Plasma-laser characterization by electrostatic mass quadrupole analyzer

A study of laser ablation of different metals (aluminium, zinc, tantalum and lead), in vacuum, by using 3 ns Nd:YAG laser radiation, at 532 nm wavelength, is reported. Laser pulse, at intensities of the order of 10⁹ W/cm², produces high non-isotropic emission of neutrals and ionic species. Mass quadrupole spectrometry, associated to electrostatic ion deflection, allows estimation of the energy distributions of the emitted species within the plume as a function of the incident laser energy. Neutrals show typical Boltzmann distributions while ions show Coulomb-Boltzmann-shifted distributions. The plasma characterization is rationalized in terms of kinetic energies of ejected particles, ion, electron and neutral temperatures, ion charge states, and plasma density. A special regard is given to the parameters which regulate the plasma temperature: the boiling point, the electron density and the ionization potentials of the ablated elements. The ion acceleration processes occurring inside the plasma due to the high electrical field generated in the non-equilibrium plasma conditions are presented and discussed.     

Si nanocrystals formation by a new ion implantation device

Metallic and non-metallic ion beams can be used to modify the properties of wafer surfaces if accelerated at moderate energies. We developed a new “implantation machine” able to generate ions and to accelerate them up to 80 kV. The ion generation is achieved by a laser-plasma source which creates plasma in expansion. The device consists of a KrF excimer laser and a generating vacuum chamber made of stainless steel. The laser energy was 45 mJ/pulse with a power density of 2.25 × 10⁸ W/cm². The target was kept to positive voltage to accelerate the produced ions. The ion dose was estimated by a fast polarised Faraday cup. This machine was utilised to try synthesizing silicon nanocrystals in SiO₂ matrix. Preliminary results of Si nanocrystals formation and the glancing-angle X-ray diffraction analyses are reported.     

Polymer processing by a low energy ion accelerator

Ion implantation is a process in which ions are accelerated toward a substrate at energies high enough to bury them just below the surface substrate in order to modify the surface characteristics. Laser-produced plasma is a very suitable and low cost technique in the production of ion sources. In this work, a laser ion source is developed by a UV pulsed laser of about 10⁸ W/cm² power density, employing a C target and a post ion acceleration of 40 kV to increase the ion energy. In this work, we implanted C ions on ultra-high-molecular-weight-polyethylene (UHMWPE) and low-density polyethylene (LDPE). We present the preliminary results of surface property modifications for both samples. In particular, we have studied the modifications of the surface micro-hardness of the polymers by applying the “scratch test” method as well as the hydrophilicity modifications by the contact angle measurements.     

Comparison of Pd plasmas produced at 532 nm and 1064 nm by a Nd:YAG laser ablation

A comparison between the laser ablation of a palladium target in vacuum, by using 1064 nm and 532 nm Nd:YAG laser wavelengths, with an intensity of about 10⁹ W/cm², is reported. Nanosecond pulsed ablation produces high non-isotropic emission of neutrals and ions. For both wavelengths, mass quadrupole spectrometry and time-of-flight measurements allow estimation of the atomic and ionic species emitted from the plasma and of their energy distributions.Ions show Coulomb-Boltzmann-shifted distributions depending on their charge state. Surface profiles of the ablated craters permitted to study the ablation threshold and yields of palladium in vacuum vs. the laser fluence. The plasma temperature and density was 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 inside the plasma.     

