H.L. Zhao

First results of the use of a continuously flowing lithium limiter in high performance discharges in the EAST device

As an alternative choice of solid plasma facing components (PFCs), flowing liquid lithium can serve as a limiter or divertor PFC and offers a self-healing surface with acceptable heat removal and good impurity control. Such a system could improve plasma performance, and therefore be attractive for future fusion devices. Recently, a continuously flowing liquid lithium (FLiLi) limiter has been successfully designed and tested in the EAST superconducting tokamak. A circulating lithium layer with a thickness of <0.1 mm and a flow rate ~2 cm3 s−1 was achieved. A novel in-vessel electro-magnetic pump, working with the toroidal magnetic field of the EAST device, was reliable to control the lithium flow speed. The flowing liquid limiter was found to be fully compatible with various plasma scenarios, including high confinement mode plasmas heated by lower hybrid waves or by neutral beam injection. It was also found that the controllable lithium emission from the limiter was beneficial for the reduction of recycling and impurities, for the reduction of divertor heat flux, and in certain cases, for the improvement of plasma stored energy, which bodes well application for the use of flowing liquid lithium PFCs in future fusion devices.

Investigations of LHW-plasma coupling and current drive at high density related to H-mode experiments in EAST

Two important issues in achieving lower hybrid current drive (LHCD) high confinement plasma in EAST are to improve lower hybrid wave (LHW)-plasma coupling and to drive the plasma current at a high density. Studies in different configurations with different directions of toroidal magnetic field (Bt) show that the density near the antenna is affected by both the radial electric field induced by plasma without a LHW (Er_plasma) in the scrape off layer (SOL), and the radial electric field induced by LHW power (Er_LH) near the grill. Investigations indicate that Er × Bt in the SOL leads to a different effect of configuration on the LHW-plasma coupling and Er_LH × Bt accounts for the asymmetric density behaviour in the SOL observed in the experiments, where Er is the total radial electric field in the SOL. Modelling of parametric instability (PI), collisional absorption (CA) and scattering from density fluctuations (SDF) in the edge region, performed considering the parameters of high density LHCD experiments in EAST, has shown that these mechanisms could be responsible for the low current drive (CD) efficiency at high density. Radiofrequency probe spectra, useful for documenting PI occurrence, show sidebands whose amplitude in the case of the lithiated vacuum chamber is smaller than in the case of poor lithiation, consistently with growth rates from PI modeling of the respective reference discharges. Since strong lithiation is also expected to diminish the parasitic effect on the LHCD of the remaining possible mechanisms, this appears to be a useful method for improving LHCD efficiency at a high density.

Lower hybrid current drive experiments with different launched wave frequencies in the EAST tokamak

EAST has been equipped with two high power lower hybrid current drive (LHCD) systems with operating frequencies of 2.45 GHz and 4.6 GHz. Comparative LHCD experiments with the two different frequencies were performed in the same conditions of plasma for the first time. It was found that current drive (CD) efficiency and plasma heating effect are much better for 4.6 GHz LH waves than for the one with 2.45 GHz. High confinement mode (H-mode) discharges with 4.6 GHz LHCD as the sole auxiliary heating source have been obtained in EAST and the confinement is higher with respect to that produced previously by 2.45 GHz. A combination of ray-tracing and Fokker-Planck calculations by using the C3PO/LUKE codes was performed in order to explain the different experimental observations between the two waves. In addition, the frequency spectral broadening of the two LH wave operating frequencies was surveyed by using a radio frequency probe.

Effect of gas puffing from different side on lower hybrid wave-plasma coupling in experimental advanced superconductive tokamak

Effect of gas puffing from electron-side and ion-side on lower hybrid wave (LHW)-plasma is investigated in experimental advanced superconductive tokamak for the first time. Experimental results with different gas flow rates show that electron density at the grill is higher in the case of gas puffing from electron-side; consequently, a lower reflection coefficient is observed, suggesting better effect of puffing from electron-side on LHW-plasma. The difference in edge density between electron- and ion-side cases suggests that local ionization of puffed gas plays a dominant role in affecting the density at the grill due to different movement direction of ionized electrons and that part of gas has been locally ionized near the gas pipe before diffusing into the grill region. Such difference could be enlarged and important in ITER due to the improvement of plasma parameters and LHW power.

Effort of lower hybrid current drive experiments toward to H-mode in EAST

Lower hybrid current drive (LHCD) is an effective tool to achieve high confinement (H-mode) plasma in EAST. To utilize LHCD for accessing H-mode plasma, efforts have been made to improve LHW (lower hybrid wave)-plasma coupling and current drive capability at high density. Improved LHW-plasma coupling by means of local gas puffing and gas puffing from the electron side is routinely used during EAST operation with LHCD. High density experiments suggest that low recycling and high LH frequency are preferred for LHCD experiments at high density, consistent with previous results in other machines. The effect of LHCD on the current profile in EAST demonstrates that it is possible to control the plasma profile by optimizing the LHW spectrum. Repeatable H-mode plasma was obtained by LHCD and the maximum density during H-mode with the combination of 2.45 GHz and 4.6 GHz LH waves was up to 4.5 × 1019 m−3.

