This work presents a robust methodology for effectively distinguishing between the seed island and the onset of the neoclassical tearing mode (NTM) in the EAST tokamak. Unlike previously employed methods, the width of the seed island is carefully regulated by gradually ramping up the currents in the resonant magnetic perturbation (RMP) coils. Remarkably, the seed island phase can be sustained for several hundred milliseconds, providing ample time to examine the nonlinear dynamics of NTM threshold physics. This study investigates the plasma response in terms of various factors, including the plasma rotation, the electron density, and the electron temperature, from the initial formation of the seed island to the saturation of the NTM island. Through extensive statistical analysis of NTM triggering events, it is revealed that both the critical width of the seed island and the critical RMP currents are positively correlated with beta p. Moreover, the relationship of the transition time associated with mode penetration to beta p differs from the dependence of the transition time for NTM triggering on beta p. The growth rates associated with these two nonlinear phenomena show similar characteristics. Finally, reduced magnetohydrodynamic (MHD) modeling replicates the two nonlinear bifurcation states observed in the experiment. It is noteworthy that the RMP-induced NTM islands are locked to the static magnetic perturbation. This scenario differs from the natural excitation of NTMs, which occurs due to transient MHD phenomena, such as sawtooth crashes, where the triggering source is a rotating magnetic perturbation. This leads to the formation of rapidly rotating islands at a natural frequency. Furthermore, the stability of these islands is expected to be related to the polarization current effect. Nevertheless, this experiment sets a new course for the comprehensive investigation and understanding of NTM threshold physics, providing valuable insights for future tokamak design and operational strategies

Direct evidence of micro-turbulence effect on the onset of neoclassical tearing mode (NTM) is reported for the first time in this letter. A puzzling positive correlation between critical width of seed island of NTM and normalized plasma pressure beta_p is first observed employing a novel method for clearly separating the processes of seed island and the onset of NTM in the EAST tokamak. Different from the methods developed before, the width of the seed island is well controlled by slowly ramping up the current in resonant magnetic perturbation coils. It is revealed that the positive correlation is mainly attributed to the enhancement of perpendicular transport by micro-turbulence, which overcomes the destabilizing effect of beta_p on the onset of NTM. Reduced magnetohydrodynamics (MHD) modeling well reproduced the two states of nonlinear bifurcations observed in this experiment by including the finite transport effect. This result provides a new route for understanding multi-scale interaction in plasma physics.

Scintillators, which are more tolerant of neutrons or γ-rays than semiconductors, are a promising candidate for soft X-ray (SX) diagnostics in high neutron flux environments such as JT-60SA or ITER. Although scintillators are tolerant of radiations, neutrons and γ-rays can cause scintillation light and become noise on SX signals. Therefore, a method to estimate the temporal effect by the radiations on SX signals and an appropriate design of the radiation shield based on the estimation are required. In previous studies, it has been proposed for estimating the effect by the radiations to calculate the absorption powers due to SXs, neutrons, and γ-rays in scintillators assuming that amplitudes of scintillation light are proportional to the absorption powers. In this study, an experimental examination of this proposal is conducted in the Experimental Advanced Superconducting Tokamak (EAST). It is shown that the proposal may be valid in the examination of EAST. In addition to results in EAST, initial results of a multi-channel scintillator-based SX diagnostic in the Large Helical Device (LHD) are introduced. Although a scintillator-based SX diagnostic in LHD observes oscillations of SXs by magnetohydrodynamic (MHD) phenomena successfully, the observed temporal effect on SX signals by neutrons or γ-rays is more significant than the expected effect, which is estimated by calculating the absorption powers. One of the possible reasons for the contradiction between the results in EAST and LHD is unexpected γ-rays around the scintillators in LHD. Although the temporal effect by the radiations is significant in the current system of LHD, the degradation of amplitudes of SX signals after the deuterium plasma experiments is not observed with the current level of the fluence. The scintillator-based SX diagnostic in LHD may work as a diagnostic to research MHD instabilities in deuterium plasma experiments without additional maintenance during an experimental campaign by making the pinhole larger or setting an additional radiation shield.

