Temporal and spatial study of differently charged ions emitted by ns-laser-produced tungsten plasmas using time-of-flight mass spectroscopy

Ding WU(吳鼎),George C-Y CHAN,Xianglei MAO(毛向雷),Yu LI(李裕),Richard E RUSSO,Hongbin DING (丁洪斌),* and Vassilia ZORBA,3,*

1 School of Physics,Dalian University of Technology,Dalian 116024,People’s Republic of China

2 Lawrence Berkeley National Laboratory,Berkeley,CA 94720,United States of America

3 Department of Mechanical Engineering,University of California,Berkeley,CA 94720,United States of America

Abstract Tungsten (W) is an important material in tokamak walls and divertors.The W ion charge state distribution and the dynamic behavior of ions play important roles in the investigation of plasma–wall interactions using laser-ablation-based diagnostics such as laser-induced breakdown spectroscopy and laser-induced ablation spectroscopy.In this work,we investigate the temporal and spatial evolutions of differently charged ions in a nanosecond-laser-produced W plasma in vacuum using time-of-flight mass spectroscopy.Ions with different charge states from 1 to 7 (W+ to W7+) are all observed.The temporal evolutions of the differently charged ions show that ions with higher charge states have higher velocities,indicating that space separation occurs between the differently charged ion groups.Spatially-resolved mass spectroscopy measurements further demonstrate the separation phenomenon.The temporal profile can be accurately fitted by a shifted Maxwell–Boltzmann distribution,and the velocities of the differently charged ions are also obtained from the fittings.It is found that the ion velocities increase continuously from the measured position of 0.75 cm to 2.25 cm away from the target surface,which indicates that the acceleration process lasts through the period of plasma expansion.The acceleration and space separation of the differently charged ions confirm that there is a dynamic plasma sheath in the laser-produced plasma,which provides essential information for the theoretical laser-ablation model with plasma formation and expansion.

Keywords: laser ablation,temporal and spatial evolution,time-of-flight mass spectroscopy,spatial separation,differently charged ions

1.Introduction

The basic nanosecond laser-ablation processes,including plasma formation and expansion,have been thoroughly investigated using theoretical [1–8] and experimental methods [9–12] over the last two decades.During the plasma formation process,electrons,ions,and atoms in the plasma expand away from the ablation surface,producing strong optical emission,including continuum radiation and line emission.The plasma parameters,such as the atomic,ionic,and electron-number densities and their temperatures are essential for the purpose of chemical analysis,and especially important for the laser-induced breakdown spectroscopy(LIBS)theoretical model.Furthermore,knowledge of the related species’ spatial and temporal evolutions in the plasma,including those of electrons,atoms,and ions,is necessary in order to better understand the plasma behaviors and improve the laser-ablation theoretical model.Moreover,velocity or kinetic energy distributions are also desired parameters for a full description of the plasma.The temporal evolution of ions has been measured using a Faraday cup (FC) to investigate the velocity or kinetic energy distribution,ion angular distributions,charge loss,etc.,during the expansion [13–18].Langmuir probes (LP) also have been used to measure the electron temperature,electron density,and plasma behaviors,such as plume splitting or plasma oscillations[9,11,19–21].In these investigations,differently charged ions have not been directly verified.The temporal and spatial evolutions of ions,in particular for differently charged ions during the plasma expansion,are necessary to provide the basic information to complete the laser–material interaction models.

In recent years,the LIBS technique has been developed to be an important remote,in situmonitoring diagnostic tool for investigating plasma–wall interaction (PWI) processes such as wall erosion,impurity deposition,impurity migration,and plasma fuel retention[22–25]in tokamaks.However,differently charged ions for the LIBS analysis have not received much attention,as reduced pressure or vacuum conditions are required for their observation.For the diagnosis of the plasma–wall interaction using the LIBS technique,an investigation of the temporal and spatial evolutions of the differently charged ions is of benefit for evaluating the potential of utilizing the optical emissions of the differently charged ions.Moreover,when laser ablation is carried out during a tokamak discharge pulse,the ablated species will penetrate the tokamak edge plasma,leading to species re-ionization and re-excitation and further producing characteristic line emissions,which is known as laser-induced ablation spectroscopy (LIAS) [26,27].Absolute quantitative analysis can be carried out,based on the re-emitted characteristic line intensity with absolute calibration and inverse photon efficiency.Therefore,it also has been proposed as a unique method for measuring the fuel retention and impurity migration during the tokamak discharge process [23].Before the LIAS measurements in tokamaks,an assessment of the influence of differently charged ions is a critical process for obtaining reliable data for the absolute quantitative analysis.Hence,investigating the temporal and spatial behavior of laser-produced wall-related elemental tokamak plasmas has become necessary.

