Physical Sciences

One set of electrodes is used for both transmission and reception.

A modified Penning-Malmberg trap that could store a small cloud of antiprotons for a relatively long time (weeks) has been developed. This trap is intended for use in research on the feasibility of contemplated future matter/antimatter-annihilation systems as propulsion sources for spacecraft on long missions. This trap is also of interest in its own right as a means of storing and manipulating antiprotons for terrestrial scientific experimentation.

The use of Penning-Malmberg traps to store antiprotons is not new. What is new here is the modified trap design, which utilizes state-of-the-art radio-frequency (RF) techniques, including ones that, heretofore, have been used in radio-communication applications but not in ion-trap applications.

A basic Penning-Malmberg trap includes an evacuated round tube that contains or is surrounded by three or more collinear tube electrodes. A steady axial magnetic field that reaches a maximum at the geometric center of the tube is applied by an external source, and DC bias voltages that give rise to an electrostatic potential that reaches a minimum at the center are applied to the electrodes. The combination of electric and magnetic fields confines the charged particles (ions or electrons) for which it was designed to a prolate spheroidal central region. However, geometric misalignments and the diffusive cooling process prevent the steady fields of a basic Penning-Malmberg trap from confining the particles indefinitely.

In the modified Penning-Malmberg trap, the loss of antiprotons is reduced or eliminated by use of a "rotating-wall" RF stabilization scheme that also heats the antiproton cloud to minimize loss by matter/antimatter annihilation. The scheme involves the superposition of a quadrupole electric field that rotates about the cylindrical axis at a suitably chosen radio frequency.

The modified Penning-Malmberg trap (see Figure 1) includes several collinear sets of electrodes inside a tubular vacuum chamber. Each set comprises either a single metal tube or else a tube that is segmented into four electrodes that subtend equal angles about the cylindrical axis. The output of an RF signal generator is fed through a 90° hybrid coupler and then through two baluns to generate four replicas of the signal at relative phase shifts of 0°, 90°, 180°, and 270° (see Figure 2). These signal replicas are fed through –6-dB directional couplers, then via coaxial cables to the vacuum chamber. The signal is then routed to a phase cancellation network, which filters out the drive signal with the difference representing the plasma interaction. Inside the vacuum chamber, twisted-pair wires feed the signals from the coaxial cables to the four electrodes of each segmented electrode tube.

It is not necessary to use a different set of electrodes for monitoring the antiproton cloud. Instead, the –6-dB directional couplers are used to receive the signal that emanates from the antiproton cloud when the cloud interacts with the applied signal. The received signal can be routed to either a spectrum analyzer or a network analyzer.

This work was done by William H. Sims and James Martin of Marshall Space Flight Center, and Raymond Lewis of RLewis Co. For further information, access the Technical Support Package (TSP) free on-line at under the Physical Sciences category. This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel
Marshall Space Flight Center
(256) 544-0021.

Refer to MFS-31780.

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