Effect of the radial electric field on lower hybrid plasma heating in the FT-2 tokamak

Conditions for efficient ion heating in the interaction of lower hybrid waves with plasma are experimentally determined. Experiments show that efficient lower hybrid heating stimulates a transition to the improved confinement mode. The formation of internal and external transport barriers is associated with strong central ion heating, which results in a change of the radial electric field E _r and an increase in the shear of the poloidal plasma velocity. The improved confinement mode in the central region of the discharge is attained under the combined action of lower hybrid heating and an additional rapid increase in the plasma current. A new mechanism for the generation of an additional field E _r is proposed to explain the formation of a transport barrier.     

Formation of an internal transport barrier in the ohmic H-mode in the TUMAN-3M tokamak

In experiments on studying the ohmic H-mode in the TUMAN-3M tokamak, it is found that, in high-current (I _p～120–170 kA) discharges, a region with high electron-temperature and density gradients is formed in the plasma core. In this case, the energy confinement time τ _E attains 9–18 ms, which is nearly twice as large as that predicted by the ELM-free ITER-93H scaling. This is evidence that the internal transport barrier in a plasma can exist without auxiliary heating. Calculations of the effective thermal diffusivity by the ASTRA transport code demonstrate a strong suppression of heat transport in the region where the temperature and density gradients are high.     

Fast ion loss-driven H-mode transition in RF discharge plasmas of the Uragan-3M torsatron

In the l = 3 Uragan-3M (U-3M) torsatron with a helical divertor the plasma is produced and heated by RF field in the ω ≤ ω _ci range of frequencies. A two-temperature ion perpendicular energy distribution with a suprathermal tail sets in with heating. If the heating power is high enough, a spontaneous transition to an H-like confinement mode is observed. Recently, it has been supposed that the transition is connected with hotter and suprathermal ions (common name “fast ions“, FI) loss. The objective of this work is an experimental elucidation of the real link between the H-transition and FI loss. To do this, a transient regime of the RF discharge with two H-mode states is chosen, and the evolution is followed of electron density, FI content in the confinement volume, FI outflow to the divertor and edge potential. On the basis of juxtaposing of these processes, a conclusion is made that the H-mode transition in U-3M is really driven by FI loss. Possible mechanisms resulting in the transition are discussed, among them the ion orbit loss and the radial drift of helically-trapped ion orbits seem most probable.     

Formation of an internal transport barrier and magnetohydrodynamic activity in experiments with the controlled density of rational magnetic surfaces in the T-10 Tokamak

Results are presented from experiments on the formation of an internal electron transport barrier near the q = 1.5 rational surface in the T-10 tokamak. The experiments were carried out in the regime with off-axis electron cyclotron resonance (ECR) heating followed by a fast plasma current ramp-up. After suppressing sawtooth oscillations by off-axis ECR heating, an internal transport barrier began to form near the q = 1.5 rational surface. In the phase of the current ramp-up, the quality of the transport barrier improved; as a result, the plasma energy confinement time increased 2–2.5 times. The intentionally produced flattening of the profile of the safety factor q(r) insignificantly affected magnetohydrodynamic activity in the plasma column in spite of the theoretical possibility of formation of substantial m/n = 3/2 and 2/1 magnetic islands. Conditions are discussed under which the flattening of the profile of the safety factor q near low-order rational surfaces leads to the formation of either an internal transport barrier or the development of an island magnetic structure induced by tearing modes.     

Formation of electron transport barriers under ECR control of the q(r) profile in the T-10 tokamak

Abstract-the formation of transport barriers under electron cyclotron resonance heating and current drive in the t-10 tokamak is studied. in regimes with off-axis co-eccd and q _L <4 at the limiter, a spontaneous transition to improved confinement accompanied by the formation of two electron transport barriers is observed. the improvement resembles an L-H transition. It manifests itself as density growth, a decrease in the D_α emission intensity, and an increase in the central electron and ion temperatures. Two deep wells on the potential profile (the first one at r/a _L ≈0.6, where a _L is the limiter radius, and the second one near the edge) arise during the transition. the internal barrier is formed when dq/dr～0 with q≈1 in the barrier region.     

