Combined effects of ambient gas pressures and magnetic field on laser plasma expansion dynamics

In this work,we investigated the influence of air gas pressures on the expansion features of nanosecond laser ablated aluminum plasma in the absence and presence of a nonuniform magnetic field using fast photography.A particular emphasis was given to the plume dynamics(shape,size) with the combined effects of ambient gas pressures and an external magnetic field.Free expansion,sharpening effect,and hemi-spherical structures of the aluminum plasma were observed without a magnetic field under different gas pressures.Analysis of the resulting plume images with the combined effects of air gas pressures and a magnetic field show significant changes,such as plume splitting,elliptical geometry changes,radial expansion,and plume confinement.Furthermore,the total size of the plasma plume with a magnetic field was measured to be smaller than the plasma plume without a magnetic field at several background pressures.     

Effect of distances between lens and sample surface on laser-induced breakdown spectroscopy with spatial confinement

Spatial confinement can significantly enhance the spectral intensity of laser-induced plasma in air.It is attributed to the compression of plasma plume by the reflected shockwave.In addition,optical emission spectroscopy of laser-induced plasma can also be affected by the distance between lens and sample surface.In order to obtain the optimized spectral intensity,the distance must be considered.In this work,spatially confined laser-induced silicon plasma by using a Nd:YAG nanosecond laser at different distances between lens and sample surface was investigated.The laser energies were 12 mJ,16 mJ,20 mJ,and 24 mJ.All experiments were carried out in an atmospheric environment.The results indicated that the intensity of Si (I) 390.55 nm line firstly rose and then dropped with the increase of lens-to-sample distance.Moreover,the spectral peak intensity with spatial confinement was higher than that without spatial confinement.The enhancement ratio was approximately 2 when laser energy was 24 mJ.     

Influence of sample temperature on the expansion dynamics of laser-induced germanium plasma

In this paper, we investigated the influence of sample temperature on the expansion dynamics and the optical emission spectroscopy of laser-induced plasma, and Ge was selected as the test sample. The target was heated from room temperature(22 °C) to 300 °C, and excited in atmospheric environment by using a Q-Switched Nd:YAG pulse laser with the wavelength of 1064 nm. To study the plasma expansion dynamics, we observed the plasma plume at different laser energies(5.0, 7.4 and 9.4 mJ)and different sample temperatures by using time-resolved image. We found that the heated target temperature could accelerate the expansion of plasma plume. Moreover, we also measured the effect of target temperature on the optical emission spectroscopy and signal-to-noise ratio.     

A 7 T Pulsed Magnetic Field Generator for Magnetized Laser Plasma Experiments

A pulsed magnetic field generator was developed to study the effect of a magnetic field on the evolution of a laser-generated plasma.A 40 kV pulsed power system delivered a fast(~230 ns),55 kA current pulse into a single-turn coil surrounding the laser target,using a capacitor bank of 200 nF,a laser-triggered switch and a low-impedance strip transmission line.A one-dimensional uniform 7 T pulsed magnetic field was created using a Helmholtz coil pair with a 6 mm diameter.The pulsed magnetic field was controlled to take effect synchronously with a nanosecond heating laser beam,a femtosecond probing laser beam and an optical Intensified Charge Coupled Device(ICCD) detector.The preliminary experiments demonstrate bifurcation and focusing of plasma expansion in a transverse magnetic field.     

Spatial Resolution Measurements of C,Si and Mo Using LIBS for Diagnostics of Plasma Facing Materials in a Fusion Device

Recently,a laser-induced breakdown spectroscopic(LIBS) system has been developed for in situ measurements of the chemical compositions of plasma facing materials(PFMs)in the Experimental Advanced Superconducting Tokamak(EAST).In this study,a LIBS system,which was used in a similar optical configuration to the in situ LIBS system in EAST,has been developed to investigate the spatial distribution of PFM elements at 1CP4 Pa.The aim of this study was to understand the nature of the spatial distribution of atoms or ions of different elements in the plasma plume and optimize the signal to background ratio for the in situ LIBS diagnosis in EAST.The spatial profiles of the LIBS signals of C,Si,Mo and the continuous background were measured.Moreover,the influence of laser spot size and laser energy density on the LIBS signals of C,Si,Mo and H was also investigated.The results show that the distribution of the C,Si and Mo peaks' intensities first increased and then decreased from the center to the edge of the plasma plume.There was a maximum value at R≈1.5 mm from the center of the plasma plume.This work aims to improve the understanding of ablating plasma dynamics in very low pressure environments and give guidance to optimize the LIBS system in the EAST device.     

