Dichroic filters to protect milliwatt far-infrared detectors from megawatt ECRH radiation

Abstract

Dichroic filters have been used to shield effectively the far infrared (FIR) detectors at the interferometer/polarimeter on TEXTOR. The filters consist of metal foils with regular holes, the hole diameter, the mutual spacing and the thickness of the foils are chosen to transmit radiation at the design frequency with transmission >90%. The attenuation at the low frequency end of the bandpass filter is about 30 dB per octave, the high frequency transmission is between 20% and 40%. The filters have been used to block the stray radiation from the megawatt microwave heating beam to the detectors of the FIR interferometer, operating with power on the detector in the milliwatt range. If required, the low frequency attenuation can be still enhanced, without compromising the transmission in the passband. The FIR interferometer used for plasma density and position control is no longer disturbed by electromagnetic waves used for plasma heating.

A quasi-optical microwave interferometer for the XuanLong-50 experiment

“…An issue associated with the free-space scheme is the large circular openings of 100 mm in diameter on the shielding box wall to pass the 140 GHz microwave beam (figure 3 and figure 7), which can allow the stray 28 GHz ECRH microwave to pass through and interfere with the system. A dichroic filter [7,8] is designed to transmit the 140 GHz microwave and block the 28 GHz microwave at the same time. It is essentially a thin metal plate with a 2D array of small holes arranged in an equilateral triangular pattern.…”

Robust phase estimation for signals with a low signal-to-noise-ratio

“…The plasma induced phase shift is in many magnetic fusion experiments measured by Mach-Zehnder interferometers. A typical set-up is given in Figure1 [11]. A laser beam is splitted into three beams.…”

“…Simplified schematic of Mach-Zehnder interferometer set-ups for plasma density measurements at tokamak devices (adapted from[11]) Effect of signal-filtering with bandpass of 9.5 kHz -10.5 kHz on signal. The upper time traces display the raw signal, the lower time traces provide the signal after band-pass filtering.…”