Downhole neutron generators and methods to auto tune downhole neutron generators

A downhole neutron generator comprising:

• a downhole neutron generator tube comprising:
• a filament having a coating that releases hydrogen isotopes when the filament is heated;
• a target foil;
• and
• an acceleration tube for accelerating ions of the hydrogen isotopes, wherein neutrons are generated when ions of the hydrogen isotopes are accelerated through the acceleration tube, and into the target foil;
• a reservoir control supply electrically connected to the filament and operable to supply a range of voltages to the filament to initiate neutron generation;
• and
• a processor operable to:
• determining whether an ion beam is oscillating;
• and
• in response to a determination that the ion beam is oscillating:
• determine proportional, integral, and derivative parameters of a proportional-integral-derivative controller that is operable to adjust an amount of power supplied to generate ions;
• and
• adjust the voltage based on the proportional, integral, and derivative parameters.

A downhole logging tool, comprising:

• a downhole neutron generator operable to transmit neutrons into a downhole formation to investigate the downhole formation, the downhole neutron generator comprising:
• a filament having a coating that releases hydrogen isotopes when the filament is heated;
• a reservoir control supply electrically connected to the filament and operable to supply a range of voltages to the filament to initiate neutron generation;
• and
• a processor operable to:
• determining if an ion beam is oscillating;
• in response to a determination that the ion beam is oscillating:
• determine proportional, integral, and derivative parameters of a proportional-integral-derivative controller that is operable to adjust an amount of power supplied to generate ions;
• and
• adjust the voltage based on the proportional, integral, and derivative parameters.