Microwave interferometry is an effective and reliable way to measure line integrated plasma electron density. An extremely low noise heterodyne interferometer has been developed for routine operation in Sino-UNIted Spherical Tokamak (SUNIST) and acts as the prototype design of a microwave inteferometer for Experimental Advanced Superconducting Tokamak (EAST) in the near future. The system has been designed with optimum frequency deduced from detailed calculation in real geometry and discharge parameters. In contrary to traditional heterodyne interferometers, the application of a single sideband modulator (SSBM) eliminates the necessity of the second millimeter wave (MMW) oscillator, which averts the intermediate frequency (IF) stability problem aroused by the two high frequency oscillators in the traditional heterodyne configuration. A pair of specially designed spot focusing antenna is also applied to boost the signal to noise ratio (SNR), whose near field scan data displays excellent focusability. The bench test data and plasma electron density measurement results from SUNIST discharges consistently show excellent performance of the interferometer, which is expected to be suitable for steady-state plasma experiments due to the high stability.

A coherent mode (CM) in the edge pedestal region has been observed on different fluctuation quantities, including density fluctuation, electron temperature fluctuation and magnetic fluctuation in H mode plasma on the Experimental Advanced Superconducting Tokamak (EAST) tokamak. Measurements at different poloidal positions show that the local poloidal wavenumber is smallest at the outboard midplane and will increase with poloidal angle. This poloidal asymmetry is consistent with the flute-like assumption (i.e. k// similar to 0) from which the toroidal mode number of the mode has been estimated as between 12 and 17. It was further found that the density fluctuation amplitude of the CM also demonstrated poloidal asymmetry. The appearance of a CM can clearly decrease or even stop the increase in the edge density, while the disappearance of a CM will lead to an increase in the pedestal density and density gradient. Statistical analysis showed there was a trend that as the CM mode amplitude increased, the rate of increase of the edge density decreased and the particle flux (Gamma(div)) onto the divertor plate increased. The CM sometimes showed burst behavior, and these bursts led bursts on Gdiv with a time of about 230 mu s, which is close to the time for particle flow from the outer midplane to the divertor targets along the scrape-off layer magnetic field line. This evidence showed that the CM had an effect on the outward transport of particles.

Microwave interferometry has been widely employed to provide reliable line averaged electron density measurement on plasma devices. For a vertically installed interferometer on a tokamak, the refraction problem, which distorts the beam path and aggravates power loss at the receiving antenna, may become significant if taking the cross section shape into account. Increasing the frequency of the probing microwave can alleviate the distortion, but at the expense of losing the density resolution. To seek for an optimized frequency, previous calculations are mainly based on the cylindrical column geometry which grossly underestimates the deflection of the beam path induced by the plasma shape, and empirical suggestions indicating ne0/nc = 1/2 ∼ 1/3 may not always be the appropriate option. Here a single ray tracing method is applied to estimate the final horizontal deviation at the receiving antenna, which is supposed to represent the level of power loss. The calculation is carried out under the real tokamak geometry in Sino-UNIted Spherical Tokamak (SUNIST) with the cross section parameters obtained from the equilibrium reconstruction, and the result indicates that for a target density of 1.2 × 10(19) m(-3), a frequency of at least 100 GHz is desirable to reduce the power loss to an acceptable level. This would be helpful for the design of a vertically installed interferometer on SUNIST.

The scalings of divertor heat and particle flux widths have been systematically performed on the Experimental Advanced Superconducting Tokamak (EAST) in radio-frequency (RF) heated L-mode plasma regime under various divertor configurations. The widths were calculated from the measurements of divertor Langmuir probe (LP) arrays and the reciprocating LP diagnostic on the low-field side. A strong inverse scaling of particle and heat flux widths with plasma current I-p (equivalently the poloidal field B-p) has been demonstrated. The measurements of divertor LPs show that the power decay length exhibits lambda(q,div)=4.97I(p)(-0.94) (lambda(q,div)=1.46B(p)(-1.15)), in good agreement with the particle decay length lambda(js)=4.74I(p)(-1.02) (lambda(js)=134B(p)(-1.21)). Similar trend measurements have also been demonstrated by the reciprocating LPs. In addition, the scaling with 1(p) (.B-p) appears to be insensitive of the divertor configurations in EAST. The study of heat and particle flux width scaling in EAST is useful for the extrapolation to the future tokamaks such as ITER. (C) 2015 Elsevier B.V. All rights reserved.

