High Energy Short Pulse Research Articles

Laser ion acceleration in the high laser energy and high laser intensity regimes

New laser facilities able to deliver either ultra high energy short pulses or ultra high intensity pulses are being constructed and will open new and exciting opportunities for laser ion acceleration. The interaction of a high intensity short pulse with underdense, near-critical and overdense targets has been studied using 2D Particle-In-Cell simulations in these regimes. In the ultra high energy regime, proton beams with maximum energies of hundreds of MeV and a high number of high energy protons could be accelerated using thin solid foils or low density targets. In the ultra high intensity regime, radiation losses will start affecting laser ion acceleration using thin overdense targets for intensities higher than 10 22 W/cm 2 , and produce very energetic ions.

Investigation on Laser Etch-Machining Underwater Based on Thermal-Mechanical Effect

The method of high-energy short pulse laser etching underwater based on thermal - mechanical effect was put forward. In the experiment system with mask and salt solution , heat affected zone was visible decrease and the image of mask was micro-copied at the workpiece surface. Owing to the photochemical reaction and thermal-mechanical coupling effect, uniform line width of 120μm can be obtained.

Improved two-micron lasers for treating glaucoma and reducing skin wrinkles

Thulium (Tm) lasers operating in the 2 m IR spectral range are interesting for many applications, including spectroscopy, remote sensing, photomedicine, optical communications, and metrology. For instance, in photomedicine, 2 m lasers may be used in ophthalmology to treat glaucoma (high pressure in the eye) by iridotomy, in which holes are made in the iris. Another ophthalmic 2 m laser application is to cut a hole in the back of the capsule that contains the lens of the eye (capsulotomy). Lasers at this frequency are also used in dermatology to reduce skin wrinkles. An interesting feature of such 2 m lasers is that water (the main component of human tissue) is absorbed very efficiently, more than at 1 m, which is produced by the wellknown Nd:YAG (neodymium:yttrium aluminum garnet) laser. Saturable absorbers (SAs) can be employed to realize both passive mode-locking and passive Q-switching of such lasers. As the intensity increases in the laser cavity, the SAs become more and more transparent until the absorption saturates. This is called absorption bleaching. Energy absorbed by the SAs is released in a laser pulse, and the same process starts again. By using the mode-locked technique, it is possible to obtain ultrashort (femtosecond) pulse durations, depending on the breadth of the gain bandwidth of the material. For ophthalmic applications, the extremely short illumination time of a femtosecond pulse will produce less heating of ocular tissue. Consequently, a mode-locked Tm laser will be particularly suitable for iridotomy. On the other hand, passive Q-switching, which is simpler than active Q-switching, can be used to realize compact, robust, and inexpensive sources of high-energy short pulses in this wavelength range. As a result, the Q-switched Tm laser with Figure 1. Autocorrelation trace (a) and laser spectrum (b) of a modelocked thulium in potassium lutetium double tungstate laser using a single-walled carbon nanotube-saturable absorber. a.u.: Arbitrary units. FWHM: Full width at half-maximum.

The experimental plan for cryogenic layered target implosions on the National Ignition Facility—The inertial confinement approach to fusion

Ignition requires precisely controlled, high convergence implosions to assemble a dense shell of deuterium-tritium (DT) fuel with ρR>∼1 g/cm2 surrounding a 10 keV hot spot with ρR ∼ 0.3 g/cm2. A working definition of ignition has been a yield of ∼1 MJ. At this yield the α-particle energy deposited in the fuel would have been ∼200 kJ, which is already ∼10 × more than the kinetic energy of a typical implosion. The National Ignition Campaign includes low yield implosions with dudded fuel layers to study and optimize the hydrodynamic assembly of the fuel in a diagnostics rich environment. The fuel is a mixture of tritium-hydrogen-deuterium (THD) with a density equivalent to DT. The fraction of D can be adjusted to control the neutron yield. Yields of ∼1014−15 14 MeV (primary) neutrons are adequate to diagnose the hot spot as well as the dense fuel properties via down scattering of the primary neutrons. X-ray imaging diagnostics can function in this low yield environment providing additional information about the assembled fuel either by imaging the photons emitted by the hot central plasma, or by active probing of the dense shell by a separate high energy short pulse flash. The planned use of these targets and diagnostics to assess and optimize the assembly of the fuel and how this relates to the predicted performance of DT targets is described. It is found that a good predictor of DT target performance is the THD measurable parameter, Experimental Ignition Threshold Factor, ITFX ∼ Y × dsf 2.3, where Y is the measured neutron yield between 13 and 15 MeV, and dsf is the down scattered neutron fraction defined as the ratio of neutrons between 10 and 12 MeV and those between 13 and 15 MeV.

