Why must a vapour diffusion pump be started with a mechanical forepump?

A vapour diffusion pump cannot function at atmospheric pressure and must be started with a mechanical forepump to reach an initial pressure of approximately 0.1 torr or less.1

This mechanical backing is essential because the diffusion pump operates by creating high-velocity vapour streams that entrain gas molecules and transfer them through successive stages. The mechanical forepump serves two critical roles:1
* Initial Evacuation: It reduces the system pressure to a level where the diffusion pump can begin its operation safely without damaging the pump fluid.1
* Gas Removal: During normal operation, the diffusion pump compresses gas and transfers it toward the fore-vacuum side, where the mechanical forepump must continuously remove the gas to maintain the pressure gradient necessary for the diffusion process to continue.1

Without the mechanical forepump, the diffusion pump would be unable to exhaust the gas it collects, preventing the attainment of high vacuum levels.1