Study On The Dynamics And Power Threshold Of L-H Transition In The EAST Tokamak

The physical mechanism of L-H transition stands as a 35-year unsolved problem in the field of fusion plasma physics,which is of significant importance for the realization of steady-state H-mode operation.The engineering design of ITER is based on high-performance H-mode operation,and the initial heating power at the beginning of ITER operation only be slightly above the power predicted by international tokamak scaling.A large uncertainty of accessing H-mode still exists for ITER.As additonal heating systems will cost much for ITER and future fusion devices,thus accessing the H-mode as a low heating power as possible is a crucial issue,and the research into the H-mode close to the threshold condition is necessary.The goal of this thesis is to study the dynamics and power threshold of the L-H transition,mainly by the high spatial-temporal resolving reciprocating probe system.EAST is the first full-superconducting tokamak with ITER-like configuration and dominant radio-frequency(RF)heating,as well as ITER-like tungsten divertor.The L-H transition power threshold studies on EAST have significant implications for ITER.Experiments on the dynamics and power threshold of the L-H transition have been car-ried out on the EAST tokamak,and the main results are listed in follow:(1)The L-H transition power threshold,which is achieved by low hybrid wave(LHW),ion cycltron range of frequency(ICRF)or neutral beam injection(NBI)alone,is close to the international tokamak scaling with medium line-averaged electron density ne>2.0 ×1019.The scaling law shows reliability for many tokamaks,as well as for EAST.The low density dependence of power threshold,namely an increase below a min-imum density about 2.0 x 1019,was first demonstrated on EAST with LHW and ICRF heating.The underlying physics mechanisms remain largely unknown,and future ex-periments are required to reveal the role of heating schemes.This result suggests that more heating power may be required for ITER to access H-mode at at the beginning of ITER operation under a low density range.(2)The effects of lithium wall-coating on the L-H transition power threshold have been investigated.Without lithium wall-coating,H-mode can hardly be achieved.With a low level injection of lithium<500 g in total,an significant reduction of low Z im-purities(C,O,H)has been found.Stable and sustainable H-modes are obtained.With a high level injection of lithium>500 g in total,a reduction of power threshold was observed together with reduced edge neutral density.These findings indicates that low recycling regime with reduced edge neutral desity and low Z impurities enabled by lithium wall-coating facilitates H-mode access,both for the C wall and Mo/C wall in EAST.(3)The role of divertor configuration and the ion ?B drift direction on the L-H transition behavior and power threshold has been investigated.A minimum power threshold is found in the double null(DN)configuration,both for the C wall and Mo/C wall with lithium wall-conditioning.For the single null(SN)configuration,the power threshold is lower for the ion ?B drift directed away from the primary X-point than for the opposite of the drift direction.The magnitude and time evolution of the scrape-off layer(SOL)plasma parallel flow are consistent with the Pfirsh-Schluiter flow on the low field side midplane.These flows tend to reverse direction when the magnetic field is reversed.For a W-shaped divertor,when the path connection length of the SOL parallel flow is short,i.e.,particles-mainly from cross-field transport at the low field side-are flowing to the outer divertor target through SOL,it will facilitates particle exhausts and impurity screeming as well as the H-mode access since the pumping is sufficient at outer divertor region.It has been proposed for the first time,that the Pfirsh-Schluter flow dominated SOL parallel flow plays an crucial role on the L-H transition power threshold.However,when the upper divertor was upgraded to the ITER-like tungsten diver-tor,the effect of the ion ?B drift direction on the L-H transition power threshold in upper single null(USN)configuration is quite different from the power threshold de-pendence found in the C wall or Mo/C wall.The power threshold is lower for the ion?B drift directed toward the primary X-point.With up-down asymmetry divertors,the parallel flow in SOL is always directed upwards for both field directions.However,the magnitude of the flow is quite different,which may be caused by the field-dependent Pfirsh-Schuter near the low field side midplane.Besides,the L-H transition power threshold is much higher for quasi-snowflake divertor configuration under similar target plasma conditions,compared with the stan-dard divertor configurations.This may result from the differences regarding the edge neutral density,since the strike-point is located further away from the divertor region in quasi-snowflake divertor configuration.The appearance and behavior of the L-H transition show a clear dependence on the divertor configuration and the pumping capability.Most 'dithering L-H' transitions were obtained with DN configuration and with reduced pumping,while most 'sin-gle step' L-H transitions were obtained with the ion ?B drift away from the primary X-point.In addition,the ditheing shows different characteristics for different configu-rations.Results from DIII-D and Alcator C-Mod show that power threshold is lower when the ion ?B drift direction towards the primary X-point.However,it shows similar or even smaller values of L-H power threshold with the ion ?B drift direction away from the primary X-point.On the other hand,a minimum power threshold is also observed in DN configuration on ASDEX-U,MAST and NSTX.These findings suggest that there are still underlying variables,such as edge neutral density,divertor geometry etc.,play role in the transition physics.In contrast to the other tokamaks,EAST has its extraordi-nary characteristics,which makes it an unique platform for the L-H transition studies.(4)The L-H transition dynamics with limit cycle oscillations(LCOs)close to the power threshold has been investigated.It is found that both the turbulence-driven zonal flow and the mean flow play role in the L-H transiton,determining the transition dy-namics as well as the L-I-H or L-I-L transition.(5)Sometimes,turbulent-driven shear flow does not exist prior to the L-H transi-tions and/or the Reynold Stress is found too small accounting for the driven shear flow.It has been observed for the first time,that turbulence radial wavenumber spectral shift and turbulence structure tiltes,within tens of milliseconds prior to and across the L-H transition,coincident with the evolution of E x B flow shear.It shows that turbulence can be quickly suppressed by dispersion when turbulence radial wavenumber spectral shifts to small-scale structures.The E x B flow shear,enhanced during the rise of pressure gradient,then continues the turbulence radial wavenumber spectral shifting.These observations provide a new approach for the theory of turbulence suppression in determing the L-H transition.(6)The magnetic perturbations of the LCOs before L-H and H-L transition has been analyzed,showing m/n=1/0 magnetic perturbation structures.