High-speed Streak Research Articles

Wavelet pattern and self-sustained mechanism of gaseous detonation rotating in a coaxial cylinder

Gaseous detonation propagating in a coaxial cylinder was studied for hydrogen/oxygen/nitrogen mixtures numerically and experimentally. The numerical method used was based on the three-dimensional Euler equations with detailed finite-rate chemistry. Smoked foil measurement was performed to record the cellular structure of the detonation propagation. The results show that the calculated streak picture is in qualitative agreement with the picture recorded by a high speed streak camera from published literature. The three-dimensional flow field induced by the continuously rotating detonation is visualized and the distinctive features of the rotating detonations are clearly depicted. Due to the curvature of the annular tube, the size of cellular pattern along the concave wall is smaller than that along the convex wall. This implies that the detonation wave near the concave wall is convergent and therefore is stronger than that near the divergent convex wall. Also, the numerical simulations show three-dimensional rotating detonation structures, where display the special combined detonation.

The influence of residual charge on surface discharge propagation

The influence of residual charge on propagation of surface discharge is experimentally investigated using high-speed framing and streak cameras, and an electrostatic probe. When consecutive impulse voltages are applied 25 times with a change in polarity, the propagation length of the surface discharge increases gradually from 79 to 164 mm and hardly converges. Under such a condition, the potential gradient in the surface leader channel decreases with the consecutive number of impulses, while that in the surface streamer remains constant and the value is 0.5–0.6 kV mm−1. With the residual charge of a previous opposite polarity discharge on an insulator, the propagation velocity increases to three to eight times as large as that of the surface discharge on a clear insulator without any residual charge. The peak current of the surface discharge with a residual charge also becomes much higher than that without a residual charge.

Late-Stage Vortical Structures and Eddy Motions in a Transitional Boundary Layer

A high-order direct numerical simulation of flow transition over a flat-plate at a free stream Mach number 0.5 is carried out. Formation and development of three-dimensional vortical structures, typically shown as A-vortices, hairpin vortices and ring-like vortices, are observed. Numerical results show that there is a strong downdraft motion of fluid excited by every ring-like vortex in the late-stage of the transition process. At two sides of the vortical structure centerline, the downdraft motions induced by the ring-like vortex and the rotating legs superimpose. This is responsible for the appearance of a high-speed streak associated with the positive spike observed in a previous investigation and the appearance of a high-shear layer in the near wall region.

Intensity correlation measurement system by picosecond single shot soft x-ray laser

We developed a new soft x-ray speckle intensity correlation spectroscopy system by use of a single shot high brilliant plasma soft x-ray laser. The plasma soft x-ray laser is characterized by several picoseconds in pulse width, more than 90% special coherence, and 10(11) soft x-ray photons within a single pulse. We developed a Michelson type delay pulse generator using a soft x-ray beam splitter to measure the intensity correlation of x-ray speckles from materials and succeeded in generating double coherent x-ray pulses with picosecond delay times. Moreover, we employed a high-speed soft x-ray streak camera for the picosecond time-resolved measurement of x-ray speckles caused by double coherent x-ray pulse illumination. We performed the x-ray speckle intensity correlation measurements for probing the relaxation phenomena of polarizations in polarization clusters in the paraelectric phase of the ferroelectric material BaTiO(3) near its Curie temperature and verified its performance.

Opening Electrical Contacts: The Transition from the Molten Metal Bridge to the Electric Arc

This paper presents a comprehensive explanation of the formation of the electric arc between opening contacts in a current carrying electric circuit. As the contacts begin to open a molten metal bridge forms between them. The rupture of this bridge and the initial formation of the electric arc are studied in both atmospheric air and vacuum using experiments to determine the direction of metal transfer between the contacts as a function of time after the rupture of the molten metal bridge. High speed streak photography is also used to show the rupture of the molten metal bridge and the initial formation of the electric arc. Analysis of these data show that a very high-pressure, high-temperature metal vapor zone exists between the contacts after the rupture of the molten metal bridge. Under this condition a pseudo-arc forms where current is carried by metal ions and an anomalous, high net transfer of metal to the cathodic contact occurs. The pressure in this region decreases rapidly and there is a transition to the usual electric arc, which still operates in the metal vapor. In this arc the current is now mostly carried by electrons. The data shows that there is still a net transfer of metal to the cathode, but now its volume is a function of the arcing time.

