Longer Pulse Width Research Articles

Optimum Effect of Asymmetric Laser Pulse Shape on Relativistic Laser-Plasma Wake Field

Effects of asymmetric laser pulse shapes with longer pulse width in falling than in rising on relativistic laser-plasma wake fields is studied. It is found that there exist an optimum pulse width ratio of in falling to in rising which excite the maximum magnitude of wake field potential. An approximate power law for this optimum ratio is obtained numerically.

Recent advances in laser therapy for the treatment of cutaneous vascular disorders.

The use of lasers for the treatment of cutaneous vascular disorders has expanded rapidly since its inception in the early 1960s. The flashlamp-pumped pulsed tunable dye laser (PDL), which is the best laser system for the treatment of port-wine stains (PWS), is based on the premise of selective photothermolysis. Recent literature demonstrates enhanced selective photothermolysis by modifying the PDL to include a longer pulse width, a longer wavelength and higher energy fluences through the use of dynamic cooling. Multiple pulse laser irradiation, a new innovative method, shows promising preliminary results in eradicating hypertrophic PWS. In the treatment of capillary haemangiomas, previous uncontrolled studies suggest benefit with early PDL therapy but a recent randomised controlled trial does not support this claim. Mixed haemangiomas respond poorly to PDL treatment, uncomplicated tumours are best left to involute spontaneously whilst life/organ-threatening lesions require early active intervention. The efficacy of lasers in the treatment of telangiectatic leg veins (TLV) remains controversial. The literature revealed many conflicting claims. Larger and properly controlled studies are necessary to better define the role of lasers in the treatment of TLV.

Axi-symmetrization of radiation pattern and two wave heating of fundamental ECRH in GAMMA 10

In GAMMA 10, the radiation pattern of a heating wave of fundamental ECRH for potential formation was not axi-symmetric but vertically elongated on the resonance surface. The non-axisymmetric radiation pattern caused a non-axisymmetric potential profile and likely induced a radial loss. Then, the radiation pattern has been axi-symmetrized. Accordingly, an almost axi-symmetric potential profile has been obtained and a large density increase during ECRH has been observed in the central cell. This implies that radial losses associated with ECRH have been substantially reduced. A new antenna has recently been installed for additional plug electron heating (second ECRH). With this antenna, a larger power and/or a longer pulse width have become available. In a most recent experiment, we have succeeded in axial plugging with a duration up to 0.15 s by sequential use of the usual ECRH and the second ECRH. Moreover, there is an indication that the second ECRH is more desirable for further reduction of the radial loss.

Resonance in scattering and absorption from large noble gas clusters

Light scattering in large noble gas clusters irradiated by intense laser pulses was studied and compared to absorption measurements. The scattering signal shows the presence of a peak, when the pulse width was varied, similar to one previously reported in absorption measurements. The peak of the scattering, however, occurs at a longer pulse width than for absorption. This result disagrees with a simple simulation and may be due to propagation or non-linear effects not included in the model.

Optimisation of welding using two Nd–YAG laser beams combined at workpiece surface

Two Nd–YAG laser beams were combined at a certain point on the workpiece surface to increase weld penetration depth. One of the beams was a pulsed laser beam, and the other was a continuous wave laser beam or a modulated laser beam. Using this combination of laser beams, a wide range of welding conditions, such as average power, peak power, and power density, could be selected. A high peak power pulsed laser beam would play a significant role in forming a keyhole, but a severe spatter loss problem could be encountered under high peak power laser conditions, thus the conditions necessary to prevent spatter loss were investigated. The greatest penetration depth is obtained under the critical conditions for spatter loss. Critical conditions for spatter loss are controlled by the peak power of a pulsed laser beam, thus deeper weld penetration is obtained using a pulsed laser beam with higher average power, that is, of longer pulse width and/or a higher repetition rate within the limit of the oscillator output. Moreover, spatter loss is reduced under conditions providing large molten zones in the weld, thus a higher peak power pulsed laser beam can be employed under such conditions. Large molten zones are obtained using a modulated laser beam of a high average power and/or low welding speeds.

