Confinement and ELM characteristics of H-mode plasmas in KSTAR

Abstract

The latest results of confinement and edge-localized mode (ELM) characteristics of Korea Superconducting Tokamak Advanced Research (KSTAR) H-mode plasmas are reported. The estimation of fast ion contribution to the total stored energy, calculated by both the NUBEAM and ASTRA simulations, and of the effective total heating power is used to derive the thermal energy confinement time (τ E,thermal), which is compared with a multi-machine database. The measured power threshold for the L–H transition (P thr) as a function of density shows a roll-over with minimum value at . KSTAR H-mode plasmas exhibit three distinctive types of ELMs: large type-I ELMs, intermediate ELMs and a mixed (type-I and small ELM peaks) ELM regime. Power scans show that the frequency of the large ELMs increases with increasing heating power, a feature of type-I ELMs. The quality of confinement is higher for type-I and mixed ELMy H-mode (H 98(y,2) ∼ 0.9–1) than for the intermediate ELM regime (H 98(y,2) ∼ 0.7). Type-I ELMs have precursor-like signals from the magnetics measurement, while the other two ELM types do not. The low-field side (LFS) profile of electron temperature (T e), from the ECE measurement, and the pedestal profile of the toroidal velocity (V t), from charge-exchange spectroscopy, show a continuous build up on the LFS during the inter-ELM period. However, the pedestal ion temperature (T i) remains unchanged for most of the inter-ELM period until it rapidly rises in the last stage of the ELM cycle (⩾70–80%). The estimated electron pedestal collisionality for a type-I ELMy regime is . The confinement and ELM characteristics for the ELM suppression discharges by the application of an n = 1 magnetic perturbation (MP) have also been investigated for each of the identified stages during the MP application. A second L–H transition during the L-mode phase after the end of first H-mode stage occurs for some discharges when the divertor configuration is restored by the plasma control system. Characteristics of this late H-mode are compared with those for the main H-mode.

A semi-analytic power balance model for low (L) to high (H) mode transition power threshold

“…1 (using parameters in Table I and C TEP evaluated from experimental data) agrees well with experimental results reported in Ref. 12, while that calculated from the ) is obtained by fitting the data points using the KSTAR parameters summarized in Table I. Semi-analytic P th shows a good agreement with experimental results, while empirical one shows a considerable deviation at low density regime.…”

“…In this work, we estimate C TEP using the KSTAR L-H transition data published in Ref. 12. Table I summarizes numerical values for various plasma parameters in Eqs.…”

“…1 represent selected experimental data points reproduced from Ref. 12. For comparison, we also show P th evaluated from the empirical scaling law, Eq.…”

“…A recent study in this line shows that P th depends primarily on average plasma density ( n), toroidal magnetic field (B T ), and plasma surface area (S) Equation (1) successfully reproduces experimental results of P th for high density regime. However, deviations of P th from the prediction of this empirical law have been observed in experiments, most notably, the increase of P th below a "roll-over" density (n min ) [12][13][14][15] (i.e., P th ' n Àa with a > 0, when n n min ). In most of the machines, the roll-over density is observed to be in the range 0.1-0.3 Â 10 20 m…”

Ion temperature and toroidal velocity edge transport barriers in KSTAR

“…CES is used to measure the carbon VI spectral line at 529.05 nm. The measurement of the ion temperature profiles has been used to classify L-and H-mode KSTAR plasmas [14,15]. The KSTAR CES system consists of a spectroscopic system looking at the KSTAR neutral beam which is modulated at 5 Hz to provide ion temperature and toroidal rotation profile measurement [16,17].…”