The Enhanced Effect of Optical Emission from Laser Induced Breakdown Spectroscopy of an Al-Li Alloy in the Presence of Magnetic Field Confinement

In this paper,the influence of magnetic field strength on laser-induced breakdown spectroscopy(LIBS) has been investigated for various pressures.The plasma plume was produced by employing Q-switch Nd:YAG laser ablation of an Al-Li alloy operating at a 1064 nm wavelength.The results indicated that the LIBS intensity of the Al and Li emission lines is boosted with an increase of magnetic strength.Typically,the intensity of the Al Ⅰ and Li Ⅰ spectral emissions can be magnified by 1.5-3 times in a steady magnetic field of 1.1 T compared with the field-free case.Also,in this investigation we recorded time-resolved images of the laser-produced plume by employing a fast ICCD camera.The results show that the luminance of the plasma is enhanced and the time of persistence is increased significantly,and the plasma plume splits into two lobes in the presence of a magnetic field.The probable reason for the enhancement is the magnetic confinement effect which increases the number density of excited atoms and the population of species in a high energy state.In addition,the electron temperature and density are also augmented by the magnetic field compared to the field-free case.     

Boiling of Potassium in Vertical Tube under Transverse Magnetic Field

The experimental study was conducted to examine the applicability of boiling potassium two-phase flow to the blanket cooling of fusion power reactors. A high flux heater pin of 44mm heating length and 6.5mm O. D. with eight thermocouples of 0.5mm O. D. was inserted from the bottom of a vertical channel which was made of a 4m long, 14.9mm I.D. and 17.5mm O. D. stainless steel tube and placed in a D.C. magnet of 50cm long poles. The experimental conditions were the heat flux: 0~67W/cm², the magnetic field strength: 0~1.8T, the Ar cover gas pressure: 1.0bar, the potassium level above heater: 1.8m, and the temperature of upper unheated section: 400°C.

In the absence of magnetic field, boiling occurred intermittently, repeating the cycles between superheating with moderate temperature fluctuation and desuperheating with condensing shock pulses. When a weak magnetic field was applied, the temperature fluctuation was enhanced by natural convection, the incipient boiling superheat was reduced, and the boiling pattern became continuous. With increasing the magnetic field strength, the fluctuation was suppressed and the incipient boiling superheat increased but tended to level off around 1.5 T. Violent incipient boiling caused by a large superheat inherent in liquid metals was mitigated by magnetohydrodynamic interaction under a transverse magnetic field of 1.0T or larger, and subsequently followed by continuous saturation boiling with small fluctuation. No burn-out of the heater pin occurred in spite of symptom of dryout within the experimental range: q=67W/cm² and B=1.5T.     

Development of a silver ion source using nanosecond pulses of a Nd:YAG laser at different wavelengths

A silver ion source was designed by focusing the fundamental and harmonics of Q-switched Nd:YAG laser pulses onto a silver target and simultaneously applying an electric potential in an argon environment. The silver ions were detected at a distance of 2 cm from the target surface using a Faraday cup ion probe after letting them pass through a retarding mesh grid (copper electrode). We aim to produce and characterize the silver ions generated by the laser radiation of different wavelengths and pulse energy, ambient gas pressure and the electrode spacing under applied electric field. In addition to this, the effect of laser radiation on plasma under vacuum and at different argon gas pressures was investigated. The velocity distribution function of the plasma emitted from the silver target was investigated under argon discharge. These measurements demonstrated clearly that the velocity distribution function and current signals depend on laser power, laser wavelength and argon pressure. We observed a ten fold increase in the plume current with increase in the applied voltage and ion velocity in the presence of a laser field. The surface morphology of the laser irradiated samples was investigated using reflection optical microscopy.     

