Intense Short Pulse Research Articles

Laser amplification by electric pulse power

It is proposed that it is possible to amplify the energy of a pulsed laser beam by imploding it inside a capillary metallic liner. If imploded with megaampere currents by the pinch effect, implosion velocities up to ∼3 × 108 cm/s can be reached, imploding a few cm long liner with an inner radius of 2 × 10−3 cm in about ∼10−10 s. If the liner radius can be imploded by 30-fold, the laser pulse would in the absence of absorption losses into the linear wall be amplified 1000-fold. Because the amplification is through the conversion from longer to shorter wave lengths, the concept offers the prospect of intense short wave length laser pulses in the far ultraviolet and soft X-ray domain. Apart from the direct drive laser beam compression by the pinch effect, an alternative indirect drive through the conversion of the electric pulse power into soft X-rays is possible as well. The limitations of this concept are the absorption losses into the liner wall, and ways to overcome these losses are presented. The most important application of the proposed laser amplification scheme might be for the fast ignition of various inertial confinement fusion schemes. An integrated fast ignition inertial confinement fusion concept using the indirect drive is also presented.

Electrochemotherapy a novel approach to the treatment of metastatic nodules on the skin and subcutaneous tissues

Electrochemotherapy (ECT) is a new treatment for metastatic nodules of solid tumors on the skin or subcutaneous tissue. ECT is a combination of a physical effect, cell membrane poration, and cytotoxic drug administration. Cell membrane poration is achieved by applying short intense electric filed pulses. Pore formation on the cell membrane allows low permeant drugs like bleomycin or cisplatin to enter the cell and thus locally increase their toxicity: up to 10.000 times for bleomycin and 80 times for cisplatin. ECT has been investigated in a multicenter study European Standard Operating Procedures for Electrochemotherapy (ESOPE) that demonstrates how by ECT over 80% of the cutaneous or subcutaneous metastatic nodules can be healed, thus confirming the results of previous studies. ECT efficacy is independent of tumor histology. The experience gathered in the ESOPE study allowed to prepare standard operating procedures that are key to the dissemination of the technology. ECT is safe effective, the treatment is completed in one session usually on an out-patient basis with minimum side-effects. ECT is cost-effective and, although palliative, it ameliorates patients' quality of life. ECT is the treatment of choice for tumors refractory to conventional treatment, can be used in form of cytoreductive therapy before conventional treatment for organ sparing and functions saving, finally can be adopted to treat hemorrhagic or painful nodules, it can be applied in previously irradiated areas.

Momentum-imaging investigations of the dissociation of D2+ and the isomerization of acetylene to vinylidene by intense short laser pulses

We present momentum images of the ionic products from the ionization of D2 and C2H2 by short laser pulses. For D2, we use a pump–probe approach to investigate the dependence of the enhanced ionization on the internuclear distance. Evidence for two (not well separated) regions of enhancement is found near internuclear distances of 6 and 10 au. In the case of acetylene, we report clear evidence for the production of both acetylene and vinylidene dications with kinetic energy releases similar to those reported earlier by core electron removal. We also find very different angular distributions for the fragments in the two channels, consistent with a finite time for the isomerization.