An intermediate phase (labeled I phase) with dithering, cycles between the L-mode and H-mode has been observed and experimentally characterized on Experimental Advanced Superconducting Tokamak (EAST). A typical characteristics of the I phase is that the D-alpha signal, edge density fluctuation level and edge radiation show several kHz periodical oscillation. The analysis shows that the dithering event is at least 2 cm inside the separatrix and extends into the scrape-off layer (SOL) region. It is found that this dithering occurs in plasma with double null (DN) or upper single null (USN) configuration and cannot be observed in plasma with lower single null (LSN) configuration where the ion B x del B drift direction is 'unfavorable', i.e. away from the X-point, in this device. The dithering cycle length (Delta t(dither)) has no clear dependence on the heating power. Both stored energy and density increase during the dithering phase and the increasing rates decrease with Delta t(dither). The evolution of density profiles during the L-I-H transition is analyzed and presented. (c) 2013 Elsevier B.V. All rights reserved.

A series of ELMy H-mode discharges were achieved on Experimental Advanced Superconducting Tokamak (EAST) with low hybrid wave (P-LHW = 0.5-1.7 MW at 2.45 GHz) and ion cyclotron resonance frequencies (P-ICRF = 0.5-2 MW) as the auxiliary heating power in 2012. In double-null (DN) configurations, the experimental power threshold with a molybdenum wall (2012) appears to be lower than that with a graphite wall (2010). For lower single null (LSN) plasmas, the lower X-point configuration has a lower power threshold, and a significant reduction (similar to 25%) in the H-mode power threshold with 3.4 cm X-point movement is achieved. The lower X-point configuration on EAST has higher dR(sep) below zero (closer to the DN configuration). Better energy confinement is observed in DN compared with single null (SN) at the same power loss. For reasons not understood, when the ion grad-B drift is in a favourable direction for H-mode access (towards the X-point) for EAST's LSN configuration, H-mode is achieved in upper single null (USN) configurations, but not in LSN.

A X-mode polarized W-band reflectometer for plasma density profile and fluctuation measurement is designed and installed on EAST. In measuring the density profile, a voltage controlled oscillator (VCO) is used as the source, allowing a high temporal resolution measurement. The density profile in a plasma with high magnetic field (3.0T) has been measured by combination of V- and W-band reflectometers. For fluctuation measurements, a frequency synthesizer is used instead of the VCO as a microwave source. The core density fluctuations during sawtooth activity are measured and analyzed.

Aimed at high-confinement (H-mode) plasmas in the Experimental Advanced Superconducting Tokamak (EAST), the effect of local gas puffing from electron and ion sides of a lower hybrid wave (LHW) antenna on LHW–plasma coupling and high-density experiments with lower hybrid current drive (LHCD) are investigated in EAST. Experimental results show that gas puffing from the electron side is more favourable to improve coupling compared with gas puffing from the ion side. Investigations indicate that LHW–plasma coupling without gas puffing is affected by the density near the LHW grill (grill density), hence leading to multi-transition of low–high–low (L–H–L) confinement, with a correspondingly periodic characteristic behaviour in the plasma radiation. High-density experiments with LHCD suggest that strong lithiation gives a significant improvement on current drive efficiency in the higher density region than 2 × 1019 m−3. Studies indicate that the sharp decrease in current drive efficiency is mainly correlated with parametric decay instability. Using lithium coating and gas puffing from the electron side of the LHW antenna, an H-mode plasma is obtained by LHCD in a wide range of parameters, whether LHW is deposited inside the half-minor radius or not, implying that a central and large driven current is not a necessary condition for the H-mode plasma. H-mode is investigated with CRONOS.

The material of limiter in HT-7 tokamak was changed from graphite to molybdenum in the last experimental campaign. The pitch angle scattering of runaway electrons due to anomalous Doppler resonance effects was observed. The experimental results agree very well with the stable boundary condition expected from the linear resistive theory but only agree with that from the nonlinear evolutionary of runaway-electron distribution theory in low electric field region. The current carried by runaway electrons is the same under different limiter conditions.

