Plasma performance enhancement and impurity control using a novel technique of argon–hydrogen mixture fueled glow discharge wall conditioning in the ADITYA-U tokamak

Effective control of impurities and precise regulation of the fueling gas are supreme prerequisites for optimal operation in any fusion device. Conventional wall-conditioning methods fall short of achieving optimal wall conditioning. Conventional wall-conditioning methods, such as vessel baking and H2/(D2)-fueled glow discharge cleaning (GDC), are generally required to remove wall-absorbed impurities in bulk after vessel venting. The excess amount of hydrogen, injected during H2 GDC, can be reduced by helium (He)-fueled GDC. However, He removal from the vessel is more challenging due to its low molecular mass, very low condensation temperature, and inert characteristics. In ADITYA-U, optimal wall conditioning cannot be achieved using H2 followed by He-fueled GDC when applied for extended periods spanning hours or days. A GDC with a mixture of argon and hydrogen (Ar–H2) is introduced in the ADITYA-U tokamak to obtain better wall conditioning than H2 followed by He GDC. In Ar–H2 GDC, long-lived ArH+ ions are formed in sufficient numbers and accelerated toward the vessel wall with high momentum. This results in the breaking of high energy bonds of impurities with the wall/plasma facing components, which is not possible by H+, H2+, H3+ ions in H2 GDC due to their lower momentum. An optimal blend ratio of Ar to H2 is established at 15%–20% for the mixture. This composition ensures that the introduction of high-Z Ar does not adversely affect tokamak plasma operations. The C- and O-containing impurities are reduced beyond the limit of the prolonged operation of H2 GDC. Relative low pressures of dominant impurities such as CO, CH4, and H2O are obtained due to the Ar–H2 GDC compared to routinely operated H2 GDC. A comparison study of H2 GDC and the developed Ar–H2 GDC is performed in terms of wall conditioning and tokamak plasma operation. The encouraging results of the Ar–H2 GDC are obtained in both wall cleaning and tokamak operation scenarios in the midsize tokamak ADITYA-U. This development and application of Ar–H2 GDC are beneficial for large-sized fusion devices, leading to improved impurity reduction, reduced operational fuel consumption (H2/D2/He), and enhanced control over fuel recycling/extraction.