Pulse line ion accelerator concept

April 3, 2006 LBNL-59492 The pulse line ion accelerator concept Richard J. Briggs Science Applications International Corporation, Alamo, California, 94507 The Pulse Line Ion Accelerator concept was motivated by the desire for an inexpensive way to accelerate intense short pulse heavy ion beams to regimes of interest for studies of High Energy Density Physics and Warm Dense Matter. A pulse power driver applied at one end of a helical pulse line creates a traveling wave pulse that accelerates and axially confines the heavy ion beam pulse. Acceleration scenarios with constant parameter helical lines are described which result in output energies of a single stage much larger than the several hundred kilovolt peak voltages on the line, with a goal of 3-5 MeV/meter acceleration gradients. The concept might be described crudely as an “air core” induction linac where the PFN is integrated into the beam line so the accelerating voltage pulse can move along with the ions to get voltage multiplication. 1. Introduction In the “Pulse Line Ion Accelerator” (PLIA) concept described in this paper a ramped high voltage pulse is applied at the input end of a helical pulse line structure. The resulting traveling wave pulse on the line can accelerate an ion bunch to energies much greater than the peak voltage applied to the line. It should also be possible to achieve an axial acceleration gradient of several MeV per meter with realistic helix parameters. The development of this concept was originally motivated by a proposal to use moderate energy intense ion beams to heat matter to regimes of interest for studies of High Energy Density Physics (HEDP) and Warm Dense Matter (WDM) [ 1 ] . In this approach, very short pulses (~ a nanosecond or less) of “medium mass” ions with energies slightly above the Bragg peak are focused to mm scale spot sizes onto thin target “foils”, which may in fact be foams with mean densities of order 10% solid density or less. The high degree of uniformity and efficiency with volumetric and shock-less heating of the target to temperatures of order 1 to 10 eV are attractive features in this approach. The concept presented in this paper is an excellent fit to the accelerator requirements for this HEDP/WDM application. A helical pulse line inserted into a large bore 5 - 10T superconducting solenoid can transport and accelerate a singly charged Ne or Na ion bunch with a total charge of order 0.1 to 1 micro-coulombs, an axial bunch length of 10’s of cm, and a radius of a few cm. The required axial compression to bunch lengths of order one cm in a neutralized drift compression region following the accelerator is then only a factor of 10-30, which greatly eases the requirements on longitudinal emittance, velocity tilt, and drift distance.

Hot electron generation by laser-cone interaction

The hot electron generation by irradiation of intense short laser pulses onto cone targets is studied by using 2D PIC simulation. It is shown that the high energy electrons from the cone target are characterized with three effective temprature, while two-temperature for plane target. The electrons composing most energetic component are generated at the cone tip where laser field is intensified by cone-guiding, and electrons for middle component are generated at cone wing which is not observed for plane target. The acceleration processes at cone wing, cone tip, and rear surface are explained.

Anomalous transmission of high contrast relativistically intense short pulses through thin metal foils

The frequency doubled laser pulses from the T3 laser system at the Center for Ultrafast Optical Science of the University Michigan 1 (with energy up to 1 J, a pulse duration of 400 fs, the wavelength is 0.53 μm and the maximum intensity is 2.7 x 10 19 W/cm 2 ) has been used to measure the light transmitted through thin metal targets. The intensity contrast ratio of the laser pulse was better than 10 -9 which is low enough to suggest the interaction with solid density plasma without preplasma. We observed the transmittance of the laser pulse through the aluminum foil with thickness up to 4 μm and found that this radiation is polarized and centered at 0.53 μm. We found that for 0.8 μm thick foils the transmission was anomalously high (∼10 -2 %) and can't be explained by the skin effect for relativistic pulse.

Electron transport dependence on target surface conditions and laser spot shape

The interaction of intense short pulse radiation with thick Al foils is studied as a function of the absorption region density profile and the laser spot shape. Absorption of the light generates relativistic hot electrons near the critical surface. When the density profile is steep with micron scale lengths, and a small spot (8 μm FWHM) the hot electrons are retained near the surface while undergoing strong lateral surface transport through intense thermoelectric magnetic fields. Alternatively, with mild, ∼30 μm, scale length initial profiles the light beam bores a hole in the corona, wraps it with B-field, extinguishes the lateral hot electron flow, and sends a reduced fraction of hot electrons forward in filaments. Finally, broadening the spot to 40 μm and totally flattening it gives strong forward-directed hot electron penetration, which, if achievable, could serve effectively for Fast Ignition and radiography.

Dynamics of relativistic laser pulses in plasmas

The dynamics of intense laser pulses in plasmas are investigated both theoretically and numerically. The linear growth and nonlinear saturation of relativistic stimulated Raman scattering of plasmons are investigated by means of a nonlinear dispersion relation and via direct Vlasov simulations. We observe acceleration of electrons up to ultra-relativistic energies by a positive electrostatic potential that is created by intense short laser pulses.

