Zonal flows and transient dynamics of the L-H transition

We elucidate the role of zonal flows in transient phenomena observed during L-H transition by studying a simple L-H transition model which contains the evolution of zonal flows, mean ExB flows, and the ion pressure gradient. Zonal flows are shown to trigger the L-H transition and cause time-transient behavior through the self-regulation of turbulence before a mean shearing, due to a steep pressure profile, secures a quiescent H mode. Surprisingly, this self-regulation lowers the power threshold for the ultimate transition to a quiescent H-mode state.     

Influence of peripheral radial electric field on L-H transition in T-10 tokamak

In this paper the peripheral plasma behaviour during and after L-H transition on the tokamak T-10 is considered. Abrupt changes of Scrape-Off Layer (SOL) plasma parameters (ion saturation current, floating potential, electron temperature) are observed in regimes with transition to the improved plasma confinement. Such changes begin (5–150) ms prior to the L-H transition. The time delay of a transition depends on auxiliary plasma heating powerP _aux and plasma currentI _p (at constant toroidal magnetic field inductionB _t). Presented at the Workshop on the Role of Electric Fields in Plasma Confinement and Exhaust, Budapest, 18–19 June, 2000. This work is supported by Ministry of Atomic Energy of Russia (contract 69F) and by Ministry of Science and Technology of Russia (Federal Program “Controlled Thermonuclear Fusion and Plasma Processes”).     

Simulations of the L-H transition dynamics with different heat and particle sources

It is crucial to increase the total stored energy by realizing the transition from a low confinement(L-mode) state to a high confinement(H-mode) state in magnetic confinement fusion. The L–H transition process is simulated by using the predictive transport code based on Weiland's fluid model. Based on the equilibrium parameters obtained from equilibrium fitting(EFIT) in the experiment, the electron density n e, electron temperature T e, ion temperatures T i, ion poloidal V p,and toroidal momenta V t are simulated self-consistently. The L–H transition dynamic behaviors with the formation of the transport barriers of ion and electron temperatures, the electron density, and the ion toroidal momenta are analyzed. During the L–H transition, the strong poloidal flow shear in the edge transport barrier region is observed. The crashes of the electron and ion temperature pedestals are also observed during the L–H transition. The effects of the heating and particle sources on the L–H transition process are studied systematically, and the critical power threshold of the L–H transition is also found.     

Increased nonlinear coupling between turbulence and low-frequency fluctuations at the L-H transition

The nonlinear coupling between small scale high-frequency turbulence and larger scale lower-frequency fluctuations increases transiently in transitions to improved confinement in the DIII-D tokamak. This increase starts before the rapid turbulence suppression and E x B shear-flow development in the region that becomes the H-mode transport barrier/shear flow region. After the transition, the coupling returns to L-mode levels. These results are consistent with expectations for spontaneous transitions to improved confinement triggered by a turbulence-driven sheared flow.     

Probability and logical structure of statistical theories

A characterization of statistical theories is given which incorporates both classical and quantum mechanics. It is shown that each statistical theory induces an associated logic and joint probability structure, and simple conditions are given for the structure to be of a classical or quantum type. This provides an alternative for the quantum logic approach to axiomatic quantum mechanics. The Bell inequalities may be derived for those statistical theories that have a classical structure and satisfy a locality condition weaker than factorizability. The relation of these inequalities to the issue of hidden variable theories for quantum mechanics is discussed and clarified.     

Mean and oscillating plasma flows and turbulence interactions across the L-H confinement transition

A complex interaction between turbulence driven E × B zonal flow oscillations, i.e., geodesic acoustic modes (GAMs), the turbulence, and mean equilibrium flows is observed during the low to high (L-H) plasma confinement mode transition in the ASDEX Upgrade tokamak. Below the L-H threshold at low densities a limit-cycle oscillation forms with competition between the turbulence level and the GAM flow shearing. At higher densities the cycle is diminished, while in the H mode the cycle duration becomes too short to sustain the GAM, which is replaced by large amplitude broadband flow perturbations. Initially GAM amplitude increases as the H-mode transition is approached, but is then suppressed in the H mode by enhanced mean flow shear.     

A negative biasing experiment to test numerical calculations basedon a neoclassical L-H transition model in stellarators

Numerical calculations based on Shaing's L-H transition theory in stellarators and performed by Dahi et al. determine viscosity as a function of ion flow speed in the interchangeable module stellarator (IMS). The calculations predict local maximums in viscosity at flows corresponding to Mach numbers -2 and -10, but not elsewhere. The local peaks manifest themselves as jumps in flow speed, and as regions of high radial electric field (E_r) shear. By inducting flows swept between Mach numbers ±5, an electron injection biasing probe revealed a jump at -2, but none at +2, in agreement with the numerical results. A series of flow profiles at constant bias confirmed this result. Altogether, these data agree well with the numerical calculations and provide support for Shaing's L-H transition model as applied to stellarators     

