Design and Simulation of Sheet Electron Beam for Gridless Inductive Output Tube

Inductive output tubes (IOTs) achieve MW-range operation by employing gridless beam modulation. Density modulation is achieved on the cathode surface using non-intercepting anodes. A switching voltage applied to these anodes results in beam gating and modulation at the desired frequency. For high-power operation of these devices, a large aspect ratio sheet electron beam (SEB) offers certain advantages over annular and multi-beam setups -- namely, the ability to achieve a high current density with lower cathode loading, lower focusing magnetic field requirements, and high interaction efficiency. For these reasons, it is of interest to investigate the viability of a sheet-beam operated gridless IOT. To this end, an 80 kV, 15 A sheet beam is designed and simulated using the particle tracking solver in CST Studio. The cathode dimensions are 4.5 mm x 40 mm. To simulate the modulation effect, the focusing anode is maintained at a 1.25 kV difference from the cathode. The particle source is set to space-charge limited, with non-zero initial electron energy. Magnetic focusing is achieved with solenoid. A uniform 0.67 T axial magnetic field is applied axially across the 55 mm drift length. The beam current is tuned by varying the anode-cathode gap. Cathode loading is 8.33 A/cm
²
, and beam perveance is lower than $7 \times 10^{-7} \mathrm{~A} / \mathrm{V}^{3 / 2}$. The compressed beam dimensions are 2.54 mm x 37.5 mm, with an aspect ratio of 14.2:1. The undesired curling of the sheet beam edges (the diocotron effect), is minimized here by using a high vertical fill factor (∼87% and precise tunnel shaping to maintain uniform space-charge potential at the edges. Particle monitors at multiple locations show a variation in beam uniformity at the edges lower than 0.05 mm. The simulation results show that the beam is suitable for an IOT.