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Measurement of magnetic field fluctuations and diamagnetic currents within a laser ablation plasma interacting with an axial magnetic field
Abstract
The guiding of laser ablation plasmas with axial magnetic fields has been used for many applications, since its effectiveness has been proven empirically [L. Gray et al., J. Appl. Phys. 53(10), 6628 (1982); J. Wolowski, Laser Part. Beams 20(01), 113 (2002); M. Okamura et al., Rev. Sci. Instrum. 81, 02A510 (2010); Y. Tsui et al., Appl. Phys. Lett. 70(15), 1953 (1997); C. Pagano and J. Lunney, J. Phys. D: Appl. Phys. 43(30), 305202 (2010)]. For more sophisticated and complicated manipulations of the plasma flow, the behavior of the magnetic field during the interaction and the induced diamagnetic current in the plasma plume needs to be clearly understood. To achieve the first milestone for establishing magnetic plasma manipulation, we measured the spatial and temporal fluctuations of the magnetic field caused by the diamagnetic current. We showed that the small fluctuations of the magnetic field can be detected by using a simple magnetic probe. We observed that the field penetrates to the core of the plasma plume. The diamagnetic current estimated from the magnetic field had temporal and spatial distributions which were confirmed to be correlated with the transformation of the plasma plume. Our results show that the measurement by the magnetic probe is an effective method to observe the temporal and spatial distributions of the magnetic field and diamagnetic current. The systematic measurement of the magnetic field variations is a valuable method to establish the magnetic field manipulation of the laser ablation plasma.
... Therefore, the interaction is not simple, and a structure should be formed in the plasma. In fact, a structure has been observed in the plasma plume in which the ions in the outer region converge due to the magnetic field like a magnetic lens and the ions in the inner region are guided as a magnetic nozzle [31]. The results indicate that although the longitudinal magnetic field can enhance the ion flux by reducing the transverse expansion, a sophisticated parameter setup is needed to realize wellcontrolled LISs [32]. ...
... The interaction process between the ablation plasma and the magnetic field has been studied using a magnetic probe. The results of the plasma guiding experiments indicate that the diamagnetic current induced by the interaction plays an important role in the guiding mechanism of a longitudinal magnetic field [31]. ...
Research activities in Japan relevant to particle beam inertial fusion are briefly reviewed. These activities can be ascended to the 1980s. During the past three decades, significant progress in particle beam fusion, pulsed power systems, accelerator schemes for intense beams, target physics, and high-energy-density physics research has been made by a number of research groups at universities and accelerator facilities in Japan. High-flux ions have been extracted from laser ablation plasmas. Controllability of the ion velocity distribution in the plasma by an axial magnetic and/or electric field has realized a stable high-flux low-emittance beam injector. Beam dynamics have been studied both theoretically and experimentally. The efforts have been concentrated on the beam behavior during the final compression stage of intense beam accelerators. A novel accelerator scheme based on a repetitive induction modulator has been proposed as a cost-effective particle-beam driver scheme. Beam-plasma interaction and pulse-powered plasma experiments have been investigated as relevant studies of particle beam inertial fusion. An irradiation method to mitigate the instability in imploding target has been proposed using oscillating heavy-ion beams. The new irradiation method has reopened the exploration of direct drive scheme of particle beam fusion.
A magnetic thrust chamber concept in a laser fusion rocket is suitable for controlling the plasma flow, and it has an advantage in that thermalization with wall structures in a thrust chamber can be avoided. Rayleigh-Taylor instability would occur at the surface expanding plasma, and it would lead to the degradation of thrust efficiency, which would result from diffusion of the plasma through an ambient magnetic field. A three-dimensional hybrid particle-in-cell code has been developed to analyze the plasma instability in the magnetic thrust chamber and to estimate the thrust efficiency. It is found that the instability would not have serious effects on the thrust efficiency; thrust efficiency in terms of momentum obtained here amounts to 65%. The effects of varying parameters on the thrust efficiency are also studied. The thrust efficiency seemed to reach its maximum value around θc = 50 deg, where θc is an angle subtended from the initial plasma position at the z axis to the solenoidal coil and its dependence on magnetic field energy produced by the coil is found to be weak for the cases studied here.