Study of plasma confinement in the L-2M stellarator during the formation of an edge transport barrier

A plasma confinement mode characterized by the formation of an edge transport barrier (ETB) was discovered in the L-2M stellarator after boronization of the vacuum vessel wall. The transition into this mode is accompanied by a jump in the electron temperature by 100–200 eV at the plasma edge and a sharp increase in the gradient of the electron temperature T _e in this region. The threshold power for the transition into the ETB confinement mode with an increased electron temperature gradient is P _thr ^?Te = (60 ± 15)n _e [10¹⁹ m^?3] kW. The formation of the ETB manifests itself also in a substantial change in the electron density profile. A density peak with a steep gradient at the outer side forms at the plasma edge. The threshold power for the transition into the ETB confinement mode corresponding to a substantial increase in the plasma density gradient near r = a is P _thr ^?Te = (67 ± 9)n _e [10¹⁹ m^?3] kW, which agrees to within experimental error with the threshold power for the transition into the ETB confinement mode determined from the sharp increase in the gradient of the electron temperature T _e . The value of P _thr for the L-2M stellarator agrees to within 25% with that obtained from the tokamak scaling. In the ETB confinement mode, the plasma energy W and the energy confinement time τ _E determined from diamagnetic measurements increase by 20–30% as compared to those obtained from the stellarator scaling for the confinement mode without an ETB. When the heating power increases by a factor of 2–3 above the threshold value, the effects related to improved energy confinement disappear.     

Characteristic properties of fluctuating particle fluxes near the last closed flux surface in the course of lower hybrid heating and during a transition into the improved confinement mode in the FT-2 tokamak

Results are presented from experimental studies of the time behavior of the transport processes in the edge plasma of the FT-2 tokamak during auxiliary lower hybrid heating when an internal transport barrier and then an external transport barrier form in the plasma. An analysis of the data on turbulent particle transport in the edge plasma shows that the radial electric field generated inside the plasma column during auxiliary heating plays an important role in both the formation of a transport barrier and the suppression of anomalous transport at the plasma periphery in the postheating phase of the discharge. The mechanism for the formation of a negative radial electric field E _r near the last closed flux surface after the end of the lower hybrid heating pulse is considered. Fluctuation spectra of the particle density and poloidal electric field are presented that characterize the process of suppression of microturbulence at the plasma periphery. The experimental data were obtained with the use of movable multielectrode Langmuir probes.     

Calculation of the energy confinement time and power threshold for L-H transitions in a tokamak with allowance for the parameters of the transport barrier

The ASTRA-ETL code is used to simulate L-H transition scenarios and calculate the energy confinement time and the threshold power of the L-H transition as functions of the averaged electron density 〈n〉, the averaged magnetic field B, the neutral density n _n , and the neutral temperature T _n , as well as the values of T _Se , T _Si , and n _S at the separatrix. It is shown that the linear dependence of the threshold power of the L-H transition on the averaged electron density, Q _L-H∝〈n〉, is associated with an increase in the viscosity of a poloidally rotating plasma due to charge exchange and is governed exclusively by an increase in the neutral density n _n . When the averaged electron density 〈n〉 is low, the threshold power rises because T _Si and T _Se increase. The accuracy of predictions for the power threshold of the L-H transition can be improved if the scaling of Q _L-H versus 〈n〉 and B is derived by processing experimental data from discharges with close parameter values at the separatrix. The hysteresis effect during an L-H-L transition triggered by varying the input power is modeled. The global energy confinement time τ_E is shown to increase linearly with 〈n〉 in the range 〈n〉<3.6×10¹⁹ m^?3 and to saturate at higher electron densities; this behavior is found to be characteristic of the Ohmic, L-, and H-modes. The saturation is associated with the fact that losses via the ion channel (when the transport coefficients are density-independent) dominate over losses via the electron channel. The dependence of τ _E on the input power is determined from the calculated database and is found to be τ _E =0.12Q _L-H ^?0.46 at a fixed averaged electron density 〈n〉. In the simulations of the L-H transition, the energy confinement time τ _E increases by a factor of 2 only if the thermal diffusivity inside the transport barrier is lower than that in the central plasma by a factor of more than 6.     