Tungsten is an important material of the tokamak wall,which has been proposed to form the divertor in the Experimental Advanced Superconducting Tokamak(EAST)[28]and the plasma-facing components (PFCs) of the International Thermonuclear Experimental Reactor (ITER) [29].Further to our previous work [30],here,we study nanosecond laser ablation at a laser energy of about 10 GW cm?2of W in vacuum to understand differently charged ions’ emissions.In the previous work,we only investigated the time-integrated mass spectroscopy of the laser ablation of W using a linear time-of-flight mass spectrometer(TOF-MS).Although differently charged ions have been observed at a low resolution,the physics of differently charged ion diffusion during plasma expansion is unclear.In this work,the dynamics of differently charged ions(from 1+to 7+),including both the temporal and spatial evolutions for W are investigated.Moreover,a reflectron time-of-flight (TOF) (as opposed to linear) configuration is used to compensate for the initial kinetic energy spread in these investigations,which consequently improves spectral resolution and better distinguishes the differently charged ions.The velocities of the differently charged ions at different positions from the target surface were obtained using a shifted Maxwell–Boltzmann distribution(SMB),and the mechanisms of the generation of the differently charged ions and their acceleration are also discussed.

2.Experimental setup

Figure 2.The extraction configuration of the LA-TOF-MS instrument (left) and the power supply parameters used (right).

A time-of-flight mass spectrometer (TOF-MS) with an orthogonal acceleration and reflectron configuration was used to measure the ionic species formed during laser ablation(figure 1).The basics and a detailed description of the instrument can be found in[31]and in our recent work[32].Briefly,pulses emitted by a Nd:YAG laser operated at a wavelength of 1064 nm with a pulse duration of 7 ns are focused by a plano-convex fused silica lens (100 mm focal length,10 mm diameter) onto a pure W target.The size of the W target is 10 mm in diameter and 2 mm in thickness with a purity of>99.95%.The laser energy used to generate the W plasma is about 42 mJ,with a fluctuation of less than 5%,and the laser spot size is estimated to be 0.27 mm according to the laser-ablation crater.Hence,the laser power density focused on the target surface is estimated to be about 10 GW cm?2.The system is operated at an air gas base pressure of ～4×10?6Pa.The total flight distance from the extraction exit to the MCP detector is about 1.9 m.

Figure 2 shows a schematic illustration of the extraction configuration of the laser-ablation time-of-flight mass spectrometer (LA-TOF-MS) system (left).The values of the power supply parameters optimized according to the signal also are shown in figure 2(right).The W target is installed on a sample rod between two repeller plates.The pulsed voltages applied to the two repeller plates (POS and NEG) are controlled by two high-voltage MOSFET (metal–oxide–semiconductor field-effect transistor) push-pull switches (Behlke,HTS 31-GSM).All other voltages applied in the system are direct-current voltages (DCV); their parameters are shown in figure 2 (right).After the laser ablation,the generated ionic species are accelerated by the high pulsed voltage and removed from the extraction hole.The diameter of the hole is 1.5 mm,allowing ions to be extracted.The pulsed voltage width is set as 0.2 μs and the delay time of the applied pulsed voltage respect to the laser pulse is set to 0.1–10 μs,which is achieved using a digital generator (DG645).The vertical distancedeis manually controlled by moving the load-lock valve with a resolution of 0.5 mm.Seven different vertical distancesde(0.75,1.0,1.25,1.5,1.75,2.0,and 2.25 cm)from the sample target surface are used to study the temporal and spatial dynamics of the plasma.The distance from the focal lens to the target surfacedfremains the same to guarantee that the laser power density is fixed after changing the vertical distancede.In this manner,the ionic species’ distribution versus time and space can be obtained.For each spatial position on the target,thirty laser shots are first applied to clean the W target,and the next 60 shots are used to measure the time evolution mass spectroscopy.The mass spectra are recorded for each laser shot and for each distance,and the experiments are repeated five times to measure the errors.

