Abstract: A open--structure high--gradient accelerating tube was designed and constructed.This operates at 750 kV across two quasi--constant field accelerating gaps whichwere made of titanium alloy electrodes, corresponding to an average gradient of30 kV/cm. Pulsed proton beam currents of 50--200 mA was accelerated through thetube. It has been operating in the 750 kV pre--injector since 1982. The effect ofvacuum cleanliness on deconditioning rate was also discussed.