Tungsten ions produced by infrared pulsed laser irradiation

Energetic ions have been obtained irradiating a tungsten target with a Q-switched Nd:Yag laser, 1064?nm wavelength, 9?ns pulse width, 900?mJ maximum pulse energy and power density of the order of 10¹⁰?W/cm². The laser-target interaction induces a strong metal etching with production of plasma in front of the target. The plasma contains neutrals and ions with high charge state. Time-of-flight measurements are presented for qualitative analysis of the ion production. A cylindrical electrostatic ion analyzer permits measuring of the yield of emitted ions, the charge state of detected ions and the ion energy distribution. Measurements indicate that, at a laser fluence of the order of 100?J/cm², the charge state may reach 9+ and the ion energy reaches about 5?keV. The ion energy distribution is given as a function of the charge state. Experimental results indicate that an electrical field is developed along the normal to the plane of the target surface, which accelerates the ions up to high velocity. The ion velocity distributions follow a “shifted Maxwellian distribution”, which the author has corrected for the Coulomb interactions occurring inside the plasma.     

Non-equilibrium plasma production by pulsed laser ablation of gold

A gold target has been irradiated with a Q-switched Nd:Yag laser having 1064?nm wavelength, 9?ns pulse width, 900?mJ maximum pulse energy and a maximum power density of the order of 10¹⁰?W/cm². The laser–target interaction produces a strong gold etching with production of a plasma in front of the target. The plasma contains neutrals and ions having a high charge state. Time-of-flight (TOF) measurements are presented for the analysis of the ion production and ion velocity. A cylindrical electrostatic deflection ion analyzer permits measurement of the yield of the emitted ions, their charge state and their ion energy distribution. Measurements indicate that the ion charge state reaches 6⁺ and 10⁺ at a laser fluence of 100?J/cm² and 160?J/cm², respectively. The maximum ion energy reaches about 2?keV and 8?keV at these low and high laser fluences, respectively. Experimental ion energy distributions are given as a function of the ion charge state. Obtained results indicate that electrical fields, produced in the plume, along the normal to the plane of the target surface, exist in the unstable plasma. The electrical fields induce ion acceleration away from the target with a final velocity dependent on the ion charge state. The ion velocity distributions follow a “shifted Maxwellian distribution”, which the authors have corrected for the Coulomb interactions occurring inside the plasma.     

Ion implantation induced by Cu ablation at high laser fluence

High energy laser plasma-produced Cu ions have been implanted in silicon substrates placed at different distances and angles with respect to the normal to the surface of the ablated target. The implanted samples have been produced using the iodine high power Prague Asterix Laser System (PALS) using 438 nm wavelength irradiating in vacuum a Cu target. The high laser pulse energy (up to 230 J) and the short pulse duration (400 ps) produced a non-equilibrium plasma expanding mainly along the normal to the Cu target surface. Time-of-flight (TOF) technique was employed, through an electrostatic ion energy analyzer (IEA) placed along the target normal, in order to measure the ion energy, the ion charge state, the energy distribution and the charge state distribution. Ions had a Boltzmann energy distributions with an energy increasing with the charge state. At a laser fluence of the order of 6 × 10⁶ J/cm², the maximum ion energy was about 600 keV and the maximum charge state was about 27+.In order to investigate the implantation processes, Cu depth profiles have been performed with Rutherford backscattering spectrometry (RBS) of 1.5 MeV helium ions, Auger electron spectroscopy (AES) with 3 keV electron beam and 1 keV Ar sputtering ions in combination with scanning electron microscopy (SEM). Surface analysis results indicate that Cu ions are implanted within the first surface layers and that the ion penetration ranges are in agreement with the ion energy measured with IEA analysis.     

