Investigations of microwave and THz radiation losses in CVD diamond and chemically modified diamond
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Investigations of microwave and THz radiation losses in CVD diamond and chemically modified diamond 1a
1a
T. A. Scherer , D. Strauss , A. Meier1a, Y.-L.Mathis1b, V. Judin1b , W. Müller-Sebert2, W. Smirnov2,C. Nebel2 1
Karlsruhe Institute of Technology KIT, Herrmann-von-Helmholtz-Platz 1, D-76344 EggensteinLeopoldshafen, Germany, [email protected] (corresponding author); a: IMF-1, b: ANKA
2
Fraunhofer-Institut für Angewandte Festkörperphysik (IAF); D-79108 Freiburg, Germany
ABSTRACT To counteract plasma instabilities like Neoclassical Tearing Modes (NTM-modes) in nuclear fusion reactors (JET, ITER, DEMO) high power microwaves are used for the Electron Cyclotron Resonance Heating (ECRH) and for the plasma current drive (CD). The foreseen power level for ITER (Cadarache, France) is Ptot = 24 MW at f = 170 GHz. Each transmission line is designed for a maximum of 2 MW power. The vacuum and tritium barrier to the ITER vacuum vessel is realized by a CVD diamond disk window assembly. Diamond has an extremely high thermal conductivity of about k = 2100 W/Km and a very low loss tangent of tan δ < 10-5 for this frequency and shows therefore a very small microwave absorption. The normalized absorbed power A=Pabs/P0 can be calculated as A = (f/c) • • (1+εr‘) • tan δ • t (with the rule of thumb estimate: (f/c)=0.5 mm-1; • (1+εr‘) = 20; tan δ =10-5; A=10-4 • t [mm]); i.e. each t = 1 mm thickness of diamond absorbs Pabs = 100 W of Po = 1 MW microwave power transmitted through the CVD diamond window with an effective tanδ of 10-5. INTRODUCTION To understand the loss mechanisms at the surface and in the bulk material of diamond, the determination of the loss tangent in dependence of the frequency in the range of several GHz up to THz and the dependence of the temperature and radiation power is essential. The main absorption for ultra-low loss diamond material (sp3) is due to the included sp2-carbon at the surfaces during the CVD growing process. These graphite like carbon acts as an additional microwave absorber and increases the loss tangent. To understand the behaviour of the bonding properties of a CVD diamond surface, different chemical atomar/molecular surface finishing is foreseen for a series of experiments. In a first step a treatment of diamond with hydrogen in a plasma process showed a contact angle of about 82° with water. The measurement of the electrical conductivity with the Van-der-Pauw method (4 circular point electrodes on top) shows a resistance of the order of several 104 Ω. This resistance is responsible for higher microwave losses at the diamond surface (10-3 < tanδ < 10-4 ; at the frequency of f =170 GHz) [4-29]. In a second step the influence of an oxygen terminated surface was investigated. The loss tangent was reduced to the limit of the resonator measurement setup with values in the range of several 10-6. First results in a high resolution Fabry-Perot resonator will be presented. To estimate the transmittance behavior at higher frequencies up to the THz range, recent measurements by using
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