Transient ion disturbances in traveling wave tubes

It is known that the presence of ions in a traveling wave tube (TWT) can lead to periodic variations in the output power, phase and the body (or helix) current. This has been referred to in the open literature as ion noise and, in internal memoranda, as helix jitter and ticking. There have been several proposed mechanisms but it is generally understood to be due to an ion induced relaxation oscillation. Recently we have observed a different form of helix jitter and while it is still observed as a small variation on the TWT output, it is not periodic. We refer to this as "random jitter" since its random nature is a defining characteristic. A study was undertaken to understand and eliminate this source of noise. It was found that random jitter was due to the spurious release of extremely small amounts of trapped gas inside the TWT. This leads to an increase in the population of trapped ions that, in turn, leads to a sudden de-trapping event. This was readily observed as disturbances in the body current, output power, and phase of the RF signal. These spiky events have a relatively fast rise (/spl sim/1 ms) and slow (/spl sim/500 ms) decay characteristic.