Laser Blow-off Research Articles

Edge Thomson Scattering in RFX-mod: Operation and first measures

During the 2009 experimental campaign the Edge Thomson Scattering System started the operations, after a commissioning period. The main modification has been the adoption of interferential filters spectrometers with APD detectors, similar to those used on the main TS. Other improvements were the optimization of the laser input path and of the collection optics, which increased the signal level of about 50 %.With the present setup, temperature and density measures show good compatibility with those of the main TS diagnostic; density data have been calculated after Rotational Raman Scattering calibration, a method which is not affected by impurity deposition on the collection window. Present operational limits of the diagnostic are discussed as well as short terms planned operations, focusing in particular on laser multi-pulse issues and laser sharing with the impurity injection system by laser blow-off.

Effect of ambient gas on the expansion dynamics of plasma plume formed by laser blow off of thin film

A study has been carried out to understand the influence of ambient gases on the dynamics of laser-blow-off plumes of multi-layered LiF–C thin film. Plume images at various time intervals ranging from 100 to 3000 ns have been recorded using an intensified CCD camera. Enhancement in the plume intensity and change in size and shape occurs on introducing ambient gases and these changes are highly dependent on the nature and composition of the ambient gas used. Velocity of the plume was found to be higher in helium ambient whereas intensity enhancement is greater in argon environment. The plume shapes have maximum size at 10−2 and 10−1 Torr of Ar and He pressures, respectively. As the background pressure increases further (>10−2 Torr: depending on the nature of gas), the plume gets compressed/focused in the lateral direction. Internal structure formation and turbulences are observed at higher pressures (>10−1 Torr) in both ambient gases.

Parametric study of expanding plasma plume formed by laser-blow-off of thin film using triple Langmuir probe

The triple probe technique has been used to study the parameters of flowing plasma plume formed by laser-blow-off (LBO) of a multicomponent LiF–C target. The advantages of triple Langmuir probe over the single probe have been discussed with regard to the present study. Our results show that triple probe is better suited for parametric measurements close to the target. The spatiotemporal evolutions of electron density and temperature were measured in different ambient environments and for various laser fluences. The angular distribution of electron density and temperature was also measured. It was found that the presence of ambient gas drastically affects the electron density and temperature of the LBO plume. Large enhancement in ne and Te at intermediate pressures (10−2 Torr) is expected due to increase in collisional processes. On the other hand, laser fluence does not affect the observed ne and Te significantly. In vacuum, a new phenomenon, i.e., an oscillation in the trailing portion of the plasma plume at a closer distance from the target, is also reported.

Disruption control on FTU and ASDEX upgrade with ECRH

The use of ECRH has been investigated as a promising technique to avoid or postpone disruptions in dedicated experiments in FTU and ASDEX Upgrade. Disruptions have been produced by injecting Mo through laser blow-off (FTU) or by puffing deuterium gas above the Greenwald limit (FTU and ASDEX Upgrade). The toroidal magnetic field is kept fixed and the ECRH launching mirrors have been steered before every discharge in order to change the deposition radius. The loop voltage signal is used as disruption precursor to trigger the ECRH power before the plasma current quench. In the FTU experiments (Ip = 0.35–0.5 MA, Bt = 5.3 T, PECRH = 0.4–1.2 MW) it is found that the application of ECRH modifies the current quench starting time depending on the power deposition location. A scan in deposition location has shown that the direct heating of one of the magnetic islands produced by magnetohydrodynamic (MHD) resistive instabilities (either m/n = 3/2, 2/1 or 3/1) prevents its further growth and also produces the stabilization of the other coupled modes and the delay of the current quench or its full avoidance. Disruption avoidance and complete discharge recovery are obtained when the ECRH power is applied on rational surfaces. The modes involved in the disruption are found to be tearing modes stabilized by a strong local ECRH heating. The Rutherford equation has been used to reproduce the evolution of the MHD modes. In the ASDEX Upgrade experiments L-mode plasmas (Ip = 0.6 MA, Bt = 2.5 T, PECRH = 0.6 MW ∼ POHM) the injection of ECRH close to q = 2 significantly delays the 2/1 onset and prolongs the duration of the discharge: during this phase the density continues to increase. No delay in the onset of the 2/1 mode is observed when the injected power is reduced to 0.35 MW.

