Transverse Instability Research Articles

Asymmetric influence of the amplitude-dependent tune shift on the transverse mode-coupling instability

At the 3 GeV ring of the MAX IV Laboratory, a fourth-generation ring-based synchrotron light source, an asymmetric influence of the sign of the amplitude-dependent tune shift (ADTS) on the transverse mode-coupling instability has been observed. Measurements of the instability, in dedicated single-bunch experiments at low chromaticity, revealed a significant dependence of the dynamics of the instability above threshold on the sign of the ADTS. While for a negative sign of the ADTS, the crossing of the instability does not lead to a loss of beam current, a positive sign results in the loss of 40% or more of the beam current at the threshold. In order to investigate the observed asymmetry, the systematic measurement of beam dynamics above the threshold has been conducted in combination with particle tracking simulations with 2 and theoretical calculations of the Landau damping due to the ADTS. The findings point toward an influence of the Landau damping in combination with the low synchrotron frequency, which indicates that this effect could become relevant in the future low-emittance electron storage rings. Published by the American Physical Society 2024

Effective mitigation of photodarkening-induced transverse mode instability in a monolithic fiber laser oscillator by 450 nm laser irradiation

Effective mitigation of photodarkening-induced transverse mode instability in a monolithic fiber laser oscillator by 450 nm laser irradiation

Theoretical Study on Transverse Mode Instability in Raman Fiber Amplifiers Considering Mode Excitation.

Raman fiber lasers (RFLs), which are based on the stimulated Raman scattering effect, generate laser beams and offer distinct advantages such as flexibility in wavelength, low quantum defects, and absence from photo-darkening. However, as the power of the RFLs increases, heat generation emerges as a critical constraint on further power scaling. This escalating thermal load might result in transverse mode instability (TMI), thereby posing a significant challenge to the development of RFLs. In this work, a static model of the TMI effect in a high-power Raman fiber amplifier based on stimulated thermal Rayleigh scattering is established considering higher-order mode excitation. The variations of TMI threshold power with different seed power levels, fundamental mode purities, higher-order mode losses, and fiber lengths are investigated, while a TMI threshold formula with fundamental mode pumping is derived. This work will enrich the theoretical model of TMI and extend its application scope in TMI mitigation strategies, providing guidance for understanding and suppressing TMI in the RFLs.

Hydrodynamics of liquid–liquid parallel flow in novel microextractors: Review

Parallel flows on microfluidic platforms enable continuous liquid–liquid operations and inline separation of effluent streams, bearing immense scope in integration of miniaturized separation processes. However, these flows face major challenges including low mass transfer efficiency due to lack of transverse convection and flow instability at low flow rates, which undermine their operating range and utility. The limitations have inspired dedicated research, delving into the fundamentals of fluid flow and transport mechanism and exploring novel configurations of microextractors. The current article summarizes the hydrodynamics of parallel flows and relevant process intensification strategies in microfluidic extractors, evolving from the use of straight to curved and helical geometries, besides elucidating unique secondary flow patterns observed in-state-of-the-art designs. It includes exclusive sections addressing various aspects of parallel flows: (i) flow inception and theoretical modeling of flow fields and phase hold up, (ii) challenges concerning interfacial stability and flow intensification, (iii) curvature effects in planar curved geometries, and (iv) curvature cum torsional effects in unique multi-helical configurations. The theoretical perspective of this review presents a roadmap that can provide further insights into design modifications for developing improved integrated microextractors based on parallel flows.

Highly polarized energetic electrons via intense laser-irradiated tailored targets

A method for the generation of ultrarelativistic electron beams with high spin polarization is put forward, where a tightly focused linearly polarized ultraintense laser pulse interacts with a nonprepolarized transverse-size-tailored solid target. The radiative spin polarization and angular separation is facilitated by the standing wave formed via the incident and reflected laser pulses at the overdense plasma surface. Strong electron heating caused by transverse instability enhances photon emission in the density spikes injected into the standing wave near the surface. Two groups of electrons with opposite transverse polarization emerge, antialigned to the magnetic field, which are angularly separated in the standing wave due to the phase-matched oscillation of the magnetic field and the vector potential. The polarized electrons propelled into the plasma slab are focused at the exit by the self-generated quasistatic fields. Our particle-in-cell simulations demonstrate the feasibility of highly polarized electrons with a single 10 PW laser beam, e.g., with polarization of 60% and charge of 8 pC selected at energy of 200 MeV within 15 mrad angle and 10% energy spread. Published by the American Physical Society 2024

Super broadband light source: a promising master oscillator source for high brightness and high-power fiber amplifiers

