Diagnostic Development For Parallel Wave-number Measurement Of Lower Hybrid Waves And Relevant Experiment In EAST

In order to realize the required temperature of nuclear fusion and the steady state tokamak operation,Radio Frequency(RF)waves and Neutral Beam Injection have become essential means for plasma auxiliary heating and non-inductive current.A number of experiments have proved that the lower hybrid current drive(LHCD)is known to have the highest efficiency among the existing various RF waves current drives,which is particularly suitable for the current distribution required by the advanced tokamak that generates long pulses and medium normalized ?.Unfortunately,a number of tokamaks report lower than expected LHCD efficiency in the high density plasmas.A number of mechanisms responsible for this reduction in current drive efficiency have been proposed and validated through detailed modeling work.Recent studies show that the n? spectrum broadening induced by parametric decay in stability(PDI)and turbulence scattering,which occurs in the scrape-off layer(SOL)near the antenna mouth,may be one possible candidate for understanding the LH density limit.However,at present,almost all the investigation on LHCD experiments of frequency spectrum,and then the wave-number spectrum is obtained by theoretical calculations,lacking the direct measurements.In this paper,an eight-channel radio frequency(RF)magnetic probe diagnostic system for 4.6 GHz LH waves has been developed to characterize n? in the SOL of the EAST.A cylindrical shield with a rectangular slit in the front face encloses the loop probe such that the magnetic probe is sensitive only to the wave magnetic field along the longer dimension side of the slit.In the experiment,the silt is oriented such that the probe is sensitive to the wave magnetic field oscillation that is normal to the background magnetic field.Thus,with an array of probes,it is used not only to deduced the dominant n? but also to evaluate the wave number spectrum by conducting the spatial Fourier analysis.The wave frequency measured at 4.6 GHz is down-converted to 20 MHz in order to directly digitize the signals and to minimize unwanted frequency mixing.Then,the down-converted signals is directly input to the fast speed digitizer.At present,the diagnostic system has been designed,tested,installed and has been successfully applied in the lower hybrid current drive experomrnts.The experimental results obtained by the diagnostic system are consistent with the theoretical predictions.It provides an effective method for further LHCD study at high density in EAST.