We present the results of our experimental studies on a discharge in the Hefei Tokamak-7 (HT-7) device with a specific instability whose emergence coincides with a sudden jump in electron cyclotron emission (ECE) that shows the transverse energy of the plasma. It is shown that, in this slide-away discharge regime, the current is carried by a relatively small group of runaway electrons and that the period of high-frequency small-amplitude oscillations varies from 1 to 2 ms, which depends on the value of the loop voltage in ohmic discharges. In low hybrid current drive (LHCD) plasmas, the period of high-frequency small-amplitude oscillations in the ECE signal during the whole period of anomalous Doppler instability (ADI) is about 2 ms. It is also found that the ADI in ohmic and LHCD discharges can restrain the magnetic oscillations.

A dust capture experiment was conducted in HT-7, a medium-sized superconducting tokamak of the Chinese Academy of Sciences. An aerogel was used to intercept fast particles propagating along the ion flow. The particles produced sizable impact craters, which were characterized and measured using 3D computer tomography and a seed growing algorithm. Captured particles were also photographed and measured. This allowed the determination of average impact yields, which were approximately more than four times the particle (projectile) mass. We provide evidence that the particle velocities may reach the hypervelocity regime, i.e. v(p) >= O(1 km s(-1)).

Runaway production is observed to be enhanced in lower hybrid current driven (LHCD) plasmas during ion Bernstein wave (IBW) heating as compared with the LHCD only plasma in the HT-7 tokamak. The distortion of the electron distribution function is the effect of the quasilinear diffusions of two types of waves. IBWs modified the distribution function of the electrons by helping to fill the so-called lower hybrid wave (LHW) spectral gap for low parallel velocity. Thus the LHW was significantly coupled to the fast electrons produced by the IBW, and partial LHW power was absorbed on the first pass without significant n∥-upshift. The synergy interaction of two types of waves results in high electron parallel energy and enhanced quasilinear diffusion, which is favorable for the production of runaway electrons. This is directly related to the improvement of plasma performance in the operation mode of simultaneous injection of LHW and IBW power.

The first plasma discharges were successfully achieved on the experimental advanced superconducting tokamak (EAST) in 2006. The sawteeth behaviours were observed by means of soft x-ray diagnostics and ECE signals in the EAST. The displacement and radius of the q = 1 surface was studied and compared with the result of equilibrium calculation. The density sawtooth oscillation was also observed by the HCN laser interferometer diagnostics. The structure of the EAST operational region was studied in detail. Plasma performance was obviously improved by the boronization and wall conditioning. It was observed that lower qa and a wider stable operating region is extended by the GDC boronization.

Lower hybrid current drive (LHCD) experiments have been carried out to achieve high performance for long pulse operation in the HT-7 superconducting tokamak. Multifaceted asymmetric radiation from the edge (MARFE) phenomena is summarized and studied in the HT-7 tokamak. MARFE during LHCD discharge often occurs at the value of Zeff1/2fGW in range of 0.6–0.9. These MARFEs generally appear to have the same characteristics as high fGW density MARFEs and are locally stable throughout the LHCD pulse. The MARFE instability typically occurs at a fraction 33% of the nGW density during LHCD discharge, and it can often terminate the long pulse discharges in the HT-7 tokamak.

An electrostatic dust detector has been successfully developed to measure dust event in situ and in real time on the HT-7 tokamak. For measuring dust near the edge plasmas and preventing interference of electrons and ions, the shielding plates were designed and installed around the dust detector. The electric signal of dust has been successfully measured during LHCD discharges on HT-7 tokamak. The measured dust signal was in good agreement with bursts appeared on multi-channel Hα radiation and on multi-channel ECE diagnostics. Diagnostics of the spectrum and the measurement of impurity emission during dust bursts were studied in detail. It is interesting that there is a delay between dust bursts and CIII line emission. It is observed that the delay time between dust signal and measured CIII line emission is about 0.3ms in the HT-7 tokamak.

The incoherent radiation emanating out of the tokamak plasmas gives vital information about the electron temperature. Electron cyclotron emission (ECE) is a powerful diagnostic tool for the measurement of electron temperature due to many advantages such as the high spatial and temporal resolutions. This paper presents the sixteen channel superheterodyne radiometer as an electron cyclotron emission diagnostic to measure the electron temperature on HT-7 tokamak.