Helium Gas Jet Research Articles

Laser-driven ion and electron acceleration from near-critical density gas targets: Towards high-repetition rate operation in the 1 PW, sub-100 fs laser interaction regime

Ion acceleration from gaseous targets driven by relativistic-intensity lasers was demonstrated as early as the late 1990s, yet most of the experiments conducted to date have involved picosecond-duration, Nd:glass lasers operating at low repetition rate. Here, we present measurements on the interaction of ultraintense (≈1020Wcm−2, 1 PW), ultrashort (≈70fs) Ti:Sa laser pulses with near-critical (≈1020cm−3) helium gas jets, a debris-free targetry with the potential for future compatibility with high (≈1 Hz) repetition rate operation. We provide evidence of α particles being forward accelerated up to ≈2.7−MeV energy with a total flux of ≈1011sr−1 as integrated over >0.1−MeV energies and detected within a 0.5−mrad solid angle. We also report on on-axis emission of relativistic electrons with an exponentially decaying spectrum characterized by a ≈10−MeV slope, i.e., five times larger than the standard ponderomotive scaling. The total charge of these electrons with energy above 2 MeV is estimated to be of ≈1nC, corresponding to ≈0.1% of the laser drive energy. In addition, we observe the formation of a plasma channel, extending longitudinally across the gas density maximum and expanding radially with time. These results are well captured by large-scale particle-in-cell simulations, which reveal that the detected fast ions most likely originate from reflection off the rapidly expanding channel walls. The latter process is predicted to yield ion energies in the MeV range, which compare well with the measurements. Finally, direct laser acceleration is shown to be the dominant mechanism behind the observed electron energization. Published by the American Physical Society 2024

Investigation of terahertz radiation generation from laser-wakefield acceleration

We investigate the generation of terahertz (THz) radiation from laser-wakefield acceleration (LWFA) in a helium gas jet. We consider a three-dimensional setup incorporating a realistic gas density distribution and use particle-in-cell simulations to study the interaction of a femtosecond intense laser pulse with the gas medium. Our results show that LWFA can efficiently produce THz radiation. In the simulations, we use multiple probes to record the electric and magnetic fields arising from the interaction. In addition, we compare the results of fixed and moving window simulation boxes used to capture electromagnetic fields in the THz range. We demonstrate that a moving window with a 600 μm width can be significantly useful for THz studies. We further analyze the spectrum of spatially and temporally resolved electromagnetic radiation and its emission angle. Our results are consistent with experimental data. Our findings provide valuable insights into the potential of LWFA as a strong source of THz radiation.

Laser Wakefield Photoneutron Generation with Few-Cycle High-Repetition-Rate Laser Systems

Simulations of photoneutron generation are presented for the anticipated experimental campaign at ELI-ALPS using the under-commissioning e-SYLOS beamline. Photoneutron generation is a three-step process starting with the creation of a relativistic electron beam which is converted to gamma radiation, which in turn generates neutrons via the γ,n interaction in high-Z material. Electrons are accelerated to relativistic energies using the laser wakefield acceleration (LWFA) mechanism. The LWFA process is simulated with a three-dimensional particle in cell code to generate an electron bunch of 100s pC charge from a 100 mJ, 9 fs laser interaction with a helium gas jet target. The resultant electron spectrum is transported through a lead sphere with the Monte Carlo N-Particle (MCNP) code to convert electrons to gammas and gammas to neutrons in a single simulation. A neutron yield of 3×107 per shot over 4π is achieved, with a corresponding neutron yield per kW of 6×1011 n/s/kW. The paper concludes with a discussion on the attractiveness of LWFA-driven photoneutron generation on high impact, and societal applications.

Enhanced betatron x-ray emission in a laser wakefield accelerator and wiggler due to collective oscillations of electrons

We report x-ray emission caused by betatron motion of electrons in a laser wakefield accelerator driven by high intensity ($\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$) laser pulse interaction with a 4-mm long helium gas-jet and study the role of collective oscillation of electrons as well as plasma density. The collective oscillations of electrons, inferred from the observed wavy pattern in the electron beam spectrum (up to 300 MeV) led to enhance peak x-ray flux which was found to be further enhanced at higher plasma densities. A similar correlation between the enhancement in x-ray emission and collective oscillation was also observed in nitrogen gas target. Possible physical pictures for the above observations are also suggested and discussed. For the helium gas jet target, the typically observed x-ray source brightness is $\ensuremath{\ge}1\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }\text{ }\mathrm{photons}/\mathrm{s}\text{ }{\mathrm{mm}}^{2}\text{ }{\mathrm{mrad}}^{2}$ in 0.1% BW, with a critical energy of $35\ifmmode\pm\else\textpm\fi{}5\text{ }\text{ }\mathrm{keV}$. We have also demonstrated the application of the betatron x-ray source in high-resolution imaging.