Study of plasma fluctuations in the Tuman-3m tokamak using microwave reflectometry with an obliquely incident probing beam

Plasma fluctuations in the Tuman-3M tokamak are studied experimentally by analyzing backscattered radiation for different angles of incidence of the probing beam from the normal to the cut-off surface. The poloidal rotation velocity of the plasma fluctuations is determined from the Doppler shift of the reflected radiation spectrum measured on the edge of the tokamak during the transition to the H-mode. It is shown that, before the transition to the H-mode, the rotation velocity can be estimated quantitatively from the spectral shift or from the rate at which the phase of the reflected signal grows. The experimental data obtained during the transition to the H-mode provide evidence for the onset of a sheared poloidal flow. The shear makes it difficult to correctly estimate the poloidal rotation velocity in the improved confinement regime. The main mechanisms responsible for the broadening of the backscattered radiation spectra are considered. The turbulent diffusion coefficients determined under the assumption that the spectral broadening is diffusive in character are found to be close to those determined from the charged-particle balance.     

Reversed-shear experiments in the T-10 tokamak

Results from T-10 experiments in regimes with nonmonotonic plasma current profiles are presented. The possibility of controlling the current profile j(r) by electron-cyclotron current drive is demonstrated experimentally. Nonmonotonic q profiles with the reversed shear are obtained in which the q _min value varies in a wide range, q _min=1–2.3. It is shown that the current profiles with q _min～2 (in this case, there are two resonant magnetic surfaces q=2 in the plasma) can cause the onset of MHD instabilities. The possibility of the formation of an internal transport barrier in reversed-shear discharges in the T-10 tokamak is analyzed. In T-10, electron transport is governed by short-wavelength electron turbulence. As a result, there is no clear evidence of the formation of an inner transport barrier in these experiments.     

Magnesium uptake by soybeans

Magnesium contents of soybean (Glycine max) roots increase and the K and Ca contents decrease with increased MgCl₂ concentrations in ambient solutions. The Mg uptake is inhibited when both Ca and K are present in the solution, but not by K or Ca alone. Chloride uptake, which is very low from the MgCl₂ solution, is greatly enhanced by the presence of K. The selectivity against Mg imparted by K + Ca appears to be at an external barrier for cation uptake as shown by its dependence on the presence of Ca in the external solution. The Ca content of roots is influenced only slightly by changes in external Ca concentrations from 10⁻⁴ to 10⁻²m, but that of shoots is greatly enhanced as the Ca concentration is increased or the K concentration is decreased. These effects on Ca contents are explained as arising from transport to the shoot without involvement of vacuoles of root cells.     

Testing of the spectroscopic method for location of water microleakages in ITER at the L-2M stellarator

Results of testing of a possible method for location of water microleakages in the cooling system of the first wall and vacuum chamber of ITER are presented. The method consists in spectroscopic detection of the emission lines of atoms and ions of the Xe additive dissolved in water. These lines are excited when the water with dissolved Xe contacts the plasma. The high electron cyclotron resonance heating (ECRH) power deposited in a relatively small plasma volume in the L-2M stellarator (P = 0.5 MW, V = 0.24 m³, and the specific heating power ??2 MW/m³) makes it possible to achieve plasma parameters close to those in the edge plasma of ITER for different operating modes, including the H-mode with an edge transport barrier. In test experiments, several lines of Xe ions were revealed suitable for detection of xenon in plasma with parameters close to those in the edge plasma of ITER at leakage rates at a level of ??10^?6 Pa m³ s^?1 and spatial resolution of ??0.5 cm.     

