Function Of Pulse Repetition Rate Research Articles

Microvessel rupture induced by high-intensity therapeutic ultrasound\u2014a study of parameter sensitivity in a simple in vivo model

BackgroundSafety analyses of transcranial therapeutic ultrasound procedures require knowledge of the dependence of the rupture probability and rupture time upon sonication parameters. As previous vessel-rupture studies have concentrated on a specific set of exposure conditions, there is a need for more comprehensive parametric studies.MethodsProbability of rupture and rupture times were measured by exposing the large blood vessel of a live earthworm to high-intensity focused ultrasound pulse trains of various characteristics. Pressures generated by the ultrasound transducers were estimated through numerical solutions to the KZK (Khokhlov-Zabolotskaya-Kuznetsov) equation. Three ultrasound frequencies (1.1, 2.5, and 3.3 MHz) were considered, as were three pulse repetition frequencies (1, 3, and 10 Hz), and two duty factors (0.0001, 0.001). The pressures produced ranged from 4 to 18 MPa. Exposures of up to 10 min in duration were employed. Trials were repeated an average of 11 times.ResultsNo trends as a function of pulse repetition rate were identifiable, for either probability of rupture or rupture time. Rupture time was found to be a strong function of duty factor at the lower pressures; at 1.1 MHz the rupture time was an order of magnitude lower for the 0.001 duty factor than the 0.0001. At moderate pressures, the difference between the duty factors was less, and there was essentially no difference between duty factors at the highest pressure. Probability of rupture was not found to be a strong function of duty factor. Rupture thresholds were about 4 MPa for the 1.1 MHz frequency, 7 MPa at 3.3 MHz, and 11 MPa for the 2.5 MHz, though the pressure value at 2.5 MHz frequency will likely be reduced when steep-angle corrections are accounted for in the KZK model used to estimate pressures. Mechanical index provided a better collapse of the data (less separation of the curves pertaining to the different frequencies) than peak negative pressure, for both probability of rupture and rupture time.ConclusionThe results provide a database with which investigations in more complex animal models can be compared, potentially establishing trends by which bioeffects in human vessels can be estimated.

Preparation and characterization of zirconium films for first mirror application in fusion devices

Abstract The nanostructured zirconium (Zr) films deposited on Al 2 O 3 substrate are obtained by pulsed laser deposition (PLD) for the application of first mirror. Structural features, optical properties and surface morphologies of as-grown Zr films are systematically investigated as a function of pulse repetition rate. It is found that the Zr films show a typical hexagonal close packed structure and all deposited films exhibit a very smooth surface. There are no voids and folds on the surface of Zr films. The root mean square roughness (RMS) values increase with increasing pulse repetition rate. The variation of pulse repetition rate has no obvious effects on the reflectivity because of the smooth film surface. Up to the wavelength of 800 nm, the reflectivity is higher than 70%, which is excellent for the application of first mirror.

Development of CZTS thin films solar cells by pulsed laser deposition: Influence of pulse repetition rate

► Development of CZTS thin film solar cell by PLD . ► Structural optical compositional and morphology properties of CZTS thin films. ► Influence of pulse repetition rate on physicochemical properties CZTS thin films. High-quality Cu 2 ZnSnS 4 (CZTS) thin films were synthesized by pulsed laser deposition as a function of pulse repetition rate onto the SLG substrates. Influence of pulse repetition rate onto the structural, morphological, compositional and optical properties have been investigated for as-deposited and annealed thin films. X-ray diffraction study shows transformation of amorphous to crystalline phase after tuning pulse repetition rate and annealing of samples. FESEM images of thin films show increase in grain size upon annealing. Films are nearly stoichiometric deposited at 10 Hz repetition rate has been confirmed with the help of EDAX and XPS analysis. The direct band gap energy of the deposited CZTS thin films are in the solar energy range. The performance of solar cell based on CZTS absorber layer has been tested and the efficiency is about 2%.

