The production of laser-induced plasmas, which ordinarily occurs only at high laser powers, occurs at far lower laser powers using resonant and collisional phenomena in alkali metal vapors. For example, 10 torr of Na vapor can be significantly ionized to produce a bright white ball with as little as 1 mW of focused cw dye laser power (e.g., at the 3p → 4d transitions at 568.3 and 568.8 nm). We first discuss the alkali metal vapors in general (1) and their important ionization processes (2), especially those occurring at photon energies below the atomic ionization energy. We then discuss the striking wavelength dependences of ionization. For example, cw laser irradiation of these vapors gives completely different behavior at slightly different wavelengths (Stwalley 1990): 10 torr of sodium vapor at 568.2 nm, where the vapor shows only molecular fluorescence and is sufficiently quiescent that highly stable, narrow-band, optically pumped lasers based upon the fluorescent transitions can easily be constructed (Bahns 1983; Bahns et al. 1983; Verma et al. 1983); and, at 568.3 nm, where a “quasiresonant” laser-induced plasma forms and strong emission (including violet excimer bands, a potential laser) is produced (Koch & Collins 1979; Bahns et al. 1989a; Bahns et al. 1989b). We will analyze and reinterpret the classic subthreshold ionization studies (3) and survey recent laser-induced plasma studies (4). We also discuss dissociative recombination and its selectivity for producing strong excimer emission (5) the predicted electronic spectra of the alkali metal molecular ions (6).

The possibility of the creation and the application prospects of the laser-electron X-ray generator based on Thomson scattering of laser radiation on a bunch of relativistic electrons are considered. Such a generator fills the existing gap between X-ray tubes and synchrotron radiation sources, which is several orders of magnitude in terms of the brightness, average intensity, size, and also in the construction and running costs.

Air breakdown by avalanche ionization plays an important role in the electron beam and microwave propagations. For high electric fields and short pulse applications one needs avalanche ionization parameters for modeling and scaling of experimental devices. However, the breakdown parameters, i.e., the ionization frequency vs E/p (volt. cm−1. Torr−1) in air is uncertain for very high values of E/P. We review the experimental data for the electron drift velocity, the Townsend ionization coefficient in N2 and O2 and develop the ionization frequency and the collision frequency for momentum transfer in air. We construct the E/p vs Pτ diagram and show that our results are in better agreement with the most recent short pulse air breakdown experiments, compared to those predicted by the expression of Felsenthal & Proud (1965). This is because they extrapolate an expression for the drift velocity, linear in E/p, to high values of E/p. Experimentally the drift velocity varies as (E/p)½ in the region of E/p > 100.

A low energy pulsed X-ray source with 17 J electrical input energy has been constructed based on the vacuum spark configuration triggered by the pseudospark electron beam. With such a low input electrical energy, the frequently studied X-ray emitting vacuum spark plasma is not obtained. Instead, this configuration gives rise to a point-like electron beam-target pulsed X-ray source at the tip of the anode. Emission of X-ray is observed during the pre-breakdown phase. The X-ray spectrum is dominated by the characteristic line emission of the anode material. The total X-ray energy emission from the source can be estimated by assuming it to be a monochromatic point source. It is found to have an X-ray production efficiency of 0.1%, giving an average X-ray dosage of 18 mGy per pulse. X-ray radiography of small biological sample is demonstrated to illustrate a potential application of the present X-ray source.

Proposed laser fusion power plant concepts suffer from the huge size and expense of the lasers needed for compression and ignition. In a 1969 study (classified in 1970 and declassified in 2007), the idea to use chemical high explosives for the pumping of megajoule lasers was explored. Apart from being less expensive by orders of magnitude, such lasers are expected to be much more compact, and with their large energy, output could simultaneously drive several thermonuclear micro-explosion chambers. Because of its topical importance, I accepted the journal's invitation to publish a previously classified work, but with new unpublished ideas, with the previously classified paper put into the appendix.

