Magnetically Enhanced Propellant Isolator for Ion Sources

A magnetic field inhibits the diffusion of electrons.

John H. Glenn Research Center, Cleveland, Ohio

A magnetically enhanced, high-voltage propellant isolator has been conceived for incorporation into materials-processing or space-based ion systems. The high-voltage isolator is needed to provide electrical isolation between the ion source, typically at high voltage, and the gas-feed system.

Heretofore, it has been the usual practice to provide the required electrical isolation by installing ceramic breaks (section of ceramic tubes) between grounded gas-feed lines and the high-voltage plasma source, as shown in the upper part of Figure 1. Unfortunately, at operating pressures between 13 Pa and 13 kPa and kilovolt potentials, the short ceramic spacer can often fail due to electrical breakdown of the feed gas, thereby causing a highly conductive plasma discharge to form inside the ceramic. Such breakdowns effectively short the ion source to near ground potentials, thus preventing high-voltage operation.

A magnetically enhanced, high-voltage isolator, as shown in the lower part of Figure 1, consists of a conventional ceramic isolator immersed in a strong transverse magnetic field. The strong, transverse field is provided by commercially available rare-earth magnets. The magnetic field increases the breakdown voltage at a given isolator internal pressure, thereby widening the range of operating conditions over which breakdown does not occur.

The primary purpose of the magnetic field is to severely restrict the motion of electrons parallel to the axis of the isolator. To first approximation, electrons are constrained to move in circles about magnetic-field lines and can diffuse across the transverse field only by collision with neutral molecules or ions. To inhibit the diffusion of electrons along the axis so as to prevent avalanche formation and breakdown, the magnetic field must be strong enough such that the electron cyclotron frequency (which is proportional to the magnetic-field strength) greatly exceeds the frequency of collisions between electrons and neutrals.

A prototype magnetically enhanced isolator was constructed by placing rare-earth magnets on opposite sides of a conventional isolator. The transverse magnetic field associated with this configuration was 2.6 kG on centerline. In tests at various gas pressures, the breakdown potential of the magnetically-enhanced isolator were found to exceed those of the same isolator without magnetic enhancement (see Figure 2). The greatest increase in breakdown potential was achieved at low pressures; this was expected because under those conditions, electron motion is sufficiently magnetized due to the lower electron-neutral collision frequency.

 

Figure 1. A Magnetically Enhanced Propellant Isolator, which isolates the gas-feed line from the ion source, is basically a conventional isolator tube sandwiched between permanent magnets that generate a strong, transverse field.

Figure 2. The Breakdown Potential applied between the ends of a prototype isolator tube similar to that of Figure 1, with and without magnetic enhancement, was measured as a function of xenon gas pressure. These plots have the form of typical Paschen breakdown curves.

 

This work was done by John E. Foster of Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Physical Sciences category.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4 —8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16749.

 

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