A. S. Sakharov

Linear theory of resonance self-modulation of an intense laser pulse in homogeneous plasma and plasma channels

The analytical solutions describing the linear stage of the intense-laser-pulse self-modulation, which results in a strong plasma wakefield excitation, are studied in terms of the paraxial approximation. The attention is focused on phase relations that were ignored in the previous studies. It is shown that the value of the phase velocity of the plasma wake wave differs from the pulse group velocity so that under some specific conditions, the relativistic factor corresponding to the phase velocity can be substantially less than that for the group velocity. This may be important for the particle acceleration in the self-modulated laser wakefield accelerator.

Effect of the Magnetic Field on the Resonant Particle Acceleration

The acceleration of charged particles trapped by a potential wave in a magnetic field is investigated as applied to the problem of the generation of fast particles in a laser plasma. The conditions for unlimited particle acceleration are determined, and the spectra of fast particles are found.

Displacement of the electron cyclotron resonance heating region and time evolution of the characteristics of short-wavelength turbulence in the 3D magnetic configuration of the L-2M stellarator

Reflection of the heating extraordinary microwave incident obliquely onto the surface of the electron cyclotron resonance (ECR) at the second harmonic of the electron gyrofrequency in the 3D magnetic configuration of the L-2M stellarator was studied experimentally. The plasma was heated using two gyrotrons with a total power of 600–700 kW, the specific heating power being 2.4–2.8 MW/m3. The displacement of the ECR region in the course of heating was monitored by measuring the phase of the reflected extraordinary wave. It is found that the growth of the plasma density is accompanied by the displacement of the ECR heating region from the center of the plasma column toward its periphery. The coefficient of reflection of the heating microwave beam from the ECR region was measured. The spectra of short-wavelength (ks ≈ 30 cm−1) plasma density fluctuations were explored by analyzing backscattered microwave radiation. A tenfold increase in the energy of short-wavelength density fluctuations and the growth of the spectral density of fluctuations in the frequency range of 0.3–1.5 MHz were observed.

Effect of microwave reflection from the region of electron cyclotron resonance heating in the L-2M stellarator

In experiments on electron cyclotron resonance (ECR) heating of plasma at the second harmonic of the electron gyrofrequency in the L-2M stellarator, the effect of partial reflection of high-power gyrotron radiation from the ECR heating region located in the center of the plasma column was revealed. The reflection coefficient is found to be on the order of 10−3. The coefficient of reflection of an extraordinary wave from the second-harmonic ECR region is calculated in the one-dimensional full-wave model. The calculated and measured values of the reflection coefficient are found to coincide in order of magnitude.

Self-modulation of high-intensity laser pulses in underlense plasmas and plasma channels

The analysis is carried out for the basic regimes of the self-modulational instability of high-intensity (I∼1017−1018u2009W/cm2) laser pulses in underdense (ω0≫ωp) plasmas. The conditions under which these basic regimes dominate, growth rates corresponding to these regimes, and phase velocity of the plasma wave excited due to the instability are discussed in relation to the previous and possible future experiments on laser acceleration of electrons in the configuration utilizing the self-modulation of laser pulses.

Kinetic Simulations of EC Plasma Heating and Current Drive in the L-2M Stellarator

The ECHLAB code, intended for a self-consistent numerical analysis of the evolution of the electron distribution function and the spatial structure of the electromagnetic field during EC plasma heating in a stellarator, is described. The results from calculations of plasma heating and current drive under conditions corresponding to experiments on EC plasma heating by an X2-mode in the L-2M stellarator are presented. It is shown that, at the existing level of microwave power, the energy deposition region displaces only slightly during heating. The energy is mainly absorbed by relatively fast passing electrons. The influence of locally trapped electrons on the efficiency of current drive is insignificant.

Radial structure of the wakefield excited during the self-modulation of a laser pulse in a plasma

A study is made of the structure of the wakefield excited in the linear stage of the self-modulation of a high-power laser pulse in a homogeneous underdense plasma. It is shown that the fronts of the wake wave are curved and the profile of the wakefield amplitude differs strongly from the intensity profile of the laser pulse. The diffraction effects are found to play a key role in the formation of the transverse profile of the wakefield.

Spectra from stimulated Raman scattering of short relativistically strong laser pulses in an underdense plasma

A set of equations describing large-angle stimulated Raman scattering (SRS) of a short, relativistically strong laser pulse propagating in an underdense plasma is derived and investigated numerically. It is shown that the SRS spectrum depends strongly on the pulse shape. If a pulse with a sharp leading edge excites a strongly nonlinear wake wave, the scattering occurs in relativistic electron flows and is accompanied by the Doppler frequency shift. When the electron flow is directed oppositely to the pulse propagation direction, the frequency upshift is maximum for the direct-backward SRS and decreases with decreasing scattering angle.

Reflection and backscattering of microwaves under doubling of the plasma density and displacement of the gyroresonance region during electron cyclotron resonance heating of plasma in the l-2M stellarator

Reflection and backscattering of high-power (400 kW) gyrotron radiation creating and heating plasma at the second harmonic of the electronic cyclotron frequency in the L-2M stellarator have been investigated experimentally. The effect of the displacement of the gyroresonance region from the axis of the plasma column under doubling of the plasma density on the processes of reflection and backscattering of microwave radiation has been examined. A near doubling of short-wavelength (k⊥ ≈ 30 cm–1) turbulent density fluctuations squared is observed. The change in the energy confinement time under variations of plasma parameters and characteristics of short-wavelength turbulence is discussed. A discrepancy between the measured values of the reflection coefficient from the electron cyclotron resonance heating region and predictions of the one-dimensional model is revealed.

Strong Local Interaction of Microwave Discharges With Solid Dielectrics in Vacuum

Excitation of microwave discharges by pulsed microwave radiation (≤2 MW, 1.95 GHz, 1-10 μs) on dielectric surfaces in vacuum (10-6 torr) was studied experimentally. Different stages of a surface microwave discharge were observed: secondary-electron-emission microwave discharge (multipactor), surface microwave breakdown (filamentary microwave discharge), and plasma-flare microwave discharge. It is found that, in the stage of microwave breakdown (which lasts for ~0.1 μs), ≥70% of the incident microwave power is absorbed by a dense plasma filament with a diameter of ~100 μm, an electron density of up to 2 × 1018 cm-3, and an electron temperature of about 2 eV. Strong interaction of the dense plasma of the filamentary microwave discharge with the dielectric leads to local destruction of the dielectric surface in the form of a long thin erosion track (l ≈ 6 cm, d ~ 100 μm). The coefficient of microwave power absorption by a plasma filament in a rectangular waveguide is calculated using an electrodynamic model that does not involve expansion in the waveguide modes and takes into account reflections of the scattered wave from the waveguide walls by introducing an array of filament mirror images. It is shown that the coefficient of microwave power absorption by the filamentary discharge plasma can reach 70% and more, which agrees well with the experimental data.