Microwave And Interferometer Diagnostics Prepared For First Plasma Operation Of Wendelstein 7-X

Abstract

For the first test operation of W7-X, only a limited set of seven diagnostics is considered to be mandatory. Among these are the Dispersion Interferometer for the electron density measurement and the ECE radiometer. Dispersion Interferometry using a 10.6 µm CO 2-laser and its second harmonic with no need for a reference line is widely independent of mechanical vibrations and drifts and thus developed as a candidate for steady state density control. The 32-channel ECE radiometer probing the electron temperature profile using Gaussian optics and a single broadband mixer is dedicated to track ECRH heating and perform first transport studies. Three reflectometers are being prepared to use the first plasmas for their test operation namely a Doppler reflectometer with an optimized Gauss telescope optics, a Doppler reflectometer with a fast steering antenna with no movable in-vessel parts capable for a fast scan of the turbulence Kspectrum, and a conventional correlation reflectometer probing the separatrix region.

The Thomson scattering diagnostic at Wendelstein 7-X and its performance in the first operation phase

“…For discharges with the ECRH resonance on the magnetic axis, the electron temperature is centrally peaked. Temperature profiles measured with the Thomson scattering diagnostic and with an electron cyclotron emission (ECE) diagnostic [38] are compared for the plateau phases of two discharges in figure 11. The ECE data include both sides of the profile, the low field side (LFS) and the high field side (HFS), averaged over the indicated time intervals.…”

Overview of diagnostic performance and results for the first operation phase in Wendelstein 7-X (invited)

“…It measures the 2nd harmonic X-mode emission in the frequency band 126 GHz-162 GHz along a sightline defined by Gaussian optics with a 32-channel heterodyne radiometer located outside the torus hall. 16 Microwave stray radiation resulting from non absorbed ECRH power is cut out of the spectrum by a 1 GHz wide waveguide Bragg reflection notch filter with depth >40 dB showing extremely steep edges and an insertion loss of only 3-5 dB outside this frequency band. 17 The radiometer is absolutely calibrated with an overall uncertainty of ∼10%.…”

The Set of Diagnostics for the First Operation Campaign of the Wendelstein 7-X Stellarator

“…The line of sight averaged electron density will be provided by a single channel dispersion interferometer [13,14] equipped with a Field Programmable Gate Array (FPGA) based data analysis. In OP1.1 we aim at demonstrating the real time capabilities of the W7-X dispersion interferometer setup, [15,16] but without the gas feedback loop yet being implemented. The time resolution for the line of sight integrated density measurements is 20 µs.…”

“…The electron temperature will be measured by a 32 channel Electron Cyclotron Emission (ECE) diagnostic applying a heterodyne radiometer followed by a filter bank [16]. A slim Gaussian beam characteristics with radius w 0 = 20 mm at its waist in the plasma centre, minimizes the emitting volume for each frequency interval, thereby maximizing the radial resolution.…”

“…The above system is complemented by an additional small horn antenna, which is integrated into the heat shield, providing a view along the same line of sight from the high-field side, thereby allowing to also measure the emission from supra-thermal and current driven electrons. [16] 2.2.7 Electron cyclotron absorption (ECA) and stray radiation diagnostic Especially in OP1.1, with starting W7-X for the very first time, initially relatively high impurity influx levels need to be expected and may lead to increasing strong energy losses and thereby decreasing electron temperatures in the course of a discharge, which could result in early discharge termination. To provide a means for an early detection of the transition towards such a scenario, the ECRH wall protection tiles opposite to the ECRH ports in module 1 and 5 (AEA11, AEE10, AEA51, AEE51) have been equipped with 2 × 63 open waveguide antennas.…”