These tubes are still used because solid state just doesn't work at these power levels. This is a lower power device. The real power thyatrons are ceramic and metal container types.
This is a pentode filled with hydrogen gas. The gas fill allows for quick turnon and turnoff of high current. In operation this would be the circuit would have a very high magnetic pulsing field (safety warning mentions don't come near it with a pacemaker) and also x-rays. Out of circuit, don't drop it.
Love large glass objects.
