Cathode-in-cathode high-power microwave (HPM) vacuum tube source and method of alignment

A high-power microwave (HPM) vacuum tube source, comprising:

• a magnetic configured to produce a magnetic field;
• a field emission (FE) cathode configured to emit an annular high-power primary electron beam in discrete pulses;
• a cylindrical RF generating tube along a longitudinal axis configured to interact with the primary electron beam to generate or amplify a pulsed RF signal;
• a thermionic emission (TE) cathode positioned inside and coaxial with the FE cathode, said TE cathode configured to emit a low-power surrogate beam as a continuous beam;
• and adjustment mechanisms responsive to position measurements of the surrogate beam fore and aft of the cylindrical RF generating tube to adjust the alignment of the FE cathode, cylindrical RF generating tube and magnet until the position measurements satisfy a coaxial alignment tolerance.

A method of aligning a high-power microwave (FPM) vacuum tube source, said source including a magnet to produce a magnetic field, a field emission (FE) cathode configured to emit an annular high-power primary electron beam, and a cylindrical RE generating tube configured to interact with the primary electron beam to generate or amplify an RF signal, the method comprising:

• roughly coaxially aligning the FE cathode and cylindrical RF generating tube such that a cathode face appears to be visually centered within an internal bore of the cylindrical RF generating tube when viewed from a far end of the cylindrical RF generating tube;
• roughly aligning the magnet such that the magnetic field is approximately coaxial with the FE cathode and the cylindrical RF generating tube by visually adjusting the magnet until the exterior of the cylindrical RE generating tube appears to be concentric within a bore of the magnet;
• precisely coaxially aligning the FE cathode, cylindrical RF generating tube and magnetic field with the FE cathode deactivated by,
• (a) positioning a thermionic emission (TE) cathode inside the FE cathode, said TE cathode configured to emit a low-power surrogate electron beam coaxially with the primary electron beam, wherein surrogate electron beam has a diameter less than or equal to 1/10th of the diameter of the primary electron beam and an average power less than 1/1,000th the peak power of the primary electron beam;
• (b) positioning a fluorescent target between the FE cathode and the cylindrical RF generating tube, drawing a vacuum on the source, activating the TE cathode to emit the surrogate electron beam to strike the fluorescent target causing it to fluoresce in a spot, visualizing the spot through the cylindrical RF generating tube to measure the position of the surrogate electron beam and adjusting at least translational positions of the primary cathode or cylindrical RF generating tube to center the spot on the fluorescent target;
• (c) breaking vacuum and removing the fluorescent target;
• (d) positioning the fluorescent target aft of the cylindrical RF generating tube, drawing a vacuum on the source, activating the TE cathode to emit the surrogate electron beam to strike the fluorescent target causing it to fluoresce in a spot, visualizing the spot to measure the position of the surrogate electron beam and adjusting at least an angular position of the magnet to center the spot on the fluorescent target;
• and
• repeating steps (b)-(d) until measured position of the surrogate electron beam satisfies a coaxial alignment tolerance.