Pulse Slippage Research Articles

Modulation-slippage trade-off in resonant four-wave upconversion

Following up on a proposal to use four-wave mixing in an underdense plasma at mildly relativistic laser intensities to produce vastly more energetic x-ray pulses [Malkin and Fisch, Phys. Rev. E 101, 023211 (2020)], we perform the first numerical simulations in one dimension to illustrate amplification of a short high frequency seed through four-wave mixing. We illustrate how parasitic processes including phase modulation and spatial pulse slippage limit the amplification efficiency. Although the regimes studied were not where the optimal efficiencies were expected, these regimes do expose the basic physical processes at play, while still yielding not insignificant spectral power upshift.

Pulse Slippage of Resonant Third Harmonic Generation in Electron-Hole Plasma**Supported by a financial grant from CSIR, New Delhi, India, under Project No. 03(1438)/18/EMR-II

Effect of pulse slippage on resonant third harmonic generation of a short pulse laser in electron-hole plasma in the presence of wiggler magnetic field has been investigated. The group velocity mismatch of the third harmonic pulse and the fundamental pulse is significant in electron hole plasma. As the third harmonic pulse has higher group velocity than that of fundamental pulse, therefore, it moves faster than the fundamental pulse. It gets slipped out of the domain of fundamental pulse and its amplitude saturates. Phase matching condition is satisfied by applying wiggler magnetic field, which provides additional angular momentum to the third harmonic photon to make the process resonant. Enhancement in the efficiency of third harmonic generation of an intense short pulse laser in electron-hole plasma embedded with a magnetic wiggler is seen.

Influence of density ripple on pulse slippage of third harmonic generation in plasma

Influence of density ripple on pulse slippage of third harmonic generation has been investigated. When highly intense short range laser pulse incident on plasma, it imparts oscillatory velocity to electrons and exert a ponderomotive force at twice the frequency of incident laser beam. It induces oscillatory velocity at second harmonic frequency 2ω which further couples with density ripple resulting in density oscillations at 2ω. Density oscillations at 2ω, couple with the oscillatory velocity at ω, to produce non-linear current density at 3ω which drives the third harmonic generation. In order to provide phase matching condition between fundamental and third harmonic pulses, density ripple in plasma is introduced which offers the required momentum to the third harmonic photon to make the process resonant. At resonance, amplitude of the third harmonic pulse is found to be maximum for optimized values of density ripple factor. It is observed that the peak value of third harmonic pulse shifted towards higher value of normalized distance z' for higher values of normalized time t'. It is also seen that on account of the group velocity mismatch between pump and third harmonic pulses, the third harmonic pulse slips out of the domain of pump pulse.

Terahertz radiation generation by pulse slippage of Cosh-Gaussian lasers in a corrugated magnetized plasma

The combined effects of pulse slippage and the transverse magnetic field are studied on terahertz radiation excitation by nonlinear beating of two cosh-Gaussian (ChG) laser pulses propagating in a corrugated plasma. The beating lasers exert nonlinear ponderomotive force on plasma electrons. The oscillating electrons couple with corrugations present in the plasma and resonantly excite a nonlinear current (at different frequencies) which drive the terahertz wave at proper phase matching conditions. As the group velocity of THz radiation is higher than the group velocity of beating lasers, the THz pulse slips forward the pump lasers, and its saturation takes place. The effects of THz wave frequency, the decentred parameter (of beating lasers), the periodicity of the density structure, and the applied dc magnetic field are studied on THz emission. An efficiency of ∼10−4 is achieved for a laser intensity of ∼2×1015 W/cm2 (at a laser wavelength of ∼10.6 μm for the CO2 laser).

Generation of “gigantic” ultra-short microwave pulses based on passive mode-locking effect in electron oscillators with saturable absorber in the feedback loop

A periodic train of powerful ultrashort microwave pulses can be generated in electron oscillators with a non-linear saturable absorber installed in the feedback loop. This method of pulse formation resembles the passive mode-locking widely used in laser physics. Nevertheless, there is a specific feature in the mechanism of pulse amplification when consecutive energy extraction from different fractions of a stationary electron beam takes place due to pulse slippage over the beam caused by the difference between the wave group velocity and the electron axial velocity. As a result, the peak power of generated “gigantic” pulses can exceed not only the level of steady-state generation but also, in the optimal case, the power of the driving electron beam.