Coincident Rutherford Backscattering Spectrometry: A novel technique for measuring charge state distributions in violent ion-atom collisions

Charge-state distributions in violent ion-atom collisions were investigated using a novel combination of traditional Rutherford backscattering spectrometry (RBS), time-of-flight (TOF) coincidence, and position-imaging techniques. The combination is termed Coincident Rutherford Backscattering Spectrometry (CRBS). A special apparatus was built in which the backscattered and recoil ions are time and charge state correlated. CRBS measurements for 0.5 and 0.6 MeV He⁺-Ar collisions are presented. From the recoil ion-projectile ion coincidence measurements of the charge state distributions, it was observed that backscattered projectile ions of the same charge state correlate with different recoil ion charge states and vice versa, indicating that any particular charge state may result from different reaction channels. Moreover, the Ar recoil-ion and He projectile-ion correlation exhibits a strong dependence on the projectile beam energy. An energy deposition model was attempted to account for some of the recoil ion charge state distributions. The model qualitatively accounts for the distributions and confirms that energy loss of a backscattered projectile due to its interaction with the target electrons is very small compared to that due to its interaction with the target nucleus.     

Separation of potential and kinetic electron emission from Si and W induced by multiply charged neon and argon ions

The total secondary electron emission yields, γ_T, induced by impact of the fast ions Ne^q+ (q = 2-8) and Ar^q+ (q = 3-12) on Si and Ne^q+ (q = 2-8) on W targets have been measured. It was observed that for a given impact energy, γ_T increases with the charge of projectile ion. By plotting γ_T as a function of the total potential energy of the respective ion, true kinetic and potential electron yields have been obtained. Potential electron yield was proportional to the total potential energy of the projectile ion. However, decrease in potential electron yield with increasing kinetic energy of Ne^q+ impact on Si and W was observed. This decrease in potential electron yield with kinetic energy of the ion was more pronounced for the projectile ions having higher charge states. Moreover, kinetic electron yield to energy-loss ratio for various ion-target combinations was calculated and results were in good agreement with semi-empirical model for kinetic electron emission.     

In situ determination of radon concentration and total gamma radiation in Kastel Gomilica, Croatia

To determine current radiation background of the environment at the “Giricic” location in Kastel Gomilica, Croatia, in situ measurement of radon concentration (²²²Rn and ²²⁰Rn) in an open atmosphere on a ground level and at the height of 1.5 m has been made as well as total gamma radiation at the height of 1 m in an energy range of 15 keV to 2 MeV. The researched location was divided in three specific parts: (i) regulated area with the bottom ash and flying ash in the basis (“old” depot), (ii) unregulated area with waste materials, including bottom ash and flying ash, in the basis (“new” depot), (iii) uncontaminated area with no waste materials deposited on. Average radon concentration on a ground level was 213 Bq/m³ for the “old” depot, 214 Bq/m³ for the “new” depot and 59 Bq/m³ for the uncontaminated area and at the height of 1.5 m 20 Bq/m³ for the “old” depot, 34 Bq/m³ for the “new” depot and 26 Bq/m³ for the uncontaminated area. Average total gamma radiation values in selected energy range were 109.92 cps (counts per second) for the “old” depot, 357.76 cps for the “new” depot and 65.97 cps for the uncontaminated area. For selected radionuclides (²¹⁴Pb, ¹³⁷Cs, ²²⁸Ac, ^234mPa, ⁴⁰K and ²¹⁴Bi) average gamma radiation values at characteristic energies have been determined as well.     

Collision cascade temperature

Interaction of a projectile with a solid has been considered in detail. It has been found that any collision cascade generated by a projectile can be characterized by the average kinetic energy of cascade atoms that represents an “instantaneous temperature” of the cascade during its very short lifetime (10⁻¹² s). We refer to this value as the “dynamic temperature” in order to emphasize the fact that cascade atoms are in a dynamic equilibrium and have a definite energy distribution. The dynamic temperature defines the electron distribution in the cascade area and, hence, the ionization probability of sputtered atoms. The energy distribution of cascade atoms and, as a consequence, the dynamic temperature can be found experimentally by measuring the energy distribution of sputtered atoms. The calculated dynamic temperature has been found to be in good agreement with the experimental data on ion formation in the case of cesium and oxygen ion sputtering of silicon. Based on the developed model we suggest an experimental technique for a radical improvement of the existing cascade sputtering models.     

