Reversed-field Configurations Research Articles

Modeling for compression of field-reversed configurations by an imploding liner

This article proposes a one-dimensional physical model to investigate the compression of reversed-field configurations (FRCs) by an imploding cylindrical liner. In this model, axial contraction of FRCs is included and parallel thermal conduction is considered as well as the radial, approximately in the open field line region of FRCs. Comparison with Spencer's analytic model of FRCs adiabatic compression shows similar results. Modeling results also indicate that classical transport model is preferred in the magnetized target fusion regime and axial contraction plays an important role in the dynamics of compression of FRCs using an imploding liner.

Normal modes propagation in a helical free-electron laser with an axial guide field

Following up the detailed three-dimensional analysis of the particle dynamics in a free electron laser with a helical wiggler and a uniform axial guide magnetic field, achieved by Rhimi et al. [1], we study the motion of the electrons in the neighborhood of the ideal trajectory. The normal modes <img src="http:/img/fbpe/bjp/v31n3/shace.gif">± of the quadratic Hamiltonian are analyzed numerically in both the normal and reversed-field configurations for different values of the ratio <img src="http:/img/fbpe/bjp/v31n3/20eq01.gif">w/<img src="http:/img/fbpe/bjp/v31n3/20eq01.gif">0. The electron trajectories are thus discussed. It is found that pseudo-circular and elliptic trajectories are characterizing both group-I and reversed-field group-II modes while the motion in the normal group-II mode is much more complicated.

Electron trajectories in a helical wiggler with a uniform axial guide field: a comparison of numerical results with results obtained using an approximate but analytical method

A comparison is made between two methods of calculating electron trajectories in a magnetic field consisting of a helical wiggler and a uniform axial guide field. The code SOLITUDE, developed by Gouard, was used to calculate with high precision the trajectories of electrons whose initial positions and velocities were chosen at random from a Gaussian distribution. After propagation through an adiabatic entry, the positions and velocities at the point where the wiggler field becomes steady were used as input to an analytical calculation developed by Donohue and Rullier. Although the calculation is analytic, the method involves truncating the true Hamiltonian, and is therefore not exact. Despite this, generally good agreement is found between the trajectories generated by the two methods, in both the normal and reversed-field configurations. The analytical approach provides physical insight into certain unusual aspects of some of the numerically generated trajectories. Several illustrative examples are compared in detail.

Observations of frequency chirping and phase of a free electron laser amplifier

We report observations of frequency chirping and phase of a free electron laser amplifier operating in the Raman regime. The FEL is driven by a mildly relativistic electron beam (750 kV, 25 ns) subjected to a combined axial magnetic field and a helical wiggler field. The input into the FEL amplifier is provided by a high power magnetron tuned to a frequency of 33.39 GHz. Phase and frequency shifts are measured both as a function of time and interaction length. It is found that in the Group I regime of FEL operation the output frequency is upshifted by ∼ 100 MHz, but smaller upshifts are observed in the Group II or the Reversed Field configurations.

Reversed Field Configuration in Force-Free State

Two reversed field configurations in force-free state of rot B =α B have been considered; one is a straight cylindrical configuration with α( r ) dependent on radial coordinate r and the other is a toroidal one. The cylindrical configuration is obtained by choosing functional form of α( r ) in such a way that it closely match experimental results. The toroidal configuration is analyzed by using an expansion procedure. A new description of F-\( \varTheta \) diagram has been introduced in order to explain experiments.

Recent results of OHTE experiments

OHTE reversed-field configurations with separatrix were produced, and high-beta confinement observed. Magnetic measurements identified m = 1 and m = 0 activity in both OHTE and RFP discharges, and B ̃ /B ∼ S − 1 2 . After careful adjustment of applied vertical field, both OHTE and RFP discharges were sustained well beyond the resistive decay or configuration time.

Helical Symmetric Reversed Field Configurations with Minimum Potential Energy

The Helical symmetric reversed field configurations with minimum potential energy has been studied by applying the variational principles to the cylindrical plasma of finite plasma pressure.The helical symmetric magnetic field with finite pressure gradient, which is derived from the MHD equilibrium equation, can't satisfy the condition of minimum potential energy obtained based on the Woltjer's invariance.An axisymmetric RFP which satisfies the Woltjer's invariance, can only confine the pressure gradient plasma without destroying the MHD equilibrium.

Numerical simulation of the anomalous transport process in radially compressed reversed-field configurations

A one-dimensional quasi-equilibrium MHD code has been used to study the effect of heat transport in belt-pinch-like reversed-field configurations compressed by an imploding cylindrical conducting liner. The code incorporates classical and anomalous transport coefficients, including drift ion cyclotron and lower hybrid instabilities with fluctuation amplitudes limited by shear and finite-beta stabilization. For a reactor-size imploding-liner configuration, heat conduction produces flat-topped temperature profiles and peaked densities. Scaling the reactor size down by a factor of five leads to a considerable increase of anomalous heat losses. A high-field, low-density buffer region near the wall is effective as a heat insulator, provided the field is sufficiently sheared there.

Application of Plasma Focus Device to Compression of Toroidal Plasma

A new concept of compressing a toroidal plasma using a plasma focus device is considered. Maximum compression ratio of toroidal plasma is determined merely by the initial density ratio of the toroidal plasma to a sheet plasma in a focus device because of the Rayleigh-Taylor instability, An initiation senario of plasma-liner is also proposed with a possible application of this concept to the creation of a burning plasma in reversed field configurations, i.e., burning plasma vortex.

Theoretical studies of the formation and adiabatic compression of reversed-field configurations in imploding liners

The formation and compression of e-beam-induced reversed-field theta-pinch configurations are studied theoretically in an MHD treatment. Taking into account classical collisional resistivity plus anomalous resistivity derived from microinstabilities, it is shown that plasmas can be created with parameters suitable for liner compression experiments. Adiabatic compression of the quasi-equilibrium plasma/flux mixture is described by using Lagrangian co-ordinates. Results are given for a sample (unoptimized) initial configuration. In the final state, the ratio of plasma to total energy and the total D-T rate are 30% and 20%, respectively, of the values which would be obtained if all the compressional energy went into heating plasma,i.e. assuming negligible magnetic-field-filled volume.

Permanent Aspects of the Earth's Non-dipole Magnetic Field over Upper Tertiary Times

Summary Investigation of collected palaeomagnetic results from continental igneous rocks and from oceanic sediment+ores, shows a persistent off-centre displacement of the effective dipole source of the main field over Quaternary and Recent times (about the past two million years). This dipole displacement is 191 f38 km northward along the rotational axis. Further evidence suggests a similar enduring displacement during all of Upper Tertiary time. The effect of this result on palaeomagnetic interpretations is discussed. The evidence also suggests that we might search for significant differences between the time-average normal and reversed field configurations, and for other internal manifestations of the north-south asymmetry. A consistently world-wide eastward declination of 3.3 k 1.0 has been found for the time-average Upper Tertiary and later field, but no satisfactory explanation has been proposed.