High Intensity Short Pulse Research Articles

Observation of Raman forward scattering and electron acceleration in the relativistic regime

Raman forward scattering (RFS) is observed in the interaction of a high intensity (>10/sup 18/ W/cm/sup 2/) short pulse (<1 ps) laser with an underdense plasma (n/sub e//spl sim/10/sup 19/ cm/sup -3/). Electrons are trapped and accelerated up to 44 MeV by the high-amplitude plasma wave produced by RFS. The laser spectrum is strongly modulated by the interaction, showing sidebands at the plasma frequency. Furthermore, as the quiver velocity of the electrons in the high electric field of the laser beam becomes relativistic, various effects are observed which can be attributed to the variation of electron mass with laser intensity.

Nonlinear propagation effects in FIR optically pumped amplifiers

The nonuniform pump intensity distribution perpendicular to the propagation direction causes an index of refraction and gain inhomogeneity of the medium with signal frequency dispersion, for instance, in the amplifier type optically pumped lasers. The resulting waveguiding and gain focusing caused gain line distortion, which is investigated experimentally and theoretically in a methanol model material with a comparatively low pumping power level. The narrowing of the spectral region of amplification is observed with a decreasing pump beam size. This observation is interpreted as the result of propagation of the signal FIR beam in a waveguide induced by the pumping beam. This waveguiding is predicted to be a dominant mechanism in the amplifier type FIR lasers pumped by focused, high intensity short pulses.

Generation and propagation of high intensity short acoustic pulses in dispersive media

Experiments on laser generation of acoustical pulses, amplitude 10 MPa and duration 1 μs, and their propagation in different liquids have been carried out. Propagation was measured in both pure liquids and structured media. Nonlinearity and dispersion caused evolution of the propagating signals. Some theoretical treatment of the results indicate the possibility of obtaining data on spectral features of the nonlinear response of the medium.

Studies on electroporation of thermally and chemically treated human erythrocytes

Erythrocytes can be transiently permeabilized in isotonic solution by the action of high intensity short electrical pulses. The consequent hemolysis of cells was found to depend on the temperature conditions during and after the pulse treatment. Application of electrical pulses to erythrocyte suspension at increasing temperatures (4–43 °C) progressively decreased the proportion of hemolysed cells. Post pulse incubation of electroporated erythrocytes at temperatures between 4 and 43 °C yielded increasing percentage of resealed cells. The rate of resealing of electropulsed cells increased significantly with the increase in pulse temperature as well as after pulse incubations. In addition, oxidative modification of membrane proteins by reaction of their sulfhydryls with N-ethylmaleimide and p-chlormercuribenzoate altered remarkably the electrical response of erythrocytes indicating involvement of chemically modified protein moieties in the membrane electroporation. Results of this investigation have demonstrated that the physical organization of membrane and alterations in membrane components determine the magnitude of cellular response to external electrical pulses.

Photobleaching with a subnanosecond laser flash

In standard fluorescence recovery after photobleaching (FRAP) applications for measuring lateral diffusion rates and adsorption/desorption kinetics of fluorescent molecules at biological or model membranes, irreversible bleaching is induced by a bright excitation flash of at least millisecond time scale. It has been presumed that the bleaching event is of a low probability and the significant bleached population that develops during the flash results from each molecule undergoing thousands of excitation/deexcitation cycles before a bleaching event occurs. In some FRAP experiments, notably polarized FRAP (PFRAP) for measuring molecular rotational diffusion rates, it is desirable to use much shorter (subnanosecond) bleaching pulses. However, subnanosecond pulses are shorter than the fluorescence lifetime, so that any fluorophore will experience at most only one visit to the excited state during the bleaching pulse. If bleaching occurs only by the same processes as in slower FRAP experiments, one would thereby expect only minimal bleaching regardless of the bleach intensity. Moreover, the ability of fast polarized pulses to imprint an anisotropic orientational pattern in the postbleach unbleached fluorophore, an ability essential for PFRAP, is not at all guaranteed, particularly if two-photon processes are involved in high-intensity short bleach pulses. In this study, bleaching depths are measured as a function of subnanosecond pulse intensity on a small labeled protein covalently immobilized on fused silica. We show that bright subnanosecond laser flashes do indeed produce significant bleaching, that both two photon effects and reversible bleaching are involved, and that polarized bleaching does produce an anisotropic orientational pattern of unbleached fluorophore. We also postulate a theoretical molecular state model which semiquantitatively accounts for the experimentally observed dependence of reversible bleaching on bleaching pulse intensity.

