Pedestal Current Research Articles

Core-Pedestal Plasma Configurations in Advanced Tokamaks

Several configurations for the core and pedestal plasma are examined for a predefined tokamak design by implementing multiple heating/current drive (H/CD) sources to achieve an optimum configuration of high fusion power in a noninductive operation while maintaining an ideally magnetohydrodynamic (MHD) stable core plasma using the IPS-FASTRAN framework. IPS-FASTRAN is a component-based lightweight coupled simulation framework that is used to simulate magnetically confined plasma by integrating a set of high-fidelity codes to construct the plasma equilibrium (EFIT, TOQ, and CHEASE), calculate the turbulent heat and particle transport fluxes (TGLF), model various H/CD systems (TORIC, TORAY, GENRAY, and NUBEAM), model the pedestal pressure and width (EPED), and estimate the ideal MHD stability (DCON). The TGLF core transport model and EPED pedestal model are used to self-consistently predict plasma profiles consistent with ideal MHD stability and H/CD (and bootstrap) current sources. In order to evaluate the achievable and sustainable plasma beta, varying configurations are produced ranging from the no-wall stability to with-wall stability regimes, simultaneously subject to the self-consistent TGLF, EPED, and H/CD source profile predictions that optimize configuration performance. The pedestal density, plasma current, and total injected power are scanned to explore their impact on the target plasma configuration, fusion power, and confinement quality. A set of fully noninductive scenarios are achieved by employing ion-cyclotron, neutral beam injection, helicon, and lower-hybrid H/CDs to provide a broad profile for the total current drive in the core region for a predefined tokamak design. These noninductive scenarios are characterized by high fusion gain (Q ~ 4) and power (Pfus ~ 600 MW), optimum confinement quality (H98 ~ 1.1), and high bootstrap current fraction (fBS ~ 0.7) for Greenwald fraction below unity. The broad current profile configurations identified are stable to low-n kink modes either because the normalized pressure is below the no-wall limit or a wall is present.

Role of the pedestal current on the stability of non-ideal ballooning modes

On the basis of a three-field flute-reduced magnetohydrodynamic model, which mainly describes the edge instabilities by shielding a major part of the J × B force in the flute reduction, we study the stability of ballooning modes in the edge pedestal, highlighting the role of an equilibrium parallel current gradient. This effect, which is designated as the current gradient driven (CGD) term in this paper, is shown to have an influence on the stability of finite-n pedestal ballooning modes due to the existence of a highly localized bootstrap current. An analysis in the ideal limit shows that the CGD term destabilizes the ballooning modes regardless of the sign of its gradient, especially near the stability boundaries. An inclusion of the finite Larmor radius (FLR) effect via ion diamagnetic flow and finite resistivity results in a coupling of the FLR effect and the current gradient. In this particular regime where the deviation from the ideal stability is considerable, this coupling effect is shown to dominate stability in intermediate n (20<n≤40) modes. Here, n is the toroidal mode number. Stability analyses using a series of model pedestal equilibria indicate that an increase in a bootstrap current can move the most unstable position from the pedestal middle to the bottom and the negative gradient of the bootstrap current at the pedestal bottom leads to further destabilization of intermediate n modes.

Influence of plasma shaping effect on peeling-ballooning instability

The influence of plasma shaping effect on peeling-ballooning mode is studied linearly and nonlinearly using the three-field module of BOUT++. Without considering the variations of pressure pedestal and bootstrap current induced by shaping, our simulations demonstrate that shaping effect on magnetic surfaces also influences peeling-ballooning instability significantly. The shape of growth rate spectrum tends to shift to a lower toroidal mode number () with the increase of elongation (), and the growth rates become smaller for all modes with the increase of triangularity () in the linear simulation. Further analysis demonstrates that peeling-ballooning mode is stabilized mainly by the increased magnetic shear () for elongation and the decreased normalized pressure gradient () for triangularity. In the nonlinear simulation, the elongation can lead to a larger pedestal collapse while the triangularity plays an opposing role. That is because the dominance of low mode that arises with the increase of and decrease of can lead to a larger energy loss. The results indicate that the effect of triangularity on the magnetic surface is quite important since it leads to ELM mitigation without any variations of pressure pedetal and bootstrap current, which provide an innovative mechanism to explain the ELM mitigation phenomenon due to plasma shaping.