Numerical Simulation of a Two-Dimensional Helium Gas Jet with Side-Jets Formation

Side jets are generated when the jet density is smaller than that of the ambient gas. It has been suggested that the side jets are related to self-excited oscillations due to the local absolute instability that occurs when the density ratio is less than 0.7, and vorticity generation due to baroclinic torque caused by the cross product of density gradient and pressure gradient. In addition, it has been suggested that a side jet is also formed by a pair of streamwise vortices with different rotation directions. First of all, in order to confirm previous hypotheses, we have tried the formation of side jets in a helium gas jet by a numerical simulation using ANSYS Fluent. As a result, it is shown that a side jet is formed between a streamwise vortex pair.

Characterisation of supersonic gas jets for different nozzle geometries for laser-plasma acceleration experiments at SPARC_LAB

Plasma-based technology promises a tremendous reduction in size of accelerators used for research, medical, and industrial applications, making it possible to develop tabletop machines accessible for a broader scientific community. The use of high-power laser pulses on gaseous targets is a promising method for the generation of accelerated electron beams at energies on the GeV scale, in extremely small sizes, typically millimetres. The gaseous target in question can be a collimated supersonic gasjet from a nozzle. In this work, a technique for optimising the so generated plasma channel is presented. In detail, a study on the influence of the nozzle throat shape in relation to the uniformity and density of the generated plasma profile is reported. These considerations are discussed first of all from a theoretical point of view, by means of a stationary one-dimensional mathematical model of the neutral gas, thus exploiting the possibility of comparing the properties of the output flow for different nozzle geometries. This is combined with an experimental approach using interferometric longitudinal density measurements of the plasma channel. The latter is generated by a high-power laser pulse focused on a helium gasjet, in the SPARC_LAB laboratories.

(Invited) Stabilizing Effect of Impinging Plasma Jet on the Water Surface

Year by year, plasma processing with atmospheric pressure plasma jets is increasingly gaining attention and diversifying into various scientific and industrial fields; plasma jet systems have the merit of being able to release plasma into target liquids or solids. Thus, naturally, part of plasma jet research has directed towards investigating interfacial phenomenon between plasma–liquid or –solid and understanding underlying principles thereof. Since impinging plasma jet on the liquid surface has been considered as a basis of plasma jet processing, for example, active efforts to assess the transport of electrons and other reactive species from plasma to liquid have been made. However, despite its scientific and practical significance, surprisingly little attention has been given to the stabilizing effect of plasma jets on plasma–liquid interfaces thus far. In this contribution, we demonstrate the stabilization of liquid instabilities by weakly ionized gas for the case of a helium gas jet impinging on water, which profoundly affects the water free surface. Special attention is given to the interfacial dynamics relevant to electrohydrodynamic (EHD) gas flow, so-called electric wind, induced by the momentum transfer from accelerated charged particles to neutral gas under a strong electric field. A plasma jet consisting of periodic pulsed ionization waves, called plasma bullets, exerts more force via electrohydrodynamic flow on the water surface than a neutral gas jet alone, resulting in cavity expansion without destabilization. Furthermore, both the bidirectional EHD gas flow and parallel electric field to the gas–water interface caused by plasma interacting ‘in the cavity’ render the surface more stable. This case study demonstrates the dynamics of liquids subjected to a plasma-induced force, offering new insights into physical processes and revealing an interdependence between weakly ionized gases and deformable dielectric matter, including plasma–water systems, and their potential stabilization.

Pulsed laser-plasma gamma radiation source for radiography

We have performed experiments on the generation of relativistic electron beams in helium gas jets (electron density ∼1020 cm−3) using a femtosecond Ti : sapphire laser. Electron beams with a total charge of 0.2−0.5 nC, a maximum electron energy of up to 60 MeV, and a divergence of up to 300 mrad are recorded. The generated electron beams are converted into bremsstrahlung X-rays using a Ta plate 2 mm thick. The average dose of bremsstrahlung radiation in the air at a distance of 1 m from the source was 5.4 ±1.7 mrad per pulse. The emitting region size measured by the blurriness of the X-ray image is about 650 μm (FWHM).

Numerical simulation of a helium gas and two-dimensional jet

Numerical simulation of a helium gas and two-dimensional jet

Self-focusing and self-compression of intense pulses via ionization-induced spatiotemporal reshaping.

Ionization is a fundamental process in intense laser-matter interactions and is known to cause plasma defocusing and intensity clamping. Here, we investigate theoretically the propagation dynamics of an intense laser pulse in a helium gas jet in the ionization saturation regime, and we find that the pulse undergoes self-focusing and self-compression through ionization-induced reshaping, resulting in a manyfold increase in laser intensity. This unconventional behavior is associated with the spatiotemporal frequency variation mediated by ionization and spatiotempral coupling. Our results illustrate a new regime of pulse propagation and open up an optics-less approach for raising laser intensity.