Enhanced production of fast multi-charged ions from plasmas formed at cleaned surface by femtosecond laser pulse

We present atomic, energy, and charge spectra of ions accelerated at the front surface of a silicon target irradiated by a high-contrast femtosecond laser pulse with an intensity of 3×10¹⁶ W/cm², which is delayed with respect to a cleaning nanosecond laser pulse of 3-J/cm² energy density. A tremendous increase in the number of fast silicon ions and a significant growth of their maximum charge in the case of the cleaned target from 5+ to 12+ have been observed. The main specific features of the atomic, energy, and charge spectra have been analyzed by means of one-dimensional hydrodynamic transient-ionization modeling. It is shown that fast highly charged silicon ions emerge from the hot plasma layer with a density a few times less than the solid one, and their charge distribution is not deteriorated during plasma expansion.This revised version was published online in August 2005 with a corrected cover date.     

Characterisation of laser-produced tungsten plasma using optical spectroscopy method

This paper describes results of spectroscopic investigation of laser-produced tungsten plasma. The laser intensity on the target surface reached up to 30 GW/cm² depending on the focusing conditions. Optical spectra emitted from plasma plumes which were formed under vacuum conditions in front of the tungsten target due to the interaction of Nd-YAG laser pulses (1.06 μm, 0.5 J), were characterised by means of an optical spectrometer (λ/Δλ= 900) in the wavelength range from 300 to 1100 nm. The spectra were recorded automatically with the use of a CCD detector with exposition time varied from 100 ns to 50 ms. On the basis of WI and WII lines it was possible to estimate electron temperature and electron density which corresponded to the expansion phase of the plasma. T_e and N_e were measured as 1.1 eV and 8×10¹⁶ cm^-3, respectively. The spectra collected by the ion energy analyser showed that the plasma included tungsten ions up to 6+ ion charge. Signals from the ion collector allowed to estimate the average value of ion energy of tungsten as 4.6 keV. Basing on this value the electron temperature corresponding to the initial stage of the plasma formation was estimated to be about 320 eV. Optical microscope investigation showed that laser irradiation caused structural changes on the surface of the target.     

Improvement of the efficiency of a glow discharge-based ion emitter with oscillating electrons

Ion emission from the plasma of a low-pressure (≈5×10⁻² Pa) glow discharge with electrons oscillating in a weak (≈1 mT) magnetic field is studied in relation to the cold hollow cathode geometry. A hollow conic cathode used in the electrode system of a cylindrical inverted magnetron not only improves the extraction of plasma ions to ≈20% of the discharge current but also provides the near-uniform spatial distribution of the ion emission current density. The reason is the specific oscillations of electrons accelerated in the cathode sheath. They drift in the azimuth direction along a closed orbit and simultaneously move along the magnetic field toward the emitting surface of the plasma. A plasma emitter with a current density of ≈1 mA/cm² over an area of ≈100 cm² designed for an ion source with an operating voltage of several tens of kilovolts is described.     

Energy analysis of protons emitted from Nd:YAg laser-generated plasmas

Hydrogenated targets have been irradiated in vacuum with the pulsed Nd:YAg laser at intensities of the order of 10¹⁰ W/cm². The laser-generated plasma, produced by the interaction with the solid, emits protons and other ions along the normal to the target surface. Ion collectors and ion energy analyzer were used to measure the current, the angular emission and the energy distributions of the emitted protons. Time-of-flight measurements, Coulomb–Boltzmann-distributions and the fits of experimental data were also used in order to evaluate the equivalent ion plasma temperature and the ion acceleration developed in the non-equilibrium-pulsed plasma.     

Acceleration of heavy multicharged ions up to 1 MeV from a cleaned solid target irradiated by a femtosecond laser pulse with an intensity of 10<Superscript>16</Superscript> W/cm<Superscript>2</Superscript>

A noticeable increase in the charge and energy of ions accelerated from a solid tungsten target irradiated by a femtosecond laser pulse with an intensity higher than 10¹⁶W/cm² has been found when the target surface is precleaned by a nanosecond laser pulse with an energy density of 3 J/cm². Tungsten ions with charges up to +29 and energies up to 1 MeV were detected in this case, while the charge and energy of tungsten ions from a target with an uncleaned surface do not exceed +3 and 12 keV, respectively.     