Detecting the ejection of particles from the free surface of a shock-loaded sample

The details of the ejection of 20- to 200-µm particles with a velocity of 1.0-1.5 km/s from the surfaces of lead and steel samples with a roughness of 5-40 µm (Rz 5-Rz 40) when shock waves with an amplitude of 15 and 27 GPa reach them are visualized with a high-speed streak camera with a CCD matrix and pulsed laser illumination at a pulse duration of 4 ns. The size and velocity distributions of the particles are obtained.

A versatile high-resolution x-ray imager (HRXI) for laser-plasma experiments on OMEGA

A high-resolution x-ray imager (HRXI) devoted to laser-plasma experiments combines two state-of-the-art technologies developed in France: a high-resolution x-ray microscope and a high-speed x-ray streak camera. The resulting streaked imager achieves spatial and temporal resolutions of approximately 5 microm and approximately 10 ps, respectively. The HXRI has recorded enhanced spatial and temporal resolution radiographs of indirectly driven targets on OMEGA. This paper describes the main features of the instrument and details the activation process on OMEGA (particularly the alignment). Recent results obtained on joint CEA/LLE radiographic OMEGA experiments will also be presented.

A novel compact high speed x-ray streak camera (invited)

Conventional in-line high speed streak cameras have fundamental issues when their performance is extended below a picosecond. The transit time spread caused by both the spread in the photoelectron (PE) "birth" energy and space charge effects causes significant electron pulse broadening along the axis of the streak camera and limits the time resolution. Also it is difficult to generate a sufficiently large sweep speed. This paper describes a new instrument in which the extraction electrostatic field at the photocathode increases with time, converting time to PE energy. A uniform magnetic field is used to measure the PE energy, and thus time, and also focuses in one dimension. Design calculations are presented for the factors limiting the time resolution. With our design, subpicosecond resolution with high dynamic range is expected.

Experimental study on large-scale streak structure in the core region of turbulent plane Couette flow

A physical experiment confirmed the existence of a large-scale streak structure in the core region of turbulent Couette flow at Re=Uwh∕ν=3750. In the low- or high-speed streak, large-scale wavy-like structures exist, the streamwise and spanwise wavelength of which are (40−65)h and (4−5)h. The low (high) -speed streak pattern is a sequence of wavy and nonwavy (tilted line or irregular) patterns. The large-scale structure is reconstructed using the wavelet components of the streamwise fluctuating velocity that have high energy on a large scale. The reconstructed flow field shows the existence of large-scale structures with a tilted pattern. The tilting angle is ±(3°−5°) against the flow direction. In some places, they constitute a weakly wavy pattern and in other places a simple tilted pattern. The weakly wavy structures appear intermittently in space along the streamwise direction. The rough estimation of the streamwise separation between large-scale structures is 300h.

