A high power magnetron for air breakdown studies

Abstract

A hybrid inverted coaxial magnetron (HICM) is being developed to produce high peak power levels at S band. This paper discusses its experimental performance with a short 40 nsec pulse forming network the results of a preliminary air breakdown study, and modifications to extend to a longer 250 nsec pulse width. In the short pulse mode the magnetron peak power levels are on the order 500 to 800 MW. The frequency of 3.24 GHz has been measured by gating the pulse and using a solid-state dispersive line. The large peak electric field <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_{p} = 5 \times 10^{4}</tex> V/cm, which this system generates (without resonant cavities), gives a unique capability to study microwave interaction and energy deposition in air breakdown plasma. The breakdown of air for pressures up to 100 Torr has been observed by monitoring the density of the resulting plasma and its light output. The plasma density at 10 Torr is approximately 100 n <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> ; n <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> is the critical plasma density where the microwave frequency equals the plasma frequency. The pulse width of the HICM will be extended to 250 nsec with a new pulse forming network which allows the examination of the hydrodynamic breakdown effects. A particularly interesting feature of the magnetron modification for longer pulse operation is the use of a novel high-voltage insulator. A newly developed circular waveguide TE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</inf> mode coupler allows axial seperation of two lengths of the eight inch output waveguide inside a larger diameter tube. The result is an effective dc isolation with very efficient r.f. transmission.