Basic relations are presented to calculate isotope enrichment and depletion factors in a supercooled flow of gaseous QF6 molecules mixed with Ar (or another carrier gas) that is coaxially or cross-wise illuminated with a laser beam. Isotope-selective laser excitation of iQF6 molecules retards their condensation on walls and during cluster growth. This yields an isotope-enriched gas and isotope-depleted wall deposits under subsonic flow conditions and coaxial laser irradiation. Similarly, enriched QF6 gas and depleted particle clusters are formed in supercooled supersonic expansion flows that are coaxially or cross-wise irradiated. Measured isotope changes for 32SF6/34SF6 and 235UF6/238UF6 are shown to be in agreement with calculated values.

The interaction of near infrared femtosecond laser pulses with a Cu based alloy (brass) in ambient air at atmospheric pressure and under different laser conditions was investigated. The effects of laser fluence and number of pulses on surface morphology and ablation rate were studied using scanning electron microscopy (SEM) and optical microscopy. Ablation rates were found to rapidly increase from 83 to 604 nm/pulse in the fluence range 1.14–12.21 J/cm2. At fluence >12.21 J/cm2, ablation rates increased slowly to a maximum (607 nm/pulse at 19.14 J/cm2), and then decreased at fluence higher than 20.47 J/cm2 to 564 nm/pulse at 24.89 J/cm2. Large amounts of ablated material in a form of agglomerated fine particles were observed around the ablation craters as the number of laser pulses and fluence increased. The study of surface morphology shows reduced thermal effects with femtosecond laser ablation in comparison to nanosecond laser ablation at low fluence.

The properties of plasma (proton) block driven by the laser-induced skin-layer ponderomotive acceleration (S-LPA) mechanism are discussed. It is shown that the proton density of the plasma block is about a thousand times higher than that of the proton beam produced by the target normal sheath acceleration (TNSA) mechanism. Such a high-density plasma (proton) block can be considered as a fast ignitor of fusion targets. The estimates show that using the S-LPA driven plasma block, the ignition threshold for precompressed DT fuel can be reached at the ps laser energy ≤ 100 kJ.

Primary storages based on a linear transformer scheme were developed long ago. In this scheme, the secondary turn only has to be insulated from the high output voltage. Seven years ago at the High Current Electronics Institute (HCEI) a primary storage based on a linear transformer scheme and called the Linear Transformer Driver (LTD) stage was designed. In LTD stages, the primary turn, the storage capacitors with the switches, the core, and the outer conductor of the secondary turn are integrated into the stage cavity representing one separate building block of the primary storage. The body of the LTD cavity keeps ground potential during the shot allowing us to assemble them in series or in parallel depending on load requirements. Such flexibility of the storage structure and high output power of the LTD stages allows us to replace for some applications the traditional water line technology with LTD-based primary storages that are connected directly to the load (Direct Drive Scheme—DDS). In this article, we present the design of several LTD stages developed at HCEI and give examples of high-power energy storages produced by using the LTD technology.

Surface plasmon enhanced electron acceleration is a recently discovered efficient particle acceleration phenomenon in the nanoscale-confined field of surface electromagnetic waves. For the generation and spatial/spectral control of keV-energy electrons generated, this way few-cycle laser pulses can be utilized particularly well. We present numerical results based on a simple model of this phenomenon analogous to the three-step model of high harmonic generation. We identify those parameter regimes where the emitted electron beam is highly directional and monoenergetic opening the door to novel ultrafast applications and methods.

Enhancement of Kα yield from a metal foil covered with metal rods of submicron sizes, with low aspect ratio, irradiated by a p-polarized femtosecond laser pulses of about 1017 W/cm2 intensity is demonstrated. Obtained relative increase in Kα X-ray line emission is explained with a model of vacuum heating of fast electrons.

