The injector will deliver electrons into the Hefei Advanced Light Facility (HALF) storage ring with full energy top-up injection operation mode. It is installed in a semi-underground tunnel and consists of two parts: a linear accelerator approximately 192 meters in length and a beam transport line about 138 meters long (The transport line length was strategically designed to accommodate the building layout.). The electron beam is generated and accelerated to 2.2 GeV in the linear accelerator, then directed into the transport line via horizontal bending magnets. Finally, two vertical bending magnets at the end of the transport line elevate the beam by 4.7 meters to bring it into alignment with the storage ring’s beam plane.

In the linear accelerator, the thermionic-cathode DC high-voltage electron gun generates electron bunches with an initial pulse width of ~1.0 ns. The beam first undergoes pulse compression in a 476 MHz sub-harmonic pre-buncher (PB) before entering the buncher (B), where the pulse width is further reduced to ~10 ps (FWHM) while simultaneously accelerating the beam to approximately 10 MeV. Subsequently, 20 accelerating units (labeled L01 to L20) operating at a frequency of 2856 MHz further boost the beam energy to the design value. Each accelerating unit can provide an energy gain of 120 MeV and consists of an RF unit and an accelerating segment. The accelerating segment comprises two traveling-wave accelerating tubes, each about 3.1 meters in length. The RF unit includes an 80 MW peak-power klystron and a solid-state modulator that supplies power to it. Among the 20 accelerating units, one is maintained as an online hot backup. The schematic layout of the linear accelerator is presented in Figure 1.