WHAM Impurities Mitigation and Performance Enhancement

Impurities in any fusion system can be detrimental to plasma performance. In a mirror machine, special considerations on reducing the impact of impurities must be made due to the topology of the magnetic field. Given WHAM has a small center cell to plasma diameter ratio, there are additional challenges arising from fast neutral charge exchange losses from recycled neutrals from the wall. Therefore, special attention is being given to mitigate impurities and improve the overall health of the WHAM plasma. Some of the steps underway are discussed here: High-Z plasma facing components are being actively upgraded with lower-Z materials like carbon. These changes aim to reduce both the radiative and charge-exchange power losses to improve plasma power balance. Ti gettering has been used in different plasma systems to increase vacuum pumping capacity. It has a high capture cross section for both impurities and H isotopes, significantly improving machine performance [1]. Currently WHAM is being upgraded with Al (aluminum) liners to coat with Ti in the center cell. Another avenue being developed is cold sprayed Ta which has a large D capture cross section, 2x times higher than that of polycrystalline W [2]. Being blister resistant, unlike W, it is a promising material for pumping out fast neutrals. Ta pumps are being designed and tested to further control fast neutrals and the resulting charge exchange losses. Bremsstrahlung measurements are being conducted to determine the Z-effective during plasma shots. Complementary line radiation analysis is identifying the present impurities. Collisional-radiative modelling is being done to estimate the line radiation component to radiative power losses. All these three together will facilitate in getting a clearer picture of the different radiative power loss channels and in developing the next energy loss mitigation steps.

References

[1] J. Winter J. Nucl. Mats. 1990 176 14

[2] M. Ialovega et al 2025 Nucl. Fusion 65 076042