Stimulated Raman scattering (SRS) and transverse mode instability (TMI) effects are crucial limitation factors for further power scaling of high-power fiber amplifiers with near-diffraction-limited beam quality. It is an important research direction to carry out laser system optimization from the perspective of seed construction. In this work, we experimentally investigated the impact of utilizing different seed lasers on the SRS and TMI characteristics in high power ytterbium-doped fiber amplifiers. Both the phase modulated single frequency fiber laser (PMSFL), fiber oscillator laser (FOL), and superfluorescent fiber source (SFS) as a typical type of temporally stable broadband light source (SBLS) are employed as seed lasers. The experimental results indicated that the SRS intensity in the fiber amplifier is influenced by the properties of the seed laser source. Notably, the SRS threshold of the SBLS could be comparable to that of a low-noise PMSFL source. There is a significant increase in TMI threshold when employing SBLS as a seed laser in high-power fiber amplifiers, which is 3.86 times that of the injected seed laser with a 3 dB narrow linewidth of 0.74 nm. Considering the combined threshold characteristics of SRS and TMI, it can be found that SBLS is a promising master oscillator source for high brightness and high-power fiber amplifiers. Our work could provide a good reference for the selection of a master oscillator source, and we believe that it is an important direction to go beyond the power limitation of high-power fiber lasers by constructing a light source with broader spectral linewidth and temporal stability.

Transversal spectral instability of periodic traveling waves for the generalized Zakharov–Kuznetsov equation

Transversal spectral instability of periodic traveling waves for the generalized Zakharov–Kuznetsov equation

High-frequency hydrogen combustion dynamics driven by local flame displacement and multidimensional thermoacoustic interactions

Knowledge of the underlying physical mechanisms responsible for the triggering of high-frequency transverse combustion dynamics is of fundamental importance in the development of heavy-duty gas turbine combustors, aircraft engine afterburners, and bipropellant liquid rocket engines. Detailed information about three-dimensional thermoacoustic interactions and local flame dynamics, however, remains largely unknown and unanticipated, mainly because high-amplitude transverse mode instabilities are challenging to excite and detect in well-controlled sub-scale laboratory environments. To overcome this impasse, here we exploit a spatially tailored rectangular injector assembly consisting of ten equidistant horizontal slit nozzles to eliminate the complications of out-of-plane flame dynamics characterization. A total of 56 datasets of self-induced instabilities were acquired over a wide range of operating conditions to understand spatiotemporal phase dynamics and important mode shapes, in conjunction with 2D Rayleigh angle reconstruction and phase-resolved OH PLIF-based local flame front identification. Experimentally, we show that high-frequency transverse instabilities are excited only under high temperature and high thermal power conditions, manifested as non-evanescent pressure fluctuations at 6.50 kHz strongly coupled to the second-order tangential mode of the rectangular combustion chamber. Two vertically-oriented pressure nodal planes and the characteristic phase transition perpendicular to the horizontal slit injector direction are accurately measured and reconfirmed by Helmholtz simulations in terms of their interpositions and spatial orientation. Remarkably, the periodic formation of co-propagating coherent structures and concomitant local flame displacement/pinch-off are revealed to play an important role in driving the high-frequency hydrogen combustion dynamics.

Transverse instability of traveling wave created by longwave Marangoni convection in the liquid layer covered by insoluble surfactant

We consider the Marangoni convection in a liquid layer heated from below. The liquid interface is covered by insoluble surfactant that plays an active role in the pattern formation together with inhomogeneity of temperature along the interface and surface deformability. In the vicinity of the onset of Marangoni convection, besides different kinds of stationary patterns (hexagons, rolls, squares), the appearance of wave patterns is possible. We analyze the transverse instability of the single traveling wave (TW) using generalizations of the complex Ginzburg–Landau equation (CGLE).

Radiographic Assessment of Transverse Tarsometatarsal Instability Complicated by Metatarsus Adductus in Hallux Valgus Patients.

Objective evaluations of transverse tarsometatarsal (TMT) hypermobility/instability are lacking. This study aims to radiographically explore the relationship between transverse TMT instability and metatarsus adductus (MA) in hallux valgus (HV). This study retrospectively analyzed 207 feet with varying degrees of HV, employing the distance between the first and second metatarsals (M1-2 distance) to assess transverse TMT instability of the first ray. Participants were categorized into MA and non-MA groups. It was found that the M1-2 distance significantly increased with the hallux valgus angle (HVA) and metatarsus adductus angle (MAA), demonstrating significant differences between the MA and non-MA groups. The measurement of M1-2 distance showed high reliability, and its cutoff value was determined to be 4.05 mm. Additionally, the results suggest that the widening of the M1-2 distance may be a predisposing factor for MA in HV patients, highlighting its role in the pathogenesis of this foot condition. These findings highlight the need for a comprehensive assessment of TMT instability on both the axial and sagittal planes for the surgical planning of HV, particularly when complicated by a large MAA. Based on these insights, reoriented first-TMT arthrodesis might be recommended for HV with significant MA to address potential multiplanar instability.

