Feasibility of applying the lower cut-off frequency for the density radial coverage extension in EAST reflectometry measurement

Abstract

The extraordinary mode (X-mode) lower cut-off frequency is proposed to be utilized in the reflectometry diagnostic on ITER for the electron density profile measurement, which is a trade-off between extreme plasma parameters and the accessible probing frequency. On the contemporary experiments, the lower cut-off frequency can be identified at the probing frequency below the electron cyclotron frequency (fce) under certain plasma conditions. We provide here, for the first time, the experimental validation of the usage of the lower cut-off frequency for the density profiles via the reflectometry measurement on EAST. The corresponding group delay of lower cut-off frequency is evolved continuously with the upper one, revealing a reasonable radial coverage extension of reflectometry measurement toward the plasma core. It is concluded that the lower cut-off frequency can be used as a supplementary to the upper one in the density profile inversion process, which is of particular interest in the high magnetic field and/or density discharge to extend the radial coverage of reflectometry measurement.

Observation of local density increase during pellet homogenization on EAST

“…The boundary between these two reflections is defined by a zero-density layer within the V-band, which is characterized by a hopping in group delay when the probing frequency equals the electron cyclotron frequency ( f probing = f ece ). Specific application regarding utilization of the lower cutoff frequency during density profile inversion on EAST has been described detailedly in [26]. As shown in figure 3(b), the Q-band sub-system demonstrates a lower cut-off reflection and a gradual increase in signal intensity with the increasing plasma density after PI.…”

“…This calculated group delay for the lower cut-off reflection signal, expected to fall within the frequency range of 0-35 GHz, is subsequently juxtaposed with the experimental data obtained from the Q-band reflectometer (33-50 GHz). The method for contrasting the upper and lower cut-off reflections is outlined in detail in [26]. Remarkably, the lower cut-off reflection signal detected in the Q-band demonstrates a remarkable concordance with the upper cut-off reflection signal, distinguished by a low group delay.…”

“…Figures 3(b)-(d) present the group delay spectra observed by X-mode polarization reflectometer at Q-, V-and W-band, respectively. In the high-density experiments under strong magnetic fields conditions on EAST, Q-band sub-system is utilized for receiving the lower cut-off reflections [26], while V-and W-band sub-systems are employed for the upper cutoff reflections. The boundary between these two reflections is defined by a zero-density layer within the V-band, which is characterized by a hopping in group delay when the probing frequency equals the electron cyclotron frequency ( f probing = f ece ).…”