Ablation characteristics of Au, Ag, and Cu metals using a femtosecond Ti:sapphire laser

Femtosecond laser ablation of metallic bulk crystals of Au, Ag and Cu was experimentally studied with laser pulse widths ranging from 120 fs through 800 fs at a center wavelength of 780 nm for micro-machining applications. Two different ablation regimes were found in terms of the laser fluence. The characteristic length of different ablation regimes was explained in terms of the optical skin depth and thermal diffusion length; it was determined by the peak electron temperature in the two-temperature model. The lateral feature of the two ablation regimes is discriminated by the amount of particles accumulated by the evaporation process. Ablated particle was observed less in the lower fluence regime than in the higher fluence regime, but there was no significant difference on the ablated surface. The parameters used in the two-temperature model, are discussed in order to model the ultrashort pulsed laser ablation process theoretically. It is shown that the obtainable range of the lower fluence regime is enhanced with the shorter pulse lasers, because the ablation etch rate is decreased with longer pulse width.

Treatment of leg telangiectasia using a long-pulse dye laser at 595 nm

Pulsed dye lasers (PDL) operating at 585 nm wavelength and 0.45 msec pulsewidth offer effective treatment for port wine stains (PWS). Vessels in leg telangiectasias are larger than those in PWS. Longer pulsewidths and wavelengths may improve clearance of these larger vessels. Twenty patients were treated using PDL at 595 nm and 1.5 msec. Vessel diameters ranging from 0.635 to 1.067 mm were treated using energy densities of 15 and 18 J/cm2, and a 2 x 7 mm elliptical spot. Telangiectasia clearance and complications were scored at 6-weeks and 5-months following the single treatment. Results demonstrated > 50% clearance by 6 weeks in 11/26 (42.3%) patients using 15 J/cm2, and 6/13 (45.2%) using 18 J/cm2. By 5 months > 50% clearance was noted in 18/34 (53.0%) using 15 J/cm2, and 11/17 (64.7%) using 18 J/cm2. Complications were minor and infrequent. The long-pulse PDL may have a role in treating leg telangiectasias.

Control of Nucleation Process in CVD by Stratified Structural Change of High-Voltage Ultrashort Pulse Plasma.

Active control of gas phase and surface reactions in CVD processes of diamond thin films has been performed by use of a high-voltage and ultrashort pulse plasma. Effects of applying this pulse plasma on the nucleation density in CVD of diamond thin film have been examined by superimposing ultrashort high-voltage pulses on the usual DC plasma, and the nucleation density has been found to be significantly enhanced by this method. This effect is more marked at longer pulse widths. Optical measurements of the time-dependent change of the plasma structure show that the pulse plasma has a well-defined stratified structure with strong radical emission intensity in the positive column, where high-density radicals are being formed near the anode. This phenomen on results in the increase of instantaneous supersaturation of key radicals on the anode substrate and then the increase of nucleation density of diamond particles.

Responses of medullary raphe neurons to electrical and chemical activation of vagal afferent nerve fibers.

