This method is superior to cleaning by baking.

Marshall Space Flight Center, Alabama

Radio-frequency-generated plasma has been demonstrated to be a promising means of cleaning the interior surfaces of a Penning-Malmberg trap that is used in experiments on the confinement of antimatter. {Such a trap was reported in "Modified Penning-Malmberg Trap for Storing Antiprotons" (MFS-31780), NASA Tech Briefs, Vol. 29, No. 3 (March 2005), page 66.) Cleaning of the interior surfaces is necessary to minimize numbers of contaminant atoms and molecules, which reduce confinement times by engaging in matter/antimatter-annihilation reactions with confined antimatter particles.

A modified Penning-Malmberg trap like the one described in the cited prior article includes several collinear ring electrodes (some of which are segmented) inside a tubular vacuum chamber, as illustrated in Figure 1. During operation of the trap, a small cloud of charged antiparticles (e.g., antiprotons or positrons) is confined to a spheroidal central region by means of a magnetic field in combination with DC and radio-frequency (RF) electric fields applied via the electrodes.

In the present developmental method of cleaning by use of RF-generated plasma, one evacuates the vacuum chamber, backfills the chamber with hydrogen at a suitable low pressure, and uses an RF-signal generator and baluns to apply RF voltages to the ring electrodes. Each ring is excited in the polarity opposite that of the adjacent ring. The electric field generated by the RF signal creates a discharge in the low-pressure gas. The RF power and gas pressure are adjusted so that the plasma generated in the discharge ( see Figure 2) physically and chemically attacks any solid, liquid, and gaseous contaminant layers on the electrode surfaces. The products of the physical and chemical cleaning reactions are gaseous and are removed by the vacuum pumps.

This cleaning method is much more aggressive than is the standard baking of ultrahigh-vacuum systems; adsorbed gases are removed much faster and more nearly completely.

The cleaning is also superior to that of a system in which plasma is generated outside the apparatus to be cleaned and made to flow through the apparatus. In contemplated further development, the method would be extended to afford a capability for plasma cleaning of, not only the electrodes, but also the interior wall of the vacuum chamber. For the purpose of cleaning the wall, it would likely be necessary to modify the electrical connections and electrical operating parameters to optimize the array of electrodes as an antenna for generating plasma between the electrodes and the wall.

This work was done by William Herbert Sims III, James Martin, and J. Boise. Pearson of Marshall Space Flight Center and Raymond Lewis ofRLewis Co., and Wallace E. Fant of Cortez III. For further information, access the Technical Support Package (TSP) free on-line at unvw.techbriefs.com/tsp under the Physical Sciences category.

This invention is owned by NASA, and a patent application has been filed. For further information, contact Sammy Nabors, MSFC Commercialization Assistance Lead, at sammy.a.nabors@nasa.gov. Refer to MFS-31825.