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Gradual or abrupt transitions from a coherent mode to quasi-coherent mode (QCM, a type of trapped electron mode turbulence) are observed in the core region of low-density plasmas in Korea Superconducting Tokamak Advanced Research. The coherent mode is initiated in the vicinity of the q = 1 surface by off-axis electron cyclotron resonant heating (ECH), and gradually or abruptly evolves into the QCM as the collisionality is slowly or rapidly increased through electron density control. When the collisionality further increases above a critical level, the QCM is changed into broadband turbulence, which has been preferentially observed in tokamak plasmas. The observed evolution of the QCMs suggests that (1) QCMs can originate from a coherent mode in ECH applied low-collisionality plasmas and (2) quasi-coherent spectra of QCMs are closely related with the collisionality in a finite range ('collisional broadening of the wavenumber and frequency'). The coherent mode appears to be a metastable electrostatic mode at the rational surface excited by off-axis ECH. [ABSTRACT FROM AUTHOR]

A dual-frequency microwave imaging reflectometry system was commissioned to measure both coherent and turbulent electron density fluctuations in KSTAR plasmas. Imaging of the density fluctuations is achieved with an array of 16 vertically aligned detectors and two X-mode probe beam frequencies (tunable over 78–92 GHz between plasma discharges). The system provides the capability of fluctuation measurements with poloidal wavenumbers (kθ) up to ∼3 cm−1 at the maximum sampling rate of 2 MHz. Following extensive laboratory tests, the system was further tested with known coherent density fluctuations during the precursor oscillation of the m/n = 1/1 internal kink mode. The phase information of the reflected beam was compared with the precursor oscillation of the electron temperature measured by an electron cyclotron emission (ECE) radiometer. Density fluctuation levels (δne/ne) at two radial positions separated by the inversion radius (inside and outside) were comparable to temperature fluctuation levels (δTe/Te) from ECE signals. Subsequently, two correlation analysis methods were applied to turbulent fluctuation measurements in a neutral beam heated L-mode plasma to determine the mean poloidal rotation velocities of density fluctuations at two radial positions. The measured mean poloidal velocities were ∼8.4 km s−1 at r/a ∼ 0.6 and ∼5 km s−1 at r/a ∼ 0.7 in the clockwise direction, which differed by 1–2 km s−1 with the projected poloidal velocities from the toroidal rotation velocity measured by charge exchange recombination spectroscopy. [ABSTRACT FROM AUTHOR]

We present experimental observations suggesting that non-diffusive avalanche-like transport events are a prevalent and universal process in the electron heat transport of tokamak plasmas. They are observed in the low confinement mode and the weak internal transport barrier plasmas in the absence of magnetohydrodynamic instabilities. In addition, the electron temperature profile corrugation, which indicates the existence of the shear flow layers, is clearly demonstrated as well as their dynamical interaction with the avalanche-like events. The measured width of the profile corrugation is around , implying the mesoscale nature of the structure. [ABSTRACT FROM AUTHOR]

The bifurcation of perpendicular mode velocity near the pedestal top () at the onset of RMP-driven edge-localized mode (ELM)-crash suppression has been directly measured by using an electron cyclotron emission imaging diagnostic on KSTAR. The ELM crashes are suppressed along with a sudden reduction of , which synchronizes with the transition into and out of the ELM-crash suppression. The change of flow appears mainly responsible for the change of , whose magnitude is the smallest near the normalized flux surface during the ELM-crash suppression. The plasma response to the RMP field was most enhanced in the vicinity of during the ELM-crash suppression and showed a hysteresis behavior with respect to resonant field strength. With these changes, a strong nonlinear coupling between turbulent eddies was observed in the ELM-crash suppression, which is an important feature of the ELM-crash suppression. [ABSTRACT FROM AUTHOR]

