What is a DC keep-alive potential used for in a TR tube?

A DC keep-alive potential in a TR (thyratron) tube plays a crucial role by maintaining a level of ionization within the tube. This potential is important for ensuring that the gas inside the tube remains in a semi-ionized state. As such, it can enhance the sensitivity of the tube to the signals it is intended to detect, as the gas needs to be partially ionized to effectively respond to changes in input.

By establishing a keep-alive potential, the tube is kept in a ready state, allowing for quicker response times to triggering signals. This involves both functions: acting as a sensitizer, increasing the likelihood of the tube responding to low-level signals, and maintaining a partial ionization level that is essential for its operation. Thus, the keep-alive potential not only enhances the sensitivity but also provides the necessary conditions for the thyratron to operate efficiently when triggered.

The option that suggests being only a sensitizer or purely for ionization doesn't capture the dual functionality provided by the DC keep-alive potential. Therefore, recognizing these combined benefits explains why the chosen answer is the most accurate.