Streak interactions and breakdown in boundary layer flows

The objective of this paper is to show that the interaction of streamwise velocity streaks of finite length can lead to turbulent breakdown in the flat-plate boundary layer flow. The work is motivated by previous numerical and experimental studies of transitional flows where the high-frequency oscillations leading to turbulence are seen to form in the region of strongest shear induced by streaks in relative motion. Therefore, a model for the interaction of steady and unsteady (i.e., slowly moving in the spanwise direction) spanwise periodic streaks is proposed. The interaction of two subsequent streaks is investigated for varying collision parameters. In particular, the relative spanwise position and angle are considered. The results show that the interaction is able to produce both a symmetric and asymmetric breakdown without the need for additional random noise from the main stream. Velocity structures characteristic of both scenarios are analyzed. Hairpin and Λ vortices are found in the case of symmetric collision between a low-speed region and an incoming high-speed streak, when a region of strong wall-normal shear is induced. Alternatively, when the incoming high-momentum fluid is misaligned with the low-speed streak in front, single quasi-streamwise vortices are identified. Despite the different symmetry at the breakdown, the detrimental interaction involves for both cases the tail of a low-speed region and the head of a high-speed streak. Further, the breakdown appears in both scenarios as an instability of three-dimensional shear layers formed between the two streaks. The streak interaction scenario is suggested to be of relevance for turbulence production in wall-bounded flows.

Dynamic properties and optical phase conjugation of two-photon pumped ultrashort blue stimulated emission in a chromophore solution

The dynamic properties of two-photon pumped blue lasing $(\ensuremath{\sim}470\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ in the solution of an organic chromophore [2-acetyl-6-(dimethylamino)naphthalene], excited by $\ensuremath{\sim}160\text{\ensuremath{-}}\mathrm{fs}$ laser pulses at $\ensuremath{\sim}775\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, have been studied. Both the forward and backward stimulated emission are enhanced by feedback from the reflection at the two optical windows of the solution filled cuvette. Under current experimental conditions, the lasing wavelengths in the forward and backward directions were almost the same, but both blueshifted compared to the fluorescence peak wavelength of the sample solution. The temporal behavior of the lasing output was recorded by a high-speed streak camera system. The multipulse structure and spectral properties of the output lasing are semiquantitatively explained. In addition, excellent optical phase-conjugation properties of the backward stimulated emission were observed; the aberration influences from an aberrator on the backward lasing beam were automatically removed.

Diagnostic system to measure spatial and temporal profiles of shock front using compact two-stage light-gas gun and line reflection method

A diagnostic system has been developed to obtain spatial and temporal profiles of shock front. A two-stage light-gas gun is used to accelerate impactors in velocity range with 4-9 km/s. The system consists of the Faraday-type electromagnetic sensors to measure impactor velocity, optical system with high-speed streak camera to measure shock-wave velocities, and the delay trigger system with self-adjustable pre-event pulse generator. We describe the specifications and performance of this system and data-analysis technique on the tilt and distortion of the shock front. Finally, we obtained the Hugoniot data of copper for system demonstration.

Streak camera observation of the crystallization on multinary compounds

Tokyo University of Agriculture and Technology, 2-24-16, Nakamachi, koganei, Tokyo 184-8588, Japan Received 24 February 2006, revised 10 April 2006, accepted 9 June 2006 Published online 4 September 2006 PACS 42.82.Bq, 68.08.De A dynamic observation system for high speed crystallization has been developed using blue LD, high speed LD driver, streak camera and precise timing controller. We succeeded in dynamic observation of the crystal growth process of phase change materials induced by laser irradiation. The time-sequential de-velopment of crystallized region differs clearly between a eutectic composition Sb

Development of Time Resolved Streak Camera for the Observation of Lightning Discharge

A new type of high-speed streak camera was developed to observe the process of lightning discharge. The camera consists of a rotating quadrate pyramid mirror and a digital video camera. Using the camera we can observe the lightning discharge in all horizontal direction and continuous observation is available without using high-speed shutter synchronizing with lightning discharge. The visual range, the time resolution and the dead time of the camera were evaluated theoretically and experimentally. From the evaluation we found that the time resolution of less than ten μs is expected at maximum rotating speed of the mirror of 200s-1. The percentage of dead time was evaluated to be less than 50%. By using the camera, we have successfully observed the lightning discharges and from the observation we have evaluated the shape, and the duration of each lightning discharge.

