Transportable antiproton traps for precision studies

Making a transportable antiproton reservoir trap

Confinement of charged particles

Charged particles can be confined in a strong magnetic field forcing the particle on a cyclotron orbit and a weak electrostatic potential to limit the motion along the magnetic field lines. This ion trap configuration is called a Penning trap. It is the essential ingredient to high-precision measurements of masses and magnetic moments of charged particles. The high stability of the superconducting magnets with a relative stability of 10-10, ultra-stable voltage sources and the long confinement times for more than one year make it the tool of choice for these precision experiments.

Antiproton storage in cryogenic vacuum conditions

Storing ions on time scales of several months requires ultra-high vacuum conditions to prevent charge-exchange reactions with residual gas. Storing antiprotons is even more challenging since the annihilation rates of antiprotons with residual gas are several orders of magnitude higher compared to charge exchange reactions. For example, a residual hydrogen pressure of about 3 10-10 mbar is required for an antiproton storage time of one second. Consequently, long term storage of antiprotons can only be performed in closed cryogenic vacuum chambers at 4.2 K or lower where the residual gas pressure is by orders of magnitude lower. In such cryogenic vacuum chambers, any residual gas freezes out while cooling the vacuum chamber down, and hydrogen diffusion in the chamber walls, which is one of the processes contribution to the residual gas pressure in room temperature vacuum chambers, is stopped by the low wall temperatures. Even residual hydrogen molecules and helium atoms in the vacuum chamber attach in monolayers to the chamber walls. This results in extremly high vacuum conditions, so that antiprotons can be trapped for more than one year and the residual gas pressure can only be measured by the loss rate of the trapped antiprotons.

The BASE reservoir trap and antiproton lifetime studies

The BASE collaboration at CERN has developed the antiproton reservoir trap technique, see here, which allows operating a single-particle precision experiment with antiprotons from an initially trapped cloud of antiprotons. The reservoir trap is the interface between the antiproton decelerator of CERN and the BASE Penning trap system for the antiproton precision measurements. Antiprotons ejected from the Antiproton Decelerator/ELENA facility of CERN are initially confined and cooled by collisions with electrons and resistive cooling to cryogenic temperatures in the BASE reservoir trap. Single particles are non-destructively separated from the antiproton cloud by voltage ramps and adiabatically transported into the traps for the precision measurements. We detect such cold antiprotons non-destructively by picking up the tiny (~ fA) image-current signal in the trap electrodes, which are generated by the oscillating motion of the trapped antiprotons. This method also allows us to count the number of trapped antiprotons in the antiproton cloud in the reservoir trap. Consequently, we can detect the annihilation of a single antiproton with residual gas in the reservoir. During the BASE physics run 2015/2016, we have confined antiprotons for 405 days in the reservoir trap and have not observed any antiproton loss due to annihilations with residual gas. Based on the equivalent single-particle antiproton storage time, we could set a lower limit on the total antiproton lifetime of 10.2 years and constrain the residual gas pressure to be less than 3 10-18 mbar. The results are described in this article about the antiproton lifetime.

Separation and extraction of single particles

For the non-destructive extraction of single particles, we transform the harmonic well of the Penning trap by voltage ramps into a W-shaped potential well, which splits the cloud of trapped antiprotons into two fractions. These two fractions are indiviually detected, so that the number of antiprotons in each fraction is determined. We apply an additional electric field to control the number of separated antiprotons on the side of the precision traps, so that eventually only a single antiproton is extracted and shuttled by voltage ramps into the other traps. The separation process can also be reversed in case more than one particle was extracted. We have demonstrated separation and merging antiproton clouds without losses with up to 100 particles. Precision measurements on single particles require ultimately only a single particle (or a few in multi-trap systems), so that the reservoir trap technique provides the ideal source for conducting antiproton precision measurements independently from the operation of CERN's antiproton deceleration facility AD/ELENA . Consequently, this technique will also be the basis of the supply of experiments from a transportable antiproton reservoir trap.