Coexisting multi-geodesic acoustic modes (GAMs), especially coexisting dual GAMs, are observed and studied through Langmuir probe arrays at the edge plasmas of the HT-7 tokamak with lithium-coated walls. The dual GAMs are named a low-frequency GAM (LFGAM) and a high-frequency GAM (HFGAM), and it is found that within the measuring range, the HFGAM propagates outwards while the LFGAM propagates both inwards and outwards with their central frequencies nearly unchanged, and both modes have maximum amplitudes at positions with radial wavenumbers close to zero; meanwhile, the two positions happen to be where the continuum GAM frequency is closest to the central frequencies of the LFGAM and the HFGAM. These characteristics are consistent with those of a kinetic GAM converted from a continuum GAM. The nonlinear couplings between the LFGAM and the HFGAM are also analysed. In this study, we observed not only the interaction between the LFGAM and the HFGAM, but also the self-coupling of the GAM with the beat frequency between them, as well as the coupling between the LFGAM and an unknown mode at ∼50 kHz. These nonlinear interactions may play important roles during the saturation process of GAMs. Additionally, amplitude correlation analyses of multi-GAMs indicate that second harmonic GAMs are probably generated from the self-interaction of fundamental GAMs.

By analyzing large quantities of discharges in the unfavorable ion B ×∇B drift direction, the I-mode operation has been confirmed in EAST tokamak. During the L-mode to I-mode transition, the energy confinement has a prominent improvement by the formation of a high-temperature edge pedestal, while the particle confinement remains almost identical to that in the L-mode. Similar with the I-mode observation on other devices, the E r profiles obtained by the eight-channel Doppler backscattering system (DBS8)[1] show a deeper edge E r well in the I-mode than that in the L-mode. And a weak coherent mode (WCM) with the frequency range of 40-150 kHz is observed at the edge plasma with the radial extend of about 2-3 cm. WCM could be observed in both density fluctuation and radial electric field fluctuation, and the bicoherence analyses showed significant couplings between WCM and high frequency turbulence, implying that the E r fluctuation and the caused flow shear from WCM should play an important role during I-mode. In addition, a low-frequency oscillation with a frequency range of 5-10 kHz is always accompanied with WCM, where GAM intensity is decreased or disappeared. Many evidences show that the a low-frequency oscillation may be a arXiv:1902.04750v3 [physics.plasm-ph]

The Madison plasma dynamo experiment (MPDX) is a novel, versatile, basic plasma research device designed to investigate flow driven magnetohydrodynamic instabilities and other high-b phenomena with astrophysically relevant parameters. A 3 m diameter vacuum vessel is lined with 36 rings of alternately oriented 4000 G samarium cobalt magnets, which create an axisymmetric multicusp that contains $14 m 3 of nearly magnetic field free plasma that is well confined and highly ionized (>50%). At present, 8 lanthanum hexaboride (LaB 6 ) cathodes and 10 molybdenum anodes are inserted into the vessel and biased up to 500 V, drawing 40 A each cathode, ionizing a low pressure Ar or He fill gas and heating it. Up to 100 kW of electron cyclotron heating power is planned for additional electron heating. The LaB 6 cathodes are positioned in the magnetized edge to drive toroidal rotation through J Â B torques that propagate into the unmagnetized core plasma. Dynamo studies on MPDX require a high magnetic Reynolds number Rm > 1000, and an adjustable fluid Reynolds number 10 < Re < 1000, in the regime where the kinetic energy of the flow exceeds the magnetic energy (M 2 A ¼ ðv=v A Þ 2 > 1). Initial results from MPDX are presented along with a 0-dimensional power and particle balance model to predict the viscosity and resistivity to achieve dynamo action. V C 2014 AIP Publishing LLC.

A Doppler reflectometer system has recently been installed in the Experimental Advanced Superconducting (EAST) Tokamak. It includes two separated systems, one for Q-band (33-50 GHz) and the other for V-band (50-75 GHz). The optical system consists of a flat mirror and a parabolic mirror which are optimized to improve the spectral resolution. A synthesizer is used as the source and a 20 MHz single band frequency modulator is used to get a differential frequency for heterodyne detection. Ray tracing simulations are used to calculate the scattering location and the perpendicular wave number. In EAST last experimental campaign, the Doppler shifted signals have been obtained and the radial profiles of the perpendicular propagation velocity during L-mode and H-mode are calculated.

One of the systems planned for the measurement of electron density in ITER is a multi-channel tangentially viewing combined interferometer-polarimeter (TIP). This work discusses the current status of the design, including a preliminary optical table layout, calibration options, error sources, and performance projections based on a CO2/CO laser system. In the current design, two-color interferometry is carried out at 10.59 μm and 5.42 μm and a separate polarimetry measurement of the plasma induced Faraday effect, utilizing the rotating wave technique, is made at 10.59 μm. The inclusion of polarimetry provides an independent measure of the electron density and can also be used to correct the conventional two-color interferometer for fringe skips at all densities, up to and beyond the Greenwald limit. The system features five chords with independent first mirrors to reduce risks associated with deposition, erosion, etc., and a common first wall hole to minimize penetration sizes. Simulations of performance for a projected ITER baseline discharge show the diagnostic will function as well as, or better than, comparable existing systems for feedback density control. Calculations also show that finite temperature effects will be significant in ITER even for moderate temperature plasmas and can lead to a significant underestimate of electron density. A secondary role TIP will fulfill is that of a density fluctuation diagnostic; using a toroidal Alfvén eigenmode as an example, simulations show TIP will be extremely robust in this capacity and potentially able to resolve coherent mode fluctuations with perturbed densities as low as δn∕n ≈ 10(-5).