Hydrodynamics of high-energy GARPUN KrF laser interaction with solid and thin-film targets in ambient air

Hydrodynamic regimes of KrF laser interaction with solid and thin-film targets in air at atmospheric and reduced pressures were investigated at the high-energy GARPUN facility. These experiments were performed with 100 J, 100 ns laser pulses in planar focusing geometry and compared with numerical simulations with the ATLANT code to verify the concept of the laser-driven shock tube (LST). Strong shock waves (SWs) are produced in the LST and the gas is accelerated to hypersonic velocity due to the deposition of laser energy. The laser beam is focused by a prism raster optical system that provides a very uniform intensity distribution at moderate laser intensities q ⩽ 1 GW cm−2 over a square spot of ∼1 cm size. Dynamics of laser-produced plasma and SWs in a surrounding gas were investigated by means of a high-speed photo-chronograph and streak camera in combination with shadow or schlieren techniques, and time and space resolved spectroscopy in the visible spectral range. Both experiments and simulations confirmed that target evaporation and blow-up of expanding plasma are the main mechanisms of UV laser–target interaction in the surrounding gas. Planar SWs with velocities up to 7 km s−1 towards the laser beam were observed in normal-density air and up to 30 km s−1 in rarefied air. Acceleration of thin CH films of 1–50 µm thickness was investigated both in free-expansion and plasma-confined regimes with the highest achieved velocities being up to 4 km s−1. The SW damping law in free space, independent of laser intensity and air pressure, could be approximated by a power law x ∼ tn with power indices n1 = 0.85–0.95 at the initial stage and n2 = 0.5–0.6 later, when the distance of the SW front from the target became comparable with the size of the irradiated spot. Instability growth at contact interfaces between ablative plasma and accelerated film, as well as between plasma and compressed air were observed, and compared for various initial irradiation non-uniformities. These non-uniformities were introduced by a grid, which was placed in front of the film target.

Nanosecond streak photography of discharge initiation on ice surfaces

This paper presents an exploratory experimental study of discharge initiation on surface of ice. The phenomena underlying these processes are, as yet, poorly understood. To elucidate the initiation process in particular, ultra high-speed streak photography was used to observe and analyze the first visible discharges and a number of parameters were investigated. It could be shown that ambient temperature, conductivity of ice surface and the presence of imperfections on the ice surface influence not only the critical voltage for discharge initiation, but also the velocity of discharge development. Some parameters could be identified, which should be investigated further. The present paper is the first stage of a long-term research effort in this new field. This study should enable further experimental investigations and mathematical simulations to gain better understanding of ice surface discharges, specifically in the context of electrical insulation integrity of support insulators in power systems operating under severe atmospheric conditions.

Measurement of temporal durations of transient collisional excitation X-ray lasers

We have measured temporal durations of Ni-like transient collisional excitation X-ray lasers (4d→4p, J=0→1) using a high-speed X-ray streak camera coupled with a flat field grating spectrometer. The pumping laser pulse consisted of a 400-ps prepulse and a 4.8-ps main pulse derived from a Nd:glass laser system with chirped pulse amplification. A quasi-traveling-wave pumping using a six-step mirror was adopted to synchronize the X-ray laser pulse and the onset of gain in a plasma medium. The X-ray laser outputs from three different materials, silver (Z=47), palladium (Z=46) and molybdenum (Z=42), showed that the X-ray laser duration became shorter as the atomic number decreased.

Experimental study and analysis of corona discharge parameters on an ice surface

The objective of this investigation is to understand the inception of ice surface corona streamers. A validated onset criterion that is useful for practical electrode geometries is presented. High-speed streak photography techniques were used to observe the streamers propagating over an ice surface. The effects of several experimental parameters such as freezing water conductivity and HV electrode radius on the streamer inception parameters of an ice surface have been examined. Empirical models have been proposed, based on elements derived from the inception parameters to account for corona streamer propagation velocity and inception voltage/field on an ice surface. Since the propagation velocity is a power function of the electric field, its value has been expressed in terms