A high charge state all-permanent magnet ECR ion source for the IMP 320 kV HV platform

A 320 kV high voltage (HV) platform has been constructed at Institute of Modern Physics (IMP) to satisfy the increasing requirements of experimental studies in some heavy ion associated directions. A high charge state all-permanent magnet ECRIS-LAPECR2 has been designed and fabricated to provide intense multiple charge state ion beams (such as 1000 eμA O⁶⁺, 16.7 eμA Ar¹⁴⁺, 24 eμA Xe²⁷⁺, etc.) for the HV platform. LAPECR2 has a dimension of ∅ 650 mm × 560 mm. The powerful 3D magnetic confinement to the ECR plasma and the optimum designed magnetic field for the operation at 14.5 GHz makes it possible to obtain very good performances from this source. After a brief introduction of the ECRIS and accelerator development at IMP, the conceptual design of LAPECR2 source is presented. The first test results of this all-permanent magnet ECRIS are given in this paper.     

Effect of spin polarization of Ni(1 1 0) surface on Auger neutralization for grazing scattering of He ions

The survival of ions during grazing scattering of keV He⁺ ions from a clean Ni(1 1 0) surface is studied as function of target temperature. We observe ion fractions in the scattered beams of typically 10⁻³ which show a slight increase with temperature of the target surface. From computer simulations of projectile trajectories we attribute this enhancement for ion fractions to effects of thermal vibrations of lattice atoms on the survival of ions in their initial charge state. Based on concepts of Auger neutralization, we discuss the role of the spin polarization of target electrons on charge transfer. We do not find corresponding signatures in our data and conclude that in the present case of Ni(1 1 0) the spin polarization has to be small.     

Dynamic dependence of the interaction potentials for grazing scattering of fast atoms from metal and insulator surfaces

For scattering of fast atoms from metal and insulator surfaces under axial channeling conditions pronounced peaks in the angular distributions of scattered projectiles are interpreted in terms of rainbow scattering. The angular position of such “rainbow peaks” are closely related to the interaction potential and its corrugation in the topmost surface region. We have scattered N and O atoms, with energies ranging from 10 to 70 keV, from clean and flat Al(0 0 1) and LiF(0 0 1) surfaces along low index axial directions in the surface plane and studied the positions of the rainbow peaks as function of the kinetic energy of the atomic projectiles normal to the surface. For the insulator surface the rainbow angle does not depend on projectile energy for constant normal energy, whereas for the metal surface we find pronounced dynamic effects. We interpret this different behaviour as arising from a projectile energy dependent contribution to the underlying interaction potentials owing to embedding the projectiles into the free electron gas in the selvedge of the surfaces, which is present for the metals but absent for insulators.     

On the emission of MoCs molecular ions from Cs irradiated molybdenum surface

The present paper deals with the emission of atomic and molecular ions from elemental molybdenum surface under Cs⁺ bombardment to explore the MCs⁺ formation mechanism with changing Cs surface coverage. Integrated count of MoCs⁺ shows a monotonic increase with increasing primary ion energy (1-5 keV). Change in MoCs⁺ intensity is attributed to the variation of surface work function ? and cesium surface concentration c_Cs due to varying impact energies. Variation of c_Cs has been obtained from the expression, c_Cs ∝ 1/(1 + Y) where Y is the elemental sputtering yield estimated from TRIM calculations. Systematic study of the energy distributions of all species emerging from Mo target has been done to measure the changes in surface work function. Changing slopes of the leading parts of Cs⁺ energy distributions suggest a substantial depletion in surface work function ? with decreasing primary ion energies. Δ? shows a linear dependence on c_Cs. The maximum reduction in surface work function Δ?_max = 0.69 eV corresponds to the highest value of c_Cs = 0.5. A phenomenological model, based on the linear dependence of ? on c_Cs, has been employed to explain the MoCs⁺ data.     

