Progress in ITER ECE diagnostic design and integration

The ITER electron cyclotron emission (ECE) diagnostic system has primary roles in providing measurements of the core electron temperature profile and the electron temperature fluctuation associated with the neoclassical tearing modes. The ITER ECE system includes a radial and oblique line-of-sight. Four 43-meter long low-loss transmission lines (TLs) are designed to transmit millimeter wave power in the frequency range of 70–1000 GHz in both X- and O-mode polarization from the port plug to the ECE instrumentation room in the diagnostic building. The measurement instrumentation includes two Fourier transform spectrometer (FTS) systems and two radiometer systems. The Indian Domestic Agency (IN-DA) and United States Domestic Agency share the responsibility. The IN-DA scope excluding instrumentation and control has passed its preliminary design review and is progressing towards the final design review (FDR). In parallel, the diagnostic integration in different areas is ongoing. Several captive components for the TLs have passed FDR and will be manufactured for installation in the tokamak building soon. A peer review meeting has been held on the prototype hot calibration source, and its integration and new thermal analysis in the diagnostic shield module are continuing. A prototype TL is being tested. A prototype polarizing Martin-Puplett type FTS, operating in the frequency range 70–1000 GHz, features an in-vacuo fast scanning mechanism and a cryo-cooled dual-channel THz detector system. Its performance has been assessed in detail against ITER requirements.

Abstract: The ITER electron cyclotron emission (ECE) diagnostic system has primary roles in providing measurements of the core electron temperature profile and the electron temperature fluctuation associated with the neoclassical tearing modes. The ITER ECE system includes a radial and oblique line-of-sight. Four 43-meter long low-loss transmission lines (TLs) are designed to transmit millimeter wave power in the frequency range of 70-1000 GHz in both X-and O-mode polarization from the port plug to the ECE instrumentation room in the diagnostic building. The measurement instrumentation includes two Fourier transform spectrometer (FTS) systems and two radiometer systems. The Indian Domestic Agency (IN-DA) and United States Domestic Agency share the responsibility. The IN-DA scope excluding instrumentation and control has passed its preliminary design review and is progressing towards the final design review (FDR). In parallel, the diagnostic integration in different areas is ongoing. Several captive components for the TLs

Introduction
The ITER electron cyclotron emission (ECE) diagnostic system [1,2] has primary roles in providing measurements of the core electron temperature profile and the electron temperature fluctuation associated with neoclassical tearing mode (NTM) instabilities. Together with the Thomson scattering systems [3], they are adequate to provide the core electron temperature profile with specifications that meet the requirements from the perspective of plasma control and physics studies. With regard to the measurement of the electron temperature fluctuation for NTM detection [4,5], ECE is the only system that has this primary role. Therefore, the specifications needed to measure these two plasma parameters are driving the system design. The ITER ECE diagnostic system also contributes to the measurements of the plasma stored energy, the radiation power in the frequency range between 70 GHz and 1 THz, the runaway electrons, the presence of H-modes and edge localized modes, the edge electron temperature profile, and the electron temperature fluctuation associated with the high frequency instability toroidal Alfvén eigenmodes.
The ITER ECE diagnostic system includes two lines-of-sight, namely, the radial and the oblique view. Both views can be used to measure the electron temperature profile when the non-thermal electrons are absent. Whereas, the oblique view is sensitive to non-thermal distortions in the bulk electron distribution function [6]. The plasma radiation will be collected by the front-end which is located inside the diagnostic shield module (DSM) #02 in equatorial port plug (EPP) #09. Two hot calibration sources are integrated into the DSM as well. Passing through the primary window on the closure plate, X-and O-mode radiation are selected and transmitted to the diagnostic room.

JINST 17 C04019
A suite of instruments has been chosen to measure the plasma radiation. It includes two Fourier transform spectrometer (FTS) systems and two radiometer systems.
In the past few years, the design effort has been put into the integration in different areas in the tokamak complex (B11). The layout of the quasi-optics in the port plug (PP) is nearly finalized. Definition of the diagnostic first wall (DFW) and the design activities on the hot calibration source bracket are ongoing. Captive ceiling supports in the gallery of B11 have passed the final design review, and will be delivered soon. Apart from the integration activities, prototypes of some critical components (the hot calibration source, the polarization splitter unit (PSU), a section of transmission line (TL), and the FTS) have been developed. The present status of ITER ECE diagnostic system will be presented in this paper. Section 2 presents the current design activities, and section 3 focuses on the prototype activities.