Improved confinement with internal electron transport barriers in RFX-mod

RFX-mod reliably operates at 1.5 MA, the highest current ever achieved on a reversed field pinch device thanks to the the feedback control on multiple magnetohydrodynamic modes.In these high current discharges magnetic topology spontaneously self-organizes in an Ohmic helical symmetry, the quasi single helicity (QSH) state, in which the magnetic dynamics is dominated by the innermost resonant mode, with the new magnetic axis helically twisting around the geometrical axis of the torus.Inside the helical structure energy confinement is enhanced and electron temperatures exceeding 1 keV are measured, with steep gradients, which identify an internal transport barrier. Separatrix expulsion and symmetric electron temperature profiles with high gradients are obtained for ratios between the dominant mode and total B above about 4%. The measured electron temperature peak involves a large fraction of the plasma cross section, corresponding to an improvement in the global electron energy confinement up to a factor 2. The steep temperature profiles obtained in QSH conditions correspond to an electron thermal diffusivity reduced by more than one order of magnitude.Perturbative experiments (pellets and impurity laser blow off injections) have been performed to study particle confinement inside and outside the thermal island. Inside the helical structure experimental evidence of main gas confinement increase has been obtained, while for impurities there is no evidence of confinement increase.

Anomalous transport of light and heavy impurities in Tore Supra ohmic, weakly sawtoothing plasmas

Experimental results on impurity transport in tokamaks are based on various techniques. We study here how the choice of the injection technique and of the analysis method influences the results. We have used three different injection techniques available in Tore Supra: laser blow-off, gas puff and supersonic molecular beam injection. We show that the long time duration of the gas puff injection compared with particle confinement time provides very limited information. The laser blow-off technique and supersonic pulsed injections give satisfactory results for diffusion but low quality convection estimates, presumably because the fast source term quenches the role of convection in the continuity equation. The best method is shown to be the combined analysis of supersonic pulsed injections and continuous puffing of a gaseous species. We obtain convection velocity profiles to an uncertainty of about 0.5 m s−1. This method is applied to ohmic, weakly sawtoothing plasmas. The diffusion coefficient is independent of the impurity charge and the convection velocity is inward. Neoclassical calculations show that these plasmas are dominated by anomalous transport. Quasilinear gyrokinetic simulations are in qualitative agreement with the above experimental results. We deduce from the simulations that convection is dominated by the curvature term, which means that no charge dependence should be expected in this situation.

Collisionless interaction of an energetic laser produced plasma with a large magnetoplasma

We have commissioned a high-energy glass-laser at the Large Plasma Device (LAPD) to study the interaction of a dense laser-produced plasma with a large (17 m) magnetized plasma. First experiments with an energy of the laser blow-off an order of magnitude higher than previous work (Gekelman et al. in J. Geophys. Res. 108(A7):1281, 2003) produced large amplitude Alfvén waves (δ B ⊥ /B 0≈15%). We will discuss the potential of this facility for collisionless laboratory astrophysics experiments.

An experimental setup to study the expansion dynamics of laser blow-off plasma plume in variable transverse magnetic field

In the present work we report generation of uniform and variable pulsed magnetic field in synchronization with pulsed plasma for the study of the expansion dynamics of laser blow-off (LBO) plasma plume. The experimental procedure for optimizing various parameters of the setup, e.g., the production of pulsed field and its synchronization with pulsed plasma and diagnostics system, is also reported. Until now the effect of magnetic field was studied using fixed field. The present setup, however, provides variable and uniform field in synchronization with the setup. A low cost time sequencing control module has been developed for the above purpose. Although the main emphasis is on the technical aspect of the setup, salient features of the effect of transverse magnetic field on the evolution features of the neutral and ionic species are also reported briefly. It is observed that LBO generated plume have a stronger correlation with the magnetic field in comparison to the conventional laser produced plasma experiments as reported earlier.

Comparative study of laser produced Li plasma plumes from thin film and solid target

The evolution features of lithium ions and neutrals generated by two different schemes viz. the laser-blow-off (LBO) of multicomponent LiF–C thin film and conventional laser ablation (referred here as LPP) from solid lithium have been studied using optical emission spectroscopic technique. The optical signal emitted by Li I (at 670.8 nm) and Li II (548.4 nm) was monitored as a function of laser fluence, ambient gas pressure, and distance “z” from the target. Apart from their similarities, some interesting differences were noticed in temporal profiles of the plumes generated by LPP and LBO both in vacuum as well as in the presence of the ambient gas. A comparative analysis of experimental results indicates that the ablation mechanism and subsequent laser-plume interaction were responsible for the observed differences in LPP and LBO plumes. Expansion features of the plume are discussed in the light of two different models.

In situ window cleaning by laser blowoff through optical fiber

The feasibility of a window cleaning system based on the laser blowoff technique is investigated to remove the impurity deposition on vacuum windows of the modified reversed field experiment fusion device. The laser pulse is sent to the window through a fused silica fiber optic (phi=1 mm), then focused on its internal surface, single shot ablating up to approximately 5 mm(2) of the impurity layer; the focused pulse is scanned across the window to clean its entire surface. The composition of the deposited layer is studied through the secondary ion mass spectrometry and profilometry techniques. Effectiveness of cleaning is analyzed in terms of quality of the cleaned spot, its dimension, repetition rate of the laser, and its wavelength. The energy damage threshold of the fiber optic is also investigated. Three different lasers (microjoule Nd:YAG, Nd:YLF, and ruby) are first tested directly on the window; then only the ruby laser beam is propagated through an optical fiber and tested.