Electrostatic shock acceleration of ions in near-critical-density plasma driven by a femtosecond petawatt laser

With the recent advances in ultrahigh intensity lasers, exotic astrophysical phenomena can be investigated in laboratory environments. Collisionless shock in a plasma, prevalent in astrophysical events, is produced when a strong electric or electromagnetic force induces a shock structure in a time scale shorter than the collision time of charged particles. A near-critical-density (NCD) plasma, generated with an intense femtosecond laser, can be utilized to excite a collisionless shock due to its efficient and rapid energy absorption. We present electrostatic shock acceleration (ESA) in experiments performed with a high-density helium gas jet, containing a small fraction of hydrogen, irradiated with a 30 fs, petawatt laser. The onset of ESA exhibited a strong dependence on plasma density, consistent with the result of particle-in-cell simulations on relativistic plasma dynamics. The mass-dependent ESA in the NCD plasma, confirmed by the preferential reflection of only protons with two times the shock velocity, opens a new possibility of selective acceleration of ions by electrostatic shock.

Modeling the electron acceleration in relativistic channels for space irradiation applications

We study the interaction of an intense short laser pulse (duration > 100 fs) with a helium gas jet (with a pressure from 1–80 bar) by performing two-dimensional particle-in-cell (PIC) simulations. The parameters of the existing setups at the CETAL PW facility are used in PIC simulations. The mechanisms of the relativistic laser channeling such as filamentary bifurcation, long-wavelength hosing, bifurcation due to long-wavelength hosing and refractive bifurcation are shown. We also findthe optimum parameters of the laser pulse and the helium gas jet for which electrons are accelerated in the direct laser acceleration regime. We obtained electrons with energies higher than 100 MeV and broad electron energy spectra features that are very useful for space irradiation simulations.

An optically multiplexed single-shot time-resolved probe of laser-plasma dynamics.

We introduce a new approach to temporally resolve ultrafast micron-scale processes via the use of a multi-channel optical probe. We demonstrate that this technique enables highly precise time-resolved, two-dimensional spatial imaging of intense laser pulse propagation dynamics, plasma formation and laser beam filamentation within a single pulse over four distinct time frames. The design, development and optimization of the optical probe system is presented, as are representative experimental results from the first implementation of the multi-channel probe with a high-power laser pulse interaction with a helium gas jet target.

Liquid dynamics in response to an impinging low-temperature plasma jet

The liquid-phase behavior induced by an atmospheric pressure plasma jet impinging onto water-based solutions was experimentally investigated. An increase in cavity size and circulation size resulting from an increase in impact force of the plasma jet with respect to a helium gas jet were observed through particle image velocimetry and cavity measurements. The increase in circulation size due to plasma discharge was larger than the increase solely due to an increase in jet impact force, suggesting additional shear along the liquid free surface evidenced by schlieren images of plasma-induced gas flow adherence after impingement. Liquid evaporation rate increased and mean liquid temperature decreased due to plasma effects on jet behavior. Plasma-induced decreases in liquid pH and temperature were contained within the circulation flow, leading to strong pH and temperature gradients within the liquid volume. Areas treated by the jet became acidic and remained more acidic than other portions of the liquid due to circulation vortices, highlighting the consequences of jet and liquid vessel parameters for plasma jet-liquid applications.

Modulating the concentrations of reactive oxygen and nitrogen species and oxygen in water with helium and argon gas and plasma jets

We employed UV–vis spectroscopy to monitor real-time changes in the oxygen tension and concentration of reactive oxygen and nitrogen species (RONS) in deionized (DI) water during treatments with helium (He) and argon (Ar) gas plasma jets. He and Ar gas jets are both shown to de-oxygenate DI water with He being more efficient than Ar, whilst the plasma jets deliver and regulate the concentrations of hydrogen peroxide (H2O2), nitrite (NO2−) and nitrate (NO3−) in DI water. The H2O2 and NO3− production efficiency varied between He and Ar plasma jets, but was similar for NO2−. Whilst DI water fully equilibrated with ambient air prior to treatment (de-oxygenated by both plasma jets) when DI water was first de-oxygenated by an inert gas jet treatment, both plasma jets were found to be capable of oxygenating DI water. These insights were then used to show how different combinations of plasma jet and inert gas jet treatments can be used to modulate O2 tension and RONS chemistry. Finally, potential further improvements to improve control in the use of plasma jets in regulating O2 and RONS are discussed.

