Design of an ech system for a small modular stellarator

Electron cyclotron heating (ECH) is one of the most common methods used for heating stellarator plasmas, providing both ionization for startup and net-current free heating during the discharge. This paper presents the design of the ECH heating system for the Stellarator of Costa Rica (SCR-1), a small modular stellarator under construction at the Instituto Tecnológico de Costa Rica (ITCR). The SCR-1 is a small (Major radius 0.238 m, mean plasma radius 0.042 m) low budget device, using a 2 period magnetic field with flat rotational transform profile (ı=0.3), a resonant strength of 8.78 × 10^-2 T and expected to achieve a 15 eV temperature with an electron line density of 3.74×10¹⁵ m^-3. The heating system will eventually be composed of two magnetrons operating in CW mode (TE₁₀) at the first harmonic frequency of 2.45 GHz. The first phase of the project will see a 2 kW system installed, with a future addition of a second 3 kW magnetron on a separate beam-line to give a nominal total of 5 kW output power. The current design focuses on the first magnetron and beam-line which will consist of a three phase power supply unit with a microcontroller for over current detection and power percentage control and an Alter Power Systems water cooled CW magnetron working at 2.45 GHz with a max power output of 2 kW. It will be carried to the torus through a waveguide with an R26/WR 340 industrial isolation system with water load and a monitoring probe for magnetron safeguard, a directional coupler measurement system and an aluminum WR 340, 3 stub tuner for impedance coupling finally to meet the torus quartz λ/2 window. The main aim of this design is to optimize the efficiency of the energy transfer process by reducing the refraction and optimal tuning of the impedance coupling; and to ensure suitable skin depth to heat the core of the plasma. We present calculations of skin depth, vector refraction ind- - ex, reflective index and complex refractive index, in a characteristic stellarator density profile. The analysis is made with the aid of software simulations to obtain the magnetic surfaces and thus optimize the selection of ECH power injection angles.