Modification of O-mode reflectometer system to detect LH waves S. - - PowerPoint PPT Presentation

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.

∇ n

Transmitter Receiver

ksc kpb kLH #1101222020 4.6GHz modulated signal

 k sc=  k pb  k LH sc= pbLH

Wave matching condition is satisfied in front of cutoff layers.

Preliminary results from 60GHz system shows the variation of power amplitude

• f 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
• f detector sensitivity.

Expanding frequency channels to study the penetration of LH waves

• 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

Density profile 1101222020

60GHz:

Currently available channel Density profile 1110106020

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.