E.E. Ashkinazi, V.G. Ralchenko, A.P. Bolshakov Fabrication of dielectric supports for travelling wave tubes by laser cutting of polycrystalline diamond wafers   E.E. Ashkinazi, V.G. Ralchenko, A.P. Bolshakov A.M. Prokhorov General Physics Institute RAS,, Moscow, Russia V.K. Balla, A.K. Mallik CSIR – Central Glass & Ceramic Research Institute, Kolkata, West Bengal, India A.F. Popovich, A.A. Khomich, M.P. Parkhomenko Institute of Radio Engineering and Electronics RAS, Fryazino, Russia GPI RAS, Moscow CGCRI, Kolata IRE RAS, Fryazino

Travelling wave tubes (TWT): construction and applications TWT is the source of high power millimeter wave radiation. Applications: tropospheric and space communication, radars. The TWT use the helix slow-wave circuit: the electromagnetic wave travels through the tube along a metal spiral that brings it into synchronism with the electron beam.     TWT features : ● wide bandwidth, ● high efficiency, ● compactness. The helix is placed in the center of a metal barrel under pressure by three dielectric rods. Undesirable heating of the helix by e-beam. The problem of effective heat dissipation via dielectric supports restricts the increase of the average output power of the device. A material for supports with high thermal conductivity (k) is needed. Goal: replace beryllium oxide BeO (k = 150 – 250 W/mK) ceramics now in use by diamond (k = 1000 – 2000 W/mK).   BeO supports

Principle of CVD process Polycrystalline diamond wafers produced by chemical vapor deposition (CVD) Diamond growth from simple methane-hydrogen mixtures in a microwave plasma Principle of CVD process 57 mm diameter diamond wafer Parallel processes: ● Etching (sp2, sp3) ● Co-deposition (sp2, sp3) Etch rate of diamond by atomic hydrogen is higher than that of graphite. ►Dominating product - diamond GPI RAS facility CGCRI facility Any physical process creating atomic hydrogen and CHx radicals potentially is able to produce diamond. CVD system ARDIS-100 2.45 GHz, 5 kW CVD system DT1800 915 MHz, 15 kW

Polycrystalline structure Typical growth conditions Gas composition: (1-7%)CH4/H2 Pressure: 80-160 Torr Flow rate: 500-1000 sccm Substrate temperature 700-1100ºC Substrate material: Si Substrate diameter 50-100 mm Growth rate: 1-6 µm/hour Randomly oriented grains. Grain size up to 100 µm for 0.5 mm thick films Raman spectrum. The narrow peak at 1332 cm-1 belongs to diamond. Free-standing transparent 0.5 mm thick diamond disk after chemical etching of Si substrate

Surface texture. Growth side. controlled by growth parameters Transparent (“white”) diamond Opaque (“black”) diamond (110) orientation (100) orientation Fracture surface columnar grains V.G. Ralchenko, et al. Diamond Relat. Mater. 23 (2012) 172

Cutting the diamond disk Why laser processing? ● Diamond is the hardest material known (H = 80-100 GPa). ● Extremely difficult to treat mechanically (e.g. sawing, polishing). ● Laser cutting is the most effective technique. Cutting the diamond disk Nd:YAG laser: λ=1.06 µm, 10 ns pulse, 10 kHz Cut surface Processing parameters Laser power 14 W. Laser beam scanning velocity 2.0 mm/s (multipass regime). Laser beam spot ≈30 µm. Kern width 120-180 µm.

Laser etching – two mechanisms ● The ablation at ~4000°C is preceded by diamond graphitization (~1700°C). ● The graphitized layer provides enhanced absorption for next pulses. ● Oxidation normally is a side effect.

Diamond rods Three identical rods of square cross sections with a length of 12mm were cut from each disc: the prototypes of supports for the TWT . The diamond plates are not polished: the roughness Ra of growth side as high as 5 to l0 µm. The opposite, nucleation side is smooth, Ra ≈ 10 nm. SEM images laser cut surface growth side up nucleation side up

Graphitization of cut surfaces: characterization by Raman spectroscopy LabRam HR840 (Horiba Jobin-Yvon ) spectrometer in a confocal configuration . Excitation wavelength 488 nm, spectral resolution of 1.0 cm-1, laser spot diameter ~1 µm. as-grown (top) surface laser cut (vertical) surface Four diamond samples of different qualities Raman spectra at cut surface are taken in two different locations. ● The diamond peak at 1332 cm-1 dissappears. ● The D and G bands at 1350 cm-1 and 1580 cm-1 belong to nanographite. ● Graphite contribute a lot to microwave radiation absorption (may increase the loss tangent by two orders of magnitude). ● Etch away the graphite layer in an air furnace at temperature of 580 - 600°C for 30 min before loss tangent measurement.

Measurements of thermal diffusivity by Laser Flash Technique (LFT) ● Delivery of laser pulse through an optical fiber to improve uniformity of irradiation on the sample. ● Software for automatic evaluation of thermal diffusivity and TC. ● Vacuum Cryostat. Measurements thermal diffusivity in the temperature range 180 – 430 K. ● LFT measures perpendicular thermal diffusivity D. Method: heat the front side by short laser pulse and monitor T(t) signal on rear side. laser beam IR detector metal film (absorber) sample Temperature evolution T(t) on rear side of the film

Thermal conductivity of CVD diamond plates as measured by LFT ● High thermal conductivity for “white” diamond films at room temperature: k = 2010 W/cmK (growth in 2.45 GHz plasma), k = 1950 W/mK (growth in 915 MHz plasma). ● k = 1340 W/mK for medium quality “gray” film; ● k < 600 W/mK for black “diamonds Thermal conductivity of best quality polycrystalline diamonds approach to that for IIa type single crystal diamond. white diamonds gray film black diamonds

Measurements of loss tangent of the diamond rods resonator method to determine permittivity and loss tangent Experimental setup ● the resonator is a rectangular waveguide with cross section a×b=7.11×3.56 mm, and length l =25.5 mm; ● oscillation mode H103; ● frequency 27.48GHz ● fit to work with small samples resonator (1) Agilent PNA-L N5230C panoramic analyzer, (2) coaxial cable, (3) coaxial-waveguide transition, (4) resonator, (5) sample to be measured, (6) diaphragm. sample Example of data sheet for samples D1 and D2 The measured tan δ ~10−3 for most thermally conductive transparent material (k =2010 W/mK at R.T.). M.P. Parkhomenko et al. Physics of Wave Phenomena 23(3), 202-208 (2015)

Summary ● Polycrystalline CVD diamond wafers with diameter up to 100 mm and thickness up to 0.6 mm are grown in a microwave plasma as starting material for highly conductive supports for TWT. ● The thermal conductivity up to 2010 W/cmK at R.T. are measured for the best quality “white’ diamond samples. ● A series of prototype diamond rods has been cut from mother disks by a Nd:YAG laser, which is the efficient way to process the diamond. ● Loss tangent tan δ as measured in a dedicated miniature resonator is of the order of 10-3 at frequency of 27 GHz.

Thank you for attention!