G. Matthieussent

Coupling between electron and ion waves in Nd‐laser beat‐wave experiments

The beating between two colinear Nd‐YLF and Nd‐YAG lasers in a homogeneous plasma generates intense relativistic plasma waves associated with a high longitudinal electric field of the order of 1 GV/m. It is shown that these electron waves couple with ion waves in the regime of modulational instability. Electric field amplitude and saturation time obtained by Thomson scattering are in agreement with theoretical predictions taking this mechanism into account.

The plasma beat-wave acceleration experiment at Ecole Polytechnique

Abstract We present an experiment for demonstrating the principle of plasma beat-wave acceleration. The beating of two Nd-laser pulses creates a relativistic plasma wave in a deuterium plasma. Electrons at an energy of 3 MeV are injected into the plasma. We observe several hundred electrons accelerated up to 3.7 MeV. This paper is mainly devoted to the description of the experimental apparatus. In the design of the apparatus, we gave particular attention to efficient electron injection and to background noise suppression. We present also some preliminary results of electron acceleration experiments.

Oblique whistler waves generated in cold plasma by relativistic electron beams

Parallel and oblique propagation of whistler waves generated by relativistic electron beams are examined. From the general dispersion equation one derives the analytical formula for the linear growth rate in the relativistic case. The growth rate is computed for realistic plasma parameters, using two distinct distribution functions for the hot electron population. The results of the present paper can explain phenomena occurring in space plasmas where relativistic electrons are present.

Laser particle acceleration: beat-wave and wakefield experiments

In a plasma, some of the energy of a high-power laser beam can be transferred to a longitudinal plasma wave with a high phase velocity. This wave can in turn accelerate relativistic charged particles to very high energies. Several mechanisms have been proposed to generate these intense electric fields and some of them have already been tested experimentally. Using the beat wave method, electric fields of 1 - 10 have been produced and electrons have been accelerated with an energy gain from 1 MeV to more than 30 MeV. Some preliminary experiments have shown that electrons can be accelerated in plasma waves generated by the wakefield method. In the case of self-modulated wakefield, electric fields larger than 100 trap electrons and eject them from the plasma with an energy up to 100 MeV. The perspectives in the near future are the production of intense and short electron beams of a few MeV and the acceleration of electrons up to 1 GeV. To reach an energy of 1 TeV and get closer to the parameters required by the high-energy physicists, one will have to test some new methods to be able to guide the laser beam over large distances.

Plasma production by multiphoton ionization: Density inhomogeneities due to ponderomotive effects

The generation of long and homogeneous plasmas by multiphoton ionization of low‐pressure gases—H2, D2, and N2 at pressures of a few Torr—with a high‐power laser at λ=0.53 μm and intensities of the order of 1015 W/cm2 has been studied. The temperature and density of the plasma are measured by Thomson thermal scattering. After the initial full ionization the hydrodynamic density evolution with time is observed. The comparison with a simple model shows that this evolution is mainly due to the ponderomotive force of the laser beam itself. Typical density variations range from 1% to 5% per 100 psec.

Nonlinear effects and chaotic behavior at plasma resonance

Resonant absorption of an electromagnetic wave (f=3.5 GHz) in a multipolar discharge (ne0≂1011 cm−3) with an adjustable density gradient is studied. The transition from a nonlinear steady‐state regime toward a chaotic one, occurring when the pump field or the gradient length is increased, is investigated experimentally and numerically.

Mach cone structure generation by the interaction of a short microwave pulse with a plasma

When a non-uniform unmagnetized plasma is irradiated by a short microwave pulse with length of the order of an ion-wave period, the travelling wavepacket of the electron plasma wave, and nonlinear large-amplitude waves in an ion-wave regime have been observed. The observed large-amplitude waves typically have two frequencies. The higher-frequency wave is an ion wakefield excited by the travelling wavepacket of the electron plasma wave, while the lower-frequency mode, also called a density cavity streamer, can be interpreted as a pseudowave induced by the supersonic ion bunch with a typical energy of 25 eV produced at the resonance layer. The experimental results are in reasonable agreement with a theoretical model based on the Mach cone structure.

ELECTRON ACCELERATION IN LASER WAKEFIELD EXPERIMENT AT ECOLE POLYTECHNIQUE

An experiment has been performed with the LULI Multi-TeraWatt Laser. The acceleration of electrons injected in a plasma wave generated by the laser wakefield mechanism has been observed with a maximum energy gain of 1.5 MeV. It has been shown that the electrons deflected during the interaction, could scatter on the walls of the experimental chamber, and fake a high-energy signal. A special effort has been given in the electron detection to separate the accelerated electrons signal from the background noise.The experimental results agree with theoretical predictions and numerical simulations when 3D effects on the electron beam are taken into account

Stochastic Behaviour of Resonant Absorption in a Plasma at Microwave Frequencies

Resonant absorption of an electromagnetic wave (f = 3.5 GHz) in a multipolar discharge (ne 1011 cm-3) with an adjustable density gradient is studied. At the resonance location, nonlinear effects, associated with the ponderomotive force due to the plasma wave, consist in large-scale profile modification, upward and downward ion acoustic wave propagation. Experimental and numerical results both show a transition from a nonlinear steady-state regime towards stochasticity when the pump field or the gradient length is increased.

Transport coefficients in relativistic collisionless plasmas

The computation of the relativistic transport coefficients in collisionless plasmas is presented. The stationary relativistic Vlasov equation is analytically solved for perturbed plasmas with respect to the global equilibrium defined by the Maxwell–Boltzmann–Juttner distribution function. The explicit expression of the distribution function is derived and the generalized collisionless transport coefficients are deduced for arbitrary plasma temperature. It is found that the relativistic effects tend to increase the value of the transport coefficients. In particular, in ultrarelativistic regimes the temperature anisotropy reaches its maximum value.