Krytron

The krytron is a cold-cathode gas-filled tube intended for use as a very high-speed switch, somewhat similar to the thyratron. It consists of a sealed glass tube with four electrodes. A small triggering pulse on the grid electrode switches the tube on, allowing a large current to flow between the cathode and anode electrodes. The vacuum version is called a vacuum krytron, or sprytron. The krytron was one of the earliest developments of the EG&G Corporation. Unlike most other gas switching tubes, the krytron conducts by means of an arc discharge, to handle very high voltages and currents (reaching several kilovolts and several kiloamperes), rather than the low-current glow discharge used in other thyratrons. The krytron is a development of the triggered spark gaps and thyratrons originally developed for radar transmitters during World War II. The gas used in krytrons is hydrogen; noble gases (usually krypton), or a Penning mixture can also be used. A krytron has four electrodes. Two are a conventional anode and cathode. One is a keep-alive electrode, placed near the cathode. The keep-alive has a low positive voltage applied, which causes a small area of gas to ionize near the cathode. High voltage is applied to the anode, but primary conduction does not occur until a positive pulse is applied to the trigger electrode ("Grid" in the image above). Once started, arc conduction carries a considerable current. The fourth is a control grid, usually wrapped around the anode, except for a small opening on its top. In place of or in addition to the keep-alive electrode some krytrons may contain a tiny amount of radioactive material (usually less than of nickel-63), which emits beta particles (high-speed electrons) to make ionization easier. The radiation source serves to increase the reliability of ignition and formation of the keep-alive electrode discharge. The gas filling provides ions for neutralizing the space charge and allowing high currents at lower voltage. The keep-alive discharge populates the gas with ions, forming a preionized plasma.