Current design layout
As shown in figure 1, the basic configuration of the entire ECE diagnostic system consists of three main parts: • "The front-end", that collects the radiation from the plasma and transmits it through to the primary vacuum windows on the closure plate. This part also includes hot sources located inside the vacuum vessel for conducting in-situ calibration of the entire system.
• "The transmission line", that transports the plasma emission in the microwave frequency range from the front-end and distributes it to the instrumentation in the diagnostic building.
• "The instrumentation", that is housed in the ECE diagnostic room includes two FTS systems covering a broad spectral range from 70 GHz to 1 THz, a low-frequency radiometer covering 122-230 GHz and a high-frequency radiometer covering 220-340 GHz [7,8].

Integration in the port infrastructure
The integration in the PP and in the ex-vessel area near the closure plate is critical from the point of view of both the schedule and the engineering challenges. Figure 2 shows the key components of the system together with the main structure of the DSM#02 in EPP#09. To meet the requirements of the system performance and the neutron shielding, it has been decided to put the two hot calibration source brackets in the same DSM bay. The primary window on the closure plate forms the boundary of the vacuum. Near the closure plate, X-and O-mode radiation is selected and divided by the PSU. A gas seal connects the primary window and the PSU, and this ensures that the entire TL can be purged. The gas seal is also able to withstand the relative displacements between the vacuum vessel and the ex-vessel components which are supported from the interspace support structure (ISS). The design of displacement compensation unit that is used to maintain the alignment is being assessed.

Integration in the gallery of B11
In the gallery of B11, many systems will be installed prior to the first plasma. Even though the ECE diagnostic is not a system for the first plasma, some of the ceiling supports will be trapped by the other systems. Therefore, those supports need to be installed well before the first plasma.
In the past two years, the United States Domestic Agency (US-DA), the Indian Domestic Agency (IN-DA), and the ITER Organization (IO) have worked together on the design. The final design review has passed. The supports are under manufacture now and will be delivered soon.

Radio frequency stray radiation protection
The stray radiation from Electron Cyclotron Resonance Heating system and Collective Thomson Scattering diagnostic system is a general issue for all microwave diagnostics [9]. It not only impacts the measurement itself but also causes damage of microwave components. Even though the detailed design is still ongoing, a strategy has been developed for the ECE diagnostic system.
(i) The stray radiation power level will be monitored and used for feedback control a fast shutter placed at the input of the PSU. The shutter is always kept closed except when ECE measurements are needed to be performed. (ii) Notch filters will be implemented in the transmission lines. (iii) The radiometer systems are designed such that the stray radiation is filtered. An additional fast shutter is placed at the entrance of the FTS.

Hot calibration source
To realise an absolute measurement, in-situ calibration needs to be carried out at two or more different radiative temperatures. Different from the conventional method of using hot/cold sources, only the hot calibration source is used in the proposed calibration strategy and the hot calibration source is capable of operating at different temperatures. This method has been demonstrated in a few operational devices such as Tore Supra [10], KSTAR [11], EAST [12], etc. Considering the requirements and constraints (compatibility with the high vacuum and high radiation environment, short-term and long-term stability, frequency range upto 1 THz, etc.), the design of the hot calibration source is challenging. Much effort has been put into the hot calibration source design and significant progress has been achieved in the past [13,14]. The hot calibration source is composed of an emitter, a heater, some shielding components, etc. The emitter is made of SiC with pyramidal shapes on the surface. This enables the emitter to have a high emissivity. After evaluating different heat transfer methods, radiative heat transfer has been selected. Improvements to increase the calibration source emissivity have also been investigated. A prototype has been designed, manufactured, and tested. The outgassing tests showed that the outgassing rates meet the vacuum requirements in ITER. Thermal analyses indicated that the design can achieve the required specifications from the perspectives of the emission surface temperature and the temperature uniformity.
A peer review meeting for the hot calibration source has been held in 2017. The US design team is continuing to work on the integration in DSM and focuses on the thermal analysis within the latest DSM environment. On one hand, the new analysis will try to verify that the performance of the hot calibration source still meets the requirements in the DSM environment. On the other hand, it would provide inputs for optimizing the design.