Achieving a robust grassy-ELM operation regime in CFETR

We have identified a robust grassy-edge localized mode (ELM) operation regime for future tokamak reactors. The regime exists within a pedestal top electron collisionality (ν*) window at high global poloidal beta (βp). The existence of an upper ν* limit for grassy-ELMs is consistent with results previously reported in experiments (Oyama et al 2010 Nucl. Fusion 50 064014), while the existence of a lower ν* limit has not been reported previously. Using EPED and BOUT + +, a theoretical model that quantitatively explains the physics of the grassy-ELMs within the window, which distinguishes them from the small mixed-ELMs at lower ν*, is presented for the first time. A peeling-ballooning stability boundary is obtained by scanning the operating density space. The change in density corresponds to a change in ν* that affects the pedestal bootstrap current. High βp leads to a strong Shafranov shift, which affects the flux surface averaged pressure drive. The two effects combine to create a peeling-dominated window in intermediate ν* buffered by ballooning-dominated regimes. Only the peeling-dominated regime shows a cyclic behavior in the perturbed pressure during the nonlinear simulation of an ELM crash, reminiscent of grassy-ELM dynamics. Similarly, the energy released across the separatrix is demonstrated to be significantly smaller. The quick recovery of the ELM crash is explainable by the rapid rise of a low n kink-peeling instability when the pedestal current Iped exceeds a threshold at high βp. It minimizes the excursion beyond marginal stability and is absent in the ballooning-dominated regime. Comparison with recent experiments over a range of βp and ν* strongly supports the physical picture proposed by the modeling.

Predict-first experimental analysis using automated and integrated magnetohydrodynamic modeling

An integrated-modeling workflow has been developed for the purpose of performing predict-first analysis of transient-stability experiments. Starting from an existing equilibrium reconstruction from a past experiment, the workflow couples together the EFIT Grad-Shafranov solver [L. Lao et al., Fusion Sci. Technol. 48, 968 (2005)], the EPED model for the pedestal structure [P. B. Snyder et al., Phys. Plasmas 16, 056118 (2009)], and the NEO drift-kinetic-equation solver [E. A. Belli and J. Candy, Plasma Phys. Controlled Fusion 54, 015015 (2012)] (for bootstrap current calculations) in order to generate equilibria with self-consistent pedestal structures as the plasma shape and various scalar parameters (e.g., normalized β, pedestal density, and edge safety factor [q95]) are changed. These equilibria are then analyzed using automated M3D-C1 extended-magnetohydrodynamic modeling [S. C. Jardin et al., Comput. Sci. Discovery 5, 014002 (2012)] to compute the plasma response to three-dimensional magnetic perturbations. This workflow was created in conjunction with a DIII-D experiment examining the effect of triangularity on the 3D plasma response. Several versions of the workflow were developed, and the initial ones were used to help guide experimental planning (e.g., determining the plasma current necessary to maintain the constant edge safety factor in various shapes). Subsequent validation with the experimental results was then used to revise the workflow, ultimately resulting in the complete model presented here. We show that quantitative agreement was achieved between the M3D-C1 plasma response calculated for equilibria generated by the final workflow and equilibria reconstructed from experimental data. A comparison of results from earlier workflows is used to show the importance of properly matching certain experimental parameters in the generated equilibria, including the normalized β, pedestal density, and q95. On the other hand, the details of the pedestal current did not significantly impact the plasma response in these equilibria. A comparison to the experimentally measured plasma response shows mixed agreement, indicating that while the equilibria are predicted well, additional analysis tools may be needed. Finally, we note the implications that these results have for the success of future predict-first studies, particularly the need for scans of uncertain parameters and for close collaboration between experimentalists and theorists.

