Cathode Sputtering

Cathode sputtering is a potential issue that can occur in the cathode of vacuum tube amplifiers. The name might sound complex, but the process itself is a straightforward result of some fundamental physics and chemistry interactions.

To understand cathode sputtering, you first need to know a little about how a vacuum tube operates. At its most basic, a vacuum tube is a device that allows current to flow from a heated element (the cathode) through a vacuum to a positively charged element (the anode). This current flow is what allows the vacuum tube to amplify signals.

However, while the term “vacuum” tube suggests that the tube is entirely empty, this isn’t completely true. There are always a few stray gas molecules left inside the tube, even after it has been sealed. These gas molecules can cause a variety of issues, one of which is cathode sputtering.

Cathode sputtering happens when these stray gas molecules become ionized by the stream of electrons flowing from the cathode to the anode. When a gas molecule is ionized, it loses or gains an electron, becoming a positively charged ion. In the case of a vacuum tube, these ions are created because the gas molecules are colliding with the stream of electrons flowing from the cathode.

Once the gas molecules have become positive ions, they are attracted to the negatively charged cathode. They accelerate towards it, and if they manage to miss the grid (another component of the vacuum tube that helps control the flow of current), they can crash into the cathode.

This crash physically damages the cathode’s surface. The process is called “sputtering” because the ions effectively sputter away or knock off small pieces of the cathode material. Over time, this can lead to significant damage to the cathode, changing its properties and potentially impacting the performance of the vacuum tube.

It’s also worth noting that sputtered material can end up on other elements within the tube, such as the grid or anode. This can create further issues, including shorts or changes in the tube’s electrical properties.