A linear charged particle accelerator is driven by an alternating voltage source operating at 10 MHz. Assume that it is used to accelerate electrons. After a few drift-tubes, the electrons attain a velocity \( 2.9 \times 10^8 \, \text{m/s} \). The minimum length of each drift-tube, in m, to accelerate the electrons further (rounded off to one decimal place) is \(\underline{\hspace{2cm}}\).