Hao Qu

Experimental study of pedestal turbulence on EAST tokamak

Turbulence in the pedestal region of the EAST tokamak has been observed and studied using reflectometry. In lower hybrid wave (LHW) or neutral beam injection (NBI) dominated heating plasma, a coherent mode (CM) was usually observed in the ELM-free phase just after the L-H transition. The CM rotated in the electron diamagnetic drift (EDD) direction in the laboratory frame with a poloidal wave number (kθ) of 0.5 cm−1–0.7 cm−1 and its frequency usually chirped from 80 – 100 kHz down to 40 – 50 kHz as the pedestal evolved. The appearance of this mode reduced the increasing rate of pedestal pressure, implying that the CM may have an effect on outward pedestal transport. This mode can exist every ten milliseconds and is finally replaced by broadband (BB) fluctuation in the later ELM-free phase. It was found that the appearance and disappearance of the CM was correlated to the pedestal pressure. In the inter-ELM phase, the pedestal turbulence is generally dominated by BB fluctuation with poloidal wave numbers from 0 to 3 cm−1 rotating in the EDD direction in the laboratory frame. Analysis shows that the pedestal pressure increasing rate dpe, ped/dt decreases with the amplitude of the BB fluctuation, implying that the BB fluctuation may play an important role in pedestal evolution. The preliminary observation on the fluctuation just inside the pedestal top is also presented.

Observation of pedestal turbulence in edge localized mode-free H-mode on experimental advanced superconducting tokamak

Two different pedestal turbulence structures have been observed in edge localized mode-free phase of H-mode heated by lower hybrid wave and RF wave in ion cyclotron range of frequencies (ICRF) on experimental advanced superconducting tokamak. When the fraction of ICRF power PICRF/Ptotal exceeds 0.7, coherent mode is observed. The mode is identified as an electromagnetic mode, rotating in electron diamagnetic direction with a frequency around 50 kHz and toroidal mode number n = −3. Whereas when PICRF/Ptotal is less than 0.7, harmonic mode with frequency f = 40–300 kHz appears instead. The characteristics of these two modes are demonstrated preliminarily. The threshold value of heating power and also the plasma parameters are distinct.

Radial and poloidal correlation reflectometry on Experimental Advanced Superconducting Tokamak.

An X-mode polarized V band (50 GHz-75 GHz) radial and poloidal correlation reflectometry is designed and installed on Experimental Advanced Superconducting Tokamak (EAST). Two frequency synthesizers (12 GHz-19 GHz) are used as sources. Signals from the sources are up-converted to V band using active quadruplers and then coupled together for launching through one single pyramidal antenna. Two poloidally separated antennae are installed to receive the reflected waves from plasma. This reflectometry system can be used for radial and poloidal correlation measurement of the electron density fluctuation. In ohmically heated plasma, the radial correlation length is about 1.5 cm measured by the system. The poloidal correlation analysis provides a means to estimate the fluctuation velocity perpendicular to the main magnetic field. In the present paper, the distance between two poloidal probing points is calculated with ray-tracing code and the propagation time is deduced from cross-phase spectrum. Fluctuation velocity perpendicular to the main magnetic field in the core of ohmically heated plasma is about from -1 km/s to -3 km/s.

Multi-channel poloidal correlation reflectometry on experimental advanced superconducting tokamak

A new multi-channel poloidal correlation reflectometry is developed at Experimental Advanced Superconducting Tokamak. Eight dielectric resonator oscillators with frequencies of 12.5 GHz, 13.5 GHz, 14.5 GHz, 15 GHz, 15.5 GHz, 16 GHz, 17 GHz, and 18 GHz are used as sources. Signals from the sources are up-converted to V band using active quadruplers and then coupled together. The output waves are launched by one single antenna after passing through a 20 dB directional coupler which can provide the reference signal. Two poloidally separated antennae are installed to receive the reflected waves from plasma. The reference and reflected signals are down-converted by mixing with a quadrupled signal from a phase-locked source with a frequency of 14.2 GHz and the IF signals pass through the filter bank. The resulting signals from the mixers are detected by I/Q demodulators. The setup enables the measurement of density fluctuation at 8 (radial) × 2 (poloidal) spatial points. A coherent mode with an increasing velocity from 50 kHz to 100 kHz is observed by using the system. The mode is located in the steep gradient region of the pedestal.