Single-Shot Measurement of Transient Phase Shift Induced by Laser Wake**Supported by the National Natural Science Foundation of China under Grant No 61377102, and the Defense Industrial Technology Development Program under Grant No B1520133010.

Based on the frequency-to-time mapping relation of the linearly chirped pulse, the temporal phase shift induced by a laser-excited wake in a helium gas jet is measured using a chirped-pulse spectral interferometry with ~ 140 fs resolution over a temporal region of 1 ps in a single shot. In this measurement, the image of the wake is obtained with one-dimensional spatial resolution and temporal resolution limited only by the bandwidth and chirp of the pulse. The ‘bubbles’ feature of the wake structure, along with multiple wakes excited by the main lobe and the side lobe of a laser focal-spot, is captured simultaneously.

Generation of 20 kA electron beam from a laser wakefield accelerator

We present the experimentally generated electron bunch from laser-wakefield acceleration (LWFA) with a charge of 620 pC and a maximum energy up to 0.6 GeV by irradiating 80 TW laser pulses at a 3 mm Helium gas jet. The charge of injected electrons is much larger than the normal scaling laws of LWFA in bubble regime. We also got a quasi-monoenergetic electron beam with energy peaked at 249 MeV and a charge of 68 pC with the similar laser conditions but lower plasma density. As confirmed by 2D particle-in-cell simulations, the boosted bunch charge is due to the continuous injection caused by the self-steepening and self-compression of a laser pulse. During the nonlinear evolution of the laser pulse, the bubble structure broadens and stretches, leading to a longer dephasing length and larger beam charge.

Helium ions acceleration by ultraintense laser interactions with foil-gas target

Laser-driven helium ion source with multi-MeV energy has an important application in the field of fusion reactor material irradiation damage. At present, the generating of high energy helium ions by relativistic ultraintense laser interacting with helium gas jet is the main scheme of laser-driven helium ion source. However, so far, this scheme has been hard to generate the helium ion beam with the characteristics, i.e., it is forward and quasi-monoenergetic and has multi-MeV in energy and high yield. These characteristics of helium ion beam are important for studying the material irradiation damage. In this paper, we propose a new scheme in which an ultraintense laser interacting with foil-gas complex target is used to generate helium ions. With this method, we perform an experiment on XingGuang III laser facility which has three laser beams with different laser durations (nanosecond, picosecond and femtosecond). In our experiment, we use a picosecond laser beam. The wavelength of this laser beam is 1054 nm and its duration is 0.8 ps. We use an off-axis parabola mirror to focus the 100 J energy of this laser beam onto a focal spot of 25 m far away. The laser intensity reaches 51018 W/cm2. The foil-gas target is composed of a copper foil with 7 m in thickness and a helium gas nozzle which is behind the copper foil. The helium gas nozzle can generate a helium gas jet with a full ionization electron density of 51019/cm3. We use the Thomson Parabola Spectrometer to record the helium ion signals and the Electron Magnetic Spectrometer to diagnose the hot electron temperature. In the experiment, the laser pulse interacts with the front surface of the copper foil and generates lots of hot electrons. These hot electrons result in the expansion of the rear surface of the copper foil. The expanding plasma accelerates the helium ions behind the copper foil. The experimental results show that the obtained helium ions are forward and quasi-monoenergetic (the peak energy is 2.7 MeV), and the total energy of the helium ions whose energies are all higher than 0.5 MeV is about 1.1 J/sr, and correspondingly the yield of helium ions is about 1013/sr. The helium ion spectrum and hot electron temperature given by particle in cell (PIC) simulation with using the experimental parameters are consistent with the experimental results. In addition, the PIC simulations also show that helium ions are accelerated by target normal sheath acceleration and collisionless shock acceleration-like mechanisms, and the maximum helium ion energy is proportional to the hot electron temperature.

Effect of mass-density on vortex formation in the initial region of a two-dimensional helium gas jet

Effect of mass-density on vortex formation in the initial region of a two-dimensional helium gas jet

Online plasma diagnostics of a laser-produced plasma

In this study, we report a laser interferometry experiment for the online-diagnosing of a laser-produced plasma. The laser pulses generating the plasma are ultra-fast (30 femtoseconds), ultra-intense (tens of Terawatt) and are focused on a helium gas jet to generate relativistic electron beams via the laser wakefield acceleration (LWFA) mechanism. A probe laser beam (λ = 800 nm) which is split-off the main beam is used to cross the plasma at the time of arrival of the main pulse, allowing online plasma density diagnostics. The interferometer setup is based on the NoMarski method in which we used a Fresnel bi-prism where the probe beam interferes with itself after crossing the plasma medium. A high-dynamic range CCD camera is used to record the interference patterns. Based upon the Abel inversion technique, we obtained a 3D density distribution of the plasma density.