The ISOLDE facility at the CERN PS Booster

The CERN ISOLDE on-line mass separator facility was reconstructed at the 1 GeV PS Booster and is now being commissioned. Protons in very short high intensity pulses (average current 2 μA) are directed on either one of the targets of the two on-line mass separators. The 60 keV ions from them are fed into a common beam transport system to which most of the experiments will be connected. The facility can supply radioactive beams of about 60 elements from the whole nuclear mass range to a community of more than 250 physicists working in the fields of nuclear, atomic, surface and solid-state physics, astrophysics and nuclear medicine. The most interesting features of the facility will be discussed and a few experiments described to illustrate the extent of the experimental programme. An outlook into future developments such as resonant laser ion sources and plans for post-acceleration of radioactive ions will be given.

Relativistic self-focusing of ultraintense laser pulses in inhomogeneous underdense plasmas.

We investigate the propagation of high-intensity short laser pulses in a plasma. A two-parameter perturbation expansion is used to consistently treat the nonlinear mass variation and ponderomotive contribution to self-focusing, including plasma inhomogeneity. An analytical expression for the critical power is given and it is found that it greatly depends on the plasma inhomogeneity. A tailoring of the preformed plasma is suggested in order to obtain a strong reduction of the critical power. The temporal evolution of the pulse and plasma dynamics are also considered.

Multipulse photon echoes in LaF3:Pr3+

Multipulse photon echoes are observed from ${\mathrm{LaF}}_{3}$:${\mathrm{Pr}}^{3+}$ and are shown to be primarily stimulated echoes. The various contributions to the echoes were measured using a three-pulse photon-echo sequence that resolved the image of the echo generated by the first two pulses, and the echo generated by the second and third pulses and that of the first and third pulses from the stimulated echo. By use of a theoretical model of Shoemaker [in Laser and Coherence Spectroscopy, edited by J. I. Steinfeld (Plenum, New York, 1978)] and relaxation theory of Hu and Hartmann [Phys. Rev. B 9, 1 (1974)], the image echo contributions to the third echo is shown to be less than 10% of the total for the pulse widths and Rabi frequencies used in the measurements. As a result, multipulse photon echoes will find limited use in measuring coherence decay until very short high-intensity pulses are used to excite the echoes.

The new CERN-ISOLDE on-line mass-separator facility at the PS-Booster

The ISOLDE on-line isotope separators have been operated since 1967 at the CERN-SC. This 600 MeV proton synchro-cyclotron had to be shut down in December 1990 after 33 years of service and it was decided to move ISOLDE to a new experimental area. The new on-line mass-separator facility is now under construction at the CERN PS-Booster. This accelerator provides an average current of about 2 μA of 1 GeV protons in very short high intensity pulses at low repetition rate. The beam can hit either one of the two target stations, the general purpose separator (GPS), a reconstructed ISOLDE-2 type machine (which can deliver beams simultaneously into three beam lines), and the high resolution separator (HRS), which is essentially the slightly modified ISOLDE-3 separator. The central GPS beam line and the HRS feed a common beam transport system to which most of the experiments will be connected. The new facility will be taken into operation in spring 1992.

New far-infrared laser lines from CH3Cl and CH3Br optically pumped with a continuously tunable high pressure CO2 laser

In this paper we report on the detection of new far-infrared laser lines from CH3Cl and CH3Br optically pumped with a continuously tunable high pressure CO2 laser. We found 80 new lines for CH3Cl and 9 new lines for CH3Br in the frequency region between 16 cm−1 and 41 cm−1, all due to stimulated Raman scattering. For the Raman gain regions bandwidths up to about 700 MHz were found. We also observed high intensity short far-infrared laser pulses of durations in the nanosecond regime.