High-density operation in tokamaks

Conclusion  In this paper we have examined various aspects regarding high-density operation in tokamaks and in particular the density limit, the plasma detachment, the MARFE formation and the fuelling efficiency. As regarding the density limit, both experimental findings and theoretical model indicate that the plasma current and the total input power are relevant in limiting the edge density that can be sustained in a tokamak discharge: radiation losses and SOL momentum and energy conservation are the underlying mechanisms. In the latest divertor experiments, operating in the detached regime, the influence of the input power seems to vanish or even disappear. Edge phenomena such as plasma detachment, occurring beyond a density threshold that can be lowered by means of impurity injection, can lead to the almost complete exhausting of the heating power by radiation which is greatly helpful for the design of the divertor plates. The compatibility of H-mode operation with this regime is still under investigation. The MARFE phenomenon, sometimes precursor of a major disruption, is now understood in terms of a radiation induced thermal instability. Finally, experiments performed in order to investigate the fuelling efficiency of the gas puffing technique have shown that at high density this technique becomes rather inefficient, thus indicating that pellet injection still remains an essential requirement to fuel the reactor plasma. The drop of the fuelling efficiency of gas-puffing at high density can be accounted for by collision phenomena taking place in the SOL.     

Shear flows at the tokamak edge and their role in core rotation and the L-H transition

Pfirsch-Schlüter fluxes in tokamaks are shown to drive strong poloidal and toroidal shear flows that are localized to the edge and scrape-off layer in the presence of temperature gradients and finite bootstrap current in the pedestal. Within a magnetohydrodynamic model, the effect of these flows on core rotation and their role in the magnetic configuration dependence of the power threshold for the low- (L-) to high- (H-)mode transition are discussed. Theoretical predictions based on symmetries of the underlying equations, coupled with computational results, are found to be in general agreement with observations in the Alcator C-Mod tokamak [Phys. Plasmas 12, 056111 (2005)10.1063/1.1876294].     

HL-2A装置上超声分子束注入触发L-H转换的实验研究

利用具有定向速度的超声分子束注入技术,研究了HL-2A装置在较低加热功率条件下实现L-H转换的等离子体放电特征,从边缘密度分布的差异比较分析了普通送气和超声分子束注入对L-H转换的影响.实验结果表明,HL-2A装置上采用超声分子束注入可直接触发L-H转换,明显降低L-H转换功率.通过对大量实验数据的分析和整理发现,利用超声分子束注入实现L-H转换的最低加热功率,比同等条件下采用普通送气实现L-H转换的最低加热功率减少约10％.     

Spatiotemporal structure of the interaction between turbulence and flows at the L-H transition in a toroidal plasma

The spatiotemporal behavior of the interaction between turbulence and flows has been studied close to the L-H transition threshold conditions in the edge region (ρ≥0.7) of TJ-II plasmas. The temporal dynamics of the interaction displays an oscillatory behavior with a characteristic predator-prey relationship. The spatial evolution of this turbulence-flow oscillation pattern has been measured, showing both radial outward and inward propagation velocities of the turbulence-flow front. The results indicate that the edge shear flow linked to the L-H transition can behave either as a slowing-down, damping mechanism of outward propagating turbulent-flow oscillating structures, or as a source of inward propagating turbulence-flow events.     

Observation of the fast potential change at L-H transition by a heavy-ion-beam probe on JFT-2M

The fast potential change near the separatrix is measured directly at the L-H transition by a heavy-ion-beam probe. The potential changes with two different time scales at the L-H transition triggered by a sawtooth crash: it drops at first with the time scale of 10--100 mus just after the arrival of the heat pulse due to the sawtooth crash. Then, it decreases again at a few 100 mus after the first drop at a time scale of about 200 mus.     

Physics of spherical tokamaks

Spherical tokamaks are a limiting case of conventional tokamaks, combining simple design with attractive physical characteristics. Being of potential importance for the controlled nuclear fusion program in their own right, spherical tokamaks also contribute much to our understanding of the physics of conventional tokamaks. For instance, they are contributing to the modeling of confinement scalings for ITER and to research on general plasma properties such as energy confinement and MHD processes. Results obtained for the first generation of spherical prototype tokamaks are reviewed and the main trends for further research are indicated. Zh. Tekh. Fiz. 69, 50–57 (September 1999)     

Edge operational regimes in tokamaks

Edge operational regimes in tokamaks are described using mostly experimental results of ASDEX Upgrade. Emphasis is put on high confinement mode (H-mode) at medium and high densities up to the density limit. Implications for the physics of H-mode and the density limit are discussed.     

First evidence of the role of zonal flows for the L-H transition at marginal input power in the EAST tokamak

A quasiperiodic Er oscillation at a frequency of <4 kHz, much lower than the geodesic-acoustic-mode frequency, with a modulation in edge turbulence preceding and following the low-to-high (L-H) confinement mode transition, has been observed for the first time in the EAST tokamak, using two toroidally separated reciprocating probes. Just prior to the L-H transition, the Er oscillation often evolves into intermittent negative Er spikes. The low-frequency Er oscillation, as well as the Er spikes, is strongly correlated with the turbulence-driven Reynolds stress, thus providing first evidence of the role of the zonal flows in the L-H transition at marginal input power. These new findings not only shed light on the underlying physics mechanism for the L-H transition, but also have significant implications for ITER operations close to the L-H transition threshold power.