Kinetic equilibrium reconstruction for the NBI- and ICRH-heated H-mode plasma on EAST tokamak

The equilibrium reconstruction is important to study the tokamak plasma physical processes. To analyze the contribution of fast ions to the equilibrium, the kinetic equilibria at two time-slices in a typical H-mode discharge with different auxiliary heatings are reconstructed by using magnetic diagnostics, kinetic diagnostics and TRANSP code. It is found that the fast-ion pressure might be up to one-third of the plasma pressure and the contribution is mainly in the core plasma due to the neutral beam injection power is primarily deposited in the core region. The fast-ion current contributes mainly in the core region while contributes little to the pedestal current. A steep pressure gradient in the pedestal is observed which gives rise to a strong edge current. It is proved that the fast ion effects cannot be ignored and should be considered in the future study of EAST.

Nonlinear MHD simulations of QH-mode DIII-D plasmas and implications for ITER high Q scenarios

In nonlinear MHD simulations of DIII-D QH-mode plasmas it has been found that low n kink/peeling modes (KPMs) are unstable and grow to a saturated kink-peeling mode. The features of the dominant saturated KPMs, which are localised toroidally by nonlinear coupling of harmonics, such as mode frequencies, density fluctuations and their effect on pedestal particle and energy transport, are in good agreement with the observations of the edge harmonic oscillation typically present in DIII-D QH-mode experiments. The nonlinear evolution of MHD modes including both kink-peeling modes and ballooning modes, is investigated through MHD simulations by varying the pedestal current and pressure relative to the initial conditions of DIII-D QH-mode plasma. The edge current and pressure at the pedestal are key parameters for the plasma either saturating to a QH-mode regime or a ballooning mode dominant regime. The influence of E × B flow and its shear on the QH-mode plasma has been investigated. E × B flow shear has a strong stabilisation effect on the medium to high-n modes but is destabilising for the n = 2 mode. The QH-mode extrapolation results of an ITER Q = 10 plasma show that the pedestal currents are large enough to destabilise n = 1–5 KPMs, leading to a stationary saturated kink-peeling mode.

Comparative investigation of ELM control based on toroidal modelling of plasma response to RMP fields

Extensive modelling efforts of the plasma response to the resonant magnetic perturbation fields, utilized for controlling the edge localized mode (ELM), help to identify the edge-peeling response as a key factor, which correlates to the observed ELM mitigation in several tokamak devices, including MAST, ASDEX Upgrade, EAST, and HL-2A. The recently observed edge safety factor window for ELM mitigation in HL-2A experiments is explained in terms of the edge-peeling response. The computed plasma response, based on toroidal single fluid resistive plasma model with different assumption of toroidal flows, is found generally larger in ELM suppressed cases as compared to that of the ELM mitigated cases, in ASDEX Upgrade and DIII-D. The plasma shaping, in particular, the plasma triangularity, contributes to the enhanced plasma response. But the shaping does not appear to be the sole factor—other factors such as the (higher) pedestal pressure and/or current can also lead to increased edge-peeling response.

Targeting the late component of the cardiac L-type Ca2+ current to suppress early afterdepolarizations.

Early afterdepolarizations (EADs) associated with prolongation of the cardiac action potential (AP) can create heterogeneity of repolarization and premature extrasystoles, triggering focal and reentrant arrhythmias. Because the L-type Ca(2+) current (ICa,L) plays a key role in both AP prolongation and EAD formation, L-type Ca(2+) channels (LTCCs) represent a promising therapeutic target to normalize AP duration (APD) and suppress EADs and their arrhythmogenic consequences. We used the dynamic-clamp technique to systematically explore how the biophysical properties of LTCCs could be modified to normalize APD and suppress EADs without impairing excitation-contraction coupling. Isolated rabbit ventricular myocytes were first exposed to H2O2 or moderate hypokalemia to induce EADs, after which their endogenous ICa,L was replaced by a virtual ICa,L with tunable parameters, in dynamic-clamp mode. We probed the sensitivity of EADs to changes in the (a) amplitude of the noninactivating pedestal current; (b) slope of voltage-dependent activation; (c) slope of voltage-dependent inactivation; (d) time constant of voltage-dependent activation; and (e) time constant of voltage-dependent inactivation. We found that reducing the amplitude of the noninactivating pedestal component of ICa,L effectively suppressed both H2O2- and hypokalemia-induced EADs and restored APD. These results, together with our previous work, demonstrate the potential of this hybrid experimental-computational approach to guide drug discovery or gene therapy strategies by identifying and targeting selective properties of LTCC.

