the high voltage elements of the tube from the operating area. The accelerator and ion source high voltages are provided by external power supplies. The control console allows the operator to adjust the operating parameters of the neutron tube. The power supplies and control equipment are normally located within 10–30 feet of the accelerator head in laboratory instruments, but may be several kilometers away in well logging instruments. In comparison with their predecessors, sealed neutron tubes do not require vacuum pumps and gas sources for operation. They are therefore more mobile and compact, while also durable and reliable. For example, sealed neutron tubes have replaced radioactive modulated neutron initiators, in supplying a pulse of neutrons to the imploding core of modern nuclear weapons. Examples of neutron tube ideas date as far back as the 1930s, pre-nuclear weapons era, by German scientists filing a 1938 German patent (March 1938, patent # 261,156) and obtaining a United States Patent (July 1941, USP#2,251,190); examples of present state of the art are given by developments such as the Neutristor, a mostly solid state device, resembling a computer chip, invented at Sandia National Laboratories in Albuquerque NM. Typical sealed designs are used in a pulsed mode and can be operated at different output levels, depending on the life from the ion source and loaded targets. Ion sources A good ion source should provide a strong ion beam without consuming much of the gas. For hydrogen isotopes, production of atomic ions is favored over molecular ions, as atomic ions have higher neutron yield on collision. The ions generated in the ion source are then extracted by an electric field into the accelerator region, and accelerated towards the target. The gas consumption is chiefly caused by the pressure difference between the ion generating and ion accelerating spaces that has to be maintained. Ion currents of 10 mA at gas consumptions of 40 cm3/hour are achievable. For a sealed neutron tube, the ideal ion source should use low gas pressure, give high ion current with large proportion of atomic ions, have low gas clean-up, use low power, have high reliability and high lifetime, its construction has to be simple and robust and its maintenance requirements have to be low. Gas can be efficiently stored in a replenisher, an electrically heated coil of zirconium wire. Its temperature determines the rate of absorption/desorption of hydrogen by the metal, which regulates