Laser Induced Plasma Micromachining (LIPMM) is a novel, tool-less micromachining process which offers machining characteristics superior to conventional laser ablation, such as multi-material capability, higher machined depth and better wall geometries. This study utilizes highly empirical methods for the purpose of a proof of concept and demonstrates the viability of using external magnetic fields in modifying the geometry and improving the aspect ratio of machined spots (up to 6) in LIPMM, which is accomplished by pulling the plasma spatially downward to machine spots with greater depth and consistent diameters, and to achieve horizontal squeezing of the plasma to create channels.
A three-axis, 2.5 mm overall diameter differential magnetic probe (also known as B-dot probe) is discussed in detail from its design and construction to its calibration and use as diagnostic of fast transient effects in exploding plasmas. A design and construction method is presented as a means to reduce stray pickup, eliminate electrostatic pickup, reduce physical size, and increase magnetic signals while maintaining a high bandwidth. The probe's frequency response is measured in detail from 10 kHz to 50 MHz using the presented calibration method and compared to theory. The effect of the probe's self-induction as a first order correction in frequency, O(omega), on experimental signals and magnetic field calculations is discussed. The probe's viability as a diagnostic is demonstrated by measuring the magnetic field compression and diamagnetism of a sub-Alfvenic (approximately 500 km/s, M(A) approximately 0.36) flow created from the explosion of a high-density energetic laser plasma through a cooler, low-density, magnetized ambient plasma.
In this article the capacitive pickup of magnetic fluctuation probes for plasma applications is studied. The nine most commonly used probe designs are compared with respect to their capacitive pickup rejection, magnetic sensitivity, and minimum plasma disturbance. For absolute calibration, well defined electric and magnetic field fluctuations are produced separately in a Faraday cup and in a Helmholtz magnetic field coil configuration, respectively. A sample measurement in a radio frequency helicon plasma demonstrates that the optimum probe design is well suited for measuring magnetic fluctuations in a plasma environment. (C) 2002 American Institute of Physics.
The angular distribution of electron temperature and density in a laser-ablation plume has been studied for the first time. The electron temperature ranges from 0.1 to 0.5 eV and is only weakly dependent on the angle in the low-intensity range studied here. In contrast, the typical ion energy is about 2 orders of magnitude larger, and its angular distribution is more peaked about the target normal. The derived values of the electron density are in agreement with the measured values of ion density.
A Laser ion source (LIS) provides high current heavy ion beams with a very simple mechanical structure. Plasma is produced by a pulsed laser ablation of a solid state target and ions are extracted by an electric field. However, it was difficult to manipulate the beam parameters of a LIS, since the plasma condition could only be adjusted by the laser irradiation condition. To enhance flexibility of LIS operation, we employed a pulsed solenoid in the plasma drift section and investigated the effect of the solenoid field on singly charged iron beams. The experimentally obtained current profile was satisfactorily controlled by the pulsed magnetic field. This approach may also be useful to reduce beam emittance of a LIS.
To create mixed species ion beam with laser pulses, we investigated charge state distributions of plasma formed from both Al-Fe alloy targets and pure Al and Fe targets placed close together. With two targets, we observed that the two kinds of atoms were mixed when the interval of two laser pulses was large enough (40 μs). On the other hand, when the interval was 0.0 μs, we observed fewer Fe ions and they did not mix well with the Al ions. The two species were mixed well in the plasma from the alloy target. Furthermore, we observed that specific charge states of Fe ions increased. From the results, it was determined that we can use two pure targets to mix two species whose difference of the drift velocity is large. On the other hand, we must use an alloy target when the drift velocities of the species are close.
An ion source for accelerators requires to provide a stable waveform with a certain pulse length appropriate to the application. The pulse length of laser ion source is easy to control because it is expected to be proportional to plasma drifting distance. However, current density decay is proportional to the cube of the drifting distance, so large current loss will occur under unconfined drift. We investigated the stability and current decay of a Nd:YAG laser generated copper
plasma confined by a solenoidal field using a Faraday cup to measure the current waveform. It was found that the plasma was unstable at certain magnetic field strengths, so a baffle was introduced to limit the plasma diameter at injection and improve the stability. Magnetic field, solenoid length, and plasma diameter were varied in order to find the conditions that minimize current decay and maximize stability.
