Application of visible bremsstrahlung to a density monitor in steady state fusion reactor

The measurement of electron density is still important in a steady state fusion reactor in the future in addition to neutron and magnetic measurements in order to control the fusion output and to sustain stable plasma discharges. In the present toroidal fusion devices, FIR interferometer and Thomson scattering diagnostics have been used for the electron density measurement as an active diagnostic method. However, it is very difficult to operate such instruments continuously over a year. The use of visible bremsstrahlung as the passive diagnostics is, then, proposed as the electron density monitor instead of such active diagnostics. Since the effective charge, Zeff, is considered to be much less than 2 in the high-density fusion reactor, the bremsstrahlung intensity, which is proportional to the square of electron density, becomes a typical indicator of the electron density. In order to check the feasibility, the line-averaged density and the square root of visible bremsstrahlung signals were compared using two kinds of high-density LHD discharges in a range of (5–10) × 1013 cm−3, such as H2 gas puff and repetitively injected H2 ice pellets discharges. A ratio of the square root of the visible bremsstrahlung signal to the density was a constant in both discharges. The visible bremsstrahlung profile was also compared with the density profile. The results obtained in the present study strongly suggested the use of the visible bremsstrahlung to the density monitor in the future reactor diagnostics.