Energy Spectrum and Charge Composition of Ions of Laser-Produced Gd and Al Plasmas

The energy spectrum and the distribution of the total charge over the ion energy are studied for plasma produced by partially coherent radiation of the neodymium glass laser with a power density of ~10¹³ W/cm² in the focusing spot incident on continuous gadolinium (Gd) and aluminum(Al) plates. The spectra of plasma ions for gadolinium and aluminumtargets are presented at laser radiation energies of 2, 7, 20 J and 1 J, respectively. For the gadolinium target, an increase in the peak amplitudes in ion spectra with increasing influencing radiation energy and manifestations of higher-energy ions are observed. The difference in the ion emission for gadolinium and aluminum targets is also recorded at close radiation energies of 2 and 1 J, respectively.     

Multiple charge states of titanium ions in laser produced plasma

An intense laser radiation (10¹² to 10¹¹ W/cm⁻²) focused on the solid target creates a hot (≥1 keV) and dense plasma having high ionization state. The multiple charged ions with high current densities produced during laser matter interaction have potential application in accelerators as an ion source. This paper presents generation and detection of highly stripped titanium ions (Ti) in laser produced plasma. An Nd:glass laser (KAMETRON) delivering 50 J energy (λ=0.53 μm) in 2.5 ns was focused onto a titanium target to produce plasma. This plasma was allowed to drift across a space of ∼3 m through a diagnostic hole in the focusing mirror before ions are finally detected with the help of electrostatic ion analyzer. Maximum current density was detected for the charge states of +16 and +17 of Ti ions for laser intensity of ∼10¹¹ W/cm⁻².     

Bi(Pb)–Sr–Ca–Cu–O layers prepared by a new method of laser ablation in air

Bi(Pb)–Sr–Ca–Cu–O layers were prepared by laser ablation in air, that is without using a vacuum chamber. At this method a much higher density of pulse energy of laser beam is required (10–30 J cm^–2) in comparison with the standard method (1–3 J cm^–2). The mentioned condition has specific effects on the nature of the transport of material from the target. As follows from the electron microprobe analysis of the layers, the starting composition of the transported material is identical with that of the superconducting phase sintered in the target; one exception represents Pb, the content of which in the transported material is lower. The image of the process is complemented by UV/VIS spectroscopy scans of the plasma plume.     

Dynamics of femtosecond laser-produced plasma ions

We investigated the ion laser-produced plasma plume generated during ultrafast laser ablation of copper and silicon targets in high vacuum. The ablation plasma was induced by ≈50 fs, 800 nm Ti:Sa laser pulses irradiating the target surface at an angle of 45°. An ion probe was used to investigate the time-of-flight profiles of the emitted ions in a laser fluence range from the ablation threshold up to ≈10 J/cm². The angular distribution of the ion flux and average velocity of the produced ions were studied by moving the ion probe on a circle around the ablation spot. The angular distribution of the ion flux is well described by an adiabatic and isentropic model of expansion of a plume produced by laser ablation of solid targets. The angular distribution of the ion flux narrows as the laser pulse fluence increases. Moreover, the ion average velocity reaches values of several tens of km/s, evidencing the presence of ions with kinetic energy of several hundred eV. Finally, the ion flux energy is confined in a narrow angular region around the target normal.     

