East Team

First Engineering Commissioning of EAST Tokamak

Experimental Advanced Superconducting Tokamak (EAST) is the first fully superconducting tokamak. The first commissioning started on Feb. 1st of 2006 and finished on March 30th of 2006 at the Institute of Plasma Physics, Chinese Academy of Sciences. It consists of leakage testing at both room temperature and low temperature, pumping down, cooling down all coils, current leads, bus bar and the thermal shielding, exciting all the coils, measuring magnetic configuration and warming up the magnets. The electromagnetic, thermal hydraulic and mechanical performance of EAST Toroidal Field (TF) and Poloidal Field (PF) magnets have also been tested. All sub-systems, including pumping system, cryogenic system, PF& TF power supply systems, magnet instrumentation system, quench detection and protection system, water cooling system, data acquisition system, main control system, plasma control system (PCS), interlock and safety system have been successfully tested.

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.

Recent advances in long-pulse high-confinement plasma operations in Experimental Advanced Superconducting Tokamaka)

A long-pulse high confinement plasma regime known as H-mode is achieved in the Experimental Advanced Superconducting Tokamak (EAST) with a record duration over 30 s, sustained by Lower Hybrid wave Current Drive (LHCD) with advanced lithium wall conditioning and divertor pumping. This long-pulse H-mode plasma regime is characterized by the co-existence of a small Magneto-Hydrodynamic (MHD) instability, i.e., Edge Localized Modes (ELMs) and a continuous quasi-coherent MHD mode at the edge. We find that LHCD provides an intrinsic boundary control for ELMs, leading to a dramatic reduction in the transient power load on the vessel wall, compared to the standard Type I ELMs. LHCD also induces edge plasma ergodization, broadening heat deposition footprints, and the heat transport caused by ergodization can be actively controlled by regulating edge plasma conditions, thus providing a new means for stationary heat flux control. In addition, advanced tokamak scenarios have been newly developed for high-performance ...

Divertor asymmetry and scrape-off layer flow in various divertor configurations in Experimental Advanced Superconducting Tokamak

Divertor asymmetry and its dependence on the ion ▿B direction has been investigated in the Experimental Advanced Superconducting Tokamak by changing the divertor configuration from lower single null (LSN), via double null (DN), to upper single null (USN) during one single discharge. Divertor plasmas exhibit the usual in-out asymmetry in particle and heat fluxes in LSN with the ion ▿B direction toward the lower X-point, favoring the outer divertor, especially at high density. The in-out asymmetry is reversed when changing the divertor configuration from LSN to USN, thus clearly demonstrating the effect of classical drifts. DN exhibits an even stronger in-out divertor asymmetry, favoring the outer divertor. A significant top-down asymmetry is also seen for DN, with greater particle and heat fluxes to the bottom divertor. In addition, the parallel plasma flow has been measured by a fast moving Mach probe at the outer midplane, which shows similar magnitude to the Pfirsch-Schluter flow. Its contribution to th...

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.

Density Profile and Fluctuation Measurements by Microwave Reflectometry on EAST

A microwave reflectometry system operating in the V-band frequency with extraordinary mode polarization has been developed on the EAST tokamak. The reflectometry system, using a voltage-controlled oscillator (VCO) source driven by an arbitrary waveform generator with high temporal resolution, can operate for the density profile measurement. The result of the bench test shows that the output frequency of the VCO has a linear dependence on time. The dispersion of reflectometry system is determined and reported in this paper. The evolution of a pedestal density profile during the L-H transition is observed by the reflectometry in H-mode discharges on EAST tokamak. A frequency synthesizer is used to replace the VCO as microwave source for density fluctuation measurements. The level of density fluctuation in the pedestal shows an abrupt decrease when the plasma enters into H-mode. A coherent mode with a frequency of about 100 kHz is observed and the mode frequency decreases gradually as the pedestal evolves.

Investigation of lower hybrid wave coupling and the related effects of ion cyclotron range of frequencies in EAST

Effective coupling for lower hybrid waves (LHWs) is achieved by adjusting the launcher position and optimizing the plasma configuration in L-mode in EAST. It is found that, compared with other divertor shapes, the plasma with double null shows better coupling performance at the same position of lower hybrid (LH) grill, especially in the case of a large safety factor near the separatrix (q95) and a large edge recycling (Dα) intensity. The ion cyclotron range of frequency (ICRF) power has a significant impact on LH wave coupling when the ICRF antenna is magnetically connected to the LH grill. The asymmetry effects in the poloidal direction on reflection coefficients are obtained with a low edge density during ICRF power application. The origin of such a relevant asymmetry with ICRF is different from LHWs. Results not only suggest that ICRF power could modify the density in the local scrape-off layer (SOL), but also indicate that density convection in the SOL could be easily obtained with a low edge density. One promising alternative for eliminating the negative impact on LHW coupling induced by ICRF is gas (D2) injection both from the electronic side and ionic side in EAST.

Current ramp-up with lower hybrid current drive in EAST

More economical fusion reactors might be enabled through the cyclic operation of lower hybrid current drive. The first stage of cyclic operation would be to ramp up the plasma current with lower hybrid waves alone in low-density plasma. Such a current ramp-up was carried out successfully on the EAST tokamak. The plasma current was ramped up with a time-averaged rate of 18 kA/s with lower hybrid (LH) power. The average conversion efficiency Pel/PLH was about 3%. Over a transient phase, faster ramp-up was obtained. These experiments feature a separate measurement of the L/R time at the time of current ramp up.

Active control of divertor asymmetry on EAST by localized D2 and Ar puffing

The divertor asymmetry in particle and power fluxes has been investigated on the EAST superconducting tokamak [S. Wu and EAST Team, Fusion Eng. Des. 82, 463 (2007)] for both single null (SN) and double null (DN) divertor configurations. D2 and Ar puffing at various divertor locations has also been explored as an active means to reduce peak target heat load and control divertor asymmetry. For SN, peak heat load on the outer divertor target is 2–3 times that on the inner divertor target under typical ohmic plasma conditions. DN operation leads to a stronger in-out asymmetry favoring the outer divertor. D2 and Ar puffing promotes partial detachment near the strike points, greatly reducing peak target heat load (over 50%), while the far-SOL divertor plasma remains attached. What is remarkable is that the particle flux is even increased away from the strike points when the B×∇B drift is directed toward the divertor target, thus facilitating particle removal.

Study on lower hybrid current drive efficiency at high density towards long-pulse regimes in Experimental Advanced Superconducting Tokamak

Significant progress on both L- and H-mode long-pulse discharges has been made recently in Experimental Advanced Superconducting Tokamak (EAST) with lower hybrid current drive (LHCD) [J. Li et al., Nature Phys. 9, 817 (2013) And B. N. Wan et al., Nucl. Fusion 53, 104006 (2013).]. In this paper, LHCD experiments at high density in L-mode plasmas have been investigated in order to explore possible methods of improving current drive (CD) efficiency, thus to extend the operational space in long-pulse and high performance plasma regime. It is observed that the normalized bremsstrahlung emission falls much more steeply than 1/ne_av (line-averaged density) above ne_av = 2.2 × 1019 m−3 indicating anomalous loss of CD efficiency. A large broadening of the operating line frequency (f = 2.45 GHz), measured by a radio frequency (RF) probe located outside the EAST vacuum vessel, is generally observed during high density cases, which is found to be one of the physical mechanisms resulting in the unfavorable CD efficien...