217 research outputs found
Equilibrium reconstruction for Single Helical Axis reversed field pinch plasmas
Single Helical Axis (SHAx) configurations are emerging as the natural state
for high current reversed field pinch (RFP) plasmas. These states feature the
presence of transport barriers in the core plasma. Here we present a method for
computing the equilibrium magnetic surfaces for these states in the force-free
approximation, which has been implemented in the SHEq code. The method is based
on the superposition of a zeroth order axisymmetric equilibrium and of a first
order helical perturbation computed according to Newcomb's equation
supplemented with edge magnetic field measurements. The mapping of the measured
electron temperature profiles, soft X-ray emission and interferometric density
measurements on the computed magnetic surfaces demonstrates the quality of the
equilibrium reconstruction. The procedure for computing flux surface averages
is illustrated, and applied to the evaluation of the thermal conductivity
profile. The consistency of the evaluated equilibria with Ohm's law is also
discussed.Comment: Submitted to Plasma Physics and Controlled Fusio
Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET
The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR
Relationship of edge localized mode burst times with divertor flux loop signal phase in JET
A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM
Refined Theory of Diamagnetic Effect in Stellarators
"The difference delta Phi between toroidal magnetic flux Phi_p through the plasma cross-section and the same flux Phi_v of a vacuum magnetic field is calculated analytically for ""conventional"" stellarators with planar circular axis. It has been done without limitations of aspect ratio, shape and position of a plasma. The results obtained show weak dependence of delta Phi/Phi_p on the geometry of equilibrium configuration. It proves that diamagnetic measurements can be considered as a reliable basis for the direct evaluation of stored plasma energy at various experimental conditions.
General Approach to the Evolving Plasma Equilibria with a Resistive Wall in Tokamaks
The dynamic problem of plasma equilibrium in a tokamak is considered taking into account the electromagnetic reaction of the vacuum vessel resistive wall. The currents induced in the wall during transient events contribute to the external magnetic field that determines the plasma shape and position. Accordingly, the plasma geometry must evolve so that the inductive excitation of the wall current would properly compensate for the resistive losses. Simultaneous consideration of these factors presents the main difficulty of the description. It is performed in a general form using the Green’s function method that guarantees the mathematical accuracy of expressions for the magnetic fields from each source. At the same time, it is desirable to minimize the related complications, which is one of the goals here. The starting point is the standard solution of the external equilibrium problem given by integral relating the poloidal magnetic flux to the magnetic field at the plasma boundary. In the evolutionary problem, the additional equations for the plasma-wall electromagnetic coupling are transformed to an equation with a similar integral over the wall, but with either the time derivative of the poloidal magnetic flux or the wall current density in the integrand. The mentioned similarity allows to use the already developed techniques, which makes this formulation compact and convenient. It provides the basis for extension of the existing analytical theory of equilibrium to the case with non-circular plasma and wall
Modeling of the rotational stabilization of tokamak plasmas with account of skin effect in the resistive wall
Sideways force due to coupled rotating kink modes in tokamaks
The possibility of generation of the rotating sideways force on the wall by the kink modes is analytically investigated. The approach is basically the same as that developed earlier in (Mironov and Pustovitov 2017 Phys. Plasmas 24 092508) for the locked modes, but now their rotation is allowed. Its main elements are ∂b/∂t ≠ 0 (described by the growth rate γ and angular rotation frequency ω of the magnetic perturbation b), resistive dissipation in the wall, and the requirement of zero sideways force on the plasma. These make the approach greatly different from those resulting in the so-called Noll's formula. The result is also different; it predicts a force an order of magnitude smaller. Nevertheless, such a force can be dangerous at the resonance frequency of the vacuum vessel. The derived relations show that the rotating force must be maximal at ωτ w = O(1), where τ w is the resistive wall time. For the faster modes it decreases roughly as ∼1/ω
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