Focused electron beam transport through a long narrow metal tube at elevated pressures in the forevacuum range

We present here our investigations of the features of focused electron beam transport in free space at elevated pressures of a few pascals. We have explored the effect of the beam accelerating voltage, operating gas pressure, and magnetic focusing upon the trajectory of beam electrons in the crossover region, in particular on the beam convergence and divergence angles. It is shown that for the forevacuum pressure range of 2–5 Pa explored, a distinctive feature of the propagation of a focused electron beam with a current of up to 20 mA at an accelerating voltage of 10–20 kV is the difference in the angles of convergence (before the focus) and divergence (after the focus). Whereas at a low pressure of 2 Pa the divergence angle is smaller than the convergence angle, as the pressure increases the divergence angle increases and for pressures greater than 5 Pa the divergence angle is greater than the convergence angle. The results obtained were used in experiments on electron beam transport through a long narrow metal tube with a diameter of 5.8–9.2 mm and length of 10–30 cm. We show that for a 30 cm long tube of 7.5 mm diameter, the focused beam transmission can exceed 70%.