The addition of cesium vapor facilitates the production of negative ions in the plasma for this the SPIDER experiment, the world’s most powerful source of negative ions, will be equipped with a cesium evaporation system
The system will consist of three cesium evaporator furnaces inserted in the plasma chamber. Cesium is essential to bring the negative ion current to the values required for the ITER particle injector. Thanks to the low extraction potential of cesium, the probability that a hydrogen atom (H) or a positive ion (H +) bouncing off the cesium-coated metal surface will capture one or two electrons and become a negative ion (H-) increases ten fold.
To test and optimize the cesium ovens, especially in terms of repeatability of the evaporated cesium flow and its characterization (flow as a function of the evaporation temperature), the CATS laboratory, CAesium Test Stand, was built.
CATS contains all the tools necessary for the management and handling of cesium, a vacuum chamber for testing the furnaces and the material for their maintenance, as well as the electronics necessary for heating the furnace and for its control. In fact, the cesium oven must operate in vacuum, at high temperature (up to 300 ° C) and must be controlled remotely since it is inaccessible during operation. Furthermore, numerous diagnostic systems dedicated to the study of cesium evaporation are available in CATS:
The SID – Surface Ionization Detector is a diagnostic tool used to measure the flow of cesium vapors. Each furnace is equipped with a SID at the injection nozzle. An additional SID (movable version) is present in CATS and allows measurements to be made in different positions inside the vacuum chamber.
LAS – Laser Absorption Spectroscopy is a diagnostic system in which a continuous laser beam passes through the vacuum chamber and, by intercepting the cesium vapor present, is attenuated by it. Based on the measurement of the decay of the transmitted light, the density of the cesium vapor crossing along the path is calculated.
Thermocouples are thermometers that are used to measure the temperature at various points on the oven and keep it at the required temperature.
The thermal imaging camera is an infrared sensitive camera that allows an image of the temperature of the various parts of the oven.
The QCM – Quartz Crystal Microbalance is an instrument to measure the mass of cesium that has deposited on the surface of the quartz crystal exposed to the vapors in the vacuum chamber.
A. Rizzolo, M. Barbisan, L. Bizzotto and others, “Characterization of the SPIDER Cs oven prototype in the CAesium Test Stand for the ITER HNB negative ion sources,” Fusion Eng. Des., vol. 146, p. 676, 2019. https://doi.org/10.1016/j.fusengdes.2019.01.053
E. Sartori, M. Barbisan, M. Fadone and others, “Diagnostics of caesium emission from SPIDER caesium oven prototype,” AIP Conf. Proc., vol. 2052, p. 040011, 2018. https://doi.org/10.1063/1.5083745
M. Barbisan, S. Cristofaro, L. Zampieri, R. Pasqualotto and A. Rizzolo, “Laser absorption spectroscopy studies to characterize Cs oven performances for the negative ion source SPIDER,” JINST, vol. 14, p. C12011, 2019. https://doi.org/10.1088/1748-0221/14/12/C12011