Table of contents

A simple helical magnetohydrodynamic equilibrium with flow, in which the vorticity is proportional to the current density, is presented. It is argued that the helical magnetic structures, known as snakes, can be described as a local transition to this equilibrium, triggered by the large temperature drop, observed when a pellet crosses the q = 1 rational magnetic surface.

In this paper mode conversion of the ordinary wave to the extraordinary wave in a cold magnetized plasma at electron cyclotron frequencies is examined. A reduced set of wave equations, accounting for both the 2D inhomogeneity of the magnetic field and an arbitrary poloidal component of the magnetic field and coupling of the ordinary wave to the extraordinary wave near the O-mode cutoff surface, is derived. An integral representation of solutions to this set of equations is found. The conversion and reflection coefficients are calculated and analysed.

A method for zonal flow study by using direct density fluctuation measurements is proposed. When ambient drift-wave turbulence is modulated by zonal flows (i.e. in the drift-wave-zonal flow systems), an envelope of the ambient density fluctuations has spectral peaks around zonal flow frequencies. A spectral peak at the geodesic acoustic mode (GAM) frequency is observed in the envelope of the ambient density fluctuations measured in edge plasma of the JFT-2M tokamak. The significant cross-bicoherence is also found between the ambient density fluctuations and its envelope in the GAM frequency. This result demonstrates that we can measure the GAM only by using density fluctuation data. This method provides a possibility of zonal flow research in burning core plasma by density fluctuation diagnostics such as microwave reflectometry.

An X-mode swept-frequency reflectometry diagnostic has recently been upgraded for density profile measurement on the JET tokamak. To overcome the extensive computing time required for full-wave simulation, a ray-tracing code has been developed for studying the 3D geometry effects on the JET reflectometry measurement of the density profile. Although such a ray-tracing code is not adequate to examine the role of plasma turbulence, comparisons with full-wave computations have shown that it is still a useful tool for simulation of the density profile measurements in the case of smooth spatial variations of the plasma parameters. The effects of the plasma geometry and the radiation pattern of the reflectometer emitting and receiving antennas on the spatial resolution of these measurements are investigated in this paper. In particular, the results point out the weak role played by the multi-dimensional effects on the reflectometry measurements performed in the equatorial mid-plane of large tokamaks such as JET.

A deconvolution of time-resolved ion collector signals was used to determine the temperature and the centre-of-mass (COM) velocity of H⁺ and C^q+ (1 ≤ q ≤ 6) ions emitted from a polyethylene target irradiated by the 3rd harmonics (λ = 438 nm) of an iodine laser up to the laser-beam intensity value of about 2 × 10¹⁶ W cm⁻². The intensity dependence of the COM energy for single charge-states is shown. A plasma block, which precedes the main ion group, is characterized. The properties of the main ion group are compared with those of fast ions produced by a CO₂ laser [1]. It is shown that the separation of peaks, i.e. their visualization in the time-resolved ion collector signal, can be characterized by the ratio of thermal velocity to COM velocity. The charge-state dependence of the COM velocity is discussed.

The resonant interaction between longitudinal and transverse plasma waves is investigated on the basis of a relativistic kinetic theory. The relativistic form of nonlinear plasma permittivity is derived assuming that initial plasma is at thermal equilibrium and described by a relativistic Maxwellian distribution. For low nonrelativistic plasma temperatures (k_BT/mc² ≪ 1), the nonlinear plasma permittivity is markedly different from the standard form provided by a classical nonrelativistic approach. The relativistic correction derived in this case is estimated to be of the order of thermal plasma parameter, μ⁻¹ = k_BT/mc², and it will vanish only for an idealized cold plasma, T → 0.

Transport theory is applied to magnetic helicity injection into plasmas with toroidal geometry. Magnetic relaxation during helicity injection can be described as hyper-resistive diffusion of the current. By using the generalized Balescu–Lenard extension of quasi-linear transport theory, it is shown that hyper-resistive diffusion is generally slow compared with heat transport. It follows that magnetic relaxation due to such turbulence tends to flatten the temperature profile, as observed in reversed-field pinches. Given flattened temperature profiles, Taylor's minimum principle for magnetic relaxation is usefully reformulated as minimum dissipation, yielding circuit equations for electrostatic helicity injection in laboratory devices such as spheromaks and tokamaks. A favorable heat pinch could benefit helicity injection into tokamaks. These results are also relevant to natural phenomena involving the generation of fields by magnetic relaxation.

The results of theoretical studies and numerical simulations of laser-driven acceleration of a flat foil up to ultrahigh velocity of the order of 1000 km s⁻¹, which corresponds to the achievement of thermonuclear temperatures due to kinetic energy transition into thermal energy at an inelastic impact, are reported. The behavior of a foil accelerated to such high velocities, in particular, the distribution of foil density, which defines thermonuclear reaction intensity, has been studied. The calculation results are compared with the results of the experiments performed on the Gekko/HIPER laser, where a laser-driven projectile achieved record-breaking velocity. The laser pulse and foil parameters responsible for acceleration of the projectile up to 'thermonuclear' velocities in a dense state have been determined.

Experiments on plasma turbulence and related transport processes generally require diagnostics with high spatio-temporal resolution. Standard tools for this purpose are statistical methods like cross-correlation analysis or conditional averaging. High spatial resolution is achieved at the expense of an asynchronous recording and a subsequent averaging process. Recently, we had shown that diagnostics arrays in combination with a super-resolution algorithm can achieve a comparably high spatial resolution [1]. This paper compares the results of both techniques applied to identical experimental conditions. It addresses the question of whether the higher efforts for spatio-temporal diagnostics are needed to study the dynamics of large-scale structures in a turbulent system and to what extent the results of statistical approaches are reliable.

Collisionless ion collection by a negatively biased stationary spherical probe in a finite shielding length plasma is investigated using the Particle in Cell code SCEPTIC, in the presence of a weak magnetic field B. The overall effect of the magnetic field is to reduce the ion current, linearly in |B| for weak enough fields, with a slope steepness increasing with the electron Debye length. The angular current distribution and space-charge buildup strongly depend on the focusing properties of the probe, hence on its potential and the plasma shielding length. In particular, it is found that the concavity of the ion collection flux distribution can reverse sign when the electron Debye length is comparable to or larger than the probe radius (λ_De ≳ r_p), provided the ion temperature is much lower than the probe bias (T_i ≪ −ZeV_p).

Real time control of the plasma elongation and plasma current using electron cyclotron resonance heating (ECRH) actuators has been demonstrated on TCV. Tokamak plasmas may be elongated by off-axis ECRH. The associated flattening of the current density profile increases the plasma's vertical stability, enabling higher elongations to be obtained, at lower currents than would be possible in Ohmic conditions. Successful experiments were carried out on TCV to control, in real time, the magnitude of the elongation and the ECRH deposition location using second harmonic ECRH power and ECRH launcher mirror angle actuators. Plasma current control in fully non-inductive plasmas using ECRH actuators to drive ECCD was also demonstrated.