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Resonant error fields with toroidal harmonics n = 1 and n = 2 in otherwise axisymmetric tokamak plasmas can drive disruptive locked modes when their strength exceeds a threshold value. This paper presents the sensitivity of error field threshold in global parameters such as line-averaged density and toroidal magnetic field as well as toroidal harmonics dependence in KSTAR L-mode plasmas. The error field threshold for mode-locking increases along with density up to a roll-over density, which coincides well with the linear ohmic confinement (LOC) to saturated ohmic confinement (SOC) density for the studied KSTAR plasmas. This non-monotonic density correlation of the error field threshold in LOC–SOC can be explained by a modification of existing theoretical scaling in SOC, while plasma rotation can also play an important role in setting the resonant error field threshold. [ABSTRACT FROM AUTHOR]

An overview of the physics of intrinsic torque is presented, with special emphasis on the phenomenology of intrinsic toroidal rotation in tokamaks, its theoretical understanding, and the variety of momentum transport bifurcation dynamics. Ohmic reversals and electron cyclotron heating-driven counter torque are discussed in some detail. Symmetry breaking by lower single null versus upper single null asymmetry is related to the origin of intrinsic torque at the separatrix. [ABSTRACT FROM AUTHOR]

Abstract: Lower hybrid current drive (LHCD) efficiency in the HT-7 tokamak is observed to increase about 30% with ion Bernstein wave (IBW) heating. The current drive efficiency as high as has been reached in HT-7 thanks to an LHCD/IBW synergy. The IBWs are significantly coupled to the fast electrons produced by the lower hybrid waves, thus results in higher fast electron photon temperature in the combined LHCD+IBW scenario. The interaction of LHCD+IBW improved current drive efficiency. [Copyright &y& Elsevier]

A high spectra resolution X-ray imaging crystal spectrometer has been implemented on J-TEXT Tokamak for the measurements of K α spectra of helium-like argon and its satellite lines. The wavelength range of K α spectra of helium-like argon is from 3.9494 Å to 3.9944 Å that includes the resonance line w, intercombination lines x and y , forbidden line z and numerous satellite lines, referenced using standard Gabriel notation. In low-density discharge, the intensity of q , r satellite lines and z lines can be significantly enhanced by non-thermal electrons. Non-thermal electrons are produced due to the low plasma density. The high hard X-ray flux from NaI detector and significant downshift electron cyclotron emissions from energetic runaway electrons also indicated that there is a large population of runaway electrons in the low-density discharge. The non-thermal part of electrons can affect the excitation/transition equilibrium or ionization/recombination equilibrium. The q line is mainly produced by inner-shell excitation of lithium-like argon, and the r line is partially produced by inner-shell excitation of lithium-like argon and dielectronic recombination of helium-like argon. [ABSTRACT FROM AUTHOR]

Abstract: The energy limit of runaway electrons in the HT-7 tokamak is investigated by measuring the synchrotron radiation originated from the runaway electrons and the hard X-ray radiation (HXR) when they hit the first wall. An upper boundary on the runaway energy can appear due to the resonance between the electron gyromotion and the magnetic field ripple in the low field side. The experimental derived maximum energy in the core is about 26 MeV, and maximum energy in the edge region is blocked to no more than 5 MeV. This resonance interaction of runaways with the nth harmonic of the magnetic field ripple can account for the observed energy gap of the runaways. [Copyright &y& Elsevier]

Abstract: In the HT-7 superconducting tokamak, long-pulse discharge was generated by synergy of high power LHCD (300–600kW, 2.45GHz) and high power IBW (200–350kW, 27MHz) heating. Recently, a new set of actively cooled toroidal double-ring graphite limiters at bottom and top of the vacuum vessel has been developed for long-pulse operation in the HT-7. Based on the understanding of plasma surface interactions, nearly full non-inductive current driven plasma (I p =120–180kA, B T =1.5–2T, T e(0)=2–4keV, and 〈n e〉=1–2.5×1019 m−3) was achieved with the operational pulse length of 1–10s. For lower performance operation, more than 60s of long-pulse plasma was obtained by the LHCD in 2003 on the HT-7 tokamak. [Copyright &y& Elsevier]

The charge-exchange neutral particles fluxes and energy distribution in IBW heated plasma were investigated in the HT-7 tokamak. The RF frequency was 30MHz and with an injecting power up to 200kW. It is observed that the plasma performance is obviously enhanced by IBW heating. The electron temperature was increased by 0.5keV and the central line averaged electron density was doubled. The neutral particle fluxes of high-energy increased and the bulk ions were heated during IBW heating. The ion temperature was increased by 0.3keV and the ion heating efficiency of (2–3) eVkW-1×1013cm-3 was achieved. The velocity distribution of charge-exchanged neutral particles appears to be Maxwellian without high-energy tail ions up to the maximum RF power. [Copyright &y& Elsevier]

Abstract: The first systematic assessment of divertor performance was made in the EAST superconducting tokamak for both single null (SN) and double-null (DN) divertor configurations under Ohmic and L-mode plasma conditions. Particle and heat fluxes are higher at outboard divertor targets than those at inboard divertor targets for both SN and DN. However, the DN divertor configuration exhibits a much stronger in-out asymmetry favoring outer divertors. DN operation also leads to an up-down asymmetry with higher particle fluxes to the divertors with their ∇ B drift toward the X-point. Reversing toroidal field direction shows a strong influence on the divertor asymmetries. The plasma exhibits a large amplitude broadband turbulence in the outer divertors for both SN and DN, which is significantly reduced in the inner divertors, especially for DN. This coincides with a broad divertor SOL profile on the outboard side. Initial comparisons are also made with SOLPS-B2/EIRENE modeling. [Copyright &y& Elsevier]