Self-generated n = 1 magnetic perturbation from coupled tearing modes results in mitigation of sawtooth crash, i.e. reduction of sawtooth period and crash amplitude by a factor of several 10 or 100 s (to a few ms and a few or 10 s eV, respectively), which is commonly observed prior to plasma disruption in the KSTAR tokamak. Quasi-3D visualization of the mitigation process shows strong correlations between the sawtooth dynamics and the tearing modes: (1) rapid destabilization of internal m/n = 1/1 mode in the core region followed by incomplete relaxation in the presence of the m/n = 2/1 and 3/1 tearing modes, where the spatial orientation of the relaxation channel is aligned with the displacement direction of core flux surfaces due to the tearing modes, and (2) more reduction of the sawtooth period and amplitude for higher amplitude tearing modes. Application of external n = 1 magnetic perturbation further reduces the sawtooth period and amplitude. The sawtooth mitigation is suggested as an additional disruption warning alarm. [ABSTRACT FROM AUTHOR]

We report on observations of bursty mode activity during early neutral beam heating in KSTAR plasmas, before current flat top while the q profile is still evolving. The magnitude of the activity increases with early beam heating, and reduces with the addition of resonant magnetic perturbation magnetic field coils. A mode analysis yields a toroidal mode number of n = 2. The mode is observed to be downward chirping in frequency, and exists for the duration of the slowing-down of the beam. Motional Stark effect constrained equilibrium reconstructions are available at adjacent time slices: we have rescaled the total current to the measured value to obtain the q profile during the mode activity. From this we have computed the mode spectrum and identified a number of candidate gap modes. Wave-particle simulations with plausible distribution functions are computed, which demonstrate that the lowest frequency mode satisfies the condition for wave drive , where is the fast ion diamagnetic drift frequency. An interesting finding is the change from exponential growth of the mode above , whereby the mode continues to nonlinearly grow at a reduced rate over a period of 100 wave periods up to final saturated amplitude. We believe that this may be because the two spatial resonances at s = 0.4 and s = 0.8 overlap for sufficiently high fast ion density, and so the phase-space volume and fast ion density available to drive the mode increases. [ABSTRACT FROM AUTHOR]

The microwave imaging reflectometer (MIR) is the leading diagnostic tool for study of density fluctuations in KSTAR. For last three years since 2014, major components such as the multi-frequency probe beam source, multi-channel detector array, signal processing electronic system, data acquisition system, and optical system have been gradually upgraded. In this paper, the detailed system upgrade with test results in the laboratory and/or plasma is given, and analysis results of a distinctive fluctuation structure referred to as the quasi-coherent mode (QCM) measured by the upgraded MIR system for an L-mode discharge are presented. Cross-coherence analysis with multiple channels shows that the QCM is localized in a core region and appears to be driven by electron temperature gradient for the discharge. [ABSTRACT FROM AUTHOR]

Broadband density fluctuations with peak frequency ranging from 150 to 400 kHz were measured using a multichannel microwave imaging reflectometer in core region of the low-density L-mode plasmas heated by neutral beam injection on KSTAR. These fluctuations have been studied by comparing the dominant mode scales estimated from the measurement with those predicted from linear gyrokinetic simulation. The measured poloidal wavenumbers are qualitatively comparable to those of the ‘fastest growing modes’ from simulations, whereas they are larger than those of the ‘transport-dominant modes’ by about a factor of three. The agreement on wavenumbers between the measurement and linear simulation (for the fastest growing modes) is probably due to sufficiently weak flow shear compared to the maximum linear growth rate. Meanwhile, the transport-dominant modes seem to be related to the fluctuations in lower frequencies (∼80–150 kHz) observed in some of the measurement. [ABSTRACT FROM AUTHOR]