A time-resolving Langmuir probe was used to investigate the expansion of tungsten plasma produced by the pulsed Nd-YAG laser (wavelength = 1064 nm, pulse duration = 10 ns). Current–voltage (IV) plots at various times after the laser shot hit the target were obtained from probe electron and ion currents. Langmuir probe theory was used to determine the temporal variation of the plasma potential, electron temperature and electron density. These plasma parameters sharply rise to a maximum value and then slowly decrease during the plume expansion. For the laser irradiance of 10.88 × 108 W/cm2, maximum values of plasma potential, electron temperature and electron density were about 24 V, 15.8 eV and 2.24 × 1016 cm−3 respectively. The measurements performed with time-of-flight electrostatic energy analyzer confirmed the presence of highly charged tungsten ions in the plasma and charge state of ions found to increase with the laser irradiance.
We are here concerned with experiments in which time-of-flight (TOF) measurements are made with particles which are vaporized, sputtered, or desorbed due to a pulsed heat source. If the emitted particle number density is low enough, the particles will disperse collisionlessly. Provided the emission is truly thermal, the velocities will then be described by a “half-range” Maxwellian, i.e. a Maxwellian with only positive velocities normal to the target, for which it is well known that the surface temperature, Ts, and the energy, Ê, defined by the peak position of the TOF spectrum are related by . More commonly the emitted particle density is high enough that near-surface collisions occur. For as few as 3 collisions per particle a Knudsen layer forms, i.e. there is a layer within a few mean free paths of the target surface in which the distribution function evolves to a “full-range” Maxwellian in a center-of-mass coordinate system. We show that for on-axis measurements the relation is replaced by , with ηK ranging from 2.52 for a monatomic species to 3.28 for a species with many accessible internal degrees of freedom. Failure to recognize the formation of a Knudsen layer thus leads to a severely overestimated value for Ts, at least for an-axis measurements.
An axial magnetic field with a maximum strength of 2.2 kG was used to confine and guide a laser produced carbon plasma around a 30 deg bend. The plasma was produced by a 25 ns KrF laser pulse at a peak intensity of 1×109 W cm−2. An array of Faraday cups positioned at the exit of the guide field was used to determine that approximately 20% of the original plasma was confined by the magnetic field and guided along a 15 cm trajectory to a 5 cm2 area.
The angular and radial variation of the ion density and electron temperature in the plasma plume produced by laser ablation of silver at fluences of 0.8–1.3Jcm−2 at 355nm have been studied using a time-resolving Langmuir probe. The angular dependence of the electron temperature and the magnitude of the ion flux, at the time when the ion flux is maximised, agree with the predictions of the self-similar isentropic model of the plasma expansion by Anisimov et al.
Results of an experiment in which an elongated plasma column is produced are presented. Plasma was generated by means of a laser pulse (~ 15 J, 1 ns) which was focused on a surface of a flat target situated inside a quasistationary magnetic trap. Plasma expanded freely along or at an angle to the magnetic field lines. Spatial characteristics of the dynamics of interaction of the plasma with the magnetic field were registered by means of various diagnostics, such as: a combined set-up of interferometry, polarimetry and shadowgraphy and also by x-ray imaging and spectroscopy in the x-ray range and visible. Interesting influence of the strong (15-20 T) magnetic field on the plasma dynamics and shape was observed. Results of numerical modelling are compared with the experimental data. The possibility of achieving population inversion and x-ray lasing conditions is discussed.
The magnetic guiding, focusing, and compression of charge and current-neutral ion cylindrical beams formed in a field-free region and propagated axially into a solenoidal field in a vacuum is described both theoretically and experimentally. It is shown that the motion of the beam can be described theoretically by either one of two models. The first applies to beams for which the magnetic skin depth exceeds half the beam radius; the second is the snowplow model which applies to plasmas having a small magnetic skin depth. The experimental apparatus and diagnostics are described, and experimental data supporting each of the models are presented. In the first case, the peak beam intensities are found at the predicted common focus for a wide range of focal lengths. In the second case, the exclusion of flux is verifed by diamagnetic measurements and the compression of the beam is shown by damage patterns. Instabilities are found similar to those observed in theta pinches.
A simple Langmuir probe technique has been used to measure the electron density, electron temperature, and plasma potential in the late stages (>5 μs) of a laser ablated plasma plume. In the plasma, formed following 248 nm laser irradiation of a copper target, in vacuum at a laser fluence of 2.5 J cm−2, electron densities of ∼1018 m−3 and temperatures of ∼0.5 eV were measured. These values are comparable with those reported previously using Faraday cup detectors and optical emission spectroscopy, respectively. © 1997 American Institute of Physics.
