MIR on EAST

Millimeter-wave imaging diagnostics, with large poloidal span and wide radial range, have been developed on the EAST tokamak for visualization of 2D electron temperature and density fluctuations. A 384 channel (24 poloidal × 16 radial)Electron Cyclotron Emission Imaging (ECEI) system in F-band (90-140 GHz) was installed on the EAST tokamak in 2012 to provide 2D electron temperature fluctuation images with high spatial and temporal resolution. A simultaneous, co-located Microwave Imaging Reflectometry (MIR) will be installed for imaging of density fluctuations by December 2017 and put into commission in the 2018 experimental campaign. This "4th generation" MIR system has eight independent frequency illumination beams in W-band (75-110 GHz) driven by fast tuning synthesizers and active multipliers.

Both of these two advanced millimeter-wave imaging diagnostic systems have applied the latest techniques. A novel design philosophy "general optics structure" has been employed for the design of the ECEI and MIR receiver optics with large aperture. The extended radial and poloidal coverage of ECEI on EAST is made possible by innovations in the design of the front-end optics. The front-end optical structures of the two imaging diagnostics, ECEI and MIR, have been integrated into a compact system, including the ECEI receiver, MIR transmitter and receiver, and share the same mid-plane port for simultaneous, co-located 2D fluctuation measurements of electron density and temperature. For friendly interface of electronics, an intelligent remote-control is utilized in the MIR electronics systems to maintain focusing at the desired radial region even with density variations by remotely tuning the probe frequencies in about 200 µs. A similar intelligent technique has also been applied on the ECEI IF system, with remote configuration of the attenuators for each channel. All electronics modules have been characterized with an identification number and self-calibrate and output feedback commands after each shot.

Fig 1: ECEI and MIR system will share the same mid-plane port for simultaneous, co-located 2D fluctuation measurements of electron density and temperature fluctuations. (source: Rev. Sci. Instrum. 87, 11D901 (2016), credit to Yilun Zhu)

1. Zoom/focus/FCA decoupled optical design

The aim of the imaging optics is to match the image plane with the layer detected in the plasma. A small mismatch exponentially increases the difficulty of interpreting the signal. The imaging optics has Three basic components for major functions,

1) Focus lens: radial image position matching

2) Zoom lens: poloidal observation matching

3) FCA: shape matching or field curvature analysis, and incident angle matching.

And two additional requirements for flexible operation:

1) Decoupling, which requires that the four basic functions are expected to be executed independently

2) Combination, which requires that the minimum number of lenses or mirrors be used to conserve the limited space available in the experimental facility.

The EAST MIR optical system has been designed to have a minimum of lenses and decouple the 3 important components.

3. Design of advanced Frequency Selective Surface Based Filters and 8-way wideband Wilkinson Power Divider

4. Development of Remote Control System with Arduino and Labview.