Polarization splitter unit
As illustrated in figure 2, the PSU selects the X-and O-mode radiation and divides the two linesof-sight into four beams. As shown in figure 3(a), the PSU is composed of a wire grid and a few reflectors. The ellipsoidal mirrors are chosen because of their better performance from the perspectives of transmission loss and heat-resistance compared to lenses. The optical parameters are basically defined by the front-end design [15] inside the PP. The overall design of the front-end and the PSU is done altogether to ensure the spatial resolution using the Gaussian beam optics. A prototype has been developed to study the performance. The mechanical specifications listed in table 1 have been tested and the requirements are met. The optical performance (beam pattern, polarization, etc.) will be tested soon.  Vacuum and leak rate Vacuum: 10 −2 mbar Leak rate: 10 −8 mbar · L/s Mirror mounts Linear movement: +/-10 mm Angular tilt: +/-3 degree Mirror focal length 300 mm +/-0.5 mm Mirror surface roughness 5-10 μm Ra (Average roughness)

Transmission line
The TL needs to be as lossless as possible because the length of each line is around 43 meters. The attenuation per unit length has been measured for three types of waveguide, namely, the circular smooth walled waveguide, the corrugated waveguide and the dielectric coated waveguide [16,17]. The circular smooth walled waveguide and the corrugated waveguide have similar attenuation (about 0.1-0.2 dB/m) for the frequency range upto around 600 GHz. But the former has better performance (∼0.25 dB/m versus ∼0.4 dB/m) above 600 GHz. Therefore, the circular smooth walled waveguide has been selected. A TL mock-up composed of the circular smooth walled waveguide with a length of around 10 meters and a few miter bends has been developed. Using the mock-up, the atmospheric absorption has been studied. The preliminary results show that the attenuation due to the atmospheric absorption is reduced in the case of an evacuated TL. Taking the attenuation for the resonance absorptions around 557/752/988 GHz as an example, it is as low as around 1 dB/m. In air, the attenuation for those peaks could be as high as 10 dB/m.
To be noted, having evacuated waveguides has an implication on the safety strategy, as isolation valves are needed along the TL from the port cell to the diagnostic building. This adds a risk to the system: if a vacuum leak is detected on the TL, safety valves must be closed and therefore stopping the whole diagnostic from operating. Considering the simplicity of using Compressed Air (CA)/dry N 2 purged TL with regard to safety requirements, and the fact that all the functional requirements are achieved, it has been decided to use CA/dry N 2 purged TL for the ITER ECE diagnostic system. It has been demonstrated that purging the TL can mitigate the atmospheric absorption [17]. Detailed assessment of the CA/dry N 2 purged TL will be conducted in the final design phase.

Fourier transform spectrometer
A FTS for ECE diagnostic applications has been designed and developed within a close collaboration between IN-DA and the supplier [18]. The most challenging is the design of the mechanically moving part. It needs to have a sufficient scan distance to achieve the required frequency resolution (<5 GHz). It also needs to have a lifetime that is comparable with ITER lifetime to ensure a high system availability. The performance of the system has been assessed in detail against the ITER requirements [19]. The achieved parameters are listed in table 2. Apart from the frequency resolution, the performance of this prototype meets all the ITER requirements. It has served as a powerful system to characterize the TL. Research and development to improve the frequency resolution is underway.

Summary
The design, research and development of the ITER ECE diagnostic system have progressed very well since the last progress updates presented [2]. The preliminary design review on the transmission line, the Fourier transform spectrometer, and the low-frequency radiometer has been completed and is near closure. The captive supports in the gallery have passed their final design review, and will be delivered soon. The integration in the port plug and other areas continues. The layout of the front-end is close to being finalized. The design of the hot source bracket and the mirror support is ongoing. Prototypes for the hot calibration source, the polarization splitter unit, a section of transmission line, and the Fourier transform spectrometer have been developed successfully and characterized.