The experimental results of the investigations on the influence
of external magnetic and electric fields on the characteristics
of a tungsten ion stream emitted from a plasma produced by the
Nd:glass laser (1 J, 1 ns) performed at IPPLM, Warsaw are
presented. A negatively biased target up to −15 kV and
a magnetic field up to 0.45 T were used in the experiment. A
set of ion collectors and an electrostatic cylindrical ion energy
analyzer located at small angles with respect to the laser beam
axis and at large distances from the target were applied for
ion measurements. The effect of an external magnetic field is
essential to plasma expansion, but the effect of the retarding
potential of the target is very weak in our experimental
conditions. The aim of the studies was to prove the possibility
of the optimization of ion beam parameters from laser-produced
plasma for the particular application as a laser ion source
coupled with the electron cyclotron resonance ion source for
particle accelerators.
The expansion of copper laser-produced plasma along a magnetic field was investigated with the aim of confining the lateral expansion of the plasma. Time-resolved optical imaging, time- and space-resolved optical emission spectroscopy and time-of-flight Langmuir ion probe measurements were used to study the plasma dynamics. Thin films of copper were deposited without, and with, the magnetic field. It was observed that the magnetic field gives rise to substantial confinement of the plasma, leading to an increase by a factor of 6 in the ion yield and 2.6 in the deposition rate.
For laser ablation plumes which are significantly ionised, Langmuir probes have proved to be a relatively simple and inexpensive tool for measuring the plume shape, ion energy distribution and electron temperature. In this paper we describe some recent work on the development of Langmuir probes for laser ablation plume diagnosis. Typically in laser ablation plasma the flow velocity is supersonic, which complicates the interpretation of the I-V probe characteristic. We describe some new work on the behaviour of a flat probe lying parallel to the plasma flow. We also compare our measurements with theoretical models of laser ablation plume expansion and draw some conclusions as to which model is more appropriate for the low temperature plasmas which arise in pulsed laser deposition.
Results of transporting a laser‐generated plasma through magnetic fields are reported. Plasma plumes have been generated in strong magnetic fields, in directions both transverse and parallel to the field. Collective effects are demonstrated by the plasma while in the high‐density state near the laser target. The formation of the plasma and its transport through an axial magnetic field enhances the relative amount of highest charge states and the lowest charge states. The focusing properties of the magnetic field near the extractor gap can prove useful in enhancing ion density at the anode aperture of the extractor gap. It is suggested that the duty cycle of laser ion sources can be extended by simply increasing the ion flight time through the magnetic field from the laser target to the extractor gap without appreciable loss of ions. Further, it is suggested that energy spread in a given ion species can be made small by using an extractor potential programmed to increase in time relative to the laser fire time.
Laser-produced plasma has been proposed to be an efficient source of ions which can be used for different applications (e.g., particle accelerators and ion implantation technology). In this article we present a study of the influence of an axial magnetic field on angular distributions of ions emitted from tungsten plasma produced using a 1 ns Nd:glass laser beam of energy 0.2–2 J at intensities of 10<sup>10</sup>–10<sup>11</sup> W/cm <sup>2</sup>. Investigations were performed at magnetic field induction of 1.2 T. An electrostatic ion energy analyzer and a set of ion collectors located at different angles with respect to the target normal were applied for measurements of the ion stream characteristics. The results of measurements of angular distributions of ion emission show that the magnetic field causes a significant increase of a current density in ions close to the magnetic field axis (parallel to the target normal). The maximum current density of ions that expand in the magnetic field and considerably lower maximum current density of freely expanding ions increase fast with an increase of laser pulse energy. Simple Monte-Carlo simulations of laser-produced ions that expand freely and those expanding in the presence of an axial magnetic field are compared. © 2004 American Institute of Physics.
A laser ion source (LIS) can easily provide a high current beam. However, it has been difficult to obtain a longer beam pulse while keeping a high current. On occasion, longer beam pulses are required by certain applications. For example, more than 10 micros of beam pulse is required for injecting highly charged beams to a large sized synchrotron. To extend beam pulse width, a solenoid field was applied at the drift space of the LIS at Brookhaven National Laboratory. The solenoid field suppressed the diverging angle of the expanding plasma and the beam pulse was widened. Also, it was observed that the plasma state was conserved after passing through a few hundred gauss of the 480 mm length solenoid field.
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