ECEI on J-TEXT

The ECE imaging system for J-TEXT was completed in 2018 and is scheduled to be installed on the J-TEXT tokamak in Wuhan, China in 2019. It is a dual array system, with one array designed for W-band (75-110 GHz) and one for F-band (90-140 GHz).

Each array consists of 24 imaging channels, although only the central 16 channels of each array are populated at this time. At the heart of the array box are two 12-element vertically staggered imaging sub-arrays, with a dedicated miniature elliptical lens (0.50 inch = 12.7 mm diameter) attached to each antenna. Facing each sub-array is a dichroic plate (a metal plate with closely spaced circular holes) which function as a high pass filter. ECE radiation from the plasma is directed to one of these dichroic plates, while local oscillator (LO) power is directed to the other. An internal beam splitter serves to split both the incoming ECE radiation and the LO power into roughly two equal parts to illuminate the two arrays. The downconverted RF signals from each antenna are amplified using low noise 2-12 GHz preamplifiers located in the back half of the array box, that are powered by an internal power supply.

A total of 16 dichroic plates have been fabricated for J-TEXT with nominal cutoff frequencies of ranging from 70 to 130 GHz. The dichroic plates function as high pass filters to (a) protect the ECEI arrays from low frequency pick-up, and (b) allow the ECEI array to operate as a single sideband mixer with LO operation near the cutoff frequency of the RF filter.

Further protection to the arrays is provided with quasi-optical notch filter plates. Each notch filter provides between 14 and 25 dB of protection against 105 GHz ECRH power used to heat the J-TEXT plasma. Arranged in stacks of 3, each filter stack provides >60 dB of rejection, with a passband loss of ≤4 dB.

Customized ECEI electronics modules were developed for J-TEXT, spanning an IF frequency range of 2.0 to 11.7 GHz. Unique to J-TEXT, the IF spacing between channels is switchable between not just two but three different values: 0.8 GHz, 1.0 GHz and 1.3 GHz. In the narrow zoom configuration, the IF channels are centered at 3.8, 4.6, 5.4, 6.2, 7.0, 7.8, 8.6, and 9.4 GHz. In the medium zoom configuration, the IF channels are centered at 3.2, 4.2, 5.2, 6.2, 7.2, 8.2, 9.2 and 10.2 GHz. In the wide zoom configuration, the IF channels are centered at 2.3, 3.6, 4.9, 6.2, 7.5, 8.8, 10.1, and 11.4 GHz.

Each ECEI module is comprised of two parts: an RF board and an IF board. Signals input to the RF board are amplified and divided into 8 parts, each of which terminates in a balanced mixer that down-converts the input RF signal to lower frequencies which are then passed on to the IF board. Once on the IF board, each of the 8 IF signals are amplified and bandpass filtered with upper cutoff frequencies selected in accordance to the RF zoom configuration: narrow zoom signals employ 180 MHz filters, medium zoom 225 MHz filters, and wide zoom 435 MHz filters. The filtered signals are then detected/rectified, video amplified, and low-pass filtered. Video bandwidth control is set by a tenth order, low-pass filter whose cutoff frequency may be remotely selected between 50, 100, 200, or 400 kHz.

Also unique to the J-TEXT ECEI system are the digitizers employed to acquire ECEI data. The NI PXIe-based digitizer system consists of a PXIe-8135 controller and multiple PXIe-6368 digitizers. Unlike conventional digitizers, each of these digitizers contains 24-bits of digital I/O. To minimize noise and improve shielding, each module is equipped with a 68-pin VHDCI/SCSI connector whose pin-out matches that of the digitizer thereby allowing standard double-shielded VHDCI cables to connect each module to its corresponding digitizer. The digital I/O signals allow the digitizers to control the internal 0-15 dB IF attenuators within each module.

Lastly, the J-TEXT ECEI controllers themselves represent a significant improvement over previous ECEI controllers in that they are remote-controlled. Rather than having front-panel switches and knobs to select the RF zoom, RF attenuator and video bandwidth settings, all of these functions are controlled via USB through an Arduino micro-controller within each 20-element controller.

Reference:

Zhu, Y., et al. "Optics design for J-TEXT ECE imaging with field curvature adjustment lens." Review of Scientific Instruments 85.11 (2014): 11D854.
Ma, X. D., et al. "Optics design for electron cyclotron emission imaging system on J-TEXT." Journal of Instrumentation 11.05 (2016): P05022.
Pan, X. M., et al. "Design of the 2D electron cyclotron emission imaging instrument for the J-TEXT tokamak." Review of Scientific Instruments 87.11 (2016): 11E106.
Zhenling, Z. H. A. O., et al. "Quasi-3D electron cyclotron emission imaging on J-TEXT." Plasma Science and Technology 19.9 (2017): 094001.
Domier, Calvin, et al. "Electron Cyclotron Emission Imaging (ECEI) System for the J-TEXT Tokamak." APS Meeting Abstracts. 2018.
Xie, X. L., et al. "Hyperbolic lens design of local oscillator optics system for electron cyclotron emission imaging on J-TEXT." Review of Scientific Instruments 89.10 (2018): 10H101.

Google Sites

Report abuse