High intensity generation of 9–13 Å x-rays from BaF2 targets

Studies of the interaction of condensed matter with short pulse (∼ 600 fs) high intensity (∼1017 W/cm2) ultraviolet (248 nm) radiation show that intense spatially compact sources for x-ray emission in the kilovolt range (0.5–1.0×1015 W/cm2) can be generated from solid targets at close to the maximum volume specific rate allowed.

Thermoelastic effects on x-ray lithography masks under pulsed irradiation

The purpose of this paper is to present computer calculations of thermoelastic effects in x-ray lithography masks caused by the absorption of high intensity short x-ray pulses. Several mask structures are considered with different substrate and absorber materials. In particular, substrates materials of Si, SiC, and diamond, and absorber materials including Au, W and Ta will be considered. The powerful analytical capabilities of NASTRAN were combined with the graphic input and output capabilities of CAEDS to perform the calculations using finite element analysis. The calculations were performed for x-ray masks operating in air, and in a He atmosphere with a 40 μm gap between the mask and the wafer. Thermoelastic stress calculations were performed for 8.3 Å, and 10 Å wavelength x-ray pulses. Results indicate a maximum temperature rise on the mask of the order of 30 °C for a 2 nanosecond exposure with a 10 mJ/cm2 x-ray pulse. The maximum temperature was obtained at the end of the x-ray pulse. The temperature decayed quickly, with a time constant depending on the thermal properties of the absorber and the substrate. The initial ambient temperature was reached after about 10 milliseconds. Mechanical static analysis showed that the maximum stress in the absorber films which was due to maximum temperature differences in the mask layers, also occurred at the end of the pulse. The magnitude of the induced thermoelastic stress was found comparable to the intrinsic stress level of the mask materials. The analysis indicates that when the pulse amplitude reaches 10 mJ/cm2, there is need for experimental study of x-ray mask distortion during exposure to short x-ray pulses (from a laser plasma or similar source). The results also point out directions for the selection of mask materials to minimize mask fatigue and distortion.

Complex second harmonic emission spectra from plasmas produced using short pulse, high intensity, 1μ m laser radiation

Complex second harmonic spectra emitted from plasmas generated using short pulse high intensity 1 μm laser radiation can be most easily explained by a generation mechanism involving a decay instability of resonance absorption operating in combination with modulation of the spectrum by return current driven ion acoustic waves.

Nonlinear Plasma Dynamics at High Intensity Laser Interaction

Some aspects of progress in laser fusion are briefly reviewed which indicate that experiments are on the threshold of entering a regime where nonlinear dynamics will become important. Recent studies of the general nonlinear force that include additional nonponderomotive terms are reviewed. Computer calculations that provide a detailed knowledge of the transfer of optical energy into a plasma corona by nonlinear forces are then described. Two key regimes are observed: 1) early production of fast moving cold blocks of plasma followed by 2) a decay of the corona where the plasma velocity obeys a Kortweg-de-Vries equation. This soliton decay corresponds to a fast thermalization of laser radiation in the corona at perpendicular incidence, i.e., without asymmetries of laser incidence as in the case of resonance absorption or striated motion. This permits a short pulse high intensity irradiation to achieve conditions equivalent to an ideal gasdynamic ablation compression.

Experiments on dynamic plastic loading of frames

Tests are described on plane frames of mild steel and titanium (commercial purity) in which high intensity short duration pressure pulses were applied transversely to the beam member either uniformly over this member or concentrated at its center. The objective was to examine applications of two estimation techniques (upper bounds on deflections and the mode approximation technique) for major response features of pulse loaded structures at large deflections, taking account of strong plastic strain rate sensitivity. Loads over a range such as to cause final deflections up to about a third of the span were applied by detonating explosive sheet. Agreement between estimated and measured final deflections was often very good (generally conservative) but the intrinsic error of the mode technique was not observed as expected.