Sucrose uptake by developing soybean cotyledons

Sucrose uptake by excised developing soybean cotyledons shows a biphasic dependence on sucrose concentration. At concentrations less than about 50 millimolar external sucrose, uptake can be described as a carrier-mediated process, with a K_m of 8 millimolar. At higher external sucrose concentrations, a linear dependence becomes apparent, which suggests the participation of a nonsaturable component in total uptake. Sucrose absorption is dependent on the presence of an electrochemical potential gradient for protons since agents interfering with the generation or maintenance of this gradient (NaN₃ or carbonylcyanide-m-chlorophenyl hydrazone) decrease sucrose transport to a level at or below that predicted from the operation of the noncarrier-mediated process alone. The saturable component of sucrose uptake is also sensitive to the sulfhydryl-modifying compounds N-ethylmaleimide and p-chloro-mercuribenzenesulfonate. The thiol-reducing agent diethioerythritol reverses fully the p-chloro-mercuri-benzenesulfonate inhibition, but not that of N-ethyl maleim de. Sucrose transport is sensitive to external pH, being decreased at high pH₀. Since sucrose-induced depolarization of the membrane potential and carrier-mediated sucrose influx show similar pH-dependence, inhibitor sensitivity, and values of K_m for sucrose, a sucrose/proton contransport process appears to operate in developing soybean cotyledon cells. Measurement of free space and intracellular sucrose concentrations in vivo suggests that the carrier-mediated process is fully saturated and that sucrose transport may be limiting for sucrose accumulation by the developing seed.     

Methylammonium Transport in Phaseolus vulgaris Leaf Slices

Methylammonium (as a nonmetabolized analog of ammonium) transport was studied in leaf slices of Phaseolus vulgaris L. var. `Hawkesbury Wonder.' The relationship of influx to external pH (6.0-10.5) shows that the influx at low external pH is a larger fraction of that at high external pH than would be expected from the pK_α of methylammonium and the assumption that only CH₃NH₂ is entering the cells. The relationship between methylammonium influx and external methylammonium concentration shows some evidence of saturation; this is a function of the transport system rather than of the (limited) methylammonium metabolism in the cells. The “equilibrium” concentration ratio for methylammonium between leaf slices and bathing medium is far higher than can be explained by the transport of CH₃NH₂ alone and the pH of the compartments involved. These three lines of evidence strongly suggest that there is an influx of CH₃NH₃⁺, possibly by a uniporter driven by the electrical potential of the cytoplasm with respect to the medium, as has been shown for other plant cells. Competitive inhibition of methylammonium influx by ammonium suggests that there is also an ammonium transport system. The significance of this for the recycling of N within the plant and for exchange of gaseous NH₃ between leaves and the atmosphere is discussed.     

Charge as a Selection Criterion for Translocation through the Nuclear Pore Complex

Nuclear pore complexes (NPCs) are highly selective filters that control the exchange of material between nucleus and cytoplasm. The principles that govern selective filtering by NPCs are not fully understood. Previous studies find that cellular proteins capable of fast translocation through NPCs (transport receptors) are characterized by a high proportion of hydrophobic surface regions. Our analysis finds that transport receptors and their complexes are also highly negatively charged. Moreover, NPC components that constitute the permeability barrier are positively charged. We estimate that electrostatic interactions between a transport receptor and the NPC result in an energy gain of several k _B T, which would enable significantly increased translocation rates of transport receptors relative to other cellular proteins. We suggest that negative charge is an essential criterion for selective passage through the NPC.     

Plans for Liquid Metal Divertor in Tokamak Compass

The COMPASS tokamak (R = 0.56 m, a = 0.2 m, B_T = 1.3 T, I_p ~ 300 kA, pulse duration 0.4 s) operates in ITER-like plasma shape in H-mode with Type-I ELMs. In 2019, we plan to install into the divertor a test target based on capillary porous system filled with liquid lithium/tin. This single target will be inclined toroidally in order to be exposed to ITER-relevant surface heat flux (20 MW/m²). Based on precisely measured actual heat fluxes, our simulations predict (for 45° inclination, without accounting for the lithium vapor shielding) the surface temperature rises up to 700°C within 120 ms of the standard ELMy H-mode heat flux with ELM filaments reaching hundreds MW/m². Significant lithium vaporization is expected. The target surface will be observed by spectroscopy, fast visible and infrared cameras. The scientific program will be focused on operational issues (redeposition of the evaporated metal, ejection of droplets, if any) as well as on the effect on the plasma physics (improvement of plasma confinement, L–H power threshold, Z_eff, etc.). After 2024, a closed liquid divertor may be installed into the planned COMPASS Upgrade tokamak (R = 0.84 m, a = 0.3 m, B_T = 5 T, I_p = 2 MA, P_in = 8 MW, pulse duration ~2 s) with ITER-relevant heat fluxes loading the entire toroidal divertor.     