MHD stability of the pedestal in ITER scenarios

The linear ideal magnetohydrodynamic (MHD) limits of the pedestal in ITER scenarios associated with the preparation and realization of the nominal fusion gain Q = 10 (inductive scenario at 15 MA/5.3 T, half-field/half-current and intermediate H-mode scenario at 10 MA/3.5 T), as well as the hybrid scenario at 12 MA/5.3 T, are investigated in this work. The accessible part of the MHD stability diagram is determined by computing the bootstrap current and self-consistently evaluating the corresponding pedestal current. This procedure shows that only a small part of peeling–ballooning diagrams is physically accessible. Uncertainties about the foreseen plasma profiles motivate studies evaluating the impact of various parameters on the pedestal limits. We have addressed issues such as the pedestal width, the global performance, pressure peaking, edge current density, internal inductance and plasma shaping. A scaling law for the maximum pedestal pressure in the ITER scenarios is proposed, highlighting that the main dependences are on the plasma current, the edge safety factor, the pedestal width and the internal inductance.

Comparison of the Effects of the Transient Outward Potassium Channel Activator NS5806 on Canine Atrial and Ventricular Cardiomyocytes

Objective: NS5806 activates the transient outward potassium current (Ito) in canine ventricular cells. We compared the effects of NS5806 on canine ventricular versus atrial tissues and myocytes. Methods: NS5806 (10 μM) was evaluated in arterially-perfused canine right atrial and left ventricular wedges. Atrial and ventricular epi- and endocardial cells were isolated by enzymatic dissociation. Current and voltage-clamp recordings were made in the absence and presence of NS5806. Results: In ventricular wedges NS5806 increased phase 1 repolarization in epi- and midmyocardial cells. A minor effect on conduction and upstroke velocity also was observed. In contrast, application of NS5806 to atrial preparations slowed upstroke velocity and reduced excitability, consistent with sodium channel block. In ventricular myocytes, NS5806 increased the magnitude of Ito by 80% and 16% in epi and endo, respectively (at +40 mV). In atrial myocytes, NS5806 increased peak Ito by 25% and had no effect on the sustained pedestal current, IKur. INa density in atrial myocytes was nearly 100% greater than in endocardial myocytes. NS5806 caused a negative shift in steady-state mid-inactivation (V1/2) for both cell types (73.9±0.27 to −77.3±0.21mV for endocardial and −82.6±0.12 to −85.1±0.11mV for atrial cells). The shift in V1/2 resulted in a reduction of INa in both cell types. However, the more negative V1/2 in atrial cells suggests that atrial cells lose excitability at more depolarized voltages than endocardial cells which may explain the greater reduction of excitability in atrial vs ventricular wedges by NS5806. Conclusion: NS5806 produces a prominent augmentation of Ito with little effect on INa in the ventricles, but a potent inhibition of INa with little augmentation of Ito in atria.

Characterization of an ultrarapid delayed rectifier potassium channel involved in canine atrial repolarization.

