We report a discovery of a fusion plasma regime suitable for commercial fusion reactor where the ion temperature was sustained above 100 million degree about 20 s for the rst time. Nuclear fusion as a promising technology for replacing carbon-dependent energy sources has currently many issues to be resolved to enable its large-scale use as a sustainable energy source. State-of-the-art fusion reactors cannot yet achieve the high levels of fusion performance, high temperature, and absence of instabilities required for steady-state operation for a long period of time on the order of hundreds of seconds. This is a pressing challenge within the eld, as the development of methods that would enable such capabilities is essential for the successful construction of commercial fusion reactor. Here, a new plasma con nement regime called fast ion roled enhancement (FIRE) mode is presented. This mode is realized at Korea Superconducting Tokamak Advanced Research (KSTAR) and subsequently characterized to show that it meets most of the requirements for fusion reactor commercialization. Through a comparison to other well-known plasma con nement regimes, the favourable properties of FIRE mode are further elucidated and concluded that the novelty lies in the high fraction of fast ions, which acts to stabilize turbulence and achieve steady-state operation for up to 20 s by self-organization. We propose this mode as a promising path towards commercial fusion reactors.

Theory and experiments have shown that electron temperature gradient (ETG) turbulence on the electron gyro-scale, k ? q e Շ 1, can be responsible for anomalous electron thermal transport in NSTX. Electron scale (high-k) turbulence is diagnosed in NSTX with a high-k microwave scattering system [D. R. Smith et al., Rev. Sci. Instrum. 79, 123501 (2008)]. Here we report on stabilization effects of the electron density gradient on electron-scale density fluctuations in a set of neutral beam injection heated H-mode plasmas. We found that the absence of high-k density fluctuations from measurements is correlated with large equilibrium density gradient, which is shown to be consistent with linear stabilization of ETG modes due to the density gradient using the analytical ETG linear threshold in F. Jenko et al. [Phys. Plasmas 8, 4096 (2001)] and linear gyrokinetic simulations with GS2 [M. Kotschenreuther et al., Comput. Phys. Commun. 88, 128 (1995)]. We also found that the observed power of electron-scale turbulence (when it exists) is anti-correlated with the equilibrium density gradient, suggesting density gradient as a nonlinear stabilizing mechanism. Higher density gradients give rise to lower values of the plasma frame frequency, calculated based on the Doppler shift of the measured density fluctuations. Linear gyrokinetic simulations show that higher values of the electron density gradient reduce the value of the real frequency, in agreement with experimental observation. Nonlinear electron-scale gyrokinetic simulations show that high electron density gradient reduces electron heat flux and stiffness, and increases the ETG nonlinear threshold, consistent with experimental observations. V C 2015 AIP Publishing LLC.

In this letter, we report the first observation of the fast response of electron-scale turbulence to auxiliary heating cessation in National Spherical Torus eXperiment [Ono et al., Nucl. Fusion 40, 557 (2000)]. The observation was made in a set of RF-heated L-mode plasmas with toroidal magnetic field of 0.55 T and plasma current of 300 kA. It is observed that electron-scale turbulence spectral power (measured with a high-k collective microwave scattering system) decreases significantly following fast cessation of RF heating that occurs in less than 200 μs. The large drop in the turbulence spectral power has a short time delay of about 1–2 ms relative to the RF cessation and happens on a time scale of 0.5–1 ms, much smaller than the energy confinement time of about 10 ms. Power balance analysis shows a factor of about 2 decrease in electron thermal diffusivity after the sudden drop of turbulence spectral power. Measured small changes in equilibrium profiles across the RF cessation are unlikely able to explain this sudden reduction in the measured turbulence and decrease in electron thermal transport, supported by local linear stability analysis and both local and global nonlinear gyrokinetic simulations. The observations imply that nonlocal flux-driven mechanism may be important for the observed turbulence and electron thermal transport.

Dispersion interferometers have been used to measure line integrated electron densities from many fusion devices. To optically suppress noise due to mechanical vibrations, a conventional dispersion interferometer typically uses two nonlinear crystals located before and after the plasma along the laser beam path. Due to the long beam path, it can be difficult to overlap the fundamental and second harmonic laser beams for a heterodyne dispersion interferometer and to focus the beams on the second nonlinear crystal located after the plasma, especially when the aperture of the nonlinear crystal is small, i.e., of the order of mm. To overcome such difficulties, a new concept of a heterodyne dispersion interferometer, a single crystal dispersion interferometer (SCDI), is developed and installed on KSTAR with the laser wavelength of 1064 nm. The concept and the optical setup of the KSTAR SCDI are discussed, as well as its first measurement during a shattered pellet injection that produces abrupt and large changes in the electron density. To demonstrate feasibility, the KSTAR SCDI measurements are also compared with those from the existing two-color interferometer.

Major parts of an IR–visible two-color interferometer (TCI) on KSTAR have been upgraded for the multi-chord operation: (1) a diode-pumped-solid-state (DPSS) laser (660 nm) replacing the former HeNe laser (633 nm), (2) vacuum-compatible vibration isolator with titanium retro-reflectors, and (3) full digital phase comparator for multi-chord real-time density signals. The commercial compact DPSS laser suits the multiple chord configuration with its strong beam power (500 mW) and long coherent length (>100 m). Ti retro-reflectors are mounted on vacuum-compatible vibration isolators. The isolators are essential for the visible beams to avoid any fringe skips due to their short wavelength, considering the speed of the mechanical vibration (up to hundreds of μm). Field-programmable-gate-array (FPGA) modules count the entire fringes fast enough with a signal output rate up to 1.25 MHz, solving the fringe skip issues. The FPGA module enables the full digital processing of the phase comparator with a CORDIC algorithm after the sampling rate of 160 MS/s for the 40 MHz intermediate frequency of each beam. The full digital signals are transferred to the main plasma control system in real-time. Stable single-input-single-output operation of the KSTAR density control was demonstrated with the TCI. The real-time density profile control is also promising in the near future, with multiple actuators such as pellets and gas puffings.