Since the first H-mode discharges in 2010, the duration of the H-mode state has been extended and a significantly wider operational window of plasma parameters has been attained. Using a second neutral beam (NB) source and improved tuning of equilibrium configuration with real-time plasma control, a stored energy of W tot ∼ 450 kJ has been achieved with a corresponding energy confinement time of τ E ∼ 163 ms. Recent discharges, produced in the fall of 2012, have reached plasma β N up to 2.9 and surpassed the n = 1 ideal no-wall stability limit computed for H-mode pressure profiles, which is one of the key threshold parameters defining advanced tokamak operation. Typical H-mode discharges were operated with a plasma current of 600 kA at a toroidal magnetic field B T = 2 T. L-H transitions were obtained with 0.8-3.0 MW of NB injection power in both single-and double-null configurations, with H-mode durations up to ∼15 s at 600 kA of plasma current. The measured power threshold as a function of lineaveraged density showed a roll-over with a minimum value of ∼0.8 MW at ne ∼ 2×10 19 m −3 . Several edge-localized mode (ELM) control techniques during H-mode were examined with successful results including resonant magnetic perturbation, supersonic molecular beam injection (SMBI), vertical jogging and electron cyclotron current drive injection into the pedestal region. We observed various ELM responses, i.e. suppression or mitigation, depending on the relative phase of in-vessel control coil currents. In particular, with the 90 • phase of the n = 1 RMP as the most resonant configuration, a complete suppression of type-I ELMs was demonstrated. In addition, fast vertical jogging of the plasma column was also observed to be effective in ELM pace-making. SMBI-mitigated ELMs, a state of mitigated ELMs, were sustained for a few tens of ELM periods. A simple cellular automata ('sand-pile') model predicted that shallow deposition near the pedestal foot induced small-sized high-frequency ELMs, leading to the mitigation of large ELMs. In addition to the ELM control experiments, various physics topics were explored focusing on ITER-relevant physics issues such as the alteration of toroidal rotation caused by both electron cyclotron resonance heating (ECRH) and externally applied 3D fields, and the observed rotation drop by ECRH in NB-heated plasmas was investigated in terms of either a reversal of the turbulence-driven residual stress due to the transition of ion temperature gradient to trapped electron mode turbulence or neoclassical toroidal viscosity (NTV) torque by the internal kink mode. The suppression of runaway electrons using massive gas injection of deuterium showed that runaway electrons were avoided only below 3 T in KSTAR. Operation in 2013 is expected to routinely exceed the n = 1 ideal MHD no-wall stability boundary in the long-pulse H-mode ( 10 s) by applying real-time shaping control, enabling n = 1 resistive wall mode active control studies. In addition, intensive works for ELM mitigation, ELM dynamics, toroidal ro...

Since the successful first plasma generation in the middle of 2008, three experimental campaigns were successfully made for the KSTAR device, accompanied with a necessary upgrade in the power supply, heating, wall-conditioning and diagnostic systems. KSTAR was operated with the toroidal magnetic field up to 3.6 T and the circular and shaped plasmas with current up to 700 kA and pulse length of 7 s, have been achieved with limited capacity of PF magnet power supplies. The mission of the KSTAR experimental program is to achieve steady-state operations with high performance plasmas relevant to ITER and future reactors. The first phase (2008–2012) of operation of KSTAR is dedicated to the development of operational capabilities for a super-conducting device with relatively short pulse. Development of start-up scenario for a super-conducting tokamak and the understanding of magnetic field errors on start-up are one of the important issues to be resolved. Some specific operation techniques for a super-conducting device are also developed and tested. The second harmonic pre-ionization with 84 and 110 GHz gyrotrons is an example. Various parameters have been scanned to optimize the pre-ionization. Another example is the ICRF wall conditioning (ICWC), which was routinely applied during the shot to shot interval. The plasma operation window has been extended in terms of plasma beta and stability boundary. The achievement of high confinement mode was made in the last campaign with the first neutral beam injector and good wall conditioning. Plasma control has been applied in shape and position control and now a preliminary kinetic control scheme is being applied including plasma current and density. Advanced control schemes will be developed and tested in future operations including active profiles, heating and current drives and control coil-driven magnetic perturbation.

The aim of the Korea superconducting tokamak advanced research (KSTAR) project is to develop a steady-state-capable advanced superconducting tokamak for establishing a scientific and technological basis for an attractive fusion reactor. Since the KSTAR mission includes the achievement of a steady-state-capable operation, the use of superconducting coils is an obvious choice for the magnet system. The KSTAR superconducting magnet system consists of 16 toroidal field (TF) and 14 poloidal field (PF) coils which include 8 central solenoid coils. Both the TF and PF coil systems use internally-cooled cable-in-conduit conductors (CICC). The TF coil system provides a magnetic field of 3.5 T at the plasma centre and the PF coil system provide a flux swing of 17 V s. The major achievement in the KSTAR magnet system development includes the development of CICC, a full size TF model coil, a background magnetic field generation coil system and the construction of a large scale superconducting magnet and the CICC test facility. TF and PF coils are at the stage of fabrication for the KSTAR completion in the year 2007.

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The KSTAR superconducting magnetic coils, which are made of cable in-conduit conductor (CICC), maintain a superconducting state with forced-flow supercritical helium (4.5 K, 5.5 bar). During current changing of the superconducting magnetic coils, AC losses are generated in the CICC due to dI/dt, and the heat generated from the loss is removed by high heat capacity supercritical helium. At the same time, reversed flow of the helium occurs due to a rapid increase of the helium temperature and momentary changing of the pressure inside the CICC. This phenomenon has been detected in all of the poloidal field (PF) coils, especially in the upper (U) and lower (L) PF1 PF4 coils. The maximum change of the magnetic field in the PF1UL PF4UL coils is located near the inlet and outlet of the helium cooling channels, and that of the PF5UL 7UL coils is located at the center of the cooling channel. The temperature variation at the helium inlet was always measured to have a time delay after each shot. In the PF1 coil tests, it was measured to have a delay of 26 sec. During the first plasma campaign, this phenomenon was more severe in the case of all PF coils operating together than for a single PF operation. In this paper, we investigated the thermal-hydraulics of this phenomenon.Index Terms-CICC, inverse helium flow, KSTAR, superconducting magnet, supercritical helium.