Characterization of the laser ablation plasma used for the deposition of amorphous carbon

The plasma produced by laser ablation of a graphite target was studied by means of optical emission spectroscopy and a Langmuir planar probe. Laser ablation was performed using a Nd:YAG laser with emission at the fundamental line with pulse length of 28 ns. In this work, we report the behavior of the mean kinetic energy of plasma ions and the plasma density, as a function of the laser fluence (J/cm²), and the target to probe (substrate) distance. The characterized regimes were employed to deposit amorphous carbon at different values of kinetic energy of the ions and plasma density. The mean kinetic energy of the ions could be changed from 40 to 300 eV, and the plasma density could be varied from 1 × 10¹² to 7 × 10¹³ cm⁻³. The main emitting species were C⁺ (283.66, 290.6, 299.2 and 426.65 nm) and C⁺⁺ (406.89 and 418.66 nm) with the C⁺ (426.65 nm) being the most intense and that which persisted for the longest times. Different combinations of the plasma parameters yield amorphous carbon with different structures. Low levels (about 40 eV) of ion energy produce graphitic materials, while medium levels (about 200 eV) required the highest plasma densities in order to increase the CC sp³ bonding content and therefore the hardness of the films. The structure of the material was studied by means of Raman spectroscopy, and the hardness and elastic modulus by depth sensitive nanoindentation.     

Schlieren diagnostics of pulsed electron beam ablation of polymethyl-methacrylate

The investigation of the interaction of pulsed electron beams with PMMA (polymethylmethacrylate) targets is reported. The electron beam of some 10^–8 s in duration is produced in a pulsed low-pressure gas discharge. The beam power density of up to 10⁸ W/cm² leads to a surface plasma formation similar to that of the pulsed laser ablation process. The propagation of the ablated material and the shock wave inside the PMMA target are observed by means of Schlieren diagnostics. An electron density gradient of over 3×10¹⁹ cm^–4 has been observed in the expanding plasma up to 1.5 s after the plasma formation. During the early stage of expansion, the expansion velocity of the plasma plume as determined by the steep electron density gradient is around 10⁵ cm/s. The pressure behind the shock front inside the PMMA target as determined from the shock velocity exceeds 0.3 Gpa.     

Generation of a dense electron beam in a thin film using an ultraintense femtosecond laser pulse

Electron dynamics in a thin target irradiated with femtosecond laser pulses at an intensity of 10²⁰ W/cm² is studied in the framework of the kinetic theory of laser plasma based on the construction of propagators (in classical limit) for electron and ion distribution functions in plasma. The calculations are performed for real densities and charges of plasma ions. Electrons are partly ejected from the target. The laser pulse energy is predominantly absorbed by electrons, and the electrons are accelerated to relatively high energies.     

Neutrals' temperature in laser-generated plasma at LNS

A Nd–YAG laser operating at 532 nm with a maximum intensity of 10¹⁰ W/cm² was used to ablate aluminium and tantalum targets placed in vacuum.

A mass quadrupole spectrometer (MQS) at high sensitivity, operating in the range of 1–300 amu, with a resolution better than 1 amu, was used to analyse the atomic emission produced by the laser ablation. The neutrals' emission produced by laser-generated plasma at INFN-LNS was investigated in terms of temperature, ablation yield, angular distribution and velocity.

The neutrals' detection through the MQS permitted to measure the mass energy distribution. Results demonstrate that the maximum temperatures of the neutral species are of the order of 100 eV. The angular emission of neutrals is peaked along the normal to the target surface, as it was observed for the ions; the ablation yield increases suddenly at low laser intensity and decreases at high laser intensity, owing to the higher ionization processes; the flow velocity follows the adiabatic expansion of the plasma in vacuum and it is of the order of 10⁴ m/s.

Measurements will be presented and discussed, according to the available models.     

Characterization of laser-generated silicon plasma

A study of visible laser ablation of silicon, in vacuum, by using 3 ns Nd:YAG laser radiation is reported. Nanosecond pulsed ablation, at an intensity of the order of 10¹⁰ W/cm², produces high non-isotropic emission of neutrals and ionic species. Mass quadrupole spectrometry, coupled to electrostatic ion deflection, allows estimation of the energy distributions of the emitted species from plasma. Neutrals show typical Boltzmann-like distributions while ions show Coulomb-Boltzmann-shifted distributions depending on their charge state. Time-of-flight measurements were also performed by using an ion collector consisting of a collimated Faraday cup placed along the normal to the target surface. Surface profiles of the craters, created by the laser radiation absorption, permitted to study the ablation threshold and ablation yields of silicon in vacuum. The plasma fractional ionization, temperature and density were evaluated by the experimental data. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated at the non-equilibrium plasma conditions. The angular distribution of the neutral and ion species is discussed.     

