Noninductive plasma current start-up using 2nd harmonic electron cyclotron resonance heating (ECRH) with oblique radio frequency (RF) injection is demonstrated in a Q-shu University experiment with steady-state spherical tokamak. A strong transition was observed in the heating and plasma current ramp-up. The initial bulk electron heating regime exhibits T ebulk ∼ 140 eV and no hard x-ray (HXR) emission with a low I p of ∼15 kA; it abruptly transitions to a regime that exhibits a low T ebulk of ∼10 eV and a strong HXR emission with a high I p of ∼50 kA. This behavior is distinctly different from that observed in previous fundamental ECRH experiments. The mechanism of the heating and current drive transition are investigated considering wave power absorption and plasma power balance. The results indicate that the transition is caused by the favorable heating of tail electrons where the RF power absorption at the 2nd harmonic increases nearly linearly with T etail , while the power transfer from the tail electrons to the bulk electrons decreases with 1/T etail 0.5 . This causes a rapid transition to a state with high T etail while reducing T ebulk towards colder ion temperature. The understanding of the transition mechanism helps to consider plasma current start-up using 2nd harmonic ECRH for tokamak reactors such as JT-60 SA and ITER.
The spatial distribution of the hydrogen atom density was evaluated in a spherical tokamak (ST) plasma sustained only with 28 GHz electron cyclotron heating (ECH). The radially resolved Hδ emissivity was measured using multiple viewing chord spectroscopy and Abel inversion. A collisional-radiative (CR) model analysis of the emissivity resulted in a ground-state hydrogen atom density of 1015–1016 m−3 and an ionization degree of 1–0.85 in the plasma.
Toroidal electron cyclotron resonance (ECR) plasma is an ECR heated plasma in an open toroidal magnetic field. The plasma contains no toroidal current and is used for the pre-ionization and non-inductive startup of a tokamak plasma. To obtain a deeper understanding of the basic properties of the plasmas produced in the spherical tokamak QUEST (Q-shu University Experiment with steady-state Spherical Tokamak), we have developed an optical emission spectroscopy system with multiple viewing chords and have used it to measure the spatial distributions of the toroidal and poloidal flow velocities of C 2+ ions. We compare the measured velocities with those calculated from the ion drift equations using the plasma parameters reported for a similar spherical tokamak, LATE (the Low Aspect Ratio Torus Experiment).
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