Energy and flux measurements of laser-induced silver plasma ions by using Faraday cup

Silver(Ag)plasma has been generated by employing Nd∶YAG laser(532 nm,6 ns)laser irradiation.The energy and flux of ions have been evaluated by using Faraday cup(FC)using time of flight(TOF)measurements.The dual peak signals of fast and slow Ag plasma ions have been identified.Both energy and flux of fast and slow ions tend to increase with increasing irradiance from 7 GW cm-2 to 17.9 GW cm-2 at all distances of FC from the target surface.Similarly a decreasing trend of energies and flux of ions has been observed with increasing distance of FC from the target.The maximum value of flux of the fast component is 21.2×1010cm-2,whereas for slow ions the maximum energy and flux values are 8.8 keV,8.2×1012 cm-2 respectively.For the analysis of plume expansion dynamics,the angular distribution of ion flux measurement has also been performed.The overall analysis of both spatial and angular distributions of Ag ions revealed that the maximum flux of Ag plasma ions has been observed at an optimal angle of～15°.In order to confirm the ion acceleration by ambipolar field,the self-generated electric field(SGEF)measurements have also been performed by electric probe;these SGEF measurements tend to increase by increasing laser irradiance.The maximum value of 232 V m-1 has been obtained at a maximum laser irradiance of 17.9 GW cm-2.     

Trapping of Free Electrons in Multipole System

In this paper, the effective Parameters in the confinement and trapping of fast electrons in plasma source Such as; plasma pressure, wall material of plasma chamber and magnetic mirror rate have been investigated with using Comsol & Geant4 code. The calculations are shown that the Multicusp magnetic field was effective the pressure less than 5?mTor, and the confinement effect becomes stronger with decreasing pressure. It is equivalent to a higher yield of output ions of plasma source. The number of fast electrons trapped in the magnetic field increases with increasing magnetic field intensity and using aluminum for wall material. Optimum conditions of confinement plasma, leading to increased the hot electron density, and ionization efficiency is increased. The results of investigations have demonstrated good correspondence with theoretical calculations, therefore there is the adequacy of the developed approach and the possibility to build more effective source ion on this basis.     

Micro-impulse and plasma plume produced by irradiating aluminum target with nanosecond laser pulses in double-pulse scheme

The micro-impulse generated by ablating an aluminum target in double-pulse laser bursts with different interpulse delays was investigated using a torsion pendulum. The plasma plume was simultaneously visualized using high-speed photography to analyze the coupling mechanism of the ablation impulse. The experiment was carried out using a pulsed laser with a pulse width of 8 ns and a wavelength of 1064 nm. The experimental results show that an impulse with an interpulse delay of 60 ns is roughly 60% higher than that with no delay between the two pulses, when the energy of both laser pulses is 50 mJ. Therefore, double-pulse schemes could enhance the ablation impulse under certain conditions. This is because the ablation of the first laser pulse changes the optical properties of the aluminum target surface, increasing the absorptivity. However, the ablation impulse is reduced with a time delay of 20 ns when the energy of both laser pulses is 100 mJ or 150 mJ. It can be concluded that the plasma produced by ablating the aluminum with the first pulse shields the second laser pulse. To summarize, the experimental results show that different delay times in a double-pulse scheme have a significant effect on the ablation impulse. The study provides a reference for the optimization of the parameters when laser ablation propulsion with a double-pulse scheme is applied in the fields of space debris removal, laser ablation thrusters, and so on.     