Initiation of ecton processes by interaction of a plasma with a microprotrusion on a metal surface

Evolution of rapid (～10 ns) Ohmic overheating of a microprotrusion on a surface in contact with a plasma by emission current is studied taking into account the energy carried by plasma ions and electrons, as well as Ohmic heating, emissive source of energy release (Nottingham effect), and heat removal due to heat conduction. Plasma parameters were considered in the range of n = 10¹⁴?10²⁰ cm^?3 and T _e = 0.1 eV?10 keV. The threshold value of energy transferred to the surface from the plasma is found to be 200 MW/cm²; above this value, heating becomes explosive (namely, an increase in the temperature growth rate (δ² T/δt ² > 0) and in passing current (δJ/δt > 0) is observed in the final stage at T ～ 10⁴ K and j ～ 10⁸ A/cm²). In spite of the fact that Ohmic heating does not play any significant role for plasmas with a density lower than 10 18 cm^?3 because the current is limited by the space charge of electrons, rapid overheating of top of microprotrusion is observed much sooner (over a time period of ～1 ns) when the threshold is exceeded. In this case, intense ionization of vapor of the wall material leads to an increase in the plasma density at the surface, and the heating becomes of the Ohmic explosion type. Such conditions for the formation of a micr?xplosion on the surface and of an ecton accompanying it can be created during the interaction of a plasma with the cathode, anode, or an insulated wall and may lead to the formation of cathode and anode spots, as well as unipolar arcs.     

Analysis of a tungsten surface irradiated by fast ions and deuterium plasma

The effects occurring on the surface of tungsten under irradiation with fast ions with an energy in the megaelectrolvolt range and with high fluxes of hydrogen (deuterium) plasma are considered. These effects are radiation damage of the surface layer of the material, its erosion and deuterium retention in it. Irradiation with helium ⁴He²⁺ (3.2–4.0 MeV) and carbon ¹²C³⁺ (10 MeV) ions is performed using a cyclotron at the National Research Center Kurchatov Institute. The thickness of the damaged layer is 3.5–6 μm. The irradiated samples are exposed to steady-state deuterium plasma using a LENTA linear plasma facility to reach a plasma ion fluence of 10²¹–10²² cm^?2. Tungsten erosion and modification of the structure of the damaged layer are analyzed at a plasma-ion energy of 250 eV. Deuterium retention in the damaged layer is studied by elastic recoil detection analysis. The deuterium concentration and its penetration depth into the material are measured. The data obtained for different kinds of fast ions used in the work are compared.     

Formation of fast multicharged heavy ions under the action of a superintense femtosecond laser pulse on the cleaned surface of a target

It is found that the mean charge of tungsten ions in a solid tungsten target cleaned from the surface layer of hydrocarbon and oxide compounds and exposed to femtosecond laser radiation with an intensity exceeding 10¹⁶ W/cm² attains 22+, while the maximum charge is 29+. The maximum energy of such ions approaches 1 MeV. The corresponding values obtained on a dirty target with the same laser pulse parameters constitute 3+, 5+, and 150 keV. The results of numerical simulation show that such a large maximum charge of ions can be attained owing to the emergence of an electrostatic ambipolar field at the sharp boundary between the plasma and vacuum. The main mechanism of ionization of ions with maximum charges is apparently impact ionization in the presence of an external quasi-static field. In addition, direct above-threshold ionization by this field can also play a significant role. It is also shown that heavy ions in a clean target are accelerated by hot electrons. This leads to the formation of high-energy ions. The effect of recombination on the charge of the ions being detected is analyzed in detail.