Effect of pulse slippage on density transition-based resonant third-harmonic generation of short-pulse laser in plasma

The resonant third-harmonic generation of a self-focusing laser in plasma with a density transition was investigated. Because of self-focusing of the fundamental laser pulse, a transverse intensity gradient was created, which generated a plasma wave at the fundamental wave frequency. Phase matching was satisfied by using a Wiggler magnetic field, which provided additional angular momentum to the third-harmonic photon to make the process resonant. An enhancement was observed in the resonant third-harmonic generation of an intense short-pulse laser in plasma embedded with a magnetic Wiggler with a density transition. A plasma density ramp played an important role in the self-focusing, enhancing the third-harmonic generation in plasma. We also examined the effect of the Wiggler magnetic field on the pulse slippage of the third-harmonic pulse in plasma. The pulse slippage was due to the group-velocity mismatch between the fundamental and third-harmonic pulses.

Resonant third harmonic generation in clusters with density ripple: Effect of pulse slippage

AbstractA model is presented for the resonant third harmonic generation of short pulse lasers in cluster plasma in the presence of density ripple. Because of ripple in cluster density and plasma electron density outside the cluster, the phase-matching condition for the third harmonic process is satisfied, leading to resonant enhancement of harmonic generation. We explore the impact of laser intensity, cluster size, and collisional frequency of electrons on the efficiency of third harmonic generation. Moreover, since the group velocity of the third harmonic wave is greater than that of the fundamental wave, it causes the slippage of the generated harmonic pulse out of the fundamental laser pulse and its amplitude increases with time.

Wiggler magnetic field assisted second harmonic generation in clusters

Wiggler magnetic field assisted second harmonic generation in clusters has been investigated theoretically. An intense short-pulse laser propagating through a gas embedded with atomic clusters, converts it into hot plasma balls. For clusters with radius less than one tenth of the laser wavelength, the nonlinear restoration force dominates, which leads to second harmonic generation. The magnetic wiggler provides the uncompensated momentum to second harmonic photon, to make the process of harmonic generation resonant. We explore the impact of laser intensity and cluster size on the efficiency of second harmonic generation. Pulse slippage of second harmonic pulse out of the domain of fundamental laser pulse has been observed on account of group velocity mismatch between the fundamental and second harmonic pulse. Enhancement in the efficiency of the second harmonic is seen for the optimum values of wiggler magnetic field.

Effect of pulse slippage on beat wave THz generation in a rippled density magnetized plasma

Beat wave excitation of terahertz radiation by nonlinear mixing of two laser pulses in a ripple density magnetized plasma is investigated allowing for the effect of pulse slippage. The lasers' ponderomotive-force-induced electron drift couples with the density ripple to produce a nonlinear current that resonantly drives the terahertz at the beat frequency. Density ripple provides the phase matching while the magnetic field, transverse to the direction of laser propagation, provides a transverse component to current density. However, the group velocity mismatch between the lasers and the terahertz radiation leads to slippage of the latter behind the laser pulses leading to saturation of the terahertz amplitude.

Resonant third harmonic generation of a short pulse laser in plasma by applying a wiggler magnetic field

We examine the effect of wiggler magnetic field on pulse slippage of short pulse laser-induced third harmonic generation in plasma. The process of third harmonic generation of an intense short pulse laser in plasma is resonantly enhanced by the application of a magnetic wiggler. The laser exerts a ponderomotive force at second harmonic driving density oscillations. The second harmonic oscillations coupled with electron velocity at the laser frequency, produces a non-linear current, driving the third harmonic. Third harmonic pulse generates in the fundamental pulse domain. However, the group velocity of the third harmonic wave is greater than the fundamental wave. Hence, the third harmonic pulse saturates strongly and moves forward from the fundamental pulse at shorter distance than the second harmonic pulse.

