Protection Filters in ECEI Systems for Plasma Diagnostics

Abstract

For plasma diagnostic imaging systems such as the electron cyclotron emission imaging (ECEI) system, spurious rf heating power may saturate or even damage the mixer arrays. Without protection, the sensitivity of the mixers can significantly decrease or in the extreme case, the diodes can even be burnt. A metallic dichroic plate is usually used to rejection the spurious rf heating power. However, as a high pass filter, the dichroic plate can not be used when the frequency of the heating power is in the middle of the frequency range of interest. Consequently, a frequency selective surface (FSS) has been introduced as a planar filter in ECEI systems. FSSs can work as low pass, high pass, and band stop filters according to the various system requirements. Also, as a thin, light, planar filter, it is very easy to mount in imaging systems. This paper will focus on the design and fabrication of the FSS notch filter applied in TEXTOR, which is used to protect the imaging array from stray 140 GHz ECRH power. The filter is used in TEXTOR due to its deep rejection, and excellent angle insensitivity. The design procedure will be presented. More FSS applications will be talked in this paper. The new fabrication technique Electro Fine Forming (EF2) technology will also be introduced. FSS filters in the millimeter wave range also have possible applications in imaging systems in other fusion machines such as KSTAR, DIIID, and LHD.