Filamentation of a relativistic short pulse laser in a plasma

An intense short pulse laser propagating through a plasma undergoes filamentation instability under the combined effects of relativistic mass variation and ponderomotive force-induced electron density depression. These two nonlinearities superimpose each other. In a tenuous plasma, the filament size scales as where ω p is the plasma frequency, a0 is the normalized laser amplitude and γ 0 is the relativistic gamma factor.

Ponderomotive acceleration of electrons in the interaction of arbitrarily polarized laser pulse with a tenuous plasma

The forward ponderomotive force associated with an intense short laser pulse, propagating in a tenuous plasma, accelerates the electrons to velocities higher than the group velocity of the laser. In this work, a simple general solution for ponderomotive acceleration is presented for arbitrary polarization. The circular polarization is more efficient than linear polarization, since the threshold laser intensity needed for electron acceleration is lower for a circularly polarized laser pulse.

Semiclassical IVR approach to rotational excitation of non-polar diatomic molecules by non-resonant laser pulses

We apply the ‘classic’ semiclassical initial value representation (SC-IVR) approach to describe rotational excitation of non-polar diatomic molecules by intense short non-resonant laser pulses. We also investigate the applicability of the quantum mechanical sudden approximation. It is found that the SC-IVR approach gives accurate rotational excitation probabilities in regimes where the sudden approximation fails. Primitive semiclassical wavefunction propagation is used to illustrate the phenomenon of angular focussing of rotation states by strong pulses.

Novel diagnostic of low- Z shock compressed material

An experiment was performed using high-energy protons, generated by an intense short laser pulse, to try to characterize in situ, the spatial and temporal evolution of a laser-driven shock propagating through a low- Z material. Radiography of the shock propagating through the low- Z transparent material could lead to density determination under compression. A first test of a proton radiography of the rear-side released state of a shocked aluminum foil has also been tried.

Pulsed Cavitational Ultrasound: A Noninvasive Technology for Controlled Tissue Ablation (Histotripsy) in the Rabbit Kidney

The optimal minimally invasive treatment for small renal masses continues to evolve. Current ablative technologies rely on thermal mechanisms for tissue destruction. However, the creation of precise lesions is limited by inhomogeneous heating/cooling due to tissue variability, perfusion effects and tissue charring. We hypothesized that nonthermal mechanical effects of ultrasound (cavitation) can be used to progressively homogenize tissue in controlled fashion with predictable results. We developed a focused annular array ultrasound system capable of delivering high intensity (greater than 20 kW/cm) short pulses (20 microseconds) of energy to a target volume. This system operates at a repetition frequency of 100 Hz, resulting in a low time averaged power output (approximately 5 W total acoustic output). Following approval from the institutional animal care committee a series of transcutaneous ablations were performed in the normal kidneys of 10 rabbits. Lesions created with a small number of pulses (10 or 100) produced scattered areas of damage characterized by focal hemorrhage and small areas of cellular injury in the targeted volume. Lesions created with greater numbers of pulses (1,000 or 10,000) demonstrated complete destruction of the targeted volume. Gross examination revealed that lesions contained a liquefied core with smooth walls and sharply demarcated boundaries. Histological examination demonstrated extensive areas of acellular debris surrounded by a narrow margin of cellular injury. This pulsed cavitational ultrasound system is capable of transcutaneous nonthermal destruction of renal tissue. Refinement of this technology for noninvasive ablation of small renal masses is currently under way.

Generation of Strong Short Terahertz Pulses via Stimulated Raman Adiabatic Passage-assisted Coherent Scattering

ABSTRACTWe analyzed the efficiency of coherent scattering of infrared radiation in atomic and molecular gases for production of intense short THz pulses, using simulated Raman adiabatic passage (STIRAP). The method is based on excitation of maximal coherence when the system is undergoing through STIRAP in IR-irradiated atomic or molecular gases (for example Rb, methanol, and others) at room temperature. By applying optical pulses in correct sequence one can generate coherence in a system during STIRAP which triggers following coherent scattering of infra-red radiation, and can produce pulses of THz radiation with pulse energies ranging from several nJ to µ-J and pulse durations from several fs to ns.