Quantum well laser with an extremely large active layer width to optical confinement factor ratio for high-energy single picosecond pulse generation by gain switching

It is theoretically shown that gain switching quantum well AlGaAs/InGaAs laser with a very large ratio of active layer thickness to optical confinement factor (∼8 µm) can result in single, high-energy (≫1 nJ) short (∼100 ps) single optical pulses at wavelengths ∼1 µm. Calculations predict that high electrical-to-optical conversion efficiency can be achieved in such structures, which require an injection current pulse of a modest amplitude of ∼10 A and a duration of 1.5–2 ns. A narrow asymmetric waveguide design is proposed as a way of implementing such a structure while maintaining good far-field properties of the single emitted mode.

Temperature Field Simulation of Nitride Hard Film Irradiated by High-Intensity Pulsed Ion Beam

Surface treatment of hard nitride film with high-intensity pulsed ion beam (HIPIB) was investigated in the present research. On considering the high energy density and short pulse duration of HIPIB source, a one-dimension physical model was built according to the structure feature of film-base sample. It was found that the irradiation of HIPIB lead to a very fast thermal recycle of heating rate 1011K/s and cooling rate up to 1010K/s. The highest temperature located at the surface of film irradiated. When using the HIPIB parameters of accelerating voltage 350kV, pulse duration 70ns and current density 60A/cm2, the surface layer of film would be melt with depth of about 0.35mm, that was verified by the experimental result along with the grain refinement effect due to the fast solidification process.

大能量窄脉宽激光脉冲受激布里渊散射输出波形分析

大能量窄脉宽激光脉冲受激布里渊散射输出波形分析

Enhancement of the Electrical Safe Operating Area of Integrated DMOS Transistors With Respect to High-Energy Short Duration Pulses

The influence of the source/body layout on the electrical safe operating area (SOA) of both vertical and lateral double-diffused metal-oxide-semiconductor transistors is experimentally investigated using a transmission line pulse system. Tradeoff between RDSon ·Area and SOA is discussed. Stripe geometries with and without (i.e., conventional design) body contact extension and cell designs with circular and oval geometries are considered. It is shown that a circular design of the source/body cell is superior with respect to SOA, compared with the conventional stripe design. The oval design is used to improve RDSon ·Area without compromising SOA performance, compared with the circular design.

Pulse shape measurements using single shot-frequency resolved optical gating for high energy (80 J) short pulse (600 fs) laser

Relevant to laser based electron/ion accelerations, a single shot second harmonic generation frequency resolved optical gating (FROG) system has been developed to characterize laser pulses (80 J, ∼600 fs) incident on and transmitted through nanofoil targets, employing relay imaging, spatial filter, and partially coated glass substrates to reduce spatial nonuniformity and B-integral. The device can be completely aligned without using a pulsed laser source. Variations of incident pulse shape were measured from durations of 613 fs (nearly symmetric shape) to 571 fs (asymmetric shape with pre- or postpulse). The FROG measurements are consistent with independent spectral and autocorrelation measurements.

High-energy soliton pulse generation with a passively mode-locked Er/Yb-doped multifilament-core fiber laser

We report the generation of high-energy short pulses from a mode-locked erbium/ytterbium-doped large-mode-area multifilament-core fiber laser operating in the purely anomalous dispersion regime. The self-starting fiber laser emits 400 mW of average output power at a pulse repetition rate of 44 MHz, corresponding to a pulse energy of 9.1 nJ. The laser produces near transform-limited output pulses with pulse duration of 1.6 ps, corresponding to 5 kW peak power. This new type of low-nonlinearity fibers demonstrates the power and energy scaling potential of fiber-based short pulse lasers in the eye-safe region.

Laser generated proton beam focusing and high temperature isochoric heating of solid matter

The results of laser-driven proton beam focusing and heating with a high energy (170J) short pulse are reported. Thin hemispherical aluminum shells are illuminated with the Gekko petawatt laser using 1μm light at intensities of ∼3×1018W∕cm2 and measured heating of thin Al slabs. The heating pattern is inferred by imaging visible and extreme-ultraviolet light Planckian emission from the rear surface. When Al slabs 100μm thick were placed at distances spanning the proton focus beam waist, the highest temperatures were produced at 0.94× the hemisphere radius beyond the equatorial plane. Isochoric heating temperatures reached 81eV in 15μm thick foils. The heating with a three-dimensional Monte Carlo model of proton transport with self-consistent heating and proton stopping in hot plasma was modeled.

