Table of contents

Nonlinear theory of Doppler reflectometry is generalized to the case of the cylindrical plasma geometry, which takes into account the plasma poloidal curvature. The registered signal power is calculated taking into account possible curvature of the probing antenna wavefront. The spectrum shift is analyzed and shown to correspond to the poloidal velocity specifically averaged over the probing wave propagation region with the weight function reflecting the geometry influence.

The resonant interaction between three longitudinal plasma waves is investigated on the basis of a relativistic kinetic theory. The corresponding nonlinear plasma permittivity is derived assuming that the initial plasma at thermal equilibrium is described by a relativistic Maxwellian distribution. Introducing the covariant variables, explicit and reduced forms of relativistically correct nonlinear permittivities are obtained which hold for arbitrary values of the complex wave-frequencies and the plasma temperatures. In the limit of nonrelativistic temperatures the covariant form of the nonlinear permittivity is expressed in terms of the plasma dispersion function which facilitates the evaluation of wave-coupling constant. In the limit of a nonrelativistic theory the covariant form is reduced to the noncovariant version which is also expressed in terms of the plasma dispersion function. The relativistic correction found in the covariant approach for nonlinear plasma permittivity is estimated to be generally small, but it could become important when superluminal plasma waves are involved in the nonlinear coupling processes.

Stability limits against external kink modes driven by large current density and pressure gradient values in the pedestal region are investigated for tokamak plasmas with separatrix. Stability diagrams for modes with different toroidal wave numbers under variations of pressure gradient and current density in the pedestal region are presented for several equilibrium configurations related to TCV. A scaling for the toroidal wave number of the most unstable mode is proposed. The influence of the plasma cross-section geometry on the stability limits is discussed.

The nonlinear propagation of incoherent magnetic field-aligned electromagnetic ion-cyclotron-Alfvén (EMICA) waves in plasmas is studied. The equations governing the nonlinear interactions between incoherent EMICA waves and ion-acoustic perturbations are then used to derive a nonlinear dispersion relation, which is appropriate for investigating the modulational instability of broadband EMICA wave-packets. The case of spectral broadening of the EMICA waves is compared with the monochromatic case. It is found that the incoherency decreases the growth rate while extended regimes for the modulational instability growth rate are allowed due to the interplay between partial coherence and the dispersive effects of the EMICA waves.

A new high-rotation (HR) mode of operation is reported for magnetized rotating plasmas. Plasma is produced by an externally applied radial electric field in the Maryland centrifugal experiment, a mirror magnetic field plasma rotating azimuthally. The HR mode is achieved by optimizing over the discharge current. Previously reported stationary or steadily declining plasma voltages are now replaced by time-increasing voltages in the HR mode, with maximum voltages significantly higher than those previously recorded. Sonic Mach numbers are close to the theoretical values required for centrifugal confinement and for suppressing large scale magnetohydrodynamic (MHD) instabilities. Plasma momentum confinement time is much longer than the MHD instability time scale and longer than in ordinary mode operation. The mode is sustained over several confinement times.

Density profile information is invaluable to plasma experiments. In particular, the edge and mantle plasma density profile is believed to influence core plasma behaviour of fusion devices such as tokamaks. In low density plasmas, light emission caused by neutral hydrogen gas injection and subsequent excitation can be used to measure the local plasma parameters. A neutral penetration model is required to evaluate plasma density from H_α light emission. This paper verifies that a simple collisionless three-dimensional neutral penetration model can be utilized in low-density, low-temperature plasmas. Since the H_α emission intensity can be linked directly to the local electron density, this model permits the reconstruction of the density profile well beyond the last closed flux surface of the plasma.

A uniform framework, based on the frequency dependent plasma response model (PRM), is proposed to study the physics and control of the resistive wall mode (RWM). The PRM is constructed, respectively, from the Fitzpatrick–Aydemir model, from a cylindrical theory with multiple RWM, and, finally, from toroidal calculations. Based on the PRM, several important aspects of the RWM physics are studied, including the interplay between active feedback and plasma rotation to stabilize the mode, the efficiency of external versus internal active coils for the mode control and the resonant field amplification effect due to a rotationally damped RWM.

The wiggler pumped ion-channel free electron laser (WPIC-FEL) is treated and the classes of possible single-particle electron trajectories in this configuration are discussed in the paper. A new region of orbital stability is seen in the negative mass regime. A kinetic description of WPIC-FEL is given. Vlasov–Maxwell equations are solved to get the linear gain in a tenuous-beam limit, where the beam plasma frequency is much less than the radiation frequency and the self-field effects can be ignored.

Experiments designed for the attainment of the hybrid advanced mode of operation in JET revealed the existence of a new form of reconnecting modes with toroidal number n = 1 and poloidal number m = 1, which have the same radial structure as sawtooth precursors but are characterized by small growth rate and low saturation amplitude. These modes develop in repetitive cycles with typical growth times of 100 ms. Temperature perturbations are not in the form of sawteeth; in fact, there is slow and mild erosion of the central temperature without any crash. Comparison with theory of m = 1 modes indicates that the slow growth observed in the hybrid regime is caused by low magnetic shear (a measure of the radial variation of field line inclination) and possibly by diamagnetic effects in the plasma core.

The linear stability of the ion-temperature-gradient (ITG) driven drift modes is investigated in an International Thermonuclear Experimental Reactor-like geometry using an advanced reactive fluid model and the ballooning mode formalism. The spectrum of stable and unstable modes and their real frequencies, growth rates and eigenfunctions are calculated for two specific magnetic flux surfaces. The effects of density and temperature gradients, temperature ratios, wave vector and geometrical quantities such as local magnetic shear (LMS), normal curvature, geodesic curvature and magnetic field on the ITG mode are discussed. It is found that the most unstable eigenfunction is extended and less unstable at the magnetic surface where global magnetic shear is reversed. Moreover, the role of positive LMS is found to be destabilizing at the reverse shear magnetic surface. However, at a positive global shear magnetic surface, the eigenmode is found to be more localized and more unstable, and its structure and stability are affected by the local behaviour of the geometrical quantities.

Plasma density fluctuations in the edge plasma of the RFX-mod device are measured through the gas puffing imaging diagnostics. Statistical features of the signal are quantified in terms of the probability distribution function (PDF), and computed for several kinds of discharges. The PDFs from discharges without particular control methods are found to be adequately described by a Gamma function, consistently with the recent results by Graves et al (J P Graves et al2005 Plasma Phys. Control. Fusion47 L1). On the other hand, pulses with external methods for the plasma control feature modified the PDFs. A first empirical analysis suggests that they may be interpolated through a linear combination of simple functions. An inspection of the literature shows that this kind of PDF is common to other devices as well and it has been suggested that it is due to the simultaneous presence of different mechanisms driving, respectively, coherent bursts and Gaussian background turbulence. An attempt is made to relate differences in the PDFs to plasma conditions such as the local shift of the plasma column. A simple phenomenological model to interpret the nature of the PDF and assign a meaning to its parameters is also developed.