3.Results and discussion

3.1.Temporally resolved mass spectroscopy of differently charged W ions

Figure 3 shows the distribution of differently charged W ions at various delay times at the measured position of 1.5 cm.The measured maximum charge state for differently charged W ions is up to 7;the W ions with higher charges appear earlier in time.The typical widths here for W+to W7+are from～0.7 μs (at a flight time of ～30 μs) to ～0.2 μs (at a flight time of ～11 μs).Moreover,the transient laser-produced plasma is highly space- and time-dependent,leading to the width of the peak varying with time.The differently charged ion intensities and relative ratios vary with time,indicating that differently charged ions play an important role in plasma formation and expansion.The differently charged ions are probably generated due to step-ionization as a result of the plasma shielding effect during the strong laser–plasma interaction process,which has been discussed and proposed in our recent work [30].During the period of nanosecond laser irradiation on the W target surface,the laser-pulse front rapidly heats the free electrons in the metal conduction band and the target surface’s temperature rises through electron–phonon interactions.When the temperature rises to the critical value required for phase changes,the target is melted,and vaporizes to initially form an dense layer in front of the surface.The vapor layer’s initial electrons continue to absorb the latter part of the laser pulse by inverse bremsstrahlung(IB),and further excitation ionization occurs,leading to the formation of the primary plasma.

Figure 3.Time-resolved mass spectra of W ions at different delay times at a position of 1.5 cm.

If the laser power density is high enough and the laser pulse does not end,the primary electrons will generate new electrons via strong IB collisions,leading to electron cascade growth.When the electron density increases to a critical value at some time,the plasma becomes opaque to laser radiation at a specific wavelength,which is known as the plasma shielding effect[33].With the continuous absorption of the laser energy,the plasma can be further ionized by the strong IB collision process,leading to the generation of differently charged ions.

3.2.Charge state distributions and temporal evolutions of W ions at different positions

We investigated the charge state distributions and time evolutions of the Wn+(n=1–7) ions at seven positions from 0.75 cm to 2.25 cm with an interval of 0.25 cm.The timeresolved mass spectra of W ions at different delay times from 0.1 μs to 10 μs were also obtained.For clarity,the results for six positions are shown in figure 4 using a color scheme.At a position of 1.0 cm,the intensity of the W1+ion is weak compared to those at longer positions,which indicates that the recombination process should be an important channel for the origin of the W1+ions.

The temporal evolutions of the area-integrated intensities for the differently charged Wn+ions (n=1–7) at a position of 1.5 cm and the W4+ions’ temporal evolutions at different positions are shown in figures 5(a) and (b),respectively.The experimental data points are fitted by time-integrated shifted Maxwell–Boltzmann distributions shown in solid lines.The velocity valuesucare obtained from the fittings,which will be described later.Except for W1+,the root-mean-square values for all other fittings are better than 0.97.It can be clearly seen in figure 5(a) that W ions with higher charged states rise to their peak intensities earlier as a result of higher velocities.This behavior indicates that spatial separation occurs between these differently charged ion groups during the plasma expansion process.

3.3.The velocities of Wn+ ions versus charge state and position

The expression for the time evolution of the ions produced by laser ablation can be represented by a shifted Maxwell–Boltzmann distribution written as a function of time,as shown below [34–37]:whereAis the normalization constant,mandLare the mass of the W ion and the vertical distance between the target surface and the extraction hole,respectively,ucis the centerof-mass velocity of the ions,andkTis the equivalent temperature describing the stream velocity spread.In our case,as the pulse width (tw) for the TOF extraction is 0.2 μs,the measured signal intensity should be time integrated.Hence,the time evolution of the ions is expressed by the time-integrated shifted Maxwell–Boltzmann (ISMB) function:

