Table of contents

Electron cyclotron current drive (ECCD) has been numerically simulated in magnetic islands caused by neo-classical tearing modes. The electron drift orbits are tracked with the Coulomb collisions and the quasi-linear diffusion by EC waves, which are simulated by the Monte-Carlo method. The EC resonance region is assumed to be located around the O-point and localized in the toroidal direction. The driven current is well confined in the helical flux tube which includes the EC resonance region. The driven current channel looks like a 'snake' in real space. As the results of the ECCD with 10 MW in a plasma with n_e = 3 × 10¹⁹ m⁻³ and T_e = 10 keV, the current drive efficiency is about 1.6 times higher than that of an axi-symmetric plasma with no magnetic island. The driven current profile tends to peak around the O-point with increasing EC wave power.

In this work elements of existing eigenmode identification analysis techniques are combined to yield an improved technique for the extraction of mode numbers in toroidal plasmas. The technique, which involves fitting Fourier-time and Fourier-spatial basis functions to magnetic perturbation data, uses singular value decomposition (SVD) to provide an optimal fit across a realistic subset of the full Fourier transform basis and selects the spatial basis with the least solution residue. The method yields best-fit mode numbers, mode amplitudes and phase. A stochastic analysis provides a null-test, yielding the probability that Gaussian noise would produce the same residue of the fit or mode amplitude. The technique quantifies eigenmode mode fits in toroidally confined magnetic systems. Our approach improves upon earlier techniques in that the frequency or mode number of degenerate modes are resolved, all magnetic coil information is used synchronously, wave-train averaging is performed, and a quantitative measure of fit is generated. In turn, weak magnetic signals with long coherence time, and eigenmodes which are degenerate in mode number or frequency are resolved, and the mode fit statistically quantified by comparison with noise. The latter measure enables automated rejection or acceptance of the mode fit, obtained by comparing the probability of the null hypothesis to the 1% confidence level. Convolution of the frequency-resolved mode amplitudes and residues with a Gaussian is used to improve the confidence of identification, reducing scatter at the expense of frequency resolution. Finally, the method is applied to magnetic fluctuation data from the mega Ampere spherical tokamak outboard Mirnov array for high frequency acquisition (OMAHA) in order to analyse strong low-frequency activity and weaker high frequency Alfvénic activity.

An electrostatic instability driven by counter-propagating tenuous proton beams that traverse a bulk plasma consisting of electrons and protons is considered. The system is spatially homogeneous and is evolved in time with a one-dimensional particle-in-cell simulation, which allows for a good statistical plasma representation. Mildly and highly relativistic beam speeds are modeled. The proton beams with a speed of 0.9c result in waves that saturate by the trapping of electrons. The collapse of the phase space holes in the electron distribution scatters these to a flat-top momentum distribution. The final electric fields are weak and the proton beams are weakly modulated. No secondary instabilities are likely to form that could thermalize the proton beams. The proton beams moving with 0.99c initially heat the bulk plasma through a three-wave interaction. Coalescing phase space holes in the bulk proton distribution arising from the saturation of ion acoustic waves transport wave energy to low wavenumbers. Highly relativistic phase space holes form in the electron distribution, which are not spatially homogeneous. The spatial envelope of these electron phase space holes interacts with the fluctuations driven by the phase space holes in the bulk protons, triggering a modulational instability. A Langmuir wave condensate forms that gives rise to strong and long electrostatic wave packets, as well as to a substantial modulation of the proton beams. The final state of the system with the highly relativistic proton beams is thus more unstable to further secondary instabilities that may transfer a larger beam energy fraction to the electrons and thermalize the proton beams more rapidly.

