Self-modulation Regime Research Articles

Statistical fluctuations of radiation in quasi-Cherenkov generators

In a nonstationary (self-modulation) regime, terahertz quasi-Cherenkov generators can operate at MHz–GHz repetition rates and megawatt power levels. Shot noise, intrinsic to electron beams, is the cause of statistical fluctuations of radiation in the generators. The shot-noise related spread of instability growth time imposes appreciable limitations on the possibility of coherent summation of electromagnetic oscillations from several quasi-Cherenkov generators operating in a nonstationary mode.

Study of the Q-switching regimes of the loop cavity of a Nd:YAG laser using an external plasma mirror

This paper discusses the self-modulation regimes of a multiloop cavity with external optical action initiated by the reflection of the laser beam’s focused radiation from the laser target plasma created by it. The self-modulation regime does not have a distinct threshold character. It is found that an intense Q-switching regime is characterized by the presence of a mode-locking regime. It is demonstrated that the self-Q-switching regime of a loop cavity can be used to automatically identify substances by the method of laser-spark emission spectroscopy.

Synchronisation of self-oscillations in a solid-state ring laser with pump modulation in the region of parametric resonance between self-modulation and relaxation oscillations

The synchronisation of the self-modulation oscillation frequency in a Nd : YAG ring laser by an external periodic signal modulating the pump power in the region of parametric resonance between self-modulation and relaxation oscillations is studied theoretically and experimentally. The characteristic features of synchronisation processes in lasers operating in the self-modulation regime of the first kind and in the regime with a doubled self-modulation period are considered. Two bistable branches of synchronisation of self-modulation oscillations are found by numerical calculation. The experimental data agree well with the numerical simulation results for one of these branches, but the other branch of bistable self-modulation oscillations was not observed experimentally.

Parametric excitation of relaxation oscillations at the subharmonic of the external modulating signal in a ring Nd : YAG laser

The dynamics of solid-state ring laser (SSRL) radiation with periodic pump modulation is studied experimentally and simulated numerically in the case of parametric excitation of relaxationoscillations at the subharmonic of a modulating signal. Parametric processes are investigated by modulating the pump in two regimes of the SSRL operation: steady-state regime of unidirectionallasing and self-modulation regime of the first kind. Significant differences in the dynamics of radiation for these regimes are found. It is established that when the laser operates in the self-modulation regime, a first-order parametric instability can result in the appearance of the dynamic chaos regime.

A proposed demonstration of an experiment of proton-driven plasma wakefield acceleration based on CERN SPS

AbstractThe proton bunch-driven plasma wakefield acceleration (PWFA) has been proposed as an approach to accelerate an electron beam to the TeV energy regime in a single plasma section. An experimental program has been recently proposed to demonstrate the capability of proton-driven PWFA by using existing proton beams from the European Organization for Nuclear Research (CERN) accelerator complex. At present, a spare Super Proton Synchrotron (SPS) tunnel, having a length of 600 m, could be used for this purpose. The layout of the experiment is introduced. Particle-in-cell simulation results based on realistic SPS beam parameters are presented. Simulations show that working in a self-modulation regime, the wakefield driven by an SPS beam can accelerate an externally injected ~10 MeV electrons to ~2 GeV in a 10-m plasma, with a plasma density of 7 × 1014 cm−3.

Plasmon-induced terahertz radiation generation due to symmetry breaking in a nonlinear metallic nanodimer

We suggest an alternative way of terahertz radiation generation utilizing the effect of spontaneous symmetry breaking in a nonlinear metallic nanodimer illuminated by light. We show that the symmetry breaking is caused by instability of plasmon eigenmodes of the system and can lead either to spontaneous magnetization of the dimer or to a periodic self-modulation regime of light scattering. We find that the modulation frequency lies in the terahertz band and may be tuned within a wide range.

Experimental observation of stochastic resonance in a solid-state ring laser in the absence of bistability

The appearance of a stochastic resonance is found in a solid-state ring laser in the absence of bistability in it. It is shown that in this laser operating in the self-modulation regime of the first kind, the noise modulation of the pump can cause a significant increase in the laser response to the periodic modulating signal. These effects observed are explained qualitatively.