Figure 5.(a) Temporal evolutions of the Wn+ ions (n=1–7) at a position of 1.5 cm; (b) temporal evolutions of the W4+ ions at different measured positions from 0.75 cm to 2.25 cm.

where η is the detection efficiency of the microchannel plate(MCP)for specific charged ions,tis the delay time,andtwis the effective time width for different charged ions.As the effective time widths are different for ions with different charge states,twis not fixed for fitting.For better fitting,only partial experimental data (0.1 μs to 2.5 μs) of the time evolution of W1+ions were applied to the fit,as the tendency at later times is irregular,for some unknown reason.The irregular tendency of the singly charged ions is probably due to collisions with the electrode repellers which confine the less-charged ions,causing the signal time to last longer and be irregular.In a future study,we will upgrade the repeller plates to grids,which will reduce the influence of the collisional reflectivity caused by the electrodes.The fitted solid lines are shown in figure 5.The center-of-mass velocityucis plotted as a function of the charge state (+1 to +7) and the positions(0.75 cm to 2.25 cm) from the target surface in figure 6.Ions with higher charge states have greater velocities,and their velocities are related to their positions.The velocities of Wn+ions increase continuously from the measured position of 0.75 cm to 2.25 cm.The maximum center-of-mass velocity obtained here is up to about (4.5±0.2)×106cm s?1.

Figures 7(a) and (b) show the spatial profiles of the areaintegrated intensities for the Wn+ions at delay times of 0.25 μs and 0.4 μs,respectively.The data in figure 7 show that W ions with more charges are distributed at the further positions,which directly demonstrates the spatial separation phenomenon between the differently charged ion groups.It has also been observed that the ions’velocity has a positive relationship with their charged state,as shown in figure 6(a).The differently charged ions’ acceleration mechanism is probably due to the transient dynamic sheath formed during the ablation process.During the nanosecond laser irradiation,electrons mainly gain energy from the laser via IB collisions,and the differently charged ions are also generated by the strong collisions.As electrons are the lightest species in the plasma,they escape from the sample surface much earlier than the ions and atoms.This causes a charge separation between the fast electrons and ions in the plasma and gives rise to an induced inner electric field.The inner electric field prevents the electrons from escaping and confines the electrons at the plasma’s outer edge[35],leading to a sheath configuration,which is called the transient dynamic plasma sheath.The spatial scale of the charge separation between the fast electrons and ions may extend to several centimeters during the laser pulse,which has been observed in investigations [38].Hence,W ions with higher charge states are accelerated with higher velocities by the dynamic plasma sheath.It can be observed in figure 7(b)that the spatial distribution of W3+is broader than those of the W ions with higher charges.The reason for this might that during the expansion process,some W3+ions may originate from the recombination of more highly charged ions,leading to the broader distribution.

Figure 6.Velocities of the Wn+ ions as a function of charge state (a) and position (b).The error bar is determined using multiple measurements.

Figure 7.Spatial profile of the area-integrated intensities for W ions at delay times of 0.25 μs(a)and 0.4 μs(b),respectively.The data points are smoothly connected for better display.Error bar is determined by using multiple measurements.

4.Conclusions

We have investigated the temporal and spatial dynamics of differently charged W ions in a nanosecond laser-produced plasma.The charge state of the W ions can be up to 7 with the current experimental parameters.The spatial separation between W ion groups with different charge states was observed,which is due to the acceleration of the plasma sheath that is present.The temporal evolution of the differently charged ions was found to follow a shifted Maxwell–Boltzmann distribution,and the center shifted velocities for the differently charged ions were also obtained.The differently charged ions’distribution and their spatial and temporal evolution results provide beneficial information for the nanosecond laser-ablation model,including information about plasma formation and expansion,which also benefits the investigation of plasma–wall interactions in the EAST tokamak using the LIBS and LIAS diagnostics.

Acknowledgments

This work was supported by the National Key R&D Program of China (No.2017YFE0301304),National Natural Science Foundation of China (No.12005034),and the China Postdoctoral Science Foundation (No.2019M661087).This research was also supported by the US Department of Energy,Office of Defense Nuclear Nonproliferation Research and Development,under contract number DE-AC02-05CH11231 at the Lawrence Berkeley National Laboratory.