1. Various intensities, frequencies, and pulse widths of electrical stimulation of vagal afferent fibers were used to assess the responses of 87 medullary raphe neurons to vagal afferent fiber input in pentobarbital sodium-anesthetized, barodenervated paralyzed cats. Thirty-seven neurons were antidromically activated from the T2-T3 segments of the thoracic spinal cord, and 40 neurons could not be antidromically activated. Neurons were located in the nucleus raphe magnus (79%) and the nucleus raphe obscurus (15%). The remaining 6% of the neurons were not found; however, their locations were comparable in depth and position on the midline with other neurons in the same animals whose locations were identified. 2. The responses of 60 neurons to electrical stimulation of vagal afferent fibers were classified as excitatory (38%), inhibitory (24%), or mixed, (7%). The mixed responses were characterized by excitation at one frequency or intensity and inhibition at another frequency or intensity. The remaining 27 neurons did not clearly respond. 3. The excitatory responses to electrical stimulation of the cervical vagus nerve were intensity and frequency dependent. Inhibitory responses were frequency dependent at lower frequencies of stimulation and both frequency and intensity dependent at higher frequencies. The mixed responses were frequency dependent. Overall, longer pulse widths produced significantly greater responses than shorter pulse widths. 4. Thirty-three neurons were tested for responses to chemical stimulation of vagal afferents with intra-atrial injections of three doses of veratridine. Twenty-one percent were excited, 55% were inhibited, and 6% had mixed responses. For the mixed responses, excitation occurred at one dose and inhibition at another. The remaining 18% of the neurons were unresponsive to veratridine. The excitatory responses were dose dependent, but the inhibitory responses were not. Three doses of phenybiguanide (PBG) were also used to chemically activate vagal afferents in 27 neurons. Eleven percent were excited, 44% were inhibited, and 4% had mixed responses. The remaining 41% were unresponsive to PBG. The excitatory and inhibitory responses were dose dependent. 5. When comparing responses in projection and nonprojection neurons, inhibition was seen significantly more often in projection neurons and excitation in nonprojection neurons. Sixty-three percent of the neurons inhibited by electrical stimulation were raphespinal neurons, and 78% of the neurons excited by vagal stimulation were nonprojection neurons. Similar observations were made with the responses to chemical activation of the vagus. 6. Neurons with lower spontaneous discharge rates were more often excited by vagal stimulation and neurons with higher rates were more often inhibited.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of rise time and pulse width on the transport and interaction of a 1 MeV, 15 kA relativistic electron beam in low pressure argon and hydrogen

A numerical study of the effect of rise time (10–30 nsec) and pulse width (30–100 nsec) on the transport and interaction of a 1 MeV, 15 kA relativistic electron beam (REB) in argon and hydrogen in the pressure range 0.05–1.0 Torr is reported. The computations have been carried out taking into account charge and current neutralization effects, plasma heating by return current, and two-stream instabilities. The results of the study show that charge transport increases for short rise times and longer pulse widths at the optimum pressure in the range 0.1–0.2 Torr. With a rise time of 30 nsec and a pulse width of 100 nsec, the 1 MeV, 15 kA REB generates a plasma with density 1–10×1021 m−3, an electron temperature of 4–20 eV, and transports with 90% efficiency at the optimum pressure.

Ultraviolet-induced photochemical damage in ocular tissues.

Exposure to ultraviolet (UV) lasers may result in pathology to either the cornea, lens or retina of the primate eye. The particular combination of exposure parameters (wavelength, peak power, pulsewidth, pulse repetition rate, and total energy delivered) determines the primary target tissue(s) in each instance. The effect may be acute or chronic, and the implicated damage mechanism may be categorized as photochemical, photoablative or thermal. This paper summarizes a number of specific cases where UV laser radiation affected one or more of the ocular tissues, describes the nature of the pathologies, and indicates what is known about the damage mechanism in each case. Ranges of exposure parameters where each ocular tissue is the most sensitive are defined. The cornea is most sensitive via a photochemical damge mechanism in the 260- to 280-nm range, where the threshold dose is approximately 5 mJ cm-2. For near-UV wavelengths (320-400 nm), different target molecules absorb the radiation and are susceptible to less efficient photochemical damage mechanism yielding corneal thresholds in the range of 10-100 J cm-2. However, we report acute cataract induction following exposure to a 337-nm nitrogen laser at 1 J cm-2 when the energy is delivered in a 10-ns pulse. Further, with longer pulsewidths (approximately 1 s) of comparable wavelength, retinal lesions were induced when 0.28 J was delivered to the eye. The data suggest that the acute lens effect is the result of a thermal mechanism, whereas the UV-induced retinal lesions result from a photochemical insult to the photoreceptors. Data presented include the action spectra for far- and near-UV-induced ocular damage, the pulsewidth and total energy dependencies of ocular thresholds, cumulative effects of repeated exposures, and repair or recovery rates.

