Modification of O-mode reflectometer system to detect LH waves S. Baek, R. Parker, S. Shiraiwa, A. Dominguez, G. Wallace,G. Kramer LHCD density limit observed below accessibility limit: SOL effect ● Indirect measurements (ECE, hard X-ray) indicates negligible penetration of ● LH waves into the plasma in relatively high density plasma Direct measurement of local LH wave field ● Material probe cannot be deeply inserted due to high temperature ● Scattering technique: Wave-matching condition ensures local measurement ● Using 60GHz O-mode reflectometer system, it was demonstrated that LH wave can ● be measured. #1101222020 ∇ n k pb 4.6GHz modulated signal k LH Transmitter Receiver k sc k sc = k pb k LH Wave matching condition is satisfied in sc = pb LH front of cutoff layers.
Preliminary results from 60GHz system shows the variation of power amplitude of scattered signals depending on the plasma density. L-mode : As plasma density increases, the power amplitude decreases, ● suggesting the presence of weaker LH electric fields in the interaction region. H-mode: stronger power amplitude is seen even higher density when ● compared to L-mode. The biggest concern is whether the power of scattered signal is in the range ● of detector sensitivity.
Expanding frequency channels to study the penetration of LH waves Density profile 1110106020 Density profile 1101222020 60GHz: Currently available channel Preparing similar setup in the higher frequency channels (75,88 GHz) to the ● 60GHz system will allow the study of LH wave propagation. Hardware modification is necessary. ● Need to change the intermediate frequency (IF) ● – Currently, IF is tuned to the baseband fluctuation measurement Gunn oscillators, mixers, filters, amplifiers ● Improve the sensitivity of the spectrum recorder ●
Hardware upgrade is necessary. Spectrum recorder can be improved to increase the sensitivity, in general. ● 100 kHz low pass filter is currently used to select frequency. ● The combination of electronic chips and the narrow low pass filter near ● zero frequency might suppress the signal heavily as I have pushed the lower limit of operating frequency region. Using higher frequency with narrow bandpass filter will also lower 1/f ● noise. 75GHz: major hardware modification is done with 60GHz system. ● Need to assess waveguide losses ● Still, it remains the question whether the input beam is strong enough ● 88/112GHz: major modification is necessary ● Gunn diode oscillators, mixers, filters, amplifier,.... ● ● The final goal is to demonstrate how far LH waves propagates inside plasma using multiple frequency channels.
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