2016-12-26T15:11:00

Ion cyclotron emission (ICE) is detected during edge localised modes (ELMs) in the KSTAR tokamak at harmonics of the proton cyclotron frequency in the outer plasma edge. The emission typically chirps downward (occasionally upward) during ELM crashes, and is driven by confined 3MeV fusion-born protons that have large drift excursions from the plasma core. We exploit fully kinetic simulations at multiple plasma densities to match the time-evolving features of the chirping ICE. This yields a unique, very high time resolution (< 1µs) diagnostic of the collapsing edge pedestal density. PACS numbers: 52.35.Hr, 52.35.Qz, 52.55.Fa, 52.55.Tn Understanding the physics of edge localised modes (ELMs) [1][2][3][4] in magnetically confined fusion (MCF) plasmas is crucial for the design of future fusion power plants. The same is true of the physics of the energetic ions born at MeV energies [5,6] from fusion reactions between fuel ions in the multi keV thermal plasma. The crash of an ELM involves impulsive relaxation of the edge magnetic field, releasing energy and particles from the plasma at levels which may not be compatible with sustained operation of the next step fusion experiment, ITER [7,8]. The confinement of fusion-born ions while they release energy, collisionally or otherwise, to the thermal plasma, was a key physics objective of the unique deuterium-tritium plasma experiments in TFTR [9] and JET [10], and will be central to ITERs research programme. Here we report an unexpected conjunction of ELM physics with fusion-born ion physic. We show how this can be exploited as a diagnostic of plasma edge density with unique, very high (< 1µs) time resolution. This is achieved through particle orbit studies combined with first principles kinetic plasma simulations that explain high-time-resolution measurements of ion cyclotron emission (ICE) from the medium-size tokamak KSTAR [11]. We show that ICE from KSTAR deuterium plasmas is driven by a small subset of the fusion-born proton population, originating in the core of the plasma and passing through the edge region where they radiate collectively through the magnetoacoustic cyclotron instability (MCI) [13][14][15][16][17][18][19][20][21][22]. The MCI can occur because of the spatially localised population inversion in velocity space that is caused by the large drift excursions of 3.0 MeV fusionborn protons on deep passing orbits. Our simulations of the MCI in its saturated nonlinear regime show that the frequency spectrum excited depends strongly on the plasma density. By comparing MCI spectra simulated at different densities with high-time-resolution measurements of ICE spectra during ELM crashes in KSTAR, we are able to infer the time evolution of the collapsing edge density at sub-microsecond resolution, which is unprecedented. Recently, ICE has been detected from the outer mid-plane of KSTAR [23-25], with spectral peak frequencies at local proton cyclotron harmonics; see e.g. Fig. 1. The only energetic protons in KSTAR plasmas are produced in deuteron-deuteron (D-D) ...

Multiscale interaction between the magnetic island and turbulence has been demonstrated through simultaneous two-dimensional measurements of turbulence and temperature and flow profiles. The magnetic island and turbulence mutually interact via the coupling between the electron temperature (T e ) gradient, the T e turbulence, and the poloidal flow. The T e gradient altered by the magnetic island is peaked outside and flattened inside the island. The T e turbulence can appear in the increased T e gradient regions. The combined effects of the T e gradient and the the poloidal flow shear determine two-dimensional distribution of the T e turbulence. When the reversed poloidal flow forms, it can maintain the steepest T e gradient and the magnetic island acts more like a electron heat transport barrier. Interestingly, when the T e gradient, the T e turbulence, and the flow shear increase beyond critical levels, the magnetic island turns into a fast electron heat transport channel, which directly leads to the minor disruption. PACS numbers:

Highly collimated, plasma-filled magnetic flux tubes are frequently observed on galactic, stellar and laboratory scales. We propose that a single, universal magnetohydrodynamic pumping process explains why such collimated, plasma-filled magnetic flux tubes are ubiquitous. Experimental evidence from carefully diagnosed laboratory simulations of astrophysical jets confirms this assertion and is reported here. The magnetohydrodynamic process pumps plasma into a magnetic flux tube and the stagnation of the resulting flow causes this flux tube to become collimated.The extreme collimation of astrophysical jets [1,2,3] and the solar corona heating mechanism [4] are two seemingly unrelated astrophysical mysteries, yet both involve collimation of magnetic flux tubes. Astrophysical observations [2,3] and simulations [1,5] indicate that bipolar plasma outflows (jets) are natural [1,6] features of young stellar objects, black holes, active galactic nuclei and even aspherical planetary nebula [7]. Although it has long been presumed [8,9] that astrophysical jets are magnetohydrodynamically driven, the standard models do not agree on a single collimation process. A similar issue exists in solar physics: solar spicules [10], prominences [11,12] and coronal loops [13] are considered to be plasma-filled filamentary magnetic flux tubes; coronal heating models [14,15] then invoke magnetic reconnection and plasma flow within such filamentary loops. However, the models explain neither the origin of the observed flows nor the extreme collimation (filamentary nature) of the observed structures.We propose that the collimation of any, initially flared, current-carrying magnetic flux tube is due to the following process [16]: a magnetohydrodynamic (MHD) force resulting from the flared current profile drives axial plasma flows along the flux tube; the flows convect frozen-in magnetic flux from strong magnetic field regions to weak magnetic field regions; flow stagnation then piles up this embedded magnetic flux, increasing the local magnetic field and collimating the flux tube via the pinch effect. Thus, the flux tube fills with ingested plasma and simultaneously becomes collimated. This paper presents direct experimental evidence for this process. We use ultra-high-speed imaging and Doppler measurements of the fast plasma flows, combined with direct density measurements before and after the filling of the flux tube.Our experimental setup [17] simulates magneticallydriven astrophysical jets at the laboratory scale by imposing boundary conditions analogous to astrophysical jet boundary conditions (Fig. 1): a disk (cathode) representing a central object such as a star, is coaxial and co-planar with an annulus (anode) representing an accretion disk. A vacuum poloidal magnetic field produced by an external coil links these two electrodes, mimicking a poloidal magnetic field threading the accretion disk. A radial electric field applied across the gap between the FIG. 1: (log color) Typical plasma discharge sequence (#6577, 2 million fps, 40 ns/...

The effect of static n ¼ 1 resonant magnetic perturbation (RMP) on the spatial structure and temporal dynamics of edge-localized modes (ELMs) and edge turbulence in tokamak plasma has been investigated. Two-dimensional images measured by a millimeter-wave camera on the KSTAR tokamak revealed that the coherent filamentary modes (i.e., ELMs) are still present in the edge region when the usual large scale collapse of the edge confinement, i.e., the ELM crash, is completely suppressed by n ¼ 1 RMP. Cross-correlation analyses on the 2D images show that (1) the RMP enhances turbulent fluctuations in the edge toward the ELM-crash-suppression phase, (2) the induced turbulence has a clear dispersion relation for wide ranges of wave number and frequency, and (3) the turbulence involves a net radially outward energy transport. Nonlinear interactions of the turbulent eddies with the coexisting ELMs are clearly observed by bispectral analysis, which implies that the exchange of energy between them may be the key to the prevention of large scale crashes.