Nuclear fusion devices feature electron cyclotron resonance heating systems for plasma heating and stabilization, with diamond windows acting as vacuum and safety confinement barriers. Diamond is the only option for long-pulse operation in the MW-range applications, given its exceptional combination of optical, thermal and mechanical properties. Dielectric measurements have qualified almost one hundred disks of artificial diamond for their integration into metallic structures to form the complete windows. Optical and mechanical investigations of the disks, together with dedicated numerical analyses of the windows, complete the characterization of these essential components in nuclear fusion.

Research on diamond has grown rapidly due to its exceptional properties, making it crucial in fields such as quantum technologies and high-power applications like Electron Cyclotron Resonance Heating (ECRH) in nuclear fusion. Accurate measurement of the dielectric loss (tanδ) in diamond is essential, especially for developing ultra-low-loss single-crystal diamond windows for fusion applications. Conventional Fabry-Perot resonators have limited resolution, while superconducting thin-film micro-strip resonators, with Q factors exceeding 106, offer improved sensitivity. We report on loss measurements on single- and polycrystalline diamond samples grown using PECVD, cloning, and HPHT techniques. The results are analyzed using the Two-Level System (TLS) model to assess dielectric losses and material characteristics.

Research on diamond has intensified due to its exceptional thermal, optical, and mechanical properties, making it a key material in quantum technologies and high-power applications. Diamonds with engineered nitrogen-vacancy (NV) centers represent a very sensitive platform for quantum sensing, while high-optical quality diamond windows represent a fundamental safety component inside Electron Cyclotron Resonance Heating (ECRH) systems in nuclear fusion reactors. A major challenge is the development of ultra-low-loss, high-optical-quality single-crystal diamond substrates to meet growing demands for quantum coherence and power handling. Traditionally, dielectric losses (tan δ) in diamonds are evaluated using Fabry-Perot microwave resonators, in which the resonance quality factors Q of the cavity with and without the sample are compared. These devices are limited to resolutions around 10^-5 by the need to keep the resonator dimensions within a reasonable range. In contrast, superconducting thin-film micro-strip resonators, with Q factors exceeding 10^6, are stated to provide higher sensitivity for assessing ultra-low-loss materials. This study examines four diamond samples grown through different processes, analyzing their dielectric losses at extreme low temperatures (sub-Kelvin) within the Two-Level System (TLS) framework. Complementary Raman spectroscopy measurements allowed us not only to associate higher nitrogen content with increased losses, but also to investigate how the different growth process influence the way these defects are incorporated in the crystal lattice.

In this work we present the first characterization measurements performed on single- and poly-crystalline diamond substrates with the use of superconducting micro-resonators operating in the GHz range.

In this work we present a study evaluating superconducting LC micro-resonators as potentially useful characterization tools for dielectric materials, with a particular focus on poly-and single crystalline diamond substrates.

Design and characterization measurements performed on kinetic inductance detectors (KIDs) produced on sapphire and polycrystalline diamond substrates are presented. Designed as part of a nuclear fusion polarimetric diagnostic, the foreseen plasma‐probing frequency of the final devices is 1.3 THz with a maximum response time under 10 ms and cross‐polarization target accuracy of 1%. These detectors are based on superconducting micro‐resonators that undergo de‐tuning upon absorption of radiation. The main characteristics of the devices include polarization sensitiveness and lumped‐elements multi‐pixel configuration produced from photo‐lithographed niobium nitride (NbN) thin films. The direct current (DC) and microwave characterization measurements highlight large differences in the thin film quality, with the superconductor deposited on diamond showing reduced critical temperature, lower critical current density, and increased values of the kinetic inductance. This difference is likely due to the higher lattice constant and thermal expansion coefficient mismatch between film and substrate in the case of diamond, with the different surface finish quality of the crystalline samples available also playing a role. The devices on both substrates show a bolometric response to THz radiation that fulfills the requirement guidelines and represent a good starting point to optimize the design for the application at hand.