Pulse slippage in high-gain, optically guided free-electron lasers

Using the solution of the fully three-dimensional dispersion relation for the fastest-growing mode an expression for slippage in an optically guided free-electron laser (FEL) amplifier is derived. The FEL operates in the exponential regime and the optical beam is guided by the electron beam. The slippage distance is less than that in the low-gain regime. The reduced slippage arises from the dispersion characteristics of the optical beam and is a manifestation of (i) gain effects and (ii) finite-spot-size effects.

Design of a compact, optically guided, pinched, megawatt class free-electron laser

A conceptual design for a compact megawatt class free-electron laser (FEL) operating at 1 /spl mu/m is presented. The proposed FEL consists of an optically guided, pinched amplifier configuration driven by a RF linac. The gain length, efficiency, electron pulse slippage, and the distance between the wiggler and first relay mirror are determined for a megawatt class design. Of particular concern in the design is the overall length of the optical system, i.e., the wiggler length and distance to the first relay mirror. In the present design, the wiggler length is /spl sim/1 m and the distance between the first relay mirror and the wiggler is determined by the average intensity damage threshold on the mirror. By focusing the electron beam, the optical beam can be pinched upon exiting the wiggler. The pinched optical beam has a reduced Rayleigh length which permits the first relay mirror to be relatively close to the wiggler. By pinching the optical beam and employing grazing incidence the first relay mirror can be located within /spl sim/3 m of the wiggler. It is shown that frequency detuning can more than double the FEL efficiency. In the present design with frequency detuning and uniform wiggler the efficiency is /spl sim/1.8%. In addition, electron-pulse slippage is shown to be substantially reduced in a high-gain amplifier.

Effect of pulse slippage on resonant second harmonic generation of a short pulse laser in a plasma

The process of second harmonic generation of an intense short pulse laser in a plasma is resonantly enhanced by the application of a magnetic wiggler. The wiggler of suitable wave number provides necessary momentum to second harmonic photons to make harmonic generation a resonant process. The laser imparts an oscillatory velocity to electrons and exerts a longitudinal ponderomotive force on them at , where ω1 and are the frequency and the wave number of the laser, respectively. As the electrons acquire oscillatory velocity at the second harmonic, the wiggler magnetic field beats with it to produce a transverse second harmonic current at , driving the second harmonic electromagnetic radiation. However, the group velocity of the second harmonic wave is greater than that of the fundamental wave, hence, the generated pulse slips out of the main laser pulse and its amplitude saturates.

Micropulse slippage and optical mode distortion in the Boeing APLE amplifier

In the Boeing Average Power Laser Experiment (APLE), the free electron laser (FEL) oscillator produces short optical pulses for input to the amplifier section. Micropulse slippage can become significant in the amplifier, competing with the effects of high gain and optical mode distortion. A two-dimensional simulation is used to study pulse slippage, and a three-dimensional simulation is used to study mode distortion. A four-dimensional simulation is used to simultaneously study these effects, and to look at the sensitivity of the efficiency to changes in various FEL parameters. We find that high gain and mode distortion effects dominate the pulse slippage effects. The amplifier is fairly insensitice to beam quality degradation and changes in the initial optical field, but a significant improvement in efficiency could be achieved by shortening the electron pulse.

Evolution of a finite pulse of radiation in a high-power free-electron laser

The development of an optical pulse of finite axial extent is studied by means of an axisymmetric time-dependent particle simulation code for different rates of tapering of the wiggler field. The results illustrate a number of physical phenomena underlying the free-electron laser mechanism. These include: suppression of the sideband instability: the role of gain focusing versus that of refractive guiding: efficiency enhancement; and pulse slippage. It is found that a significant reduction in the sideband modulation of the optical field can be achieved with a faster tapering of the wiggler parameters. Increasing the tapering rate also reduces refractive guiding, causing the optical wavefronts to become more convex, thus spreading the optical field into a larger cross section. The corresponding enhancement of the peak output power is associated with an increased lateral extent of the optical field rather than an increase in the field amplitude.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>