Generation and analysis of quasimonoenergetic electron beams by laser-plasma interaction in transitional region from the self-modulated laser wakefield to bubble acceleration regime

Generation of quasimonoenergetic electron beams in a transitional region from the self-modulated laser wakefield to bubble acceleration regime is reported. Quasimonoenergetic electron beams containing more than 3×108 electrons in the monoenergetic peak with energies of 40–60 MeV have been obtained from a plasma with an electron density of 1.6×1019 cm−3 produced by an 8 TW, 50 fs laser pulse. The generation of quasimonoenergetic electron beams is investigated by two-dimensional particle-in-cell simulations. Few periods of the plasma wave are located inside the laser pulse, because the laser pulse duration is longer than the wavelength of the plasma wave. Electrons trapped in the first period of the plasma wave can form the monoenergetic bunch, even though the trapped electrons interact directly with the laser field. The quasimonoenergetic electron beam can be obtained due to the small contribution of the direct acceleration by the laser field. This type of monoenergetic electron acceleration is different from that of both the self-modulated laser wakefield and bubble acceleration regimes, in which the trapped electrons in the plasma wave are located behind the laser pulse due to the pulse compression or fragmentation and free from the laser electric field. This result suggests a new regime for the quasimonoenergetic electron acceleration in the region between the self-modulation and bubble regime.

Stochastic effects during the action of the pump noise on bistable self-modulation oscillations in a solid-state ring laser

Nonlinear radiation dynamics of a solid-state ring laser is studied in the region of laser parameters corresponding to the parametric resonance between the self-modulation and relaxation oscillations. Bistable regions are found in which, apart from the self-modulation regime of the first kind, a stable quasi-periodic self-modulation regime exists. Temporal and spectral emission parameters of counterpropagating waves are considered in the bistable self-modulation generation regimes. The effect of noise on the bistable self-modulation oscillations is studied. It is shown that during the interaction of noise, spectral peaks split at relaxation and self-modulation frequencies.

Nonlinear stochastic effects during the action of noise on relaxation oscillations in a ring solid-state laser

The influence of the pump noise intensity on the characteristics of the fundamental relaxation frequency of the emission spectrum of a ring chip laser operating in the self-modulation regime of the first kind is studied theoretically and experimentally. It is shown that the noise effects give rise to such nonlinear stochastic phenomena as a change in the relaxation oscillation frequency, a noticeable broadening of the relaxation peak in the emission spectrum, and the resonance dependence of the amplitude of this peak on the noise intensity (the peak becomes maximal at a particular noise intensity).

Influence of the phase difference of coupling coefficients of counterpropagating waves on relaxation oscillations in the self-modulation regime of a solid-state ring laser

The effect of the phase difference of the coupling coefficients on relaxation frequencies in the emission spectrum of a solid-state ring laser operating in the self-modulation regime of the first kind is studied theoretically. A strong dependence of one of the frequencies of relaxation oscillations on the phase difference of coupling coefficients is found. The stability of the self-modulation regime is studied analytically.

Transverse dynamics and energy tuning of fast electrons generated in sub-relativistic intensity laser pulse interaction with plasmas

The regimes of quasi-monoenergetic electron beam generation were experimentally studied in the sub-relativistic intensity laser plasma interaction. The observed electron acceleration regime is unfolded with two-dimensional-particle-in-cell simulations of laser-wakefield generation in the self-modulation regime.

Controlled plasma wave generation and particle acceleration through seeding of the forward Raman scattering instability

Trapping and acceleration of background plasma electrons in plasma waves generated by low energy (<0.1J) laser pulses copropagating with much weaker (1% in intensity) seed pulses that are down shifted in frequency by ωp is investigated in the self-modulation regime by one-dimensional (1D) and 2D particle-in-cell simulations. Without the seed, the modulation for pulses of subcritical power is found to develop slowly compared to the interaction time limited by diffraction and dephasing. With the seed, on the other hand, strong primary pulse modulation, wakefield excitation, and particle trapping and acceleration can be achieved reproducibly. Moreover, in contrast to incoherent seeding mechanisms such as ionization, the seed pulse enables coherent control of the excited wakefield. The simulations show that 38 mJ seeded pulses can generate relativistic bunches of ∼0.5nC. Seeded acceleration by pulses of subcritical power (P=1∕2Pc, or 19 mJ) is demonstrated by simulations in a performed plasma channel

Unsteady beam-plasma discharge: I. Mathematical model and linear theory

A mathematical model is constructed that describes the development of the beam-plasma instability in a traveling-wave tube amplifier in the presence of a neutral gas. Steady solutions are derived for conditions of microwave discharges in a magnetized plasma-filled traveling-wave tube amplifier, and their stability is investigated. It is shown that the steady-state amplification regime may become unstable and change to the self-modulation regime. The relationships between the amplifier parameters at the instability threshold are obtained, and the frequencies of the excited ion acoustic waves are determined. The results of numerical modeling are found to agree well with the analytical results.