Identification of Plasma Boundary and Position for HL-2A Tokamak

Using the virtual-case principle, the plasma boundary, the plasma current center, and the x-point are identified for the HL-2A tokamak. The plasma current is represented by the current center and the virtual multipole moments which produce a magnetic flux in a form of polynomial. Adaptive parameters in the polynomial are determined by the least-square fit of the poloidal magnetic fields. The measurement of the magnetic field is performed using pick-up coils. The virtual-case principle is applied outside the plasma boundary. The virtual-case currents decide the position of the current center and produce a negative confinement magnetic field inside the plasma and the magnetic field generated by the plasma current outside the plasma boundary. The convergence is fast enough to get a picture between the sequent shots. The configuration reconstructed is in good agreement with the TV image taken by camera with a tangential view.     

Fabrication and absolute calibration of B-dot probe for magnetic field measurement on a high power laser facility

The fabrication and absolute calibration of a B-dot probe is employed to measure the pulsed magnetic field at the Shenguang-II high power laser facility. Copper enameled silk with a cross section diameter of 0.1 mm is used to wind the one-turn coil with a 1 mm diameter. Two coils are paired and reversely linked to their respective circuits to form a differential B-dot probe that is sealed in and protected by a quartz shell. This B-dot probe is experimentally calibrated and then used to measure the pulsed magnetic field in laser targeting experiments at the Shenguang-II high power laser facility. Signals show a high performance of this B-dot probe. The common mode noise can be effectively canceled out by the differential pair. The magnetic field of over 300 T can be extrapolated at the location close to the target.     

Linear Connected Array of Non-Adiabatic Traps

An innovative confinement concept of a linearly connected array of non-adiabatic traps is examined. A non-adiabatic trap unit consists of a cylindrical vacuum chamber with external mirror coils and a pair of inner anti-parallel coils. Positions and currents on these coils are adjusted to achieve zero magnetic fields at the center. The plasma is trapped stably in this configuration due to the “Absolute Minimum B” magnetic configuration that is created. However, like a cusp field, energy confinement of plasma in a trap unit is poor, since particles suffer random pitch-angle scattering near the central zero field region and eventually fall into the loss-cone. However, once these non-adiabatic traps are linearly connected, plasma particles escaping a unit will be effectively re-trapped within the neighboring trap due to collision-free pitch-angle scattering in the zero field region. Since the transition of a charged plasma particle from unit to a next unit is stochastic, a connected array of non-adiabatic fusion core units and similar “leak suppressor” array units with low fusion rate at both ends of the core array improves the plasma confinement sufficient to achieve reactor-grade plasmas.     

The Plasma Jet/Laser Driven Compression of Compact Plasmoids to Fusion Conditions

Magneto-inertial fusion (MIF) is based on both magnetic and inertial confinement. An embedded magnetic field is compressed along with the target plasma to achieve magnetic insulation and fusion condition. Several magnetic systems for plasma confinement may be used for laser-driven (LD) and plasma jet driven (PJ) magnetic flux compression. Estimations show the possibility in principle to realize regimes of PJMIF system with a plasma gain factor Q > 10.     

Numerical simulation of laser ablation of molybdenum target for laser-induced breakdown spectroscopic application

Laser-induced breakdown spectroscopy has been recognized as a significant tool for element diagnostics in plasma–wall interaction. In this work, a one-dimensional numerical model is developed to simulate the laser ablation processes of a molybdenum(Mo) target in vacuum conditions. The thermal process of the interaction between the ns-pulse laser with wavelength of 1064 nm and the Mo target is described by the heat conduction equation. The plasma plume generation and expansion are described by Euler equations, in which the conservation of mass density, momentum and energy are included. Saha equations are used to describe the local thermal equilibrium of electrons, Mo atoms,Mo~+ and Mo~(2+) Plasma shielding and emission are all considered in this model. The mainly numerical results are divided into three parts, as listed below.Firstly, the rule of the plasma shielding effect varying with laser intensity is demonstrated quantitatively and fitted with the Nelder function. Secondly, the key parameters of plasma plume,such as the number density of species, the propagation velocity and the temperature, are all calculated in this model. The results indicate that the propagation velocity of the plume center increased with time in a general trend, however, one valley value appeared at about 20 ns due to the pressure gradient near the target surface leading to negative plasma velocity. Thirdly, the persistent lines of a Mo atom in the wavelength range from 300 nm to 600 nm are selected and the spectrum is calculated. Moreover, the temporal evolutions of Mo's spectral lines at wavelength of 550.6494 nm,553.3031 nm and 557.0444 nm are given and the results are compared with experimental data in this work.     

