Controllability of current profiles in long duration discharges is studied by two kinds of combination of wave spectra. First, by superposition of a higher N∥ spectrum wave at 2.45 GHz on a target plasma driven by a lower N∥ one at 8.2 GHz, a hollow j(r) profile has been achieved and sustained for 20 s. The current profile can be well controlled below a threshold power of 14 kW by varying the power at 2.45 GHz. A spontaneous transition phenomenon to a peaked j(r) profile, however, has occurred above the threshold power even after a reasonable steady state condition had been obtained. Second, using two oppositely travelling waves at 8.2 GHz the total current is clearly reduced, and the j(r) profile becomes peaked when the backward travelling lower hybrid waves (BWs) are superposed on the forward lower hybrid wave at a low power of 20 kW. When the BW power is increased above 27 kW, further current reduction is suppressed, and above 34 kW the direction of the current driven by the BWs is completely reversed and the j(r) profile becomes broad. Thus, current modification by BWs shows a highly non-linear behaviour with respect to the BW power.