Density dependence of the forbidden lines in Ni-like tungsten

The magnetic-octupole (M3) and electric-quadrupole (E2) transitions between the ground state $3d^{10} ^1S_0$ and the lowest excited $3d^94s$ $(5/2,1/2)$ J = 3 and J = 2 states in the Ni-like tungsten are shown to exhibit a strong dependence on electron density $N_e$ in the range of values typical for tokamak plasmas. Remarkably, the total intensity of these overlapping lines remains almost constant, which may explain the strong emission in the 7.93 \AA line observed in tokamak experiments (Neu R et al. 1997 J Phys B {\bf 30} 5057). Utilization of the M3 and E2 line ratios for density diagnostics in high-spectral-resolution experiments is discussed as well.