Generation of vortex rings by nonstationary laser wake field

A new concept of generating quasistatic magnetic fields, vortex rings, and electron jets in an isotropic homogeneous plasma is presented. The propagation of plasma waves, generated by a relativistically intense short pulse laser, is investigated by using the kinetic model and a novel nonpotential, time-dependent ponderomotive force is derived by obtaining a hydrodynamic equation of motion. This force can in turn generate quasistatic magnetic fields, vortex rings, and electron jets. It is also shown that the vortex rings can become a means for accelerating electrons, which are initially in equilibrium. The conservation of canonical momentum circulation and the frozen-in condition for the vorticity is discussed. The excitation of the vortex waves by the modulation of the amplitude of the plasma waves is considered. These vortex waves, which generate a lower hybrid mode propagating across the generated magnetic field, are also investigated.

Satellite measurements of daily variations in soil NOx emissions

Soil NOx emission from agricultural regions in the western United States has been investigated using satellite observations of NO2 from the SCIAMACHY instrument. We show that the SCIAMACHY observations over a 2 million hectare agricultural region in Montana capture the short intense NOx pulses following fertilizer application and subsequent precipitation and we demonstrate that these variations can be reproduced by tuning the mechanistic parameters in an existing model of soil NOx emissions.

Investigation of intensity thresholds for ultrasound tissue erosion

Our previous studies have shown that short intense pulses delivered at certain pulse repetition frequencies (PRF) can achieve localized, clean erosion in soft tissue. In this paper, the intensity thresholds for ultrasound induced erosion and the effects of pulse intensity on erosion characterized by axial erosion rate, perforation area and volume erosion rate were investigated on in vitro porcine atrial wall tissue. Ultrasound pulses with a 3-cycle pulse duration and a 20-kHz PRF were delivered by a 788-kHz single element focused transducer. I(SPPA) values of 1000 to 9000 W/cm2 were tested. Results show the following: (1) the estimated intensity threshold for generating erosion was at I(SPPA) of 3220 W/cm2; (2) the axial erosion rate increased with higher intensity at I(SPPA) < or = 5000 W/cm2, while decreased with higher intensity at I(SPPA) > or = 5000 W/cm2; and (3) the perforation area and the volume erosion rate increased with higher intensity.

High energy electron bunch generation by using a plasma separator

In this paper, we propose an electron acceleration by using an intense short pulse laser and a thin slab plasma separator. When an intense short pulse laser illuminates electrons in vacuum, the electrons are accelerated by the ponderomotive force at the front of laser pulse and the electrons accelerated loose their energies at the tail of laser pulse. This is one of serious problems in the ponderomotive electron acceleration. In order to suppress the energy loss, we propose an overdense plasma separator to extract the electrons before entering the deceleration region of laser pulse. In our electron acceleration mechanism, only the laser is reflected by the overdense plasma separator and the electrons pass through the plasma separator. Consequently, the electrons can obtain a net energy after the interaction with the laser pulse.

Non-perturbative solution of the time-dependent Schrödinger equation describing H2 in intense short laser pulses

A method for solving the time-dependent Schrödinger equation describing the electronic motion of molecular hydrogen exposed to very short intense laser pulses has been developed. The fully correlated three-dimensional time-dependent electronic wavefunction is expressed in terms of field-free wavefunctions. These are obtained from a configuration-interaction calculation where the one-electron basis functions are built from B splines. The reliability of the method is tested by comparing results in the low-intensity regime to the prediction of lowest order perturbation theory. The onset of non-perturbative effects is shown for higher intensities and the validity of the single-active electron approximation is briefly discussed. Finally, the ability of the method to calculate photoelectron spectra including above-threshold-ionization peaks is demonstrated.

Intense Laser Alignment in Dissipative Media as a Route to Solvent Dynamics

We extend the concept of alignment by short intense pulses to dissipative environments within a density matrix formalism and illustrate the application of this method as a probe of the dissipative properties of dense media. In particular, we propose a means of disentangling rotational population relaxation from decoherence effects via strong laser alignment. We illustrate also the possibility of suppressing rotational relaxation to prolong the alignment lifetime through choice of the field parameters. Implications to several disciplines and a number of potential applications are proposed.