The Helm X-ray tube, composition H. van der Gugten
Shortly after the commissioning of the irradiation machine in 1914, Gaarenstroom introduces more powerfull water-cooled X-ray tubes of the "Helm" type .See drawing.
It is an ion tube, like all earlier X-ray tubes. The cathode is not heated with a filament. Electrons are released by heating caused by the bombardment of gas-ions onto the cathode. The gas, hydrogen is sealed in the glass tube and is ionised by the high voltage discharge (Townsend discharge) between the anode and cathode. At the top of the tube, there is a gas pressure regulator: hydrogen easily diffuses through heated palladium, a characteristic which is utilized to bring extra hydrogen in the tube by heating palladium with a gas flame. In an alternative construction, the regulator is connected to the high voltage and extra gas is produced by sparking. The cathode is on the left, the anticathode (anode) where the X-rays are produced, is in the middle and an auxiliary anode is on the right.
This X-ray tube is water cooled. The anode H is positioned on a hollow metal cylinder with a thick wall. Inside this, fitting exactly is the metal tube B-B. In the thick wall of the cylinder, a spiral shaped groove connects the water reservoir A and the space E behind the anode. The water in E is warmed by the anode and the heated water rises through the tube B-B to the reservoir A. Cold water sinks down through the spiral channel to E and cools the cylinder and the anode. The spherical reservoir and the tube are connected by rubber tubes to a 3 litre water container . This is sufficient cooling water to irradiate for a period of 2 hours at a tube current of 3-5 mA. The high voltage is estimated to be around 80 kV.