Study of saturation of CR39 nuclear track detectors at high ion fluence and of associated artifact patterns

The occurrence of saturation in CR39 solid state nuclear track detectors has been systematically studied as a function of the incident ion (alpha particles and laser-accelerated protons) fluence and the etching time. When overexposed (i.e., for fluences above approximately 10(8) particles/cm(2)) and/or overetched, the CR39 detectors enter a saturated regime where direct track counting is not possible anymore. In this regime, optical measurements of saturated CR39 detectors become unreliable as well, since the optical response of the saturated detectors with respect to the ion fluence is highly nonlinear. This nonlinear optical response is likely due to scattering from the surface of irregular clumping patterns which have a diameter approximately 20 microm, i.e., ten times larger than the diameter of individual tracks. These patterns, which aggregate many individual tracks, are observed to develop in highly saturated regimes. For fluences typical of high energy short pulse laser experiments, saturation occurs, inducing the appearance of artifact ringlike structures. By careful microscopic analysis, these artifact ring patterns can be distinguished from the genuine rings occurring below saturation and characteristic of low energy laser accelerated proton beams.

Development of an in situ peak intensity measurement method for ultraintense single shot laser-plasma experiments at the Sandia Z petawatt facility

Using the physical process of ultraintense field ionization of high charge states of inert gas ions, we have developed a method of peak intensity measurement at the focus of high energy short pulse lasers operating in single shot mode. The technique involves detecting ionization products created from a low pressure gas target at the laser focus via time of flight detector. The observation of high ion charge states collected by the detector yields peak intensity at the focus when compared with the results obtained from well established tunnel ionization models. An initial peak intensity measurement of 5×1016Wcm−2 was obtained for a 1.053μm center wavelength, 0.4J pulse with 1ps pulse duration focused with an f∕5.5 off-axis parabola. Experiments with multijoule level, 500fs laser pulses are on the way.

Implantation of ions produced by the use of high power iodine laser

Abstract The iodine high power Prague Asterix Laser System (PALS), emitting radiation at 438 nm wavelength (3rd-harmonic of a fundamental radiation wavelength equal to 1315 nm), was employed to irradiate in vacuum different metallic targets (Cu, Ag and Ta). The high energy (up to 230 J) short (400 ps) laser pulses produce non-equilibrium plasma expanding mainly along the normal to the target surface. Plasma contains high charge state ions, with maximum charge states of 27 + , 36 + and 49 + for Cu, Ag and Ta, respectively. Time-of-flight (TOF) measurements, performed with the use of an electrostatic ion energy analyser (IEA) placed along the target normal, indicate that the maximum recorded ion kinetic energy is higher than 900 keV for Cu and Ag ions and than 5 MeV for Ta. The laser-produced ions have been implanted into different substrates (polymers, C, Al, Si and Ti) placed at different distances and angles with respect to the target normal. In order to investigate an implantation depth, a density profile of implanted ions and an implanted dose, the samples have been analysed by using the 1.7 MeV helium Rutherford backscattering spectrometry (RBS). The energies of the ions determined with the use of the RBS analysis are in a good agreement with the ion energies measured with the use of the IEA. The results are presented and discussed giving a special attention to the potential of the ion implantation method for modifying the chemical and physical properties of the implanted materials.

Generation of high-energy (0.3 μJ) short pulses (>400 ps) from a gain-switched laser diode stack with subnanosecond electrical pump pulses

Record pulse energy (E/spl sim/0.3 /spl mu/J) is reported for a gain-switched laser diode stack with subnanosecond electrical pump pulses. Pulsewidths near 100 ps are seen to be feasible. A detailed time-resolved analysis of emitted near-field patterns is presented, which provides guidelines for further improvements in time performances. A stack design for pulsed laser diodes of high energy is discussed.

Fundamental Limitations and Topological Considerations for Fast Discharge Homopolar Machines

Fusion research experiments require high energy short duration pulses. A homopolar machine, as an inertial energy storage system, offers an attractive source of energy meeting these requirements. The Energy Storage Group at The University of Texas at Austin has investigated the fundamental limitations to the discharge time of homopolar machines of various topological configurations. This paper presents a mathematical model for fast discharge homopolar machines. Based on this model, various machine configurations are analyzed. A new configuration - the spool type machine - is also discussed, criteria for the evaluation of different alternatives are presented, and it is concluded that highly efficient (≅ 95%), high energy (≅ gigajoules), fast-discharge (≅ 5 to 30 milliseconds) homopolar inertial energy storage systems are technically feasible. Brief reference is also made to some of the experimental results obtained from the existing laboratory models.