324 Design of Double Barrier Ceramic Radio Frequency Vacuum Window Akhil Jha * , P. Ajesh, Rohit Anand, Paresh Kumar Vasava, Rajesh Trivedi, and Aparajita Mukherjee ITER-India, Institute for Plasma Research, Bhat, Gandhinagar - 382 428, India * E-mail: akhil.jha@iter-india.org ABstRACt Vacuum windows are an essential part of any radio frequency (RF) system which launches/couples RF power from an atmospheric to a vacuum environment. This paper describes the RF design of a double barrier ceramic coaxial vacuum window. Alumina 99.5% pure is considered as ceramic barrier material while inner and outer conductors are oxygen-free copper. As the initial design approach the thickness, slope, depth of ceramic in the conductor is varied and the performance of the window is studied. The design is optimised to achieve the best insertion loss, return loss response for operating frequency range up to 65MHz. Keywords: Radio frequency; Double barrier; Vacuum window; Insertion loss; Returns loss Defence Science Journal, Vol. 71, No. 3, May 2021, pp. 324-328, DOI : 10.14429/dsj.71.16744 © 2021, DESIDOC 1. IntRoDuCtIon In radio frequency (RF) systems having their interface with vacuum systems, for example, RF coupler in accelerators or RF launchers for plasma heating, vacuum windows are used to isolate the regions having atmospheric pressure and vacuum. Several Ion Cyclotron Resonance Heating (ICRH) systems in tokamaks worldwide have developed vacuum windows as per their requirement 1-4 . These windows or feed throughs utilise ceramic barriers in a cylindrical or conical shape. The choice of shape and material of the ceramic barrier depends on several factors, for example, ceramic material should have low loss tangent (less than 0.005) 5 , better thermal conductivity, minimum strength of electric feld (E-feld) line components parallel to ceramic surface 4 , minimum penetration of E-feld lines within the ceramic volume, etc. The double barrier type vacuum windows are used for enhanced protection against failure of primary vacuum facing barrier, where secondary barrier keeps the vacuum level and protects the chamber from contamination 1-2 . The ion cyclotron heating and current drive (ICH&CD) group at ITER-India is responsible for the development of RF sources for the ITER project 6 . Total 8+1(spare) RF sources will be delivered to coupler 20MW of power to ITER plasma in the frequency range of 36MHz to 60MHz 7 . The ICH&CD group is involved in the development of indigenous transmission line components and test stands of MW level power handling capability 8-9 . As a part of this development program, the ICH&CD group has initiated the development of a coaxial vacuum window for future application in indigenous ITER- like machine. RF Power handling capability of such window will be a few MW for continuous wave (CW) pulse. As the frst step of this development program, the design of a prototype double barrier vacuum window with line size 6-1/8inch EIA is initiated. The following paper discusses the RF design of this double barrier vacuum window. Currently, the material selection is aimed to improve the RF characteristics of the window; no thermal properties are taken into consideration. 2. MoDellIng AnD sIMulAtIon 2.1 For Different Inclination of Ceramic Barrier Initial modelling of double barrier vacuum window was done as shown in Fig. 1 using CST microwave studio (CSTMWS) 10 . The outer conductor (OC) and the inner conductor (IC) were assigned properties of copper (pure) while the ceramic barrier was assigned properties of Alumina (99.5%). This model was simulated for diferent lengths of ceramic barrier (L) while the thickness of the barrier (10mm) was kept constant. Figure 1. Initial model of double barrier vacuum window. Received : 01 February 2021, Revised : 13 February 2021 Accepted : 18 February 2021, Online published : 17 May 2021