Massive, long-lived electrostatic potentials in a rotating mirror plasma.

Hot plasma is highly conductive in the direction parallel to a magnetic field. This often means that the electrical potential will be nearly constant along any given field line. When this is the case, the cross-field voltage drops in open-field-line magnetic confinement devices are limited by the tolerances of the solid materials wherever the field lines impinge on the plasma-facing components. To circumvent this voltage limitation, it is proposed to arrange large voltage drops in the interior of a device, but coexist with much smaller drops on the boundaries. To avoid prohibitively large dissipation requires both preventing substantial drift-flow shear within flux surfaces and preventing large parallel electric fields from driving large parallel currents. It is demonstrated here that both requirements can be met simultaneously, which opens up the possibility for magnetized plasma tolerating steady-state voltage drops far larger than what might be tolerated in material media.

Diffraction of Gaussian and Laguerre-Gauss beams from a circular aperture using the moment expansion method.

A method based on the distribution theory is introduced to compute the Fresnel diffraction integral. It is applied to the diffraction of Gaussian and Laguerre-Gauss beams by a circular aperture. Expressions of the diffracting field are recast into a perturbation series describing the near- and far-field regions.

An ultracompact X-ray source based on a laser-plasma undulator.

The capability of plasmas to sustain ultrahigh electric fields has attracted considerable interest over the last decades and has given rise to laser-plasma engineering. Today, plasmas are commonly used for accelerating and collimating relativistic electrons, or to manipulate intense laser pulses. Here we propose an ultracompact plasma undulator that combines plasma technology and nanoengineering. When coupled with a laser-plasma accelerator, this undulator constitutes a millimetre-sized synchrotron radiation source of X-rays. The undulator consists of an array of nanowires, which are ionized by the laser pulse exiting from the accelerator. The strong charge-separation field, arising around the wires, efficiently wiggles the laser-accelerated electrons. We demonstrate that this system can produce bright, collimated and tunable beams of photons with 10-100 keV energies. This concept opens a path towards a new generation of compact synchrotron sources based on nanostructured plasmas.

Angular-momentum evolution in laser-plasma accelerators.

The transverse properties of an electron beam are characterized by two quantities, the emittance which indicates the electron beam extent in the phase space and the angular momentum which allows for nonplanar electron trajectories. Whereas the emittance of electron beams produced in a laser-plasma accelerator has been measured in several experiments, their angular momentum has been scarcely studied. It was demonstrated that electrons in a laser-plasma accelerator carry some angular momentum, but its origin was not established. Here we identify one source of angular-momentum growth and we present experimental results showing that the angular-momentum content evolves during the acceleration.

Relativistic wave-induced splitting of the Langmuir mode in a magnetized plasma.

A relativistic effect that occurs in a magnetized plasma irradiated by a circularly polarized wave is identified and analyzed: the usual plasma frequency associated with longitudinal oscillations splits into two new frequencies. We set up a Hamiltonian description of the plasma dynamic in order to identify this effect that results from the coupling between the plasma oscillation and the transverse circular motion driven by both the magnetic and wave fields. Within the small oscillations approximation, we compute for right- and left-handed polarization the two characteristics frequencies of the electron oscillations as functions of the field and wave parameters. We also describe the electron trajectories in the wave, magnetic, and restoring plasma fields. This new class of oscillations is rotational and therefore radiate suggesting a method for the diagnostics of strong static magnetic field in laser-plasma experiments.

Inductive heating and E to H transitions in capacitive discharges.

Capacitive discharges have classically been modeled in the electrostatic approximation. However, electromagnetic effects become significant if the excitation wavelength lambda and the plasma skin depth delta are not infinite. An electromagnetic model valid in the entire range of lambda and delta of practical interest is solved. We find that the plasma may either be sustained by the usual capacitive (E) field or by an inductive (H) field, and that the discharge experiences E to H transitions as the voltage between the electrodes is raised.

Enhanced plasma transport due to neutral depletion.

The dynamics of plasma and neutral gas in pressure balance are solved self-consistently to reveal the impact of neutral depletion. Analytical relations that determine the electron temperature, the rate of ionization, and the plasma density are derived. Because of the inherent coupling of ionization and transport, an increase of the energy invested in ionization can nonlinearly enhance the transport process. We show that such an enhancement of the plasma transport due to neutral depletion can result in an unexpected decrease of the plasma density when power is increased, despite the increase of the flux of generated plasma.

Relativistic kinetic theory of pitch angle scattering, slowing down, and energy deposition in a plasma.

The collisional dynamics of a relativistic electron population in a Lorentzian plasma are investigated and analyzed within the framework of kinetic theory. The relativistic Fokker-Planck equation describing both slowing down and pitch angle scattering is derived, analyzed, and solved. The analytical Green function is used to express the electron range, the range straggling, and the mean radial dispersion as a function of the plasma parameters. Compared to standard slowing down theories, the inclusion of the pitch angle scattering without any Gaussian approximation appears to be essential to calculate these quantities.

Current drive in a ponderomotive potential with sign reversal.

Noninductive current drive can be accomplished through ponderomotive forces with high efficiency when the potential changes sign over the interaction region. The effect, which operates somewhat like a Maxwell demon, can be practiced upon both ions and electrons. The current-drive efficiencies, in principle, might be higher than those possible with conventional rf current-drive techniques. It remains, however, for us to identify how the effect might be implemented in a magnetic fusion device in a practical manner.

Collisional versus collisionless resonant and autoresonant heating in laser-cluster interaction.

When a hot cluster expands, a transient matching between the plasma frequency and the laser frequency has been predicted, observed, and analyzed recently. The associate energy transfer to the electrons has been described as an enhanced collisional absorption. However, for hot plasmas the collision frequency is small and a collisionless resonant heating is more efficient. We set up and solve the problem of resonant collisional and collisionless cluster heating taking into account cluster expansion, laser pulse duration and pulse chirping. Moreover, we identify an efficient autoresonant mechanism of collisionless heating with a chirped laser pulse when the crossing between the plasmon frequency and the laser frequency is degenerate and the time derivatives of these two frequencies are equal at the crossing time. Transition between collisional regime of cluster heating and collisionless one is discussed.

Magnetic field generation through angular momentum exchange between circularly polarized radiation and charged particles.

The interaction between circularly polarized radiation and charged particles can lead to generation of magnetic field through an inverse Faraday effect. The spin of the circularly polarized electromagnetic wave can be converted into the angular momentum of the charged particles so long as there is dissipation. We demonstrate this by considering two mechanisms of angular momentum absorption relevant for laser-plasma interactions: electron-ion collisions and ionization. The precise dissipative mechanism, however, plays a role in determining the efficiency of the magnetic-field generation.