Physics of natural and artificial diamond gemstones

Diamond gemstones were very well appreciated in the antique world. Independent on the purpose of jewelry, diamond is a crystalline solid state material with excellent physical and chemical properties as a high Young modulus or a very high thermal conductivity. By doping the material with boron, electrical conductivity can be observed. This is important for electronic devices. In this talk the wide range of production of gemstones and technical applications like high frequency high power microwave transmission diamond windows for nuclear fusion power plants will be presented. Different diamond classifications, cuts and colors by impurities will be shown. A comparison of natural diamonds and artificial produces ones are topic of the discussion

Development of diamond based KIDs

Kinetic Inductance Detectors (KIDs) have proven themselves as a very versatile cryogenic detector technology capable of applications in various fields due to their flexibility of design, sensibility and ease of production. We have recently proposed a polarization sensitive Lumped Elements KID as sensor for an innovative polarimetric diagnostics based on quantum cascade lasers (QCL) for application in the nuclear fusion. Each detector unit is composed by 4 pixels arranged at the vertices of a square, each pixels being sensible to only one polarization direction. The current system is based on niobium nitride (NbN) superconductor over High Resistivity Silicon (HRSi) substrate. Such material delivers good performances but its relatively high dielectric constant and loss tangent lead to increased substrate losses. Using a transparent substrate may improve this aspect and also the radiation resistance of such devices. Diamond is the substrate of choice, being a material already widely studied and used in the fusion environment as high power microwave window, due to its outstanding optical and mechanical performances. In this work we present the preliminary design study for a diamond based Kinetic Inductance Detector and subsequent characterization measurements of the first prototypes

Characterization of Boron-doped diamond and r-plane sapphire for plasma diagnostics in future nuclear fusion reactors -A survey of electrical and dielectric properties

p-Boron-doped polycrystalline CVD diamond samples were produced and delivered by the German company Diamond Materials in Freiburg (Germany). In a first step, main properties of this candidates for diagnostic and/or heating windows in future nuclear fusion reactors were investigated. By a special measurement technique, it was possible to determine the Boron doping concentration in Diamond by measurement of the resistive properties by using the van der Pauw method. So prepared, an irradiation campaign with neutrons and/or heavy ions on these samples will follow. The second material investigated, was r-plane single crystalline sapphire. For the first characterization the dielectric properties of a 3*-wafer in dependency of the frequency in a FABRY-PEROT resonator setup was performed. Also, this is the preparation for the next irradiation experiments in this project

Basic considerations for fracture toughness measurements of MPA CVD diamond to be used in nuclear fusion

In nuclear fusion, Microwave Plasma Assisted (MPA) Chemical Vapour Deposition (CVD) polycrystalline diamond is the only material allowing for transmission of high power microwave beams (1-2 MW) in long-pulse gyrotron operations. The reason lies in the combination of extraordinary thermal, mechanical and optical properties of diamond, which is used in the shape of disks having thickness of 1 to 2 mm for windows. Being diamond a brittle material, failure to fracture is the main failure mode. Accordingly, an appropriate mechanical characterization is required as diamond plays a major safety role in fusion machines. Due to limited body of work in literature, fracture toughness measurements have to be first carried out for this material and then a design criterion for structural integrity assessment has to be applied. In this work, the preliminary activities aiming to define the optimum experimental measurement method of fracture toughness for thin diamond samples are shown and discussed. An outlook to the next steps is also given

Design and Comparison of Diamond‐ and Sapphire‐Based NbN KIDs for Fusion Plasma Polarimetric Diagnostics

In this paper we present the design and characterization measurements performed on Kinetic Inductance Detectors produced on sapphire and policrystalline diamond substrates. Designed to be part of a nuclear fusion polarimetric diagnostic instrument, 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$\%$, in accordance with the guidelines of the upcoming ITER reactor \cite{zeeland2013} \cite{Donne2007}. 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 relatively high $T_C$ of bulk NbN ($\sim$16~K) enables operation at liquid helium temperatures, simplifying the setup of the final detection system when compared to the sub-kelvin temperatures employed in astrophysical KID arrays. The DC characterization measurements performed on test strips patterned on the two substrate materials highlighted large differences in the thin films quality, with the superconductor deposited on diamond showing reduced critical temperature, lower \textcolor{red}{critical} current density and increased values of the kinetic inductance compared to the strips on sapphire. This difference is believed to be mainly due to the higher lattice constant and thermal expansion coefficient mismatch between film and substrate in the case of diamond, \textcolor{red}{with the} different surface finish quality of the crystalline samples at our disposal \textcolor{red}{also likely playing a role.} The response to microwave read-out tones of the prototypes obtained from the same films follows a similar behavior, with the devices produced on sapphire generally outperforming those on diamond. Nevertheless, diamond is a promising candidate for this kind of application, \textcolor{red}{especially considering} the advantages given by its radiation hardness. The devices on both substrates showed a response to THz radiation, bolometric in nature, that fulfills the requirement guidelines and represent a good starting point to optimize the design for the application at hand

Dielectric loss measurements of CVD diamond disks for ITER windows

Diamond disks manufactured by chemical vapor deposition (CVD) are essential elements of windows of the Electron Cyclotron Heating and Current Drive systems of fusion reactors like ITER. Diamond is selected as window material because of its high mechanical stability, high thermal conductivity and low dielectric loss. Only diamond disks with a low loss tangent guarantee a high transmission, i.e. a low absorption of microwave power in the disk. The latter results in moderate window temperatures and therefore in low thermal stresses. Hence, the measurement of the loss tangent is essential for the qualification of diamond disks for high-power windows. Dedicated measurement facilities (Fabry-Perot resonators) at KIT allow a high resolution measurement of the loss tangent at the disk centre (spherical set-up) as well as a mapping over the disk area to estimate its homogeneity (hemispherical set-up). Within a contract between F4E and KIT more than 60 diamond disks (D=70mm, t=1.11mm) produced similarly by MPA-CVD need to be qualified for their application in the ITER EC-system. The development of a dedicated test plan as well as initial results for the first disks delivered to KIT will be presented

Numerical analyses of CVD diamond windows in high power microwave applications

Nuclear fusion reactors require electron cyclotron heating and current drive (EC H&CD) systems for plasma heating and stabilization. Chemical vapor deposition (CVD) polycrystalline diamond windows on both the torus and gyrotron sides of the reactors act as confinement and/or vacuum boundaries allowing the transmission of high-power microwave beams. For example, the beam power scenarios of 1.5 MW and 2 MW are the current targets considered respectively in Wendelstein 7-X and European DEMO fusion machines. In this work, with reference to both reactors, the numerical analyses required to verify the thermal and structural performance of the windows are discussed. Experimental measurements of loss tangent in diamond provided inputs for the numerical analyses. Sensitivity studies of the windows with respect to loss tangent and other parameters were also carried out to check the temperature reserve margins of the design