It is desirable to reduce anode voltage of distributed ion pumps (DIPs) in the Cornell Electron Storage Ring (CESR) in order to suppress beam instability caused by the leakage of electric field from the DIPs and to extend the lifetime of the DIPs and DIP power supplies. The DIP pumping speed is measured as a function of anode voltage with and without the stored beam in order to determine whether the anode voltage can be reduced without sacrificing the DIP pumping speed. It is found that the DIP pumping speed remains relatively unchanged when the anode voltage changes from a "normal" operating voltage of 7.6 kV to as low as 1.8 kV when there is a stored beam in the CESR. On the other hand, the pumping speed drops rapidly as the anode voltage is decreased below 5 kV and there is no stored beam in the CESR. A simple model is used to explain the operation of DIPs under the influence of the stored beam.