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

Abstract

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 H _2 /(D _2 )-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 H _2 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 H _2 followed by He-fueled GDC when applied for extended periods spanning hours or days. A GDC with a mixture of argon and hydrogen (Ar–H _2 ) is introduced in the ADITYA-U tokamak to obtain better wall conditioning than H _2 followed by He GDC. In Ar–H _2 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 ^+ , H _2 ^+, H _3 ^+ ions in H _2 GDC due to their lower momentum. An optimal blend ratio of Ar to H _2 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 H _2 GDC. Relative low pressures of dominant impurities such as CO, CH _4 , and H _2 O are obtained due to the Ar–H _2 GDC compared to routinely operated H _2 GDC. A comparison study of H _2 GDC and the developed Ar–H _2 GDC is performed in terms of wall conditioning and tokamak plasma operation. The encouraging results of the Ar–H _2 GDC are obtained in both wall cleaning and tokamak operation scenarios in the midsize tokamak ADITYA-U. This development and application of Ar–H _2 GDC are beneficial for large-sized fusion devices, leading to improved impurity reduction, reduced operational fuel consumption (H _2 /D _2 /He), and enhanced control over fuel recycling/extraction.

Keywords

• tokamak
• wall conditioning
• plasma wall interaction