In this research, we investigate ECRH for low-energy electrons analytically and numerically, which is applicable to ECRH-assisted plasma start-up in a tokamak. From the previous experimental studies, it is wellknown that the first harmonic ECRH is more effective than the second harmonic ECRH. In this work, we developed an analytic model of ECRH in the start-up process and present comparisons of the efficiency between the first harmonic and the second harmonic. It is found that electrons gain a large energy from the waves at first harmonic resonance up to several hundred eV. However, electrons gain a large energy from the waves only up to ∼10 eV and energy-gain starts decreasing afterward at second harmonic resonance. When seed electrons are heated from the room temperature to far away above the ionization energy, seed electrons can bring about an avalanche of electrons. Thus, pre-ionization with the second harmonic can be delayed since electrons need more time to be heated up to the breakdown temperature due to the slow heating speed compared to the first harmonic ECRH.
This paper reports on experimental evidence that shows perpendicular electron cyclotron resonance heating (ECRH) can trigger classical tearing modes when deposited near a rational flux surface. The complex evolution of an m = 2 island is followed during current ramp-up in KSTAR plasmas, from its initial onset as the rational surface enters the ECRH resonance layer to its eventual lock on the wall after the rational surface leaves the layer. Stability analysis coupled to a transport calculation of the current profile with ECRH shows that the perpendicular ECRH may play a significant role in triggering and destabilizing classical m = 2 tearing modes, in agreement with our experimental observation.
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