Magnetron fields – Orientation of the applied magnetic field In a conventional cylindrical cavity magnetron, what is the orientation of the externally applied magnetic field relative to the cathode axis?

Introduction:
A cavity magnetron is a crossed-field microwave oscillator. “Crossed-field” means that the static electric field (radially outward from the central cathode to the anode cavities) and the static magnetic field are orthogonal. Recognizing the correct orientation of these fields is fundamental to understanding electron motion and spoke formation in the interaction space.

Given Data / Assumptions:

• Cylindrical geometry with a central thermionic cathode and anode block containing resonant cavities.
• Static radial electric field from cathode to anode.
• Uniform static magnetic field applied by a magnet assembly.

Concept / Approach:

The magnetic field is applied axially, i.e., parallel to the cathode (and tube) axis. With E radial and B axial, the Lorentz force causes electrons to follow curved paths with azimuthal drift (E × B motion). Proper magnetic field magnitude establishes synchronous interaction with the π-mode fields of the cavities, enabling efficient RF generation.

Step-by-Step Solution:

Verification / Alternative check:

Construction diagrams show pole pieces above and below the anode block, producing an axial B field through the interaction space.

Why Other Options Are Wrong:

Perpendicular to the axis (radial or tangential B) would not produce the standard crossed-field configuration; “inclined” is not how magnetrons are biased; claiming no magnetic field contradicts the device’s operating principle.

Common Pitfalls:

Confusing “perpendicular to E” with “perpendicular to the axis.” In a magnetron, E is radial, B is axial (parallel to axis) so E ⟂ B even though B ∥ axis.

Final Answer:

parallel to the axis of cathode.