ECH-Assisted Plasma Startup in KSTAR

Abstract

One of the major issues in fusion plasma is the development of an operation scenario for a robust and reliable plasma startup in a fully superconducting tokamak which has a limited toroidal electric field. For the International Thermonuclear Experimental Reactor (ITER), low loop voltage plasma startup using ECH pre-ionization was proposed, and many studies on this method were carried out in other devices. Korea Superconducting Tokamak Advanced Research (KSTAR) demonstrated the electron cyclotron heating (ECH)-assisted startup using 84/110-GHz second harmonic EC wave which provided the reproducibility of plasma initiation during the first and second plasma campaigns. But in KSTAR’s third plasma campaign in 2010, a comparative study of flux consumption between pure Ohmic and second harmonic ECH-assisted startup is carried out to quantify the effect of ECH assistance on plasma startup in a fully superconducting tokamak.In ECH-assisted startup using the second harmonic pre-ionization (with a ramp rate of 0.49 MA/s, prefill pressure of 1.6 × 10-5 mbar, and X-mode EC wave of 300 kW), the loop voltage is reduced from 3.7 V (0.33 V/m) to 3.2 V (0.28 V/m) as compared to the pure Ohmic startup. The flux consumption is decreased by 14 % at the end of ECH pulse. ECH-assisted ramp-up after Ohmic breakdown is proposed in order to maintain the constant radial position of plasma during current ramp-up and to minimize the production of carbon impurities by ECH. In the ECH-assisted ramp-up after Ohmic breakdown (with a ramp rate of 0.45 MA/s, prefill pressure of 1.4 × 10-5 mbar, and X-mode EC wave of 300 kW), flux consumption for reaching the current flat top (which means that loop voltage becomes almost zero) is reduced by 14 %.To study the effect of ECH-assistance on plasma startup, a pre-ionization code (0D code) is used to analyze the burnthrough and the current ramp-up using ECH pre-ionization. In conditions similar to the experiments of the ECH-assisted startup using pre-ionization, the resistive voltage is reduced from 1.31 V to 1.23 V at the middle of the ECH pulse and the flux consumption is reduced by 5.5 % at the end of ECH pulse as compared to the pure Ohmic startup. In the ECH-assisted ramp up after Ohmic breakdown, the flux consumption is reduced by 2.8 % at the end of ECH pulse. All of the simulation results show the similar behaviors as the experimental results except for the injection mode scan and the ramp-rate scan. These differences could be resulted from the failure of the plasma radial position control during the plasma startup.