SoC solution

Roadmap for developing state-of-the-art mm-wave IC technologies for fusion plasma imaging.

The groundbreaking RF SoC solution permits mm-wave fusion plasma diagnostics to solve large challenges: space inefficiency, inflexible installation, and prohibitively high cost of conventional discrete component assemblies as higher imaging resolution and data accuracy are required and achieved by significant numbers of channels. Nowadays, advances in device fabrication have recently extended the maximum operating frequency of CMOS techniques to more than a hundred gigahertz which are opening a new world in realizing SoC portfolios in fusion plasma diagnostics. Instead of occurring serious issues from the combination of various components, a low-noise, compact, and multi-function instrumentation can be implemented by integrating the entire front-end system on a single-chip paired with high-frequency packaging techniques.

The UC Davis Microwave/Millimeter Wave and Plasma Diagnostic Group (MMWPDG) team has state-of-the-art high-frequency measurement equipment and experience in designing fully customized ICs for fusion science application. Plasma diagnostics requires ultra-wideband (more than 20 GHz) operation which is approximately nine times wider bandwidth than recent commercial embodiments for communication systems. Therefore, in-house development allows the production of custom MMICs to achieve the specifications for diagnostic application with optimized performance and better efficiency. Our roadmap for incorporating state-of-the-art IC transceivers for fusion plasma diagnostics is illustrated in figure above.

Current MIR/ECEI system consists of Schottky diodes mounted directly on printed antennas with dielectric substrate lenses to down-convert received signals. Sufficiently isolating this structure from interference has proven challenging, severely limiting the quality of ECEI and MIR data. In addition, the diode conversion loss results in high noise temperatures. Currently, we are employing MMIC chip LNAs directly at the antennas to reduce the noise temperature.
Probe transmitter power has also been limited thereby limiting the number of simultaneous probe frequencies. To this end, we have successfully designed, fabricated, and tested a multi-frequency (8 tones) illumination transmitter chip using TSMC 65 nm CMOS technology. This presents a major step in developing customized integrated circuits for fusion plasma diagnostics.
The system-on-chip approach allows the entire receiver/transmitter to be packaged which not only performs better, but is more compact, more reliable. Furthermore, gaining access to design ICs with customized specification offers flexible application for diagnostics systems.

ECEI Diagnostics: 2 x 10 Waveguide Horn Array with Single MMIC Receiver Module

LCP Receiver Project with Commercial Available MMIC (2017)

The antenna array carrying Liquid Crystal Polymer (LCP) system-on-chip (SoC) technology has been demonstrated through the on-going ECEI upgrade. The commercially available receiver chips (71-76 GHz) produced by Gotmic AB in Sweden have been used in this module. Each channel is completely modularized and individually shielded. A horn antenna array with fundamental waveguide transitions provides enormous attenuation for out-of-band interference.

Moreover, using active bias controllers to design the DC power board, we are able to automatically adjust gate voltage to achieve constant bias drain current and turn on the device sequentially. This customized DC power board can achieve excellent bias stability over supply and prevent RF performance degradation due to process variation. All of these can ensure that each channel would not have considerable variation between them.

The prototype of this on-going project is now used in DIII-D ECEI system and the results are encouraging completed array wiil be installed before the end of January 2018.

RF and LO mixing configuration in the new system. Each channel is completely modularized and shielded.

Single receiver module includes the RF and DC board enclosed by alumina metal box with horn antenna in the front

W-band Receiver Project with customized MMIC (2019)

Monolithic millimeter wave “system-on-chip” technology has been employed in chip receivers in a newly developed Electron Cyclotron Emission Imaging (ECEI) system on the DIII-D tokamak for 2D electron temperature evolution diagnostics. According to ITER relevant scenarios , the ECEI system was upgraded with 15 receiver modules each with customized W-band (75 -110 GHz) chips comprising a W-Band LNA, balanced mixer, x2 LO doubler, and two IF amplifier stages in each module. The upgraded W-band array exhibits > 20 dB additional gain, x 30 improvement in noise temperature and ~ 96% receiver noise suppression. The internal 8 times multiplier chain is used to drive LO coupling. The horn-waveguide shielding house is used to avoid out-of-band noise interference on each individual module. The ECEI system has acquired 2D images for Alfven eigenmodes during ECH and precursors before edge localized mode crash. The upgraded ECEI system plays important role for absolute electron temperature evolution and fluctuation measurements for edge and core regions transport physics study.

Customized receiver chip with working frequency 75-110 GHz;
Internal low noise amplifier employed before mixer to suppress electronics noise;
Internal local oscillator (LO) chain has three frequency doubler to generate high frequency LO signal;
MMIC, RF board and DC board are assembled in the shielding house as individual receiver module;
20 modules are used to receive RF signal from plasma for different vertical fields of view.

W-band receiver chip

Customized MMIC by UC Davis

Microwave Imaging Reflectometer diagnostics: In-house design millimeter-wave integrated circuit for V-band (55-75 GHz) transmitter/receiver System-on-Chip solution

Transmitter

Microwave imaging reflectometry (MIR) is an active, radar-like plasma diagnostic technique to visualize plasma electron density fluctuations by probing density-dependent cutoff layers.
To simultaneously monitor density fluctuations on multiple cutoff surfaces, the MIR system will employ multi-frequency illumination. Each illumination frequency reflects from a different plasma cutoff layer. Thus, operating with multi-frequency illumination allows density fluctuation data to be simultaneously collected on multiple cutoff layers

The system architecture of the V-band 8-tone CMOS transmitter

V-band transmitter module in individual shielding house with waveguide input, output and 4 RF input connector

This work is based on a CMOS based multi-frequency illumination TX which expands the capabilities of microwave reflectometry as a fusion plasma diagnostic and makes the systems dramatically less expensive, more compact, and more reliable. This transmitter is able to illuminate 8 various frequencies tones simultaneously. Each of the tone provides more than 0 dBm of saturation power. In addition, the output frequency is tunable from 62 to 78 GHz to cover a wide dynamic range for the DIII-D tokamak.
In this design, the use of multiple mixers/amplifiers can significantly increase the power per frequency. The trade-off is considerable complexity and large area demands due to the number of up-converting mixers, power amplifiers, and power combiners involved. The tremendous advances in the development of CMOS technology makes it possible to integrate circuits on a single chip, miniaturize circuit designs, reduce manufacturing costs, and reach the system requirement of generating the 8 output frequencies simultaneously.

Receiver

MIR receiver system operation

V-band receiver chip

This receiver system will replace parts of the current MIR system on DIII-D tokamak to improve the received data quality, both in terms of noise temperature and immunity from stray noise pick-up as described in the following. The red block on Fig. 5 shows the expected upgrade for our new MIR system, including 12 receiver channels spaced vertically. Each of the receiver channels represents different positions (i.e., poloidally) in the vertical direction in the plasma and it down-converts 8 various frequency spurs (i.e., radial positions) to 1-10 GHz for further signal processing.
The receiver down-converts 8 independent frequency spurs located at 55-75 GHz to related 8 signals located at 1-10 GHz which are then fed into the current IF module. The IF modules amplify the received signal and perform the second down-conversion process with corresponding IF_n+510 MHz input. They are sent through narrow 140 MHz/880 MHz bandpass filters to isolate each plasma fluctuation signal. The final down conversion employs IQ mixers to generate in-phase and quadrature signals representing the plasma fluctuations. These signals are routed to the analog-to-digital converters which run at 2 M samples/s allowing imaging of plasma density fluctuations up to 1 MHz.

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