Edge localized mode control using n = 1 resonant magnetic perturbation in the EAST tokamak

A set of in-vessel resonant magnetic perturbation (RMP) coil has been recently installed in EAST. It can generate a range of spectrum, and there is a relatively large window for edge localized mode (ELM) control according to the vacuum field modeling of the edge magnetic island overlapping area. Observation of mitigation and suppression of ELM in slow rotating plasmas during the application of an n = 1 RMP is presented in this paper. Strong ELM mitigation effect is observed in neutral beam injection heating plasmas. The ELM frequency increases by a factor of 5, and the crash amplitude and the particle flux are effectively reduced by a similar factor. Clear density pump-out and magnetic braking effects are observed during the application of RMP. Footprint splitting is observed during ELM mitigation and agrees well with vacuum field modelling. Strong ELM mitigation happens after a second sudden drop of plasma density, which indicates the possible effect due to field penetration of the resonant harmonics near the pedestal top, where the electron perpendicular rotation becomes flat and close to zero after the application of RMP. ELM suppression is achieved in a resonant window during the scan of the n = 1 RMP spectrum in radio-frequency (RF) dominant heating plasmas. The best spectrum for ELM suppression is consistent with the resonant peak of RMP by taking into account of linear magnetohydrodynamics plasma response. There is no mode locking during the application of n = 1 RMP in ELMy H-mode plasmas, although the maximal coil current is applied.

Density limits investigation and high density operation in EAST tokamak

Increasing the density in a tokamak is limited by the so-called density limit, which is generally performed as an appearance of disruption causing loss of plasma confinement, or a degradation of high confinement mode which could further lead to a H → L transition. The L-mode and H-mode density limit has been investigated in EAST tokamak. Experimental results suggest that density limits could be triggered by either edge cooling or excessive central radiation. The L-mode density limit disruption is generally triggered by edge cooling, which leads to the current profile shrinkage and then destabilizes a 2/1 tearing mode, ultimately resulting in a disruption. The L-mode density limit scaling agrees well with the Greenwald limit in EAST. The observed H-mode density limit in EAST is an operational-space limit with a value of . High density H-mode heated by neutral beam injection (NBI) and lower hybrid current drive (LHCD) are analyzed, respectively. The constancy of the edge density gradients in H-mode indicates a critical limit caused perhaps by e.g. ballooning induced transport. The maximum density is accessed at the H → L transition which is generally caused by the excessive core radiation due to high Z impurities (Fe, Cu). Operating at a high density () is favorable for suppressing the beam shine through NBI. High density H-mode up to could be sustained by 2 MW 4.6 GHz LHCD alone, and its current drive efficiency is studied. Statistics show that good control of impurities and recycling facilitate high density operation. With careful control of these factors, high density up to 0.93 stable H-mode operation was carried out heated by 1.7 MW LHCD and 1.9 MW ion cyclotron resonance heating with supersonic molecular beam injection fueling.

Control of sawtooth via ECRH on EAST tokamak

Localized electron heating produced by electron cyclotron resonant heating (ECRH) system has been proven to be powerful tools for controlling sawtooth instabilities, because such system allows to directly modify the local plasma parameters that determine the evolution of sawtooth periods. In this paper, we present the experimental results carried out on experimental advanced superconducting tokamak (EAST) with regard to sawtooth period control via ECRH. The electron cyclotron heating system on EAST was capable of inject electron cyclotron wave toward certain locations inside or outside q = 1 magnetic surface on the poloidal cross section, which renders us able to investigate the evolution of sawtooth period against the ECRH deposition position. It is found that when ECRH deposition position is inside the q = 1 surface, the sawtooth oscillation is destabilized (characterized by reduced sawtooth period). So far, inside the q = 1 surface, there are not enough EAST experiment data that can reveal more detaile...

Separation of derivatized aliphatic aldehydes in beverage by nonaqueous capillary electrophoresis

A simple and mild method for the separation of aliphatic aldehydes based on a condensation reaction with 9,10-phenanthrenequinone as labeling reagent with nonaqueous capillary electrophoresis has been developed. The detection was performed with a diode array detector (DAD). A 58.5 cm (50 cm effective length) x 50 microm i.d. untreated fused-silica capillary was used. To optimize the conditions, the background electrolyte concentration, column temperature, voltage and other factors were evaluated. The results indicated that the buffer concentration had a great impact on the separation, but the influences of temperature and added additives on the resolution were not obvious. The optimized conditions were as follows: 80 mmol/L ammonium acetate, 1.4 mol/L acetic acid, voltage of 28 kV, column temperature of 20 degrees C and DAD detection at 254 nm. The samples were introduced atmospherically with the injection at 5 kPa (50 mbar) for 8 s. The results indicate that seven aliphatic aldehyde derivatives and actual samples can be achieved baseline resolution under the proposed conditions.

Study of instability driving inward particle flux during the formation of transport barriers at the edge of the HL-2A tokamak

An electrostatic coherent mode with a frequency of kHz can be observed during the formation of transport barriers in high-confinement-mode plasma in the HL-2A tokamak, using reciprocating Langmuir probes. The mode drives a strong inward particle flux measured directly with four-tip probes with values comparable to the particle flux at the striking point in the divertor, which has also been validated by the measurement of other diagnostics. Several characteristics simultaneously indicate that the mode is an ion mode excited at the edge, which plays an important role in the formation of transport barriers besides particle diffusion.