Doppler backscattering system can measure the perpendicular velocity and fluctuation amplitude of the density turbulence with intermediate wavenumber. An eight-channel Doppler backscattering system has been installed in the Experimental Advanced Superconducting Tokamak (EAST), which can probe eight different radial locations simultaneously by launching eight fixed frequencies (55, 57.5, 60, 62.5, 67.5, 70, 72.5, 75 GHz) into plasma. The quasi-optical system consists of circular corrugated waveguide transmission, a fixed parabolic mirror, and a rotatable parabolic mirror which are integrated with quasi-optics front-end of the profile reflectometer inside the vacuum vessel. The incidence angle can be chosen from 5° to 12°, and the wavenumber range is 2-15/cm with the wavenumber resolution Δk/k≤0.21. Ray tracing simulations are used to calculate the scattering locations and the perpendicular wavenumber. The dynamic range of this new eight-channel Doppler backscattering system can be as large as 40 dB in the EAST. In this article, the hardware design, the ray tracing, and the preliminary experimental results in the EAST will be presented.

Magnetic reconnection is a process of energy conversion from magnetic field to plasmas. During magnetic reconnection, topologies of magnetic fields change and magnetic energy is released to particle heating and acceleration (Birn & Priest, 2007;Yamada et al., 2010). It provides explanations for many explosive phenomena in space plasmas, such as solar flares (Masuda et al., 1994), coronal mass ejections (Lin & Forbes, 2000), and geomagnetic substorms (Baker et al., 1996;Kepko et al., 2015). Laboratory experiments have also reported occurrences of magnetic reconnection (Yamada et al., 1994).One key question for magnetic reconnection is the energy conversion. Hesse (2005, 2010) studied the energy budgets in magnetic reconnection using MHD and localized particle-in-cell simulations. They concluded that the Poynting flux from the tail lobe is converted to thermal energy through bulk kinetic energy as a mediator. Using hybrid simulations, Aunai et al. ( 2011) discovered that ions are inclined to gain thermal energy instead of kinetic energy. Q. Lu, Lu, Huang, Wu, and Wang (2013) and S. Lu, Lu, Huang, and Wang (2013) used two-dimensional (2D) particle-in-cell (PIC) simulations to study the energy conversion of electrons near the reconnection site and in the magnetic island and found that electrons are heated and form the enthalpy flux flowing toward the magnetic island. Eastwood et al. (2013) also verified that the ion enthalpy flux is dominant in the partition of energy flux through satellite observations. Experiments on energy conversion in magnetic reconnection have also been carried out (Yamada et al., 2014(Yamada et al., , 2015. They found that half of the magnetic energy is converted to particles, 2/3 of which is given to ions, and 1/3 to electrons.However, when zoomed out from the reconnection region to the global-scale phenomena, such as geomagnetic substorms, it has been found by Angelopoulos et al. (2013) that the energy conversion during substorms predominantly occurs at reconnection fronts, whereas the energy conversion at reconnection site itself is much less. Using PIC simulations, Goldman et al. (2015) also showed that energy conversion

A double-pass, radially-viewing, multichannel far-infrared (FIR) polarimeter/interferometer system is under development for current density profile and electron density profile measurements in the EAST tokamak. The system utilizes three 432.5 µm CW formic acid FIR lasers pumped by three CO 2 lasers. Each of the three FIR lasers can generate high output power of more than 30 mW per cavity. Two lasers, with slight frequency offset (∼ 1 MHz), will be made collinear with counter-rotating circular polarization in order to determine the Faraday effect by measuring their phase difference. The third laser also frequency offset, will be used as a reference providing local oscillator (LO) power to each mixer so that one can obtain the phase shift caused by the plasma electron density. Novel molybdenic retro-reflectors with shutter protection have been designed and will be mounted on the inner vessel wall in EAST. The retro-reflectors can withstand baking temperature up to 350 • C and discharge duration more than 1000 s. Vibrations and path length changes due to thermal expansion will be compensated using a He-Ne interferometer as the second color. VDI planar-diode Integrated Conical Horn Fundamental Mixers optimized for high sensitivity, typical 750 V/W, will be used. Initially a five-chord system will be installed in 2013 and an eleven-chord system will be implemented on the core region of EAST plasmas. MHz frequency response allows system to resolve fast MHD events such as tearing/neoclassical tearing, disruptions and fast-particle modes. Preliminary design will be presented.