Effect of Dye Laser Pulse Duration on Selective Cutaneous Vascular Injury

The pulsed dye laser at 577 nm, a wavelength well absorbed by oxyhemoglobin, causes highly selective thermal injury to cutaneous blood vessels. Confinement of thermal damage to microvessels is, in theory, related to the laser exposure time (pulsewidth) on selective vascular injury. This study investigates the effect of 577 nm dye laser pulsewidth on selective vascular injury. Nine Caucasian, normal volunteers received 577 nm dye laser exposures at pulsewidths of 1.5-350 microseconds to their skin. Clinical purpura threshold exposure doses were determined in each volunteer, and biopsies of threshold and suprathreshold doses were examined in each volunteer. The laser exposure dose required to produce purpura increased as pulsewidth increased in all 9 subjects (p less than 0.001). This finding corresponds to laser pulsewidths equal to or exceeding the thermal relaxation times for dermal blood vessels. Histologically, vessel damage was selectively, but qualitatively, different for short vs long pulsewidths. Pulsewidths shorter than 20 microseconds caused vessel wall fragmentation and hemorrhage, whereas longer pulsewidths caused no significant hemorrhage. The purpura noted clinically appears to be due to a coagulum of intralumenal denatured erythrocytes. At 24 h, there was marked vessel wall necrosis at all pulsewidths. The short pulsewidths may cause erythrocyte vaporization, rapid thermal expansion, and mechanical vessel rupture with hemorrhage. Long pulsewidths appear to cause thermal denaturation with less mechanical vessel damage. The selective, nonhemorrhagic, vascular necrosis caused by the long-pulsewidth dye laser may lead to a more desirable clinical outcome in the therapy of blood vessel disease processes.

Upper-limit speed of a moving object in front-surface holography using a Q-switched ruby laser

A Q-switched ruby laser, used for a light source in high-speed holography, emits light whose frequency chirps from the lower frequency side to the higher frequency side. The upper-limit speed of a moving object in high-speed holography is discussed taking this frequency chirp into consideration. Large frequency chirp rate, longer pulse width, and the introduction of an appropriate path difference between the object and reference beams permit us to increase the allowable speed of the moving object. However, the attainable speed is limited because of definite temporal overlap of the object and reference pulse beams.

Instantaneous prediction of ionospheric transmission circuits by the communication Zone Indicator ("COZI")

Detection of long-distance echoes by an oblique-incidence hf sounder with directive antenna provides a novel technique for predicting ionospheric transmission conditions to remote points. Measurements on oscilloscopic echo patterns can be correlated with (a) field strength of remote cw stations, (b) response of remote beacons synchronously triggered by the sounder, or (c) predictions based upon CRPL-Signal Corps world-wide contours of ionospheric layer heights and muf s. The results indicate that most of the echoes are due to energy backscattered at the earth, lonospherically reflected en route. Further, the transmission time (slant range) to the leading edge of the backscatter pattern measures reasonably well the skip distance (ground range) for long distances. Thus the backscatter echoes may be attributed to a ground area near and somewhat beyond skip distance to which communication is possible. A PPI echo display with a rotating directive antenna reveals in a striking manner the azimuthal variation of skip distance, giving at a glance the possible distant conmunication zones. Transmission modes, such as F-layer and sporadic-E layer reflections are readily discernible. Diurnal variation of the communication zones may be determined by comparing PPI photographs taken throughout the day. Results, recently declassified by the government, are given in a series of PPI photographs and corresponding azimuthal maps, commencing more than four years ago. Early equipment consisted of 20- kw peak power pulsers, with nominal lO0-microsecond pulse width and 20 per second repetition rate. Horizontal rhombics and multielement rotatable Yagi antennas were employed. Later, longer pulse widths up to 2 milliseconds and peak powers less than I kw were employed with results equally useful for many applications. Backscatter patterns have been observed on 9, 12, 16, and 22 mc. The first demonstration of the PPI technique is shown in a nighttime photograph taken at S . Dartmouth, Mass. on May 6, 1948 at 16 mc. A typical diurnal variation of PPI patterns is shown in a series of photographs taken at 15-minute intervals, at 22 mc, on January 19, 1950. Effect of different frequencies is shown in a series of PPI photographs taken at 12, 16, and 22 inc. F-layer and sporadic-E modes are noted. Suggested applications of the COZI technique includes assisting communication services in better circuit utilization, for implementing ionospheric prediction services, and as a new tool for ionospheric research.