A nanosecond pulsed Nd-Yag laser, operating at an intensity of about 109 W/cm2, was employed to irradiate different metallic solid targets (Al, Cu, Ta, W, and Au) in vacuum. The measured ablation yield increases with the direct current (dc) electrical conductivity of the irradiated target. The produced plasma was characterized in terms of thermal and Coulomb interaction evaluating the ion temperature and the ion acceleration voltage developed in the non-equilibrium plasma core. The particles emission produced along the normal to the target surface was investigated measuring the neutral and the ion energy distributions and fitting the experimental data with the “Coulomb-Boltzmann-shifted” function. Results indicate that the mean energy of the distributions and the equivalent ion acceleration voltage of the non-equilibrium plasma increase with the free electron density of the irradiated element.

Pulsed laser induced plasma in water produces multiple bubbles with the passage of laser pulse. Shadowgraphy and beam deflection set-up is used to study the temporal and spatial evolution of these bubbles as a function of distance from the laser focus. The formation of multiple bubbles, bubble coalescence, and their effect onto cavity dynamics is reported. Bubble radius and the corresponding velocities from shadowgraphy is used to calculate the maximum gas pressure inside the bubble using Neppiras model. The maximum pressure inside the cavity is found to be 0.4 MPa at the laser focus.

Laser induced breakdown spectroscopy is a fast non-contact technique for the analysis of the elemental composition of any sample. Our focus is to advance this technique into a regime where we use pulse energies below 100 µJ. This regime is referred to as micro-laser-induced breakdown spectroscopy or µLIBS. At present we have concentrated on two application areas : (1) The imaging of latent fingerprints and (2) the extension to laser ablation followed by laser-induced fluorescence (LA-LIF) for very high sensitivity analysis of contaminants in water. Preliminary pulse emission scaling of Na in latent fingerprints has been investigated for ~130 fs, 266 nm pulses with energies below 15 µJ. The lowest energy for reliable single shot detection of Na is approximately 3.5 µJ. A 2D map of a fingerprint on a Si wafer has been successfully demonstrated using 5 µJ pulses. In LA-LIF the detection sensitivity of micro-laser-induced breakdown spectroscopy (µLIBS) is improved by coupling it with a second resonant probe pulse. This technique was investigated for the detection of Pb at low concentrations when ablated by 266 nm, 170 µJ pulses. After a short delay the resulting plume was re-excited with a nanosecond laser pulse tuned to a specific transition of Pb. In the case of the resonant dual-pulse LIBS the limit of detection was found to be approximately 60 ppb for Pb in water for 1000 shots. It is expected that this result could be implemented with fiber or microchip lasers with multi-kHz repetition rates and fiber Bragg grating tuning elements. The results are promising for the development of portable µLIBS water monitoring systems and portable fingerprint scanners.

The high precision X-ray spectroscopy studies of plasma created from the CO2 clusters in gas jet targets by the ultrashort laser pulses (35 and 60 fs duration) were performed at the intensities IL ∼ 1017–1018 W cm−2. The spectral line shape of the H-like and He-like oxygen ions gains an asymmetry with increasing the laser pulse intensity. Theoretical modeling of the line shape shows that the asymmetry can be explained by absorption of the Doppler-shifted line radiation from the essential fraction of ions (over 10−3) with energies above 1 MeV due to photoionization of inner shells of carbon ions. The results obtained demonstrate measurement capabilities of the X-ray spectral measurements of multicharged ions accelerated during the interaction with a laser radiation.

A new method to determine the peak intensity of focused relativistic laser pulses is experimentally justified. It is based on the measurement of spectra of electrons, accelerated in the beam waist. The detected electrons were emitted from the plasma, generated by nonlinear ionization of low-density gases (helium, argon, and krypton) in the focal area of a laser beam with the peak intensity >1020 W/cm2. The measurements revealed generation of particles with the maximum energy of a few MeV, observed at a small angle relative to the beam axis. The results are supported by numerical particle-in-cell simulations of a laser–low-density plasma interaction. The peak intensity in the focal region derived from experimental data reaches the value of 2.5 × 1020 W/cm2.