Development of an ordinary mode multi-channel correlation reflectometer on EAST tokamak

An ordinary-mode polarized multi-channel correlation reflectometer has been developed on the Experimental Advanced Superconducting Tokamak (EAST). The system with four different probing frequencies (i.e., 20.4 GHz, 24.8 GHz, 33 GHz, and 40 GHz) and two poloidally spaced receiving antennas can realize both the radial correlation measurement and the poloidal correlation measurement. These diagnostics focus on the measurement of density fluctuation in the pedestal region to investigate the turbulence transport and H-mode physics on EAST. In this article, the system hardware design, the key component tests, and the system performance are shown in detail.

Effect of ∇ B drift on the H-mode power threshold in upper single null plasmas with ITER-like tungsten divertor on EAST

In this paper, we will experimentally investigate the power threshold (PL-H) in upper single null plasmas with an ITER-like tungsten divertor under different ∇B drift directions on EAST [F. Ding et al., Commissioning and PSI Behavior of the ITER-Like W/Cu Divertor in EAST 22nd PSI, Rome (2016)]. The power threshold for the low (L) to high (H) confinement mode has a clear and positive toroidal magnetic field, BT, dependence when the ∇B drift points toward the primary X-point (B×∇B↑). A factor of 2–3 increase in PL-H is observed for the ∇B drift away from the primary X-point (B×∇B↓). The edge and core impurities quantified by spectroscopy measurements show comparable levels for the transitions for both drift directions. On the other hand, it is found that the divertor Dα emission just prior to the L-H transition is lower for B×∇B↑, compared with that for B×∇B↓. The upper in-out divertor asymmetry, as manifested by particle fluxes measured by the divertor triple Langmuir probe, is most marked for B×∇B↓, and with significantly more particle flux to the outer divertor. The reversing field increases the particle flux into the upper inner and lower outer divertor, reducing the in-out asymmetry. One important distinction between the two field directions has been observed, with respect to the amplitude of the scrape-off layer (SOL) parallel flow. A dedicated experiment under similar target plasma conditions shows a lower SOL density and thus a steeper density gradient slightly inside the separatrix, where a lower PL-H is found for the B×∇B↑, compared to that for B×∇B↓. We, therefore, conclude that the field-dependent SOL plasma conditions play an important role in the transition physics.In this paper, we will experimentally investigate the power threshold (PL-H) in upper single null plasmas with an ITER-like tungsten divertor under different ∇B drift directions on EAST [F. Ding et al., Commissioning and PSI Behavior of the ITER-Like W/Cu Divertor in EAST 22nd PSI, Rome (2016)]. The power threshold for the low (L) to high (H) confinement mode has a clear and positive toroidal magnetic field, BT, dependence when the ∇B drift points toward the primary X-point (B×∇B↑). A factor of 2–3 increase in PL-H is observed for the ∇B drift away from the primary X-point (B×∇B↓). The edge and core impurities quantified by spectroscopy measurements show comparable levels for the transitions for both drift directions. On the other hand, it is found that the divertor Dα emission just prior to the L-H transition is lower for B×∇B↑, compared with that for B×∇B↓...

Data acquisition and control system for broad-band microwave reflectometry on EAST

Microwave reflectometry is a non-intrusive plasma diagnostic tool which is widely applied in many fusion devices. In 2014, the microwave reflectometry on Experimental Advanced Superconducting Tokamak (EAST) had been upgraded to measure plasma density profile and fluctuation, which covered the frequency range of Q-band (32-56 GHz), V-band (47-76 GHz) and W-band (71-110 GHz). This paper presented a dedicated data acquisition and control system (DAQC) to meet the measurement requirements of high accuracy and temporal resolution. The DAQC consisted of two control modules, which integrated arbitrary waveform generation block (AWG) and trigger processing block (TP), and two data acquisition modules (DAQ) that was implemented base on the PXIe platform from National Instruments (NI). All the performance parameters had satisfied the requirements of reflectometry. The actual performance will be further examined in the experiments of EAST in 2014.