Effect of cathode shape on the lifetime of arc discharge negative Penning ion source

Abstract

Internal Penning ion source for the production of negative hydrogen ions is commonly used in various compact cyclotrons. The ion source generally operates at high power DC discharge to provide a high negative hydrogen ion beam current. In this case, the cathode will undergo intense ion sputtering and evaporation due to fierce ion collision. Then, the sputtered material from the cathode will deposit on the inner wall of the ion source discharge chamber. These deposits may lead to slight changes in the structure of the discharge chamber and affect the ion beam extraction stability. Additionally, these deposits would accidentally break and spall, inducing sudden changes in the electric field distribution or even short circuits of the discharge. As a result, the working life of the ion source is dramatically declined. Thus, the relationship between the cathode size and the plasma state was theoretically explored in this study to prolong the ion source’s lifetime. The theory suggests that the impact power of ions on the cathode can be significantly reduced when the cooling capacity of the cathode is reduced. Meanwhile, the cathode temperature is slightly increased, and the discharge voltage declines. Experiments of 4.8 mm and 6 mm in diameter of the cathode were conducted by changing the diameter of the cathode to influence the radiation cooling efficiency. Specifically, the beam current difference was within ±15% before and after changing the cathode size in the same discharge current when the small cathode was used; the beam current of the small cathode was more than 10% (even 40%) better than that of the big one at the same operating power. Before the ion source short-circuited at the same discharge current, the small and big cathode groups operated 5536 μAh and 3000 μAh, respectively. Another test demonstrated that after working for 50 h at 1.8 A discharge current, the mass loss of small cathode was 0.0456 g, and the mass loss of big cathode was 0.0798 g.