In this work we present the study and characterization measurements of Kinetic Inductance Detectors prototypes based on NbN thin films on diamond and sapphire substrates. The detectors are polarization sensitive, have multi-pixel design and are meant to be employed as the detecting element in a polarimetric fusion plasma diagnostic system operating at a probing frequency of 1.3 THz.

In the following work, we present the first S-21 measurements of a polarization sensitive, NbN base Kinetic Inductance Detector to be employed in a low cost, small footprint polarimeter for fusion plasma diagnostics.

In this article, we present the design of a custom kinetic inductance detector (KID) tailored for polarimetric diagnostics of magnetically confined fusion plasma. The detector is of the lumped elements KID kind and is based on a crossed 2 × 2 pixel design for polarization sensitivity. Simulations have been performed to tune device parameters like resonance frequency, cross-talking and coupling with the microstrip feed line, taking into consideration the requirements of the final instrument and the field of application. The devices have been designed for optimal absorption at 1.3 THz, the chosen probing frequency for the final instrument, and they resonate in a range from 0.8 to 1.1 GHz. The characterization of the thin niobium nitride film and the first S 21 parameter measurements performed on patterned thin films deposited on 330 μm high resistivity silicon substrates are also presented.

In this paper we present the first preliminary study of a new polarimetry diagnostic system. The device foresees multiple lines of sight, so that the measure of the plasma parameters can be performed at different chords along the poloidal plane, parallel to the equatorial direction, in a single acquisition cycle. Considering the typical plasma conditions (i.e. ASDEX Upgrade) of the actual magnetic confinement machines, we need to employ sources in the range of the low (< 3) THz to have appreciable rotation angles. As source, the diagnostic foresees the use of Quantum Cascade Lasers (QCL) which represents a very promising solution, given their ability to operate at the expected frequency of 1.6 THz at 4.2 K. Since the power of the probe beam is in the order of tenths microwatts, a cryo-detector, such as kinetic inductance detector (KID), is required. This opens the field for a very compact modular machine, composed by a single cryogenic cooler encasing source, detector and the optical section. The assessment study has been performed taking into account the performances, reliability and adaptability to multiple machines, and enriched with estimates of the Faraday rotation at different conditions, using as base data coming from the ASDEX Upgrade (AUG) tokamak located at IPP in Garching.

We present a preliminary study for the design of Kinetic Inductance Detectors (KID) sensitive to polarization, which will be employed in an innovative polarimetric system for fusion plasma diagnostics applications.

In the following work, we present the study and plans for the realization of a THz QCL based Polarimeter for fusion plasma diagnostics. The device is being developed taking into consideration a reduced footprint, reliability and ease of operation and maintenance.

The future nuclear fusion power plants will require Electron Cyclotron Heating and Current Drive (ECH&CD) systems to heat up and stabilize the plasma inside the vacuum vessel. One of the key components of such systems is the Chemical Vapor Deposition (CVD) diamond window. The purpose of this device is to act as vacuum and tritium boundary while providing a high microwave transparency with minimal reflectivity. Although suited for high power microwave operation, the windows shall be internally monitored in order to properly ensure the ECH system efficiency and safety. In this paper, the latest assessment study on a set of diagnostics to be part of the window assembly is shown. The required diagnostics include arc and tritium detection, microwave stray radiation (perpendicular to the main beam and generated by cracks in the windows), pressure and disk temperature measurements. The devices must have a compact, simple and flexible layout, with a rugged design, to maximize serviceability and durability. When multiple options are possible for some of the diagnostic systems (e.g., scintillation devices vs solid state detectors for tritium detection), tradeoffs were assessed. To accommodate the diagnostics previously mentioned, a new design for the window housing was developed. As the design of the original diamond window assembly underwent further development since the beginning of this project, an update of the general layout was required. The new layout presented here integrates the updates of the windows assembly with those to the diagnostics. To validate the concepts, a test bench was developed to carry out measurements under conditions similar to the operative ones. (C) 2017 Elsevier B.V. All rights reserved.