Several types of reflectometer have been used on the JIPP TII-U tokamak to understand the phase runaway effect, which denotes the phenomenon in which the measured phase moves very fast in one direction. Homodyne, heterodyne, and ultra-narrow band reflectometers are used. Phase runaway and rapid phase variations are observed using all systems, except for the homodyne system which cannot measure phase. When the edge is measured, a relatively slow phase runaway is observed, that is caused by many fringe Jumps (2 pi phase jumps). In most cases, the amplitude of the reflected wave is well above the noise level during fringe jumps. Thus, the runaway is not a problem with the measurement system, but is a result of an interaction between the microwave and the plasma. An algorithm to remove fringe jumps has been developed, so that a phase without runaway is obtained, which probably represents macroscopic movement of the cut-off layer. The obtained phase shows that the effect of the fringe jumps on the power spectrum is serious at low frequency ranges. (C) 1997 Elsevier Science S.A.