Scintillation light produced by low-energy beams of highly-charged ions

Measurements have been performed of scintillation light intensities emitted from various inorganic scintillators irradiated with low-energy beams of highly-charged ions from an electron beam ion source (EBIS) and an electron cyclotron resonance ion source (ECRIS). Beams of xenon ions Xe^q+ with various charge states between q = 2 and q = 18 have been used at energies between 5 and 17.5 keV per charge generated by the ECRIS. The intensity of the beam was typically varied between 1 and 100 nA. Beams of highly charged residual gas ions have been produced by the EBIS at 4.5 keV per charge and with low intensities down to 100 pA. The scintillator materials used are flat screens of P46 YAG and P43 phosphor. In all cases, scintillation light emitted from the screen surface was detected by a CCD camera. The scintillation light intensity has been found to depend linearly on the kinetic ion energy per time deposited into the scintillator, while up to q = 18 no significant contribution from the ions’ potential energy was found. We discuss the results on the background of a possible use as beam diagnostics, e.g. for the new HITRAP facility at GSI, Germany.     

On the origin of the LEIS signal in TOF- and in ESA-LEIS

The ion fraction analysis of ⁴He⁺ ions backscattered from various faces of copper single crystals is performed by using time-of-flight (TOF) and electrostatic analyzer (ESA) low-energy ion scattering (LEIS) techniques. When an experiment that integrates over 2π azimuth (typical ESA-LEIS setup) is used, the yield of ions backscattered from the Cu(1 1 0) surface may be given by projectiles penetrated much deeper than just one or two monolayers. The threshold energy for reionization processes for ⁴He⁺ and Cu found earlier by TOF-LEIS is experimentally confirmed by ESA-LEIS.     

Cu and Y ion beams by a new LIS generator

In this work, we present the experimental results of a laser ion source (LIS) implemented for ion accelerators. A KrF excimer laser beam operating at 248 nm was focused on a solid target mounted inside a vacuum chamber in order to obtain the plasma. The laser energy was fixed at 11.5 mJ/pulse. The ion components of the plasma were extracted and accelerated up to 160 keV per charge state by a double gap system formed in two different stages. The beam cross section was circular, 1.5 cm in diameter. Using Cu and Y disks, as laser targets, we produced ion beams containing 1.2 × 10¹¹ ions/pulse (0.7 × 10¹¹ ions/cm²). Applying a total accelerating voltage of 60 kV we obtained an increase in ion dose up to 3.4 × 10¹¹ ions/pulse, (2 × 10¹¹ ions/cm²) for the Cu target and up to 6.3 × 10¹¹ ions/pulse (3.5 × 10¹¹ ions/cm²) for the Y target. The characterization of the plasma was performed using a Faraday cup for the electromagnetic properties, and a pepper pot system for the geometric ones. At 60 kV accelerating voltage and 5.5 mA output current the normalized beam emittance resulted in 0.22 π mm mrad for the Cu target, while under the same accelerating voltage, but with 7.4 mA output current, the normalized beam emittance resulted in 0.14 π mm mrad for the Y target.     

Equilibrium charge state distributions for boron and carbon ions emerging from carbon and aluminum targets

Equilibrium charge state distributions of boron and carbon ions through carbon and aluminum targets were measured with an energy range of 3-6 MeV. Comparisons of the data with relevant semi-empirical models for the equilibrium mean charge states and for the charge state distribution widths could provide valuable insight on the underlying mechanisms for a fast ion to lose or capture electrons. In-depth examinations of the experimental results in combination with semi-empirical models suggest that equilibrium charge state distributions are well represented by Gaussian distributions.     

Cross-section transmission electron microscopy of the ion implantation damage in annealed diamond

It has formerly been shown that low-damage levels, produced during the implantation doping of diamond as a semiconductor, anneal easily while high levels “graphitize” (above about 5.2 × 10¹⁵ ions/cm²). The difference in the defect types and their profiles, in the two cases, has never been directly observed. We have succeeded in using cross-section transmission electron microscopy to do so. The experiments were difficult because the specimens must be polished to ∼40 μm thickness, then implanted on edge and annealed, before final ion beam thinning to electron transparency. The low-damage micrographs reveal some deeply penetrating dislocations, whose existence had been predicted in earlier work.     

Angular distribution of GaAs sputtered under oblique Cs bombardment

The angular distribution of Ga and As sputtered from Gallium Arsenide (1 0 0) by a Cs⁺ ion beam was experimentally measured through a collector technique allowing modifications of the energy and incidence angle of the ion beam. The impact energy was varied in the range of 2-10 keV and the angle of incidence from 30° to 60°.The angular distributions of emitted matter are determined by means of SIMS depth profiles. Our series of experiments show an evolution of the preferential direction of emission as well as the spreading around this direction in function of the characteristics of the ion beam.The second objective is the study of the evolution of the stoichiometry of the deposit in function of the emission angle. A decrease of the As/Ga ratio around the preferential direction of emission and an increase of this ratio for oblique emission are observed for different conditions of primary bombardment. Considering that the angular distribution depends on the depth of origin, our results suggest that the Cs⁺ bombardment changes the stoichiometry of the near-surface layers of the sample with an enrichment of As in the outmost layers while the sub-surface region is impoverished in As due to preferential sputtering.     

Deuterium retention in tungsten at fluences of up to 10 D/m using D ion beams

Deuterium retention in two types of polycrystalline tungsten (PCW) was studied as a function of incident ion fluence, ion energy, and specimen temperature. (i) D retention at 300 K, as a function of D⁺ fluence, demonstrated a trend to saturation in both the Rembar hot-rolled thin foil and Plansee tungsten plate. At 500 K, new D retention results for the Plansee PCW showed an increasing trend with increasing incident D⁺ fluence without any indication of saturation, in agreement with previous results for Rembar PCW [A.A. Haasz, J.W. Davis, M. Poon, R.G. Macaulay-Newcombe, J. Nucl. Mater. 258-263 (1998) 889-895]. Even when the incident D⁺ fluence was increased to 8 × 10²⁵ D⁺/m², which is in the fluence range of plasma devices, there was still no sign of saturation. (ii) The temperature dependence results for the Plansee PCW show a decreasing trend in D retention as the temperature is increased from 300 to 500 K. These results differ from previous studies of Rembar PCW [A.A. Haasz, J.W. Davis, M. Poon, R.G. Macaulay-Newcombe, J. Nucl. Mater. 258-263 (1998) 889-895], but are similar to those seen for single crystal tungsten [M. Poon, A.A. Haasz, J.W. Davis, R.G. Macaulay-Newcombe, J. Nucl. Mater. 313-316 (2003) 199]; an explanation for the different behaviour is suggested. (iii) Varying the D⁺ energy from 100 to 500 eV/D⁺ plays a minor role in the amount of D retained, suggesting that D retention in W depends more on the W structure, incident ion fluence and specimen temperature, rather than on the incident ion energy when the energy is below the threshold for damage formation (∼960 eV for D on W).     

Modeling of heat deposition on the W/Cu movable limiter in HT-7

The thermal behavior of a directly water-cooled W/Cu movable poloidal limiter was investigated in HT-7, a medium-sized superconducting tokamak with limiter configuration, major radius R = 1.22 m, and minor radius a = 0.27 m. The W/Cu movable limiter (ML) was exposed to the plasma at various radial positions at r < a. The surface and bulk temperatures were monitored by an IR-camera and the thermocouples, respectively. The heat flux deposited on the limiter was evaluated by an ANSYS code using the measured surface temperatures as boundary conditions. It was found that the maximum heat flux incident on the ML was less than 1 MW/m² in the Ohmic discharges, but reached up to 5-7 MW/m² in the discharges with lower hybrid current drive (LHCD). A simple model was developed to understand heat transport to the W/Cu ML, taking into account the “funnel effect”.