Study on ECH-assisted start-up using trapped particle configuration in KSTAR and application to ITER

Abstract

Efficient and reliable ECH (Electron Cyclotron resonance Heating)-assisted start-up using TPC (trapped particle configuration) is demonstrated in conventional, superconducting tokamak, KSTAR and analyzed by a plasma start-up simulator. Characteristics of TPC start-up under the start-up conditions of the magnetic field quality and the prefill pressure are studied. Stable plasma start-up under ITER-relevant condition is achieved in KSTAR and the ITER start-up scenario using TPC is proposed to replace FNC (field null configuration). ECH-assisted plasma start-up has been proposed to overcome the difficulties of plasma initiation phase in future tokamaks, such as JT60-SA and ITER. Due to the limitation of superconducting solenoid coil operations, the toroidal electric field applied by an inductive way is lower than that of present tokamak devices. And large eddy current and complex ferromagnetic structure makes it difficult to control the poloidal magnetic field. So tokamak plasma initiation with ECH-assistance has been studied to supplement the limited inductive input power and to relax the criteria related to the poloidal magnetic field quality. Many researches have shown that the poloidal magnetic structure makes an efficient ECH-assisted start-up, instead of the conventional FNC. The TPC, recently developed in VEST (Versatile Experiment Spherical Torus), has shown enhanced confinement of ECH pre-ionization plasmas and based on them, efficient tokamak plasma formation with an expanded operation window is achieved. Applicability of TPC needs to be demonstrated to large, conventional, and SC tokamaks. It is noticed that TPC could help to overcome the defects of start-up based on FNC in JT60-SA or ITER, but it needs to be validated. However, there are some concerns when applying TPC to large, conventional, and SC (superconducting) tokamaks such as KSTAR. First, the PF (Poloidal Field) coils to generate the TPC field structure need to be carefully chosen with consideration of the initial charging current of PF coils. Second, the reduced particle trapping fraction effect due to the intrinsically low magnetic mirror ratio of large aspect ratio tokamaks need to be tested. Third, the operation window of the TPC poloidal field strength and initial deuterium prefill pressure needs to be identified. The plasma start-up experiments using TPC has been conducted in KSTAR for feasibility study and finding the operation regime of TPC. The plasma start-up scenario based on FNC is replaced by TPC by overlapping the mirror-like structure on the null. The 2nd harmonic, X-mode ECH system is used for pre-ionization of plasmas. 170 GHz of wave frequency with injection power of 600 kW is applied with toroidal injection angle of 20 degree, co-current direction. Applicability of TPC in conventional tokamak with improved efficiency than FNC is clearly demonstrated by experiments. The feasibility of TPC in low particle trapping ratio is analyzed through 0-d modeling, TECHP0D. The plasma confinement model is improved to represent the reduced convective loss due to the mirror effect. Based on this model, 35 % reduction of the convective loss along the magnetic field line is required to reproduce the experimental pre-ionization condition. The reduction fraction is similar value of the trapped particle fraction which is calculated by the guiding center averaged single particle calculation. Then, operation window of the TPC start-up is identified in KSTAR 2015 and 2016 campaign. With 16 shots using TPC, an operation diagram in terms of the magnetic pitch which represent magnetic field quality and the deuterium prefill pressure is obtained. The diagram shows a broader operation range of TPC for both magnetic pitch and prefill pressure than that of FNC. The magnetic pitch and the prefill pressure can be increased by 5 times and 1.5 times, respectively, compared to conventional FNC. All shots using the TPC scheme in KSTAR 2015 and 2016 campaign are used to find the operation limits of TPC. The diagram in terms of the magnetic pitch and the deuterium prefill pressure shows two operation limits of TPC

a high prefill limit that is independent of the magnetic pitch and a low prefill limit that is linearly dependent on the magnetic pitch have been found. The parameter scan experiment shows two conditions in which the plasma start-up is failed. One is in a low ionization rate state and the other is in a low ionization density state. The operation limits are analyzed by the TECHP0D code. Low pre-ionization density case, the high electron temperature is achieved due to the reduced radiation loss. The radiative barrier is overcome by only the ECH power. However, the high electron temperature makes shorter confinement time than the closed flux surface formation time and the plasma start-up is failed. These conditions are confirmed by the experimental data with a low ionization density. The low ionization rate makes energy loss by ionization reaction after solenoid swing down. The energy loss delays the rise of the electron temperature and increases the plasma resistance. As a result, the plasma start-up was failed. Applicability of TPC start-up to ITER is studied. TPC start-up with a reduced toroidal electric field to ITER-level is conducted and successful plasma formation is observed, whereas with FNC, the start-up is failed. ECH power of 700 kW is used, and the toroid electric field during the entire start-up phase is kept less than the target value of ITER of 0.3 V / m. The TPC structure can be generated by ITER PF coils of #1 and #6, even though the up-down asymmetry of position and turn-number of coils. The newly developed start-up scenario using TPC is proposed to ITER and compared to the conventional FNC based scenario. Saving the solenoid flux and stable superconducting coil operation will be expected.