Among various edge localized mode (ELM) crash control methods, only non-axisymmetric magnetic perturbations (NAMPs) yield complete suppression of ELM crashes beyond their mitigation, and thus attract more attention than others. No other devices except KSTAR, DIII-D, and recently EAST have successfully achieved complete suppression with NAMPs. The underlying physics mechanisms of these successful ELM crash suppressions in a non-axisymmetric field environment, however, still remain uncertain. In this work, we investigate the ELM crash suppression characteristics of the KSTAR ELMy H-mode discharges in a controlled multi-spectral field environment, created by both middle reference and top/bottom proxy in-vessel control coils. Interestingly, the attempts have produced a set of contradictory findings, one expected (ELM crash suppression enhancement with the addition of n  =  1 to the n  =  2 field at relatively low heating discharges) and another unexpected (ELM crash suppression degradation at relatively high heating discharges) from the earlier findings in DIII-D. This contradiction indicates the dependence of the ELM crash suppression characteristics on the heating level and the associated kink-like plasma responses. Preliminary linear resistive MHD plasma response simulation shows the unexpected suppression performance degradation to be likely caused by the dominance of kink-like plasma responses over the island gap-filling effects. [ABSTRACT FROM AUTHOR]

Multiple (two or more) flux tubes are commonly observed inside and/or near the q = 1 flux surface in KSTAR tokamak plasmas with localized electron cyclotron resonance heating and current drive (ECH/CD). Detailed 2D and quasi-3D images of the flux tubes obtained by an advanced imaging diagnostic system showed that the flux tubes are m/n = 1/1 field-aligned structures co-rotating around the magnetic axis. The flux tubes typically merge together and become like the internal kink mode of the usual sawtooth, which then collapses like a usual sawtooth crash. A systematic scan of ECH/CD beam position showed a strong correlation with the number of flux tubes. In the presence of multiple flux tubes close to the q = 1 surface, the radially outward heat transport was enhanced, which explains naturally temporal changes of electron temperature. We emphasize that the multiple flux tubes are a universal feature distinct from the internal kink instability and play a critical role in the control of sawteeth using ECH/CD. [ABSTRACT FROM AUTHOR]

A detailed study of edge-localized mode (ELM) dynamics in the KSTAR tokamak is performed using a two-dimensional (2D) electron cyclotron emission imaging (ECEI) diagnostic system. Highly coherent mode structures rotating in the poloidal view plane are routinely observed in the inter-ELM pedestal region where the optical thickness for ECE rapidly changes and the interpretation of emission intensity is complicated. To have confidence on the measurements, the observed images are compared with synthetic images of the ELM structure deduced from three-field BOUT++ simulations. The synthetic process considers instrumental effects of the ECEI diagnostic, intrinsic broadening of the ECE and background noise. The synthetic 2D images highly resemble the observed structure, providing confidence that the ELM dynamics can be visualized by ECEI. [ABSTRACT FROM AUTHOR]

We evaluate the characteristics of global energy confinement in KSTAR (τE,KSTAR) quantitatively in three ways; firstly by comparing it with multi-machine scalings, secondly by deriving multiple regression equations for the L- and the H-mode plasmas, respectively, and lastly by comparing confinement enhancement of the H-mode phase with respect to the L-mode phase in each discharge defined as Hexp. The KSTAR database exhibits τE,KSTAR of ∼0.04 to ∼0.16 s and of ∼0.06 to ∼0.19 s in L-mode and in H-mode plasmas, respectively. The multiple regression equations derived by statistical analysis present the similar dependency on PL and higher dependency on Ip compared with the multi-machine scalings, however the dependency on κ in both L- and H-mode plasmas draw the negative power dependency of κ−0.68 and κ−0.76 for H-mode and for L-mode database, respectively on the contrary to the positive dependency in all multi-machine empirical scalings. It is found that the energy confinement of both L-mode and H-mode of the discharges with Hexp > 1.5 can be well-predicted by multi-machine scalings, τE,89L and τE,92H. Apart from this, the H-mode confinement with 1.5 < Hexp < 2.0 is well-predicted by using the multi-machine empirical L-mode scaling τE,89L. [ABSTRACT FROM AUTHOR]

The accuracy in estimation of two important tearing mode stability parameters (Δ′ and wc) is improved by employing two-dimensional (2D) ECE imaging data which help one to overcome the resolution limit of conventional one-dimensional data. The experimentally measured 2D images are directly compared with synthetic ones from a tearing mode Te model to estimate the parameters and an excellent agreement is achieved. The results imply that the observed tearing mode is classically stable but has non-negligible bootstrap current drive. [ABSTRACT FROM AUTHOR]