Speaker
Description
Feedback control of radiative power from in divertor plasmas by D₂ fuel gas injection, through carbon radiation, is considered for the next operation phase of JT-60SA. The location of gas injection ports can affect the radiation response. For feedback control, the radiation profile must be converted into measurement signals, taking into account the diagnostic fields of view. In JT-60SA, resistive bolometers with both wide and narrow viewing angles per channel will be installed to evaluate divertor radiation power. The bolometer signals provide a linear measure of the total radiated power within the viewing volume [1][2]. This study aims to evaluate the response of radiation profile and bolometer signal with stepped gas injection for feedback control. In following, signal responses to stepped D₂ gas injection from different injection port locations were analyzed using the integrated divertor code SONIC. In JT-60SA, four gas injection ports are available on the poloidal cross section: (a) the upper port, (b) the lower port, (c) the inner-divertor port, and (d) the outer-divertor port. These ports enable injection of fuel or impurity gas into the vacuum vessel. For a low-density attached plasma under conditions where the electron temperature at the strike point exceeds 300 eV, the time evolution of radiation power in the outer and the inner divertor plasma was simulated with the stepped D₂ gas injection with 6.0 ×10^21 /s from four different injection ports. Among the four cases, it was found that a prompt increase in radiation power by injection from the private region port ((c) or (d)) compared to that by injection from the common flux region port ((a) or (b)). Then, it took 40 ms and 80 ms from start of stepped gas injection for the total radiation power in the outer and the inner divertor plasma to become constant, respectively. Considering that it takes 100 ms for the D2 gas to travel from the gas valve to the outlet of the injection port based on JT-60U results [3], it is estimated that it takes 140 ms – 180 ms for the radiative power to increase from the injection signal. Moreover, the time required for the radiative power to be constant depends on plasma density and gas species. To address this, additional simulations with various plasma densities and various gases, such as neon, will be presented.
[1] R.Sano et al., Rev.Sci.Instrum. 89,10E104(2018)
[2] https://www.jt60sa.org/
[3] H. Tamai et al., Fusion Eng.Des. 39–40(1998)163–167