Theory of transverse mode instability in fiber amplifiers with multimode excitations

Theory of transverse mode instability in fiber amplifiers with multimode excitations

Transverse Instability of Concentric Water Waves

Transverse Instability of Concentric Water Waves

5 kW power-level 1050 nm narrow-linewidth fiber amplifier enabled by biconical-tapered active fiber.

Effective wavelength extension is vital in the applications of high-power narrow-linewidth fiber lasers. In this work, we demonstrate a 5-kW power-level narrow-linewidth fiber amplifier at 1050 nm utilizing a homemade biconical-tapered Yb-doped fiber (BT-YDF). Up to ∼4.96 kW fiber laser is achieved with a 3 dB linewidth of ∼0.54 nm and a beam quality factor of Mx 2 = 1.46, My 2 = 1.6. The experimental comparisons reveal that BT-YDF has the advantages of improving a stimulated Raman scattering threshold and balancing transverse mode instability suppression in the fiber amplifier. This work could provide a good reference for extending the operating wavelength of high-power fiber amplifiers.

2 × 4 kW near-single-mode laser output assisted by an optimized bidirectional oscillating-amplifying integrated fiber laser configuration

Bidirectional output oscillating-amplifying integrated fiber laser (B-OAIFL) can achieve the two-ports laser amplification based on a single cavity, showcasing a promising prospect. In order to improve both the laser power and beam quality, we first simulate and optimize the stimulated Raman scattering (SRS) effect in the B-OAIFL. The simulation results show the SRS effect can be suppressed by optimizing the diameter as well as the length of the active fiber at different locations. With the guidance of theoretical and experimental analysis for the combined suppression of SRS and transverse mode instability (TMI), a near-single-mode B-OAIFL with 2 × 4 kW was demonstrated. Based on this foundation, we further devoted ourselves to the pursuit of the optimization of the structure and performance. The necessity of the configuration of side pump, which was initially introduced for its exceptional performance in stabilizing temporal chaos, was reevaluated in detail. With its negative impacts on efficiency improvement and SRS suppression were analyzed and verified, we removed this configuration and finally demonstrated a more simplified design with superior performance. A total bidirectional output of 8105 W was achieved, with an O-O efficiency of 79.6% and a near-single-mode beam quality of M A 2∼1.36,M B 2∼1.63. No signs of TMI were observed, and the signal-to-SRS suppression ratio was over 38 dB. The results still demonstrate a promising potential for power scaling based on this configuration and parameters.

Impact of thermal lensing on the TMI threshold powers in rod-type Yb-doped fiber amplifiers

The impact of thermal lensing (TL) on the transverse mode instability (TMI) threshold power in rod-type fiber amplifiers is investigated. Simulations are conducted with a 3D coupled mode analysis on a set of five scaled large pitch fiber (LPF) amplifiers. The LPF fibers are represented by surrogate step-index fibers (SIFs) with scaled cladding diameters, core diameters, and numerical apertures for a fixed normalized frequency V equal to 3.0 and scaled modal field overlap integrals with the core and cladding. It is found that thermal lensing decreases the TMI threshold powers due to increases in the TMI nonlinear gain. This gain increase is attributed to an increase in the nonlinear gain overlap integrals that occurs with the reduction in the fundamental mode effective area. Estimates for the TMI threshold power are in good agreement with measurements; however, the simulations overestimate the mode shrinkage factor. This discrepancy is tentatively attributed to the representation of the LPF by an effective SIF. These results may offer opportunities for fiber designs that increase the TMI threshold powers.

Experimental investigation of high-frequency transverse instability in Helmholtz resonator-coupled lean-premixed hydrogen combustor

Recent experimental studies suggest that lean-premixed pure hydrogen combustion plays a central role in generating self-sustained pressure oscillations, preferentially coupled with high-frequency tangential modes in land-based gas turbine combustors. The triggering of such high-frequency combustion dynamics is known to be closely connected with relatively small, flashback-resistant, characteristic nozzle dimensions, and the structural and kinematic properties of the premixed hydrogen flames. To identify important trends in the transverse modal dynamics of hydrogen flames, here we conduct detailed measurements of multislit injector-driven hydrogen flame dynamics. We analyze the measured data using the wave decomposition technique for PDF-based statistical assessments of bimodality and nodal line orientation. Experimentally, the planar-to-transverse mode transformation is revealed to be controlled by the combined influence of flame temperature and thermal power, which ultimately leads to the development of well-defined standing modes at around 4.69 kHz under the most severe test condition. Using analytic modeling and rigorous validation measurements, we also investigate Helmholtz resonator-type passive stabilization and its impact on the integrated system's thermoacoustic states with respect to the change in purge air flowrate. Our results show that the coupled system does not retain many of the initial thermoacoustic properties, but instead undergoes prominent changes in the first tangential modal dynamics, progressively evolving from a large-amplitude bimodal to a low-amplitude unimodal state.