Effect of the ponderomotive force on the development of beam-plasma instability

The amplitude of the wave generated in a plasma during the development of beam-plasma instability is nonuniform in the longitudinal direction. The ponderomotive force associated with this nonuniformity leads to a redistribution of the plasma density; as a result, the wave amplitude and its spatial distribution change. As the beam current grows, the ponderomotive force plays an increasingly important role and radically changes the mechanism by which the beam-plasma instability saturates. Ion acoustic waves generated by the ponderomotive force propagate in the direction opposite to the propagation direction of the beam, thereby ensuring distributed feedback and giving rise to a strong low-frequency self-modulation of the wave amplitude and phase. Results are presented from experimental investigations of the self-modulation regime of the beam-plasma instability in a magnetized plasma waveguide. Theoretical estimates of the parameters of the low-frequency self-modulation agree well with the experimental data.

Amplitude characteristics of a rotating ring chip laser in the presence of antiphase self-modulation oscillations

A semiclassical vector model is used to analyse a self-modulation regime of a solid-state ring laser. Expressions are obtained for the amplitude of the spectral components of counterpropagating waves, for the average intensities, and for the depth of modulation in the case of arbitrary polarisations and coupling coefficients. The amplitude and frequency nonreciprocities of the laser cavity are taken into account.

Phase shift in antiphase self-modulation oscillations in a ring chip laser

An experimental investigation was made of the phase shift between counterpropagating waves, which appeared in an asymmetric ring chip laser in an antiphase self-modulation regime of the first kind.

Dependence of the self-modulation frequency on the parameters of a solid-state ring laser

Theoretical and experimental investigations were made of the influence of the detuning of the lasing frequency from the centre of the gain line on the modulation frequency ωm in a type-1 self-modulation regime when the cavity is characterised by amplitude and frequency nonreciprocities. The moduli and phases of the coupling coefficients of counter propagating waves are arbitrary. It is shown that the amplitude nonreciprocity of the cavity has a strong influence on the nature of the dependence of ωm on the frequency detuning. A comparison with the experimental results makes it possible to estimate the difference between the phases of the coupling coefficients of a ring Nd : YAG chip laser.

Relaxation oscillations in solid-state lasers with derivative feedback

The feedback proportional to е derivative of laser intensity can affect only the system stability without changing the system steady state. In the present work by the example of a two-mode solid-state Jaser we show the possibility to excite quasi-sinusoidal self-modulation regime using only negative feedback which is proportional in this case to the derivative of the strong mode intensity l1. The region of parameters at which the stationary regime becomes unstable is found. Modulation of pump parameter being introduced in с laser equations as A=A0 (1+ξ(t)) and feedback as - K(dl1/dt). K is the feedback coefficient which we changs to gol the unstability region. At approaching the bifurcation boundary the effect of regencrative amplification of noise in the vicinity of low-frequency relaxation oscillation must be observed. In the iransfer functions (in the presence of the feedback) it looks like disappearing of high frequency peak while low frequency peak grows and narrows. The effect of regenerative amplification of noise (increase in the amplitude of the low frequency peak) is accompanicd by the effect of tolal suppression of the high frequency relaxaiion oscillation and by а significant decrease of the system sensitivity to the external perturbations.

Cavity amplitude and phase nonreciprocities of monolithic solid-state ring lasers

The 2 × 2 Jones matrix method is used in a theoretical analysis of the cavity amplitude and phase nonreciprocities of monolithic nonplanar solid-state ring lasers. An approach is developed to the optimisation of the perimeter and parameters of the cavity to ensure the necessary amplitude and phase nonreciprocities. Calculations are reported of the cavity parameters optimal for the two most important applications: unidirectional single-frequency lasing and bidirectional lasing in a self-modulation regime which is of interest in laser gyroscopy.