1. Depolarizing pulses positive to 0 mV elicit a transient outward current (Ito) and a sustained 'pedestal' current in canine atrial myocytes. The pedestal current was highly sensitive to 4-aminopyridine (4-AP) and TEA, with 50% inhibitory concentrations (EC50) of 5.3 +/- 0.7 and 307 +/- 25 microM, respectively. When the pedestal current was separated from Ito with prepulses or by studying current sensitive to 10 mM TEA, it showed very rapid activation and deactivation. We therefore designated the current IKur,d, for 'ultrarapid delayed rectifier, dog'. IKur,d inactivation was bi-exponential, with mean time constants of 609 +/- 91 and 5563 +/- 676 ms during a 20 s pulse to +40 mV. 2. The reversal potential of IKur,d tail currents are dependent on extracellular potassium concentration ([K+]o; slope, 54.7 mV decade-1). The envelope of tails test was satisfied and the current inwardly rectified at > or = +40 mV. The current was insensitive to E-4031, dendrotoxin and chloride substitution, but was inhibited by barium, with an EC50 of 1.65 mM. Lanthanum ions caused a positive shift in voltage dependence without producing direct inhibition. 3. Single-channel activity was observed in cell-attached, inside-out and outside-out patches. Upon depolarization from -50 to +30 mV, single channels had similar time constants and [K+]o dependence to whole-cell current. Channel open probability (Po) increased with depolarization in a saturable fashion and the Po-voltage relation had a half-activation voltage and slope factor similar to whole-cell IKur,d. 4. Unitary channel current was linearly related to depolarization potential to +40 mV; at more positive potentials, inward rectification occurred. The unitary conductance was 20.3 and 35.5 pS for an [K+]o of 5.4 and 130 mM, respectively. Single-channel activity was strongly inhibited by 50 microM 4-AP or 10 mM TEA. Both 4-AP and TEA decreased open time, suggesting open-channel block. 5. Selective inhibition of IKur,d with 50 microM 4-AP or 0.3-5 mM TEA prolonged canine atrial action potentials, indicating that IKur,d contributes to canine atrial repolarization. The single-channel and macroscopic properties of IKur,d have many similarities to those of currents carried by Kv3.1 cloned channels and our findings thus suggest a possible role for Kv3.1 channels in cardiac repolarization.

A flexible 128 channel silicon strip detector instrumentation integrated circuit with sparse data readout

A full-custom CMOS integrated circuit for silicon strip detector systems has been designed, fabricated, and tested. The circuit contains 128 parallel data-acquisition channels and considerable peripheral circuitry. Each channel consists of a low-noise, low-power, charge-sensitive amplifier, a multistage autobalanced comparator, an analog multiplexer, nearest-neighbor logic, priority-search logic, and a share of a position-encoding read-only memory. The analog system can substract both detector pedestal and leakage current on a channel-by-channel basis. A key feature of this design is the inclusion of on-chip sparse read-out circuitry, which allows efficient management of low-occupancy events. Designed for use at the Collider Detector Facility (CDF) at Fermilab, the circuit is suitable for large-scale silicon detector systems requiring a large, dense array of fast, low-power electronics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Applications of the Pyroelectric Vidicon

Pyroelectric vidicons (PEV) are low-cost, infrared TV camera tubes which operate at room temperature. They image in any wavelength band within the 2-400 /.1M range. For the 8-14 band, the PEV has a typical Minimum Resolvable Temperature (MRT) of less than 1C at 250 TVL. When operated with increased pedestal current, the MRT can be reduced to 0.6C at 250 TVL. This paper reviews the significant parameters of the PEV, viz., electron-beam discharge lag, sensitivity, and thermal spread in the target, and some applications. These include fire location (specifically, forest fire mapping), security, industrial uses, and medical diagnostics. Forest fire mapping has been cooperatively done by Philips Laboratories and the Canadian Forestry Service. During that exercise, local sources of burning totally obscured by smoke were readily located by the PEV system. The unique ability of the PEV to detect motion lends itself to security applications. A chopping and storage mode of operation has been developed for industrial and medical applications. With this technique, the integrated signal from several frames can be displayed as a static image. Images of a circuit board show components operating at elevated temperatures. Veinal structures in various parts of the human body have been easily imaged with this system.