Absolute and relative density measurements in a N2∕Ar dielectric barrier discharge by diode-laser absorption spectroscopy and resolved plasma emission

The population density of metastable Ar 4s′[12]° in a 70% Ar∕30% N2, short-pulsed (<15-ns full width at half maximum) dielectric barrier discharge was determined from the time-resolved diode-laser absorption as a function of pulse repetition rate for a gas pressure of 100Torr. The relative population density change of N2+ BΣu+2 and N2 CΠu3 was also determined from N2+ first negative and N2 second positive plasma emission for the same pulse repetition rates and pressure. The net power deposited per pulse was obtained from measurements of the voltage and current wave forms. The fractional energy dissipated in metastable Ar production was estimated from the power and absorption number density measurements to be roughly 20% for pulse repetition rates ⩽10kHz, decreasing to ∼10% at 30kHz for 3.5-kV applied voltage and 100Torr.

Laser-induced damage studies in GaAs

Laser-induced damage studies have been carried out on monocrystal GaAs at 1.06μm wavelength as a function of pulse repetition rate in the nanosecond regime. The single shot observed laser damage threshold is 0.9J/cm2. It has been found that the damage threshold decreases when the sample is irradiated with large number of pulses. However, the above effect is observed only when the repetition rate is higher than 1Hz. Various laser damage mechanism theories have been discussed to explain the results.

Photoablation of Gelatin With the Free-Electron Laser Between 2.7 and 6.7 µm

Photoablation in the infrared (IR) is an option for future refractive and corneal surgery; its basic principles have not yet been investigated systematically. For the first time, the free electron laser allows the dynamic study of photoablation over a wide range of wavelengths with variable combinations of pulselength and energy. The goal of this study is to use the free electron laser as a tool to describe photoablation in the IR quantitatively. We studied the function of wavelength as it is related to target material spectroscopy and the effects of corneal hydration and the pulse repetition rate. Surface absorption spectroscopy of the human cornea and of gelatin as a proven model of the cornea was performed between 2.7 and 6.7 microns. Gelatin probes of well-defined thickness (140 +/- 5 microns) and controlled hydration (wet/dry weight 1 to 4.5) served as target material. Photoablation was performed with the Vanderbilt University free electron laser (Nashville, Tenn) in September 1992 at a fluence of 1.27 J/cm2, and a macropulse of 4 microseconds, composed of 2 ps micropulses at a 2.9 GHz pulse repetition rate. Wavelength was tunable between 2.7 and 6.7 microns at stable beam profiles. Ablation experiments were performed as a function of energy, hydration, and pulse repetition rate. Ablation rates were assessed by a) perforation experiments, and b) direct measurements using confocal laser topometry (UBM, Ettlingen, FRG). Ablation rate, assessed by perforation experiments and topometry, correlated well with the corresponding measured absorbencies of the target material: maximal ablation rate at maximal target absorption, around the 3- and 6-micrometer water absorption bands. The ablation threshold at 6.2 microns was 0.7 +/- 0.05 J/cm2 (perforation) and 0.55 +/- 0.08 J/cm2 for depth measurements. Ablation rate as a function of hydration increased to 2.3 (wet/dry weight) with a decrease for higher hydrations. Ablation rate as a function of the pulse repetition rate showed an increase of up to 20 Hz, where it was found to be 60% higher. The first systematic use of free electron laser technology positively correlated ablation efficiency with target material absorption, thus identifying a "new" promising wavelength at around 6.2 microns for materials with a high water content such as corneal tissue.

