Combined KSTAR Tokomak ECEI/MIR System
Electron Cyclotron Emission Imaging (ECEI)
Electron cyclotron emission Imaging (ECEI) is a novel millimeter wave plasma diagnostic technique. It measures the electron temperature profiles and fluctuations in magnetic fusion plasma devices. In the (ECEI) approach, the single antenna in a conventional heterodyne ECE radiometer is replaced by an array of antennas. ECE radiation is collected and imaged onto a mixer/receiver array which is comprised of planar antennas with diodes. The ECEI focal plane can be swept through the plasma by sweeping the receiver frequency of the array, due to the one-to-one mapping from ECE frequency to major radius. Consequently, the 1-D radiometer is transformed into a 2-D imaging diagnostic over the minor plasma cross section
No illumination is needed for the passive millimeter wave radiometric ECEI system. Plasma radiation is imaged on to the imaging/receiver detector array by the imaging optics. The detected RF signal is down converted in the mixer array (imaging array) and goes to the IF detection system where the IF signals are eventually converted to video signals and subsequently digitized.
Microwave Imaging Reflectrometry (MIR)
Optics are added to traditional reflectometry. They are used to recover the phase information from waves reflected at the cutoff layers, which have 2-D fluctuations.
A transmitting system first illuminates the plasma. Waves with different frequencies reflect at their corresponding cutoff layer, carrying the phase information of the cutoff layers. The phase fronts of the cutoff layers are restored by imaging optics at the detector, which is an imaging array. Similar to the ECEI system, the RF signals are then down converted and go to the IF detection system.
KSTAR Combined ECEI/MIR System
The figure shows the concept of combined ECEI/MIR system. We can see that ECEI frequency range is 155-230GHz, and the MIR frequency range is 75-155GHz. Since the two frequency ranges are close but still separable, so they can share same optics and window. The two system are separated by the dichroic plate.
ECEI system shares the same plasma facing stainless steel mirrors and window within the Bay G cassette as the MIR system. Two additional mirrors are placed within the cassette to extend the plasma coverage. The preliminary ECEI optical design are shown below:
ECEI and MIR focal planes are toroidally separated by a few cm.
ECEI/MIR Receiver System
The above figure shows the conceptual structure for both the ECEI and MIR systems. The EM waves are first filtered by the quasi-optical notch filter which can protect the imaging arrays from stray ECRH power. The RF signals are then collected by the wideband antenna, and downconverted to an IF signal in the RF mixer. The IF signals then go to the IF Electronics, where they are divided into 16/32 equal parts. In each IF channel, signals with different frequencies are converted to a baseband IF signal and the detected or demodulated.
Different from other machines, KSTAR has higher frequency and wider bandwidth, which brings more difficulty. It is essential to have wide bandwidth throughout the system including the antenna, mixer, and IF Electronics and these are currently under active development at UC Davis. High frequency, high output LO sources also must be developed.
MIR Transmitting System
The KSTAR system will employ multiple illumination frequencies in order to simultaneously monitor density fluctuations on multiple cutoff surfaces. This can be extended later to 8 or 16 frequencies. Fed with an intermediate frequency (IF) input, the mixer generates radiation at frequencies of fLO and fIF, . A possible electronics configuration is shown.