Simulations of plasma behavior during pellet injection in ITER

Plasma behavior during pellet injection in ITER is investigated using a 1.5D BALDUR integrated predictive modeling code. In these simulations, the pellet ablation is described using the neutral gas shielding (NGS) model developed by Parks and Turnbull [Phys. Fluids 21, 1735 (1978)]. The NGS pellet ablation model that includes the ?B drift effect is coupled with a plasma core transport model, which is a combination of an MMM95 anomalous transport model and an NCLASS neoclassical transport model. The combination of core transport models, together with pellet model, is used to simulate the time evolution of plasma current, ion and electron temperatures, and density profiles for ITER standard type-I ELMy H-mode discharges during the pellet injection. It is found that the injection of pellet can result in either enhancement or degradation of plasma performance. The ?B drift effect on the pellet deposition is very strong in ITER. The plasma density with high field side pellets, which favorable with the ?B drift effect, is much higher and pellet can penetrate much deeper than that with low field side pellets.     

Transition into the improved core confinement mode as a possible mechanism for additional electron heating observed in the lower hybrid current drive experiments at the FT-2 tokamak

In experiments on lower hybrid current drive (LHCD) carried out at the FT-2 tokamak, a substantial increase in the central electron temperature T _e (r = 0 cm) from 550 to 700 eV was observed. A complex simulation procedure is used to explain a fairly high LHCD efficiency and the observed additional heating, which can be attributed to a transition into the improved core confinement (ICC) mode. For numerical simulations, data obtained in experiments with deuterium plasma at 〈n _e 〉 = 1.6 × 10¹⁹ m^–3 were used. Simulations by the GRILL3D, FRTC, and ASTRA codes have shown that the increase in the density and central temperature is apparently caused by a significant suppression of heat transport in the electron component. The mechanism for transition into the improved confinement mode at r < 3 cm can be associated with the broadening of the plasma current channel due to the lower hybrid drive of the current carried by superthermal and runaway electrons. In this case, the magnetic shear s = (r/q)(dq/dr) in the axial region of the plasma column almost vanishes during the RF pulse. In this study, the effect of lower hybrid waves on the plasma parameters, resulting in a transition into the ICC mode, is considered. New experimental and calculated data are presented that evidence in favor of such a transition. Special attention is paid to the existence of a threshold for the transition into the ICC mode in deuterium plasma.     

Simulation of internal transport barriers by means of the canonical profile transport model

Models with critical gradients are widely used to describe energy balance in L-mode discharges. The so-called first critical gradient can be found from the canonical temperature profile. Here, it is suggested that discharge regimes with transport barriers can be described based on the idea of the second critical gradient. If, in a certain plasma region, the pressure gradient exceeds the second critical gradient, then the plasma bifurcates into a new state and a transport barrier forms in this region. This idea was implemented in a modified canonical profile transport model that makes it possible to describe the energy and particle balance in tokamak plasmas with arbitrary cross sections and aspect ratios. The magnitude of the second critical gradient was chosen by comparing the results calculated for several tokamak discharges with the experimental data. It is found that the second critical gradient is related to the magnetic shear s. The criterion of the transport barrier formation has the form (a ²/r)d/drln(p/p _c ) > z ₀ (r), where r is the radial coordinate, a is the plasma minor radius, p is the plasma pressure, p _c is the canonical pressure profile, and the dimensionless function z _O(r) = C _O + C ₁ s (with C _0i ～1, C _0e ～3, and C _1i,e ～2) describes the difference between the first and second critical gradients. Simulations show that this criterion is close to that obtained experimentally in JET. The model constructed here is used to simulate internal transport barriers in the JET, TFTR, DIII-D, and MAST tokamaks. The possible dependence of the second critical gradient on the plasma parameters is discussed.