Q. Xu

A low aspect ratio tokamak transmutation system

A low aspect ratio tokamak transmutation system is proposed as an alternative application of fusion energy on the basis of a review of previous studies. This system includes: (1) a low aspect ratio tokamak as fusion neutron driver, (2) a radioactivity-clean nuclear power system as blanket, and (3) a novel concept of liquid metal centre conductor post as part of the toroidal field coils. In the conceptual design, a driver of 100 MW fusion power under 1 MW/m2 neutron wall loading can transmute the amount of high level waste (including minor actinides and fission products) produced by ten standard pressurized water reactors of 1 GW electrical power output. Meanwhile, the system can produce tritium on a self-sustaining basis and an output of about 2 GW of electrical energy. After 30 years of operation, the biological hazard potential level of the whole system will decrease by two orders of magnitude.

Plasma density behavior with new graphite limiters in the Hefei Tokamak-7

A new set of actively cooled toroidal double-ring graphite limiters has been developed in the Hefei Tokamak-7 (HT-7) [X. Gao et al., Phys. Plasmas 7, 2933 (2000)] for long pulse operation. The extension of operational region and density behavior with graphite (C) limiters have been studied in this paper. Extended high-density region at the high plasma current low-qa was obtained. The density profile with the C limiter was studied to compare with the previous molybdenum (Mo) limiter. The critical density of multifaceted asymmetric radiation from the edge (MARFE) onset is observed in the region of Zeff1∕2fGW=0.9∼1.2, where fGW=n¯e∕nGW. (Here n¯e is the maximum line average electron density and nGW is the Greenwald density.) Under the same injected power, the critical density of MARFE onset with the new C limiter is much higher than the previous Mo limiter.

Study of confinement and LHCD efficiency on the HT-7 tokamak

High power heating of a lower hybrid wave (LHW) was performed (PLHW ~ 800?kW at 2.45?GHz) recently in the HT-7 tokamak. Lower hybrid current drive (LHCD) efficiency is studied for different injected powers and for different densities. Improved particle confinement is observed by the application of LHCD as characterized by an increase in the central line averaged electron density and the decrease in D? emission. The particle confinement time (?p) increased by about 1.5 times. The dependence of energy confinement time (?E) on plasma density and LHW power is experimentally studied in detail.

Extension of operational limits on EAST

The first plasma has been achieved successfully in the Experimental Advanced Superconducting Tokamak (EAST). Boronization by the glow discharge (GDC) method was studied in experiments. The plasma performance was obviously improved by GDC boronization. Extension of the operational region and improvement in the plasma performance were obtained. Sawtooth discharges were observed by means of soft x-ray signals, electron cyclotron emission signals and line averaged electron density after boronization. Lower qa and more stable operation were also achieved following GDC boronization. The plasma current ramp-up rate was also improved as a result of decreased impurity content and low averaged loop voltage due to boronization. PLEASE NOTE: THERE HAS BEEN A RETRACTION PUBLISHED FOR THIS ARTICLE.

Operational region and sawteeth oscillation in the EAST tokamak

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.

Nonlinear Seepage Numerical Simulation for Super-low Permeability Reservoirs With Artificial Fractures

The fluid seepage in the super-low permeability reserves conforms to the nonlinear seepage law, and according to this characteristic, nonlinear seepage numerical simulation software has been developed. In addition, a nonlinear seepage model considering the nonlinear bending section is established. Moreover, with the combination of field and laboratory experiment data, an ideal model of diamond inverted nine-spot well pattern, is also built. Besides the simulation result of different well spacing and well array, different fracture penetration ratio and different fracture conductivity is comparatively analyzed. The result shows that the smaller the well spacing and well array is, the higher the oil recovery and water cut is; the target block has an optimal fracture penetration ratio, which is about 0.28; and artificial fracture conductivity has little effect on the oilfield development.

Particle confinement and transport coefficients in ac plasmas on the HT-7 superconducting tokamak

A quasi-steady-state alternating current (ac) plasma operation was achieved successfully on the HT-7 superconducting tokamak. It is found in ac plasmas that the particle confinement time of positive current plasma (?p ~ 17?ms) is lower than that of negative current (?p ~ 31?ms) plasma. Therefore, the particle transport coefficients are investigated in ac plasmas by the gas puff modulation method on the HT-7 tokamak. It is observed that the particle diffusion coefficient (D) for the positive plasma current case is almost the same as for the negative one, but the absolute value of inward pinch velocity (V) of positive current plasma is much lower than that of the negative one. The evolution of density profile is studied in detail in ac plasmas.

Hot spots induced by LHCD in the shadow of antenna limiters in the EAST tokamak

Hot spots induced by lower hybrid wave in experimental advanced superconducting tokamak tokamak have caused high performance experiment disruption and serious damages to the guard limiters. Experimental and theoretical analyses have been carried out to study its physical mechanism. Plasma density scan experiments indicate that the wall temperature within the hot spots enhanced by a factor of 5 and increases with the plasma density near the antenna. A lower hybrid current drive (LHCD)-only density climb experiment shows that the carbon impurity decreases to a minimum value at certain plasma density and then increases with the line averaged plasma density. A model has been developed to explain the mechanism of sputtering of graphite tiles due to hot spots as the plasma density near the LHCD antenna and the time increases. A theoretical scaling of the heat flux driven by LHCD is also presented and is consistent with the experimental scaling in the Tore Supra tokamak. The simulation results show that the total sputtering flux density has a minimum at a certain plasma density and gradually increases as the plasma density increases or decreases away from the minimum value, and the increase in parallel heat flux near the antenna would enhance the sputtering flux density. The sputtering flux density trend is qualitatively consistent with the density scan experiments. The simulated temporal evolution of sputtered flux implies that the chemical sputtering could be a candidate for the carbon impurity explosion.

IBW heating experiments in HT-7 tokamak

The Experiments with Ion Berstein wave (IBW) heating has been carried out in HT-7 superconducting tokamak. Core electron heating and peaking profile of electron temperature can be observed in the beginning 40 ms after injection of IBW. The temporary heating efficiency of IBW increases with the toroidal magnetic field (BT). The electron temperature drops gradually with the increasing electron density after 40 ms of IBW. The electron density increases by a factor up to 2.4 and the profile of electron density becomes wide. Improved particle confinement induced by IBW is one reason for the density increase. Impurity release during IBW heating is also a contribution for the density increase. Suppression of MHD by IBW is also observed. The fluctuation and transport in the SOL of HT-7 tokamak have been investigated using a Langmuir probe array. The polodial shear flow induced by IBW maybe the reason for improved particle confinement.

Microwave Reflectometry Based On Amplitude Modulation

Reflectometry is a technique, based on the radar principle, with a response mainly sensitive to the plasma regions where the microwaves are reflected. The reflectometry technique used in plasma devices measures the phase of a wave reflected by a plasma cut-off layer at a given electron density. It has become a promising experimental technique for the measurement of the electron density profile in plasma devices, due to its good spatial and temporal resolution, together with the moderate access requirements and its ability to perform profile inversion along a single line of sight. Since its first implementation on a plasma device for electron density profile measurement, the high cut-off X-mode polarization has convinced many scientists to use this technique on most of the main devices, such as ASDEX, DIII-D, Gamma 10, TJII, TFTR, W7AS etc, and it is planned for edge and scrape of layer profile determination on ITER.