Millimetre wave attenuation of prototype diagnostic components for the ITER bolometers

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Highlights

  • Attenuation of ECRH stray radiation in ITER demonstrated for bolometer prototypes.

  • Collimator with microwave reflecting grid achieves >70 dB at 170 GHz (ITER frequency).

  • For frequencies >250 GHz (ECE radiation) ceramic coating showed 40 dB attenuation.

  • Good shielding at joints of components is prerequisite to prevent microwave leakage.

  • These methods prevent the impact of ECRH stray radiation on bolometer measurements.

Abstract

Bolometers in current and future fusion devices, in particular those in ITER, are vulnerable to stray radiation from electron cyclotron resonance heating (ECRH) which results in measurement errors for plasma radiation detection. To protect the detectors from this stray radiation in the millimetre wavelength range, dedicated diagnostic components have been designed and tested. One option is to place a top plate which contains a microwave-reflecting grid onto the collimators. Another option investigated is the coating of the collimator channels using a microwave absorbing ceramic. Measurements of the mm-wave attenuation of the collimator in front of the bolometer detectors with and without top plate or coated collimator channels have been performed in the frequency range of 125–420 GHz. The attenuation factor of the collimator channels at 170 GHz (the ECRH frequency for ITER) with neither microwave grid nor coating is typically 10 dB. The coating enhances this to 40 dB and including the microwave grid yields at least an attenuation factor of 70 dB, which is sufficient to reduce the residual ECRH induced signal significantly below the one due to plasma radiation. Placing a bolometer camera (collimator connected to detector housing) inside the isotropic microwave field of the test facility MISTRAL, the attenuation factor of the full diagnostic set-up using a top plate was determined to be in the order of 45 dB. This degraded attenuation implies that particular attention has to be paid to design and quality control of the joints of diagnostic components to prevent microwave leakage.

Introduction

The total radiated power as well as the radiation emission profile on ITER will be determined by the bolometer diagnostic. A bolometer measures the plasma radiation over a wide spectral range (from soft-X to the infrared) by monitoring the temperature rise induced by deposition of photon energy in the absorber layer of the bolometer. The reference detector type chosen for ITER is the metal resistor bolometer [1], [2]. In order to determine the spatially resolved radiation profile, tomographic reconstruction routines are applied on the measurement results from many lines-of-sight (LOS). The LOS can be defined by collimators, which provide an individual aperture per detector pixel.

For measurements in ITER one has to take into account that stray radiation from electron cyclotron resonance heating (ECRH) might occur in several of the foreseen operating scenarii. In case of non-optimal absorption isotropic stray radiation up to levels of 90 kW/m2 can be expected at the bolometer surfaces located behind the gaps between blanket modules of the first wall when assuming a gap size of 30 mm [3]. The propagation of mm-waves from the entrance of the camera towards the detector is along metal surfaces with very high reflectivity. The latter also applies to the absorber. Thus, it can be assumed that an isotropic mm-wave field is generated inside the camera because of multiple reflections. The power level corresponds to the power density at the collimator opening times its area and only a fraction, corresponding to the ratio of absorber surface to the other inner surfaces, leads to additional signals. Therefore, collimators already provide a basic attenuation of stray radiation. However, the levels of power from mm-waves may be higher than the expected levels of plasma radiation and thus lead to measurement errors if not further mitigated. In addition to the openings of the collimator channels, mm-waves may enter the camera housing also at joints if the connections of components are not microwave tight, as for a non-polarized wave there is always a polarization angle which is parallel to the remaining slit and thus provides a path into the interior of the camera. Copper seals and overlapping edges are methods to reduce this effect.

In order to protect the detectors from stray radiation in the mm-wavelength range, several methods are known [4]: metal meshes, perforated metal plates and microwave absorbing coatings. In this work the latter two options have been used to manufacture prototype components of a bolometer camera suitable for ITER (Section 2) and test them in the laboratory with respect to their ability to protect the detector from incident microwave radiation in the wavelength range between 125 and 450 GHz (Section 3). Additionally, a complete set-up of a bolometer camera was tested under isotropic microwave radiation at 140 GHz in the test facility MISTRAL [5] (Section 4). From the measurement results presented conclusions with respect to the application in ITER are drawn in Section 5.

Section snippets

Collimator prototypes

The first version of the prototype collimator used for these investigations is shown on the left in Fig. 1. It consists of four viewing channels, which are split into three sub-collimators each in order to reduce the viewing cone perpendicular to the fan of LOS [6]. At the plasma facing side, a top plate is added, using overlapping edges of 5 mm length and a copper seal to prevent microwave leakage. Additionally, the top plate contains four microwave grids, one for each LOS. The cross-section of

mm-Wave attenuation in collimators

To get information on the suppression of millimetre waves for the individual viewing channels, classical transmission measurements have been performed using the linear polarized fundamental TE10 mode. Measurements with the top plate installed as well as without the top plate were performed. The set-up consisted of an ABmm Network-analyzer with a base frequency of 8–18 GHz, an active multiplier generating the 12th harmonic for the frequency range 125–225 GHz and a transition from D-band to Ka-band

mm-Wave attenuation of a bolometer camera

To test the mm-wave attenuation for a complete bolometer camera, the first version of the collimator has been mounted on a housing containing a bolometer detector and its signal connections. The joints of collimator and housing were designed with overlapping edges of 5 mm length and a copper seal to provide shielding against microwave leakage. This assembly was mounted on the end of an immersion tube which provided shielding for the signal cables when mounted in the test facility MISTRAL. This

Conclusion

The attenuation of stray radiation from heating sources in ITER has been measured for two different bolometer collimators equipped with two different methods to attenuate incoming mm-wave radiation. Depending on the methods applied, attenuation factors up to at least 70 dB have been obtained. Also, a complete bolometer camera assembly has been tested for microwave leakage in the test facility MISTRAL showing a reduced attenuation factor in the order of 45 dB.

In view of the developments for ITER

Acknowledgments

This work was supported by funds from the German Ministry for Education and Research under the Grant No. 03FUS0006. The sole responsibility for the content presented lies with the authors.

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Cited by (5)

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    A possibility to enhance the light yield by a factor of two for all LOS would be to relax the requirement on supressing stray radiation from electron-cyclotron heating. This would open the opportunity to use alternative methods for mm-wave suppression [12] than the microwave reflecting grid. However, the feasibility of this option will have to be determined considering final capabilities for mm-wave suppression and requirements on the suppression factor.

  • Current status of the design of the ITER bolometer diagnostic

    2017, Fusion Engineering and Design
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    It can be suppressed by applying a microwave coating on the inside of the collimator. As explained in detail in [7], attenuation factors up to 70 dB can be achieved. With the high operating temperatures of ITER the requirements on the support and electrical connection of the sensor are more demanding than for currently active devices.

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