Active feedback stabilization of toroidal external modes in tokamaks

Active feedback stabilization of pressure-driven modes in tokamaks is investigated by toroidal computations. Typically, the feedback does not strongly modify the plasma-generated magnetic field perturbation. Feedback with modest gain and a single coil array poloidally stabilizes substantially for a range of coil shapes. Optimum design uses narrow sensor coils not too far from the plasma and rather wide feedback coils, which may be outside the resistive wall. Complex gain, which makes the mode rotate, can decrease the gain required for stabilization, but real gain is more robust.     

Control of asymmetric magnetic perturbations in tokamaks

The sensitivity of tokamak plasmas to very small deviations from the axisymmetry of the magnetic field |deltaB/B| approximately 10{-4} is well known. What was not understood until very recently is the importance of the perturbation to the plasma equilibrium in assessing the effects of externally produced asymmetries in the magnetic field, even far from a stability limit. DIII-D and NSTX experiments find that when the deleterious effects of asymmetries are mitigated, the external asymmetric field was often made stronger and had an increased interaction with the magnetic field of the unperturbed equilibrium. This Letter explains these counterintuitive results. The explanation using ideal perturbed equilibria has important implications for the control of field errors in all toroidal plasmas.     

Nature of the isotope effect on transport in tokamaks

The reduction of energy and particle losses with the increasing mass of the hydrogen isotope is more pronounced under conditions of improved confinement when the dominant ion temperature gradient instability is suppressed and other channels of anomalous transport are of importance. In this Letter, we reconsider the dissipative trapped electron (DTE) instability by taking into account finite Larmor radius effects in the analysis of the ion response to perturbations. By applying the improved mixing length approximation in order to estimate the transport coefficients, it is demonstrated that DTE contribution is intrinsically dependent on the isotope mass and provides a plausible explanation for the isotope effect. Contrary to the common belief, it is shown that the DTE turbulence may be of importance for reactor plasmas of low collisionality.     

Triplet Josephson effect with magnetic feedback in a superconductor-ferromagnet heterostructure

We study the ac Josephson effect in a superconductor-ferromagnet heterostructure with a variable magnetic configuration. The system supports triplet proximity correlations whose dynamics is coupled to the magnetic dynamics. This feedback dramatically modifies the behavior of the junction. The current-phase relation becomes double periodic at both very low and high Josephson frequencies omegaJ. At intermediate frequencies, the periodicity in omegaJt may be lost.     

Natural velocity of magnetic islands

The phase velocity of magnetic islands is calculated in the semicollisional regime with cold ions. Two solution branches arise, corresponding to islands propagating with the ions and with the electrons. For the ion branch the phase velocity and the polarization current are small. For the electron branch, the phase velocity depends on the ratio of W, the half-width of the island, and rho(s), the Larmor radius calculated with the electron temperature. For W >rho(s) the phase velocity is larger than the electron drift velocity and the polarization current is destabilizing. For W < rho(s), the situation is reversed provided that the density and temperature gradients point in the same direction.     

Scale size of magnetic turbulence in tokamaks probed with 30-MeV electrons

Measurements of synchrotron radiation emitted by 30-MeV runaway electrons in the TEXTOR-94 tokamak show that the runaway population decays after switching on neutral beam injection (NBI). The decay starts only with a significant delay, which decreases with increasing NBI heating power. This delay provides direct evidence of the energy dependence of runaway confinement, which is expected if magnetic modes govern the loss of runaways. Application of the theory by Mynick and Strachan [Phys. Fluids 24, 695 (1981)] yields estimates for the "mode width" (delta) of magnetic perturbations: delta<0.5 cm in Ohmic discharges, increasing to delta = 4.4 cm for 0. 6 MW NBI.     

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.     

Geodesic plasma compressibility effects on the magnetic islands in a tokamak

It is shown that the stability of rotating magnetic islands in a tokamak plasma is affected by plasma compressibility related to the geodesic curvature in an inhomogeneous magnetic field. A robust contribution has been found to the Rutherford evolution equation. It is shown that the sign of the geodesic curvature contribution is opposite to the sign of the polarization term. It is suggested that this mechanism plays a crucial role in the stability of small scale magnetic islands.     

Skin effect modifications of the Resistive Wall Mode dynamics in tokamaks

We present the first evidence of the skin-effect modification of the Resistive Wall Mode (RWM) dynamics in a tokamak. The computations are performed with the CarMa code, using its unique ability of treating volumetric 3D conducting structures. The results prove that conventional thin-wall models and codes, assuming the thin equivalent wall located on the inner side of a real (thick) wall, may fail to get accurate estimates of RWM growth rates, since the inclusion of the skin effect makes the growth rates always larger than otherwise. The difference is noticeable even for the conventional slow RWMs and becomes substantial for faster modes. Some possible equivalent thin-wall modeling approaches are also discussed.     

Improvement of confinement in tokamaks by weakening of poloidal magnetic field near boundary

Theory of turbulent equipartition and experiment indicate that density, pressure, and temperature profiles follow the poloidal magnetic field profile. Therefore, it is suggested to change the magnetic geometry between core and boundary by toroidal conductors and/or plasma current. As a result, density and temperature gradients will become steeper, and stored energy will be higher with low boundary plasma parameters. The suggested new mode of confinement may substantially simplify achieving of ignition.     

On the bootstrap effect in large tokamaks with lower hybrid driven current

The amplification of lower hybrid (LH) driven current of various profiles has been found. The results strongly depend on the plasma temperature profile, on the LH current profile, and on the magnitude of the particle diffusion flux.This work was started during the visit of the author in the I. V. Kurchatov Institute of Atomic Energy, Moscow. The author thanks IAE for hospitality, and V. Parail, G. V. Pereverzev, A. Smoljakov and R. Klíma for many useful and critical discussions.     

A new method of asymmetric Abel inversion for magnetic equilibrium configuration state in tokamaks

It is difficult to obtain the asymmetrical factor along the observation direction parallel to the plasma mid-plane when the detected radiation is also in the mid-plane. This paper considers the magnetic surfaces and Grad--Shafranov shift, and develops a new method for inverse asymmetric electron density information, during magnetic equilibrium configuration in a tokamak.     

Stochastic particle acceleration in multiple magnetic islands during reconnection

A nonthermal particle acceleration mechanism involving the interaction of a charged particle with multiple magnetic islands is proposed. The original Fermi acceleration model, which assumes randomly distributed magnetic clouds moving at random velocity V(c) in the interstellar medium, is known to be of second-order acceleration of O(V(c)/c)(2) owing to the combination of head-on and head-tail collisions. In this Letter, we reconsider the original Fermi model by introducing multiple magnetic islands during reconnection instead of magnetic clouds. We discuss that the energetic particles have a tendency to be distributed outside the magnetic islands, and they mainly interact with reconnection outflow jets. As a result, the acceleration efficiency becomes first order of O(V(A)/c), where V(A) and c are the Alfvén velocity and the speed of light, respectively.