Magnetohydrodynamic simulation results related to the capillary discharge dynamics are presented. The main physical process that should be taken into account is the ablation of the capillary wall material evaporated by the heat flux from the capillary plasma. The possible applications of the capillary discharges related to the physics of the X-ray lasers and the use of the capillary plasma to provide a guiding for ultrashort high-intensity laser pulses over a distance greater than the defocusing length are discussed.

Interaction of a Nd:YAG laser, operating at wavelengths of 1064 nm (23.6 J cm−2 fluence) or 532 nm (25.9 J cm−2 fluence), and pulse duration of 40 ps, with a titanium-based medical implant Ti6Al4V alloy was studied. Surface damage thresholds were estimated to be 0.9 J cm−2 and 0.25 J cm−2 at laser wavelengths 1064 nm and 532 nm, respectively. At both laser wavelengths, the energy absorbed was mostly converted into thermal energy, forming craters, albeit about 50 times deeper at 1064 nm than at 532 nm. Periodic surface structures (PSS) were also formed with both laser wavelengths, concentric, and radial at micrometer scale (3 µm to 15 µm period), parallel at nanometer scale (800 nm period with the 1064 nm laser, 400 nm with the 532 nm laser). In the case of the 532 nm laser, the concentric structures enlarge their period with accumulating laser pulse count. These features can help roughening of the implant surface and improve bio-compatibility.

Survival of the production target in successive experiments (with a repetition rate of 1 Hz) over an extended period of time is one of the key problems encountered in designing the Super-FRS (Superconducting Fragment Separator) at the future Facility forAntiprotons and Ion Research (FAIR). Because of the difficulties involved in construction of a liquid jet metal target, it is highly desirable to employ a solid production target at the Super-FRS. However, with the high beam intensities that will be available at the FAIR, the production target may be destroyed in a single experiment due to high specific energy deposition by the beam in the target material. The level of specific energy deposition can be reduced to an acceptable value by increasing the beam focal spot area. However, the spot size is limited by requirements of achieving good isotope resolution and sufficient transmission of the secondary beam through the system. The resolving power of the fragment separator is inversely proportional to the X-dimension of the focal spot whereas the transmission depends on Y-dimension only. It has been previously shown [Tahir et al., 2005c] that an elliptic focal spot with appropriate dimensions, will fulfill the above two conditions simultaneously and will also have a large enough area to reduce the specific energy deposition to an acceptable level for certain beam intensities of interest. In this paper we present numerical simulations of thermodynamic and hydrodynamic behavior of a solid graphite target that is irradiated by 1 GeV/u uranium beam in the intensity range of 1010 –1011 ions per bunch with a bunch length = 50 ns. These simulations have been carried out using a three-dimensional computer code, PIC3D, that includes elastic-plastic effects. This theoretical work has shown that up to a beam intensity of 1011 ions/bunch, one can employ a solid target while for higher intensities the target will be destroyed due to thermal stresses induced by the beam. It has also been found that a circular focal spot leads to minimum thermal stresses as it generates minimum pressure gradients compared to an elliptic focal spot, for the same specific energy deposition. Moreover, the stress level increases with an increase in the ellipticity of the focal spot. It is therefore recommended that one should use a circular focal spot for lower intensities provided that the criteria for isotope resolution and transmission are fulfilled.

Proton ion acceleration via laser-generated plasma is investigated at relatively low laser pulse intensity, on the order of 1010 W/cm2. Time-of-flight technique is employed to measure the ion energy and the relative yield. An ion collector and an ion energy analyzer are used with this aim and to distinguish the number of charge states of the produced ions. The kinetic energy and the emission yield are measured through a consolidated theory, which assumes that the ion emission follows the Coulomb-Boltzmann-Shifted function. The proton stream is generated by thin and thick hydrogenated targets and it is dependent on the free electron states, which increase the laser absorption coefficient and the ion acceleration. The maximum proton energy, of about 200 eV, and the maximum proton amount can be obtained with thick metallic hydrogenated materials, such as the titanium hydrate TiH2.