46 / 2021-05-04 22:39:29

Control of fuel retention for a long pulse operation

Fuel retention, Long pulse discharge, Impurity powder dropper

Stable long-pulse plasma operations are required in the fusion DEMO. But active control methods during the plasma operations are restricted due to strong magnetic fields, neutron effects so on. In addition, plasma operation times are long, but maintenance times are short during the operational term. It is planned that the magnetic fields have been ramped up for more than a week.

The impurity powder dropper (IPD) system has been operated as a useful tool for the in-situ plasma controls of wall recycling, oxygen reduction, and impurity seeding in EAST [1] and LHD [2]. One of the advantages of IPD is to be able to operate under magnetic fields, and LHD-IPD has been operated under neutron irradiation environments. Therefore, it is the candidate hardware for ITER and DEMO with a real-time feedback control using another diagnostic signal. For control of wall recycling, lithium is used in EAST and boron in both EAST and LHD. The composition of lithium and boron is low Z materials, and then plasma discharges were survived when injected amounts are controlled. Both compositions have characterizations of chemical binding for hydrogen isotopes, and details of hydrogen reduction and trappings are still investigated.  

In plasma experiments after the boron powder dropping by IPD, it was observed that a reduction of oxygen and hydrogen isotopes in LHD. Obtained data shows that coated boron layers on the plasma-facing areas by IPD experiments, and plasma parameters of boron impurities and hydrogen Balmer lines with a quick response when boron particles reached plasmas in LHD and EAST. Different structures of edge plasma regions between tokamak and stellerator is well known, but the common phenomenon of these advantages by boron powder experiments is considered.

In this talk, plasma responses of wall recycling controls by boron and lithium powder dropping experiments are presented in LHD and EAST. In addition, spatial distributions observed after boron dropping to hydrogen/deuterium plasmas are shown by different diagnostics. Then real-time control techniques for long pulse discharges are discussed.

This work is supported by the LHD-PPPL collaboration, the LHD dust dropper group and the JSPS-CAS Bilateral Joint Research Projects, “Control of wall recycling on metallic plasma facing materials in fusion reactor,” 2019-2022, (GJHZ201984 and JPJSBP120197202).

[1] Z. Sun et al., Nuclear Fusion 61 (2021) 014002.

[2] F. Nespoli, N. Ashikawa et al., Nuclear Materials and Energy 25 (2020) 100842.