Dynamic Zoom High-Voltage Tracking for Electron-Beam Additive Manufacturing

Metal electron-beam powder-bed fusion over build heights exceeding 420 mm requires continuous adjustment of the focusing lens voltage from 4.2 kV to 9.8 kV to maintain focal spot diameter <180 µm as beam current varies from 2 mA to 48 mA and working distance changes by hundreds of millimeters. Dynamic zoom supplies must therefore track multiple inputs with <68 µs latency and settling to ±0.032 % while compensating for thermal lensing and space-charge effects in real time.

The lens supply uses a 320 kHz resonant push-pull converter with gallium nitride primaries driving a 1:52 ferrite transformer and single-stage quadrupler. Output voltage is regulated by primary peak-current control with a 420 kHz digital loop that achieves 142 kV/ms slew rate and 52 µs settling to 0.02 %.

Tracking inputs are fused from three sources: beam current measured at the collector, build height from the layer counter, and melt-pool vapor density inferred from secondary electron current. A 32-bit floating-point processor updates the voltage setpoint every 42 µs using a pre-characterized 3D lookup table refined by an adaptive algorithm that minimizes spot size measured during periodic focus star tests executed between layers.

Space-charge compensation adds a feed-forward term proportional to the square root of beam current, while vapor plume lensing is countered by an additional 40–220 V offset derived from real-time secondary electron fluctuation amplitude. Resulting focal spot variation remains <21 µm across full 400 × 400 × 420 mm Ti-6Al-4V builds.

Thermal lensing from absorbed metal vapor is corrected by monitoring lens electrode temperature via embedded RTDs and applying a programmed –1.8 V/°C offset, preventing the 180 µm spot growth observed in uncompensated systems during high-power hatching.

These dynamic zoom supplies routinely maintain focal spot diameter <168 µm with layer-to-layer variation <14 µm at average layer times <2.6 s, enabling as-built surface finish Ra <5.2 µm and internal channel resolution down to 180 µm in aerospace heat exchangers.