Spectral Enhancement of Laser-Induced Breakdown Spectroscopy in External Magnetic Field

In this paper the spectral enhancement of laser-induced breakdown spectroscopy (LIBS) for copper plasma in the presence of a magnetic field is investigated and the temporal-and spatial-resolved plasma emission spectra are analyzed. Experimental results show that the copper plasma atomic and ion spectra have been enhanced in the presence of the external magnetic field. In addition, the Cu I 521.82 nm spectral intensity evolution with delay time appears to have a double peak around the delay time of 2 μs, but that of Cu II 507.57 nm has a sharp decrease because of the electron-atom three body recombination process. The plasma temperature with magnetic confinement is lower than that of the case in the absence of magnetic fields. Finally, the spectral enhancement mechanisms of laser induced breakdown spectroscopy with magnetic confinement are analyzed.     

Upper Hybrid Resonance of Microwaves with a Large Magnetized Plasma Sheet

A large magnetized plasma sheet with size of 60 cmx60 cmx2 cm was generated by a linear hollow cathode discharge under the confinement of a uniform magnetic field generated by a Helmholtz Coil.The microwave transmission characteristic of the plasma sheet was measured for different incident frequencies,in cases with the electric field polarization of the incident microwave either perpendicular or parallel to the magnetic field.In this measurement,parameters of the plasma sheet were changed by varying the discharge current and magnetic field intensity.In the experiment,upper hybrid resonance phenomena were observed when the electric field polarization of the incident wave was perpendicular to the magnetic field.These resonance phenomena cannot be found in the case of parallel polarization incidence.This result is consistent with theoretical consideration.According to the resonance condition,the electron density values at the resonance points are calculated under various experimental conditions.This kind of resonance phenomena can be used to develop a specific method to diagnose the electron density of this magnetized plasma sheet apparatus.Moreover,it is pointed out that the operating parameters of the large plasma sheet in practical applications should be selected to keep away from the upper hybrid resonance point to prevent signals from polarization distortion.     

Time-of-flight and UV spectroscopy characterization of laser-generated plasma

A study of laser ablation of different targets (Al, Ti, Mo, Au and polyethylene), in vacuum, by using 3 ns Nd:YAG laser radiation, at 1064 nm wavelength, is reported. The ion emission from the plasma was monitored through time-of-flight (TOF) measurements, performed by using an ion collector placed along the normal to the target surface. The deconvolution of the IC experimental spectra with a Coulomb-Boltzmann-shifted function permitted to evaluate the equivalent ion temperature and the acceleration voltage developed inside the non-equilibrium plasma.The UV plasma emission, detected with an optical spectroscope, permitted to estimate the electronic temperature and density, to evaluate the Debye length and the temperature gradient in the laser-generated plasma plume.     

Characteristics of a kHz helium atmospheric pressure plasma jet interacting with two kinds of targets

This study investigates the influence of two types of target,skin tissue and cell culture medium,with different permittivities on a k Hz helium atmospheric pressure plasma jet (APPJ) during its application for wound healing.The basic optical–electrical characteristics,the initiation and propagation and the emission spectra of the He APPJ under different working conditions are explored.The experimental results show that,compared with a jet freely expanding in air,the diameter and intensity of the plasma plume outside the nozzle increase when it interacts with the pigskin and cell culture medium targets,and the mean velocity of the plasma bullet from the tube nozzle to a distance of 15 mm is also significantly increased.There are also multiple increases in the relative intensity of OH (A~2Σ?→?X~2Π) and O (3p~5S–3s~5S) at a position 15 mm away from nozzle when the He APPJ interacts with cell culture medium compared with the air and pigskin targets.Taking the surface charging of the low permittivity material capacitance and the strengthened electric field intensity into account,they make the various characteristics of He APPJ interacting with two different targets together.