We perform a parametric study of highly oblique solitary and periodic nonlinear stationary slow magnetoacoustic waves in the model of McKenzie and Doyle, and Stasiewicz, taking plasma pressure anisotropy into account. The model was proposed for the interpretation of linear magnetic holes, where there is almost no variation in direction of the ambient magnetic field direction. In particular, we focus on magnetic structures in high-β (ratio of kinetic to magnetic pressures) plasmas. We find that gas pressure anisotropy is crucial for the existence of solitary structures in this model for the parameter regime considered. Pressure anisotropy leads to the existence of field enhancing bright solitary structures and field depleting dark solitary structures. To explain the parametric range of the existence of solitary and periodic wave solutions of the model, a pseudo-(Sagdeev) potential is used. We obtain an analytic form for this potential in the case of low Alfvén mach waves. We find that the potential method accurately predicts the parametric range of the existence of solitary waves as calculated by direct numerical simulations.

The collisionless residual zonal flow level is calculated for toroidally axisymmetric plasmas with arbitrary aspect ratio and large elongation. For small aspect ratio, this level deviates largely from that predicted by the Rosenbluth–Hinton formula and retains a finite value. Plasma shaping such as elongation is favorable for retaining the residual zonal flow as well. The trapped particles make a major contribution to the neoclassical polarization.

Gyrokinetic calculations with the GS2 code of impurity transport in ASDEX Upgrade H-mode plasmas are presented. A method to separate the diagonal and off-diagonal terms in the quasi-linear flux computed by the gyrokinetic code for a trace impurity is introduced and applied. It is shown that in the experimental conditions of strong central electron cyclotron heating, unstable modes rotating in the electron diamagnetic direction are excited in the central region of the plasma. These are generated by the non-adiabatic response of passing electrons and feature extremely elongated eigenfunctions along the field line. The related fluctuations in the electrostatic potential generate an outward convection of the impurities. These theoretical findings are in agreement with the outward convection of laser ablated Si measured in these experimental conditions. In contrast, in the outer region of the plasma, as well as everywhere in the case of discharges without central electron heating, ion temperature gradient modes produce an impurity pinch directed inwards, which is also in agreement with the experimental observations.

A method is presented to determine all the elements of the plasma Mueller matrix with high time-resolution (about 1 µs) using only measurements of phase. This knowledge allows one to derive the pure Cotton–Mouton and Faraday effects even when polarimetric effects are large.

A theoretical study of electron trajectories and gain in a planar wiggler free electron laser (FEL) with ion-channel guiding is presented. A kinetic description is given. The linearized Vlasov–Maxwell equations are solved to investigate the growth rate in a tenuous-beam limit for perturbation about a general beam equilibrium distribution function, and generated harmonics are discussed. Linear gain corresponding to FEL resonance for the case of mono-energetic beam is derived and continuous tuning by varying the ion-channel density, for the fundamental mode, is illustrated.

The original optimization of the Columbia Nonneutral Torus (CNT) considering only volume (and error field resilience) was also successful in optimizing the stored energy. To assess the general confinement properties of a device, studies of the 1/ν neoclassical transport (effective ripple _eff) are important. For CNT the field line tracing code NEO [1] is used to compute _eff. NEO is used by the code SORSSA [2, 3] for computation of the total stored energy based on neoclassical transport.

In the reversed field pinch experiment RFX-mod a gas puffing imaging diagnostic is used to investigate the turbulence of the edge plasma. The system consists of a gas puffing nozzle and 32 optical channels to measure the He I (668 nm) line emission. The lines of sight are arranged into three fans intersecting each other in an area normal to the main magnetic field. The diagnostic system provides an analogue bandwidth of 2 MHz and all channels are simultaneously sampled at 10 Msamples s⁻¹ for the whole discharge duration (350 ms). Different inversion techniques have been applied to the data in order to obtain a 2D tomographic reconstruction of the light emission pattern from the line integrals. Comparison shows that the most precise method is based on the 2D spatial Fourier expansion, applying the singular value decomposition technique with a suitable regularization method to avoid artefacts. The high time resolution allows one to obtain a 2D image every 100 ns. Emission structures ('blobs') that move along the E × B flow emerge from the background turbulence and they are characterized by computing energy and phase of the Fourier modes.

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