Cesium-free pair-plasma creation method may aid future fusion experiments

Pair plasmas, with their symmetrically collective behavior between negative and positive ions, are different from conventional plasmas, which contain only light electrons and heavy positive ions. One way to study the physics of this peculiar form of matter is via a hydrogen pair-ion plasma, which as its name suggests, is a plasma made of positive and negative hydrogen ions of equal mass. However, the common experimental method for its creation involves cesium, which can create unwanted technical challenges in creating the plasma.

Yoshida et al. present a method to produce negative hydrogen ions by irradiating an aluminum grid with positive hydrogen ions, and subsequently creating the pair-plasma without cesium.

According to their experiments, as the negative hydrogen ions accumulate on the surface of the aluminum grid, a magnetic deflection mass separator can be used for mass-based separation and collection. The extracted negative hydrogen ions contain <10% electrons and a finite but negligible amount of other impurities. Based on their data, the authors expect their method to scale and produce negative ions in a high-density plasma setting.

Unlike the cesium-based method, which has a variety of technical problems, such as stringent maintenance requirements, the aluminum-based device uses neutral beam injection and provides a low maintenance alternative for creating negative hydrogen ions for fusion devices, such as the upcoming International Thermonuclear Experimental Reactor (ITER) in France.

Next, the authors plan to test their device in a higher density plasma and further examine the surface morphology of the aluminum grid for potential improvement.

Source: “Extraction of negative hydrogen ions produced with aluminum plasma grid,” by Masafumi Yoshida, Ryoma Watano, Yuta Morinaga, Toshimitsu Kamikawa, and Wataru Oohara, Physics of Plasmas (2021). The article can be accessed at http://doi.org/10.1063/5.0038262 .