Comments on: Interaction of a high intensity short pulse width relativistic electron beam with plasma

Okamura et al., ibid., vol. 19, p.997 (1977), performed diamagnetic loop measurements indicating that the return current is limited to the duration of the injected electron beam pulse. The present authors, Hopman et al., are not satisfied that plasma turbulence provides a reasonable explanation as to why the return beam decays so fast. They offer an alternative explanation whereby the return beam is stopped by the original electron beams induced field.

Generation of High Intensity Beams of Nanosecond Pulse Duration by Electron Linacs

Acceleration of electron beams with a very short pulse duration and high peak intensity is achieved in electron linacs. As pulse generators for very short pulse duration, a coaxial line-type pulser, a hard-tube pulser and a pulse sharpener are newly developed which can produce pulses of about 5 nanoseconds in pulse duration and about a few thousands volts in amplitude in 50-ohm load. In Tohoku linac, peak intensity of 1.3 amperes is obtained with pulse duration of 7 nanoseconds and energy of 75 MeV, while in Mitsubishi linac, 2.0 amperes, 10 nanoseconds and 6.4 MeV, respectively. Their results agree with Leiss theory. As a parameter of transient beam loading, the ratios of the energy contained in a pulsed beam to the stored energy in the accelerating structure and of the peak beam power to the peak input RF power are presented.

Simple, High Intensity Short Pulse Flashlamps

Various flashlamps were constructed using quartz envelopes and simple wire electrodes. The lamps were continuously evacuated to minimize explosive failures. Energies in the 10 to 1000 J range were discharged from low inductance capacitors through a triggered spark gap to the lamps. The lamp discharges occurred primarily in vapors ablated from the quartz envelopes and produced intense light pulses of 1 to 10 μsec duration with submicrosecond risetimes. Lamp explosion energies and peak light outputs were empirically found to be simply related to the lamp dimensions, making it possible to quickly design lamps for various applications. Successful pumping of high energy organic dye lasers demonstrated the effectiveness of these lamps in applications requiring short, high intensity pulses.

Variable-Path Magnetic Ion Buncher

Following a previously proposed method for producing short intense monoenergetic ion bunches, a complete variable-path magnetic ion buncher was constructed. Tests in 1958 showed that bunching did occur, but available ion beam intensity and instrumental resolution were too low for detailed measurements at that time. Reconstruction of the available accelerator to give a high current (≥100 μA), well-focused ion beam, and correspondingly high resolution pulse measuring equipment permitted meaningful resumption of testing by late 1960. Bunching performance in accordance with design predictions was then observed. Measurements with ½-MeV protons indicated pulse lengths ≤1 mμsec, peak currents ≥4 mA, and a bunching ratio of ≥40. Average bunched current was &amp;gt;30 μA. Although tested with ½-MeV protons, the design permits operation with protons or deuterons to more than 1 MeV. Bunching is accomplished by deflecting successive portions of a continuous monoenergetic ion beam from an electrostatic accelerator over progressively shorter paths between foci of a multifocusing 103° deflection magnet in a manner such that all ions come together essentially simultaneously at one focal point in high intensity short duration ion pulses. Sinusoidal electrostatic deflection at 8.8 Mc, linearized by cylindrical electrostatic lenses, or through harmonic synthesis, provides the required sweep.

Thermal Diffusivity Measurements on Metals at High Temperatures

A flash method for the measurement of the thermal diffusivity of metals up to 1800°C is described. A high intensity short duration light pulse is absorbed in the front surface of a thermally insulated specimen a few millimeters thick heated in an induction furnace and the resultant temperature history of the rear surface is measured by a lead sulfide cell, displayed on an oscilloscope, and photographed by a Polaroid Land camera. The thermal diffusivity of the material is determined from this temperature versus time curve provided the theoretical boundary conditions are experimentally satisfied. Measurements of the thermal diffusivity of Armco iron, molybdenum, and titanium have been made.