Recent advances in the CERN PS impedance model and instability simulations following the LHC Injectors Upgrade project

Transverse instability growth rates in the CERN Proton Synchrotron (PS) are studied thanks to the recently updated impedance model of the machine. Using this model, macroparticle tracking simulations were performed with a new method well-suited for the slicing of short wakes, which achieves comparable performance to the originally implemented method while reducing the required number of slices by a factor of 5 to 10. Furthermore, dedicated beam-based measurement campaigns were carried out to benchmark the impedance model. Until now, beam dynamics simulations based on this model underestimated instability growth rates at injection energy. Thanks to a recent addition to the impedance model, namely the kicker magnets' connecting cables and their external circuits, the simulated instability growth rates are now comparable to the measured ones even when neglecting the impact of the space charge force. Finally, the space charge force is included in simulations and its impact on the instability growth rate and intra-bunch motion is studied.

Effects of multi-dimensionality and energy exchange on electrostatic current-driven plasma instabilities and turbulence

Large-amplitude current-driven plasma instabilities, which can transition to the Buneman instability, were observed in one-dimensional simulations to generate high-energy back-streaming ions. We investigate the saturation of multi-dimensional plasma instabilities and its effects on energetic ion formation. Such ions directly impact spacecraft thruster lifetimes and are associated with magnetic reconnection and cosmic ray inception. An Eulerian Vlasov–Poisson solver employing the grid-based direct kinetic method is used to study the growth and saturation of 2D2V collisionless, electrostatic current-driven instabilities spanning two dimensions each in the configuration (D) and velocity (V) spaces supporting ion and electron phase-space transport. Four stages characterise the electric potential evolution in such instabilities: linear modal growth, harmonic growth, accelerated growth via quasi-linear mechanisms alongside nonlinear fill-in and saturated turbulence. Its transition and isotropisation process bears considerable similarities to the development of hydrodynamic turbulence. While a tendency to isotropy is observed in the plasma waves, followed by electron and then ion phase spaces after several ion-acoustic periods, the formation of energetic back-streaming ions is more limited in the 2D2V than in the 1D1V simulations. Plasma waves formed by two-dimensional electrostatic kinetic instabilities can propagate in the direction perpendicular to the net electron drift. Thus, large-amplitude multi-dimensional waves generate high-energy transverse-streaming ions and eventually limit energetic backward-streaming ions along the longitudinal direction. The multi-dimensional study sheds light on interactions between longitudinal and transverse electrostatic plasma instabilities, as well as fundamental characteristics of the inception and sustenance of unmagnetised plasma turbulence.

Requirements on double-cladding Yb-doped fiber for power scaling of diffraction-limited fiber amplifiers.

Requirements on the double-cladding Yb-doped fiber (DCYF) for power scaling of diffraction-limited fiber amplifiers are studied in this paper. By considering the limitations of thermal lens and transverse mode instability separately, it is found that the power scaling limit can be enlarged to more-than 100 kW and 80 kW, respectively, with the proper selection of pump and signal wavelengths. It is also found that the requirements on DCYF strongly depend on the wavelength and brightness of pump light. It is predicted that smaller-than 20-µm core diameter is required to achieve the 20-kW output power, as long as the 976-nm pump brightness can be high enough. The requirements on the inner-cladding diameter and cladding-to-core ratio of DCYF are also investigated.

Temperature eigenfunction basis for accelerated transverse mode instability simulation.

This work presents a model for the simulation of transverse mode instability (TMI) in rare earth doped optical fiber amplifiers. The model evaluates the internal temperature of a fiber using a superposition of a finite number of thermal eigenmodes. This simplification greatly enhances the speed of calculation with negligible impact on calculation accuracy. This new method is described and quantitatively compared to an older model that uses standard, spatially resolved FDTD to integrate the heat diffusion equation. When tested over a range of spatial and temporal resolutions, this model reduces runtime by a factor of ∼13.9 on average relative to identical simulations using the spatially resolved model.