Performance model for optical extraction from a Q-switched chemical oxygen–iodine laser

A comprehensive time-dependent model for optical extraction from a Q-switched supersonic oxygen-iodine laser is described. Gas flow is treated by using an unsteady, premixed, quasi-one-dimensional model that accounts for gas motion and expansion as well as heat release in the cavity. The model uses a simplified, temperature-dependent, chemical kinetics package that consists of several reactions among the (3)Sigma, (1)Delta, and (1)Sigma states of oxygen, atomic and molecular iodine, water, and helium. Hyperfine relaxation effects on the gain and optical extraction from the 3 ? 4 line are treated by using a simple four-level laser model. An efficient algorithm for solving the coupled medium and optical extraction equations is described. This gain model, in conjunction with a geometric model of an unstable confocal resonator, is used to examine optical extraction from the medium as a function of pulse repetition rate, duty cycle, and optical mode width. It is shown that the output power may exhibit flow-induced relaxation oscillat ons, and the conditions under which these oscillations occur are discussed.

Q-switched thulium-doped fiber laser

A tunable Q-switched thulium-doped single-mode silica fiber laser is presented for the first time. Its performance is analyzed as a function of pulse repetition rate and pumping power. The optimization of laser parameters resulted in 4 W/1 30 ns, 1 .92-jrm wavelength pulses generated at a 4-kHz repetition rate. From the measured values of small- signal attenuation and intrinsic saturation power, the thulium ion param- eters at the pump and signal wavelength are derived and used in the- oretical modeling of the laser. The moderate power required from the Ti:sapphire pump laser used in the experiment suggests that similar re- suIts can be obtained using a laser diode pump.

Sensitivity of neurons in the auditory midbrain of the grassfrog to temporal characteristics of sound. I. Stimulation with acoustic clicks

The coding of fine-temporal structure of sound, especially pulse repetition rate, was investigated on the single-unit level in the auditory midbrain of the grassfrog. As stimuli periodic click trains and Poisson distributed click ensembles have been used. The response to periodic click trains was studied in two aspects, focussing on two types of possible codes: a rate code and a synchrony code. From the iso-intensity rate histogram five basic average response rate characteristics as function of pulse repetition rate have been established: low-pass, band-pass, high-pass, bimodal and non-selective unit types. The synchronization capability, expressed in a synchronization index, was for a small majority of units non-significant and a low-pass function of pulse repetition rate for most of the other units. The rate code showed the largest diversity of response types and an enhanced selectivity to pulse repetition rate. The stimulus-response relation to Poisson distributed click ensembles was investigated by a non-linear system theoretical approach. On the basis of first- and second-order Poisson kernels possible neural mechanisms accounting for temporal selectivity were determined. A considerable fraction of units exhibited response characteristics that were invariant to changes in sound pressure level and average click rate. These units may function as feature detectors of fine-temporal structure of sound. The spectro-temporal sensitivity range of the auditory midbrain of the grassfrog appeared to be broad and not particularly tuned to the ensemble of conspecific cells.

TEMPORAL SUMMATION IN VIBROTACTILE SENSITIVITY.

The Zwislocki theory of temporal summation was tested for vibrotactile sensitivity. Vibrotactile thresholds were determined as a function of pulse repetition rate and pulse number for short pulses. Thresholds for various burst durations of sinusoidal signals were also determined. The results indicate that the theory is sufficient to predict vibrotactile thresholds for the temporal patterns used. An earlier finding, which suggested that glabrous skin contains at least two populations of mechanoreceptors, was confirmed by using contactors of varying size.

HUMAN INFORMATION TRANSMISSION WITH SEQUENCES OF SOUND PULSES AT DIFFERENT RATES

The ability of a human operator to insert information into a machine by the use of a binary key was studied. The function of the binary key was to generate sequences of regularly-occurring sound pulses as long as it was pressed. The study was divided into two component parts: 1. The ability of the human operator to count the number of pulses in sequences of regularly-occurring pulses of 1000 c.p.s. tone. 1. The ability of the human operator to use a binary key to generate desired sequences of regularly-occurring sound pulses. Both of these abilities were studied as a function of pulse repetition rate and the number of pulses in a sequence. From a comparison of the rates of information transmission using a binary key and using other means, it is concluded that the use of a binary key of the type described is a relatively inefficient way for the untrained human operator to insert information into a machine.