rotating field. The error bars mainly come from the fluctuation of the antiproton beam intensity from the AD.