Abstract
For a magnetic field of 2.5T, the electron cyclotron harmonics are spectrally well separated in W7-X as it has a large aspect ratio. Because of this advantage from the geometry of W7-X stellarator, it is easier to scan these higher harmonics (70,140,210…..GHz) compared to tokamaks with small aspect ratio. For confinement reasons, W7-X is planned to work at high plasma densities applying O2 electron cyclotron resonance heating (ECRH). For such plasmas, which already have been demonstrated in experimental campaign OP1.2a of W7-X, electron cyclotron emission (ECE) from second harmonic extraordinary mode (X2) is in cutoff. In that case, optically grey higher harmonics provide the only access to ECE signal and hence electron temperature profiles. A Michelson interferometer will be used in the next operational campaign of W7-X for broadband (50-500 GHz) ECE scan and that will be compared to modeling of ECE emission at different plasma parameters from radiation transport calculations (TRAVIS). The strong stray radiation from non-absorbed ECRH is a major concern in scanning these higher harmonics using an interferometer because this stray radiation will dominate the spectra thus masking the ECE signal. For this purpose, a multimode notch filter design based on a multilayer dielectric structure is constructed to attenuate the stray radiation at 140GHz by 50dB.
Highlights
In W7-X for various plasma conditions, the X2 mode of ECE is optically thick and it can be taken as a blackbody emission representing an electron temperature [1]
The Fourier transform of interferograms will provide the spectral information of ECE. These spectra consisting of higher harmonics of ECE will be explored as an option to get the electron temperature profiles
Strong stray radiation from non-absorbed ECRH will enter the X2 mode of ECE and it will dominate the ECE in spectra scanned using the interferometer
Summary
In W7-X for various plasma conditions, the X2 mode of ECE is optically thick and it can be taken as a blackbody emission representing an electron temperature [1]. The X2 mode is scanned using a heterodyne radiometer [2] consisting of 32 channels distributed over a frequency range of 120-160 GHz. In OP 1.2a of W7-X, the X2 mode was in cut-off at a plasma density of 1.2×20 m־3 [3]. It is further intended to work at high plasma density considering different modes of electron cyclotron resonance heating [4]. In such scenarios, ECE from optically grey higher harmonics has the potential to be used as the diagnostic tool to get the electron temperature profiles in magnetic confinement devices. The design and corresponding transmission characteristics of the filter are given in following sections
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