Generation of fast neutrals in a laser-blow-off of LiF–C film: A formation mechanism

The temporal profiles of the spectral lines LiI and LiII emitted by a laser blow off of LiF–C film were analyzed. Fast neutrals having energies of ∼310eV were observed. It was found that these fast neutrals have kinetic energies similar to that of the ablated ions. Photon production rates at 670.8nm were estimated for the recombination and charge exchange processes. We attempted to qualitatively understand the role of electron-ion recombination (two and three body recombinations) and charge exchange processes in producing observed fast neutrals. Our analysis indicates that charge exchange is the dominant process in the production of fast neutrals.

Measurements of electron temperature and density of multi-component plasma plume formed by laser-blow-off of LiF-C film

The evolution features of electrons and ions generated by laser-blow-off (LBO) of a multi-component LiF-C target have been studied using optical spectroscopic and Langmuir probe (LP) techniques. In addition to the hydrodynamic plume splitting, multi-peak structures are reported in the temporal profiles of ions measured by the LP. We propose that the observed multi-peaks arise due to the mass dependent expansion velocity of different elements present in the plasma plume. The temporal evolutions of electron density and temperature are reported in vacuum and in a 10−2 mbar argon environment. The electron temperature of the LBO plume was found to be higher than the electron temperature of laser produced plasma from solids. It was observed that the LP was perturbing the evolving plasma plume when it was placed at a short distance from the target. We have experimentally determined the distance between the target and LP beyond which it did not perturb the characteristic features of plume expansion. No previous data related to LBO plasma exist for a direct comparison with the present results.

Preliminary analysis of impurity transport in HL-2A ohmic discharges

This paper describes the behaviour of impurity transport in HL-2A ohmic discharges. In 2005, small quantities of metallic impurities (Al, Ni and Ti) were successfully injected into HL-2A plasmas by laser blow-off technique, and their progression was followed by the soft x-ray cameras with good spatial and temporal resolutions. The impurity confinement time is estimated from the characteristic decay time of the soft x-ray signal of the injected impurities, and it is about 30–60 ms. The transport coefficients of impurities (including diffusion coefficient and convection velocity) in radial different region have been derived by using a one-dimensional impurity transport code, the results present that diffusion coefficient is much smaller in the central region of plasmas than the outside of it, and it is much larger than that of neoclassical theory predictions; namely, it is anomalous.

Overview of HL-2A experiment results

Recent experiment results from the HL-2A tokamak are presented in this paper. Supersonic molecular beam injection (SMBI) with liquid nitrogen temperature propellant is used. Low temperature SMBI can form hydrogen clusters that penetrate into the plasma more deeply and efficiently. Particle diffusion coefficient and convection velocity (D = 0.5–1.5 m2 s−1 and Vconv < 40 m s−1, respectively) are obtained at the plasma periphery using modulated SMBI. Multi-probe measurements reveal the m = 0–1, n = 0 symmetries of directly measured low frequency (7–9 kHz) electric potential and field are simultaneously observed for the first time. Impurity transport is determined with the laser blow-off system and transport code. A disruption predictor has been derived based on MHD activity observations and statistical analysis. Sawtooth characteristics during ECRH are investigated and coupling between m = 1 and m/n = 2/1 modes is studied. Detachment features of HL-2A divertor are numerically and experimentally studied using the code SOLPS5.0 and measured data. The long divertor legs and thin divertor throats in HL-2A pose MHD shaping problems resulting in momentum losses even at low densities and strongly enhanced main chamber losses.

Sawtooth Activities During Impurity Injection by Laser Blow-off in HL-2A

In order to study impurity transport and their progression in theplasma, small amount of high-Z impurities were injected into HL-2Aohmic heated plasmas by laser blow-off and traced with good spatialtemporal resolution by a soft X-ray camera. An analysis of theevolution of the impurities' radiation distribution in the plasmarevealed that impurity transport in the central region wasprofoundly different from that outside of it. It was found that thesawteeth tended to be inverted on the central soft X-ray chordsignals after the impurity injection and the soft X- ray profilesexhibited discontinuities during the movement of the impurities inthe plasma central region. Detailed 2D visualization of theevolution of impurities' radiation using computer tomography hasprovided further understanding of the properties of impuritytransport in the HL-2A plasma.

Role of ambient gas and laser fluence in governing the dynamics of the plasma plumes produced by laser blow off of LiF–C thin film

The time- and space-resolved emission profiles of LiI and LiII emission lines from the laser-blow-off plumes of a multilayered LiF–C thin film have been studied using spectroscopic technique. The evolution features were analyzed in different ambient environments ranging from high vacuum to 3mbars of argon pressures and at various fluences of the ablating laser. During the evolution of the plume, a transition region was found to exist between 4 and 6mm. Here, the plume dynamics changed from free expansion to collisional regime, where the plume experienced viscous force of the medium. The enhancement observed in neutral lines, in comparison with ionic lines, is explained in terms of the yield difference in electron impact excitation and ionization processes. Substantial difference in the arrival time distribution of the plume species was observed for LiI and LiII lines at high ambient pressures. Three expansion models are invoked to explain the evolution of the plume in different ambient conditions. The laser fluence was found to control the ratio of ions and neutrals.

Synthesis of nano-structured materials by laser-ablation and their application to sensors

We describe the synthesis of nano-structured materials of ZnO and Pd by laser ablation and their applications to sensors. The synthesis of ZnO nano-wires was performed by nano-particle assisted deposition (NPAD) where nano-crystals were grown with nano-particles generated by laser-ablating a ZnO sintered target in an Ar background gas. The synthesized ZnO nano-wires were characterized with a scanning electron microscopy and the photoluminescent characteristics were examined under an excitation with the third harmonics of a Nd:YAG laser. The nano-wires with a diameter in the range from 50 to 150 nm and a length of up to 5 μm were taken out of the substrate by laser blow-off technique and/or sonication. It was confirmed that the nano-wires showed the stimulated emission under optical pumping, indicating a high quality of the crystalinity. Pd nano-particles were generated by laser-ablating a Pd plate in pure water. The transmission electron microscope observation revealed that Pd nano-particles with a diameter in the range from 3 nm to several tens of nanometers were produced. Using these nano-structured materials, we successfully fabricated sensors by the dielectrophoresis techniques. In the case of the ultraviolet photosensor, a detection sensitivity of 10 nW/cm 2 was achieved and in the case of hydrogen sensing, the response time of less than 10 s has been demonstrated with Pd nano-particles.

Effects of ambient pressure and laser fluence on the temporal evolution of 426.7 nm CII line in laser-blow-off of multilayered LiF-C thin film

The time and space resolved emission profiles of the CII line from the laser-blow-off (LBO) plumes of multilayered LiF-C thin film have been investigated using a laser-induced forward transfer technique. The evolution features of the 426.7 nm line were studied in different ambient environments ranging from high vacuum to 3 mbar of argon pressures and at various fluences of the ablating laser. It was found that many features of the plasma plume generated by the LBO method resemble that of plumes created in conventional laser produced plasma; however, a few differences were observed in their behaviour with regard to plume splitting and plume compression. A sudden change in the temporal profile was observed to occur at a distance about 4–6 mm away from the target. The variation of the plume structures observed at different fluences of the ablating laser is reported. The validity of the Rayleigh–Taylor instability, shock wave and drag force model in the present experiment are also discussed.

Updating of atomic data needed for ionization balance evaluations of krypton and molybdenum

Atomic data for both ionization and recombination of Kr and Mo ions are reviewed, the rates for these processes needing to be regularly updated following the publication of new theoretical calculations and new experimental data. Kr is used in magnetic-confinement fusion devices to produce a peripheral radiating mantle meant to spread the heat load on the plasma-facing components. In a few tokamaks Mo tiles cover the plasma-facing surfaces, acting in most cases as a plasma-column limiter. The collected atomic data represent the state of the art on the ionization and recombination data for the two considered elements. Samples of rates are proposed for both ionization and recombination along with tables of the fractional abundances at ionization equilibrium. The proposed rates should be included in codes that simulate the impurity behaviour in magnetic-confinement fusion devices, i.e., when radial transport is added to ionization and recombination to predict spatially resolved charge-state distributions to be compared with experimental results. As an example, the simulation of a Mo laser blow-off injection on the JET tokamak is re-analysed with the revised rates and multiplicative correction factors are obtained for the rate ratios recombination over ionization for L-shell ions.

Investigation of Impurity Ion Transport with Laser Blow-off inHL-2A Tokamak

Non-recycling impurities are injected into ohmic HL-2A plasma for thefirst time. The impurities of titanium and aluminium are injected in thedischarges with varying plasma density and current. The convection anddiffusion process of the injected impurity ions during the inward phaseare qualitatively investigated. The results show that the transport ofimpurities is much slower in the central region of the plasma thanoutside of it and that it is greatly enhanced during sawtooth crashes.