RTP is a relatively small tokamak, with a Major radius of 90 cm and a minor radius of 18 cm. The RTP ECE Imaging system looks at the plasma through a large viewing window (30 cm high by 18 cm wide) made of BK-7. The 16 imaging channels are spaced 11 to 13 mm apart, and cover approximately 16 cm of the plasma centered on the plasma midplane.

The antennas in the ECE imaging array are arranged in a staggered pattern of 16 vertically spaced elements arranged in two parallel (horizontally displaced) rows. Shown above are H-Plane focal plane patterns of four consecutive channels measured at 120 GHz. Here, even and odd channels are toroidally displaced by 2.5 cm. Plotted below are the E-Plane focal plane patterns at 118.75 GHz of the 14 innermost channels, illustrating the channel positions and the focal plane spot sizes as a function of channel number. Sidelobe levels, which arise from edge diffraction, increase as one moves away from the center channels.

In RTP, there is a 500 kW, 110 GHz ECRH system in which the RF power is coupled to the tokamak plasma by mirrors through the air. Hence, large levels of stray 110 GHz radiation around the tokamak hall exist which can affect not only system performance but could endanger the Schottky diode mixer array itself. All the optical components mounted on the tower are enclosed by a shielding box built to reject stray ECRH power at 110 GHz, leaving only the front surface of lens G open to collect second harmonic plasma radiation.

Any stray ECRH radiation received by lens G is rejected by the dichroic filter. To ensure mixer array safety, a dichroic plate attached to the array box functions as a high pass filter, the performance of which is shown above. In Ohmic discharges (discharges without additional ECRH heating), this filter can be removed to extend the operation frequency to below 110 GHz. The system performance is well characterized both with and without the dichroic filter and the difference is found to be small.

For results of ECE Imaging measurements on RTP, including electron temperature fluctuations, please examine the following links:

High resolution measurements of electron temperature profiles
Electron temperature fluctuation spectra
Electron temperature fluctuation dispersion relations
Electron temperature fluctuations as a function of plasma current and density
2D structure of electron temperature fluctuations

The unique features of the ECE Imaging diagnostics derive from the use of wideband, low cost monolithic and hybrid Schottky diode mixer arrays. Follow the links below to learn more about both the technology and the techniques employed in ECE Imaging.

Imaging array design and fabrication
Correlation Measurements

UC Davis has fabricated and installed multichannel ECE Imaging systems on other fusion plasma tokamaks across the world. Follow the links below for a description of the systems involved, and to sample data collected with these systems.

ECE Imaging on the TEXT-U tokamak in the U.S.A.
ECE Imaging on the TEXTOR tokamak in Germany

 Comments to: Bihe Deng