In the art of radioactive assay well logging, an assay tool is lowered into the borehole to a level of a formation to be assayed. The assay operation is then carried out by cyclically operating a neutron source so as to irradiate the formation with bursts of neutrons, the time between each burst being sufficient to allow the neutrons from the source to disappear and to allow delayed fission neutrons emitted by the formation to arrive at and be detected by a neutron detector. U.S. Pat. No. 3,686,503 to Givens, Caldwell, and Mills, Jr., describes such a system.
A pulsed neutron generator for such a system commonly takes the form of a three-element, linear accelerator tube. It includes a replenisher element which is electrically heated to boil off deuterium gas absorbed by the filament. The deuterium molecules are ionized by an ionizing section which commonly includes plates to which a positive ionization pulse is applied. The deuterium ions are then accelerated and bombard a target which included tritium molecules. The bombardment of the deuterium ions on the tritium molecules yields helium plus a supply of neutrons. One commercially available tube which is capable of such operation is the Kaman Nuclear Model A-801 Neutron Generator.
In operating such a tube it is important that the power supplied to the replenisher be correctly adjusted so that the proper amount of accelerator gas, deuterium, as described above, boils off the replenisher element. If the replenisher is overheated, too much accelerator gas boils off. In this case, ion recombination takes place in the tube. Also, arcing in the tube shortens the tube life and neutron output falls off. If too little power is supplied to the replenisher, there is not enough accelerator gas available in the tube to provide a good neutron output.
The adjustment of the power supply to the replenisher is complicated by the fact that the characteristics of the tube change as the tube ages. That is, after the tube has been in use, a greater amount of power must be supplied to the replenisher to boil off the same amount of accelerator gas. U.S. Pat. No. 3,719,827 to Charles L. Dennis describes a system in which the power supply to the replenisher element in a linear accelerator tube is automatically controlled. In this system, the time duration of the ionization pulse is compared to a reference pulse, and a control signal generated. The control signal is applied to a stepping motor. Each time the accelerator tube is ionized, the motor is advanced in one direction or the other, depending upon the comparison of the ionization pulse to the reference pulse. This motor increments a variable, autotransformer which supplies power to the replenisher. In this manner the replenisher power is adjusted to supply the correct amount of accelerator gas to the tube.