[en] Mirror end loss can be suppressed by applying an rf field at the mirror throat through low impedance coils. This method is verified to be effective for a high density plasma up to 10¹⁴ cm^-3. Experimental results shows that the rf field strength required for the plugging is differently dependent on the plasma density according to the type of coil. The mechanism of giving the density dependence is theoretically clarified for each coil. Particularly, it is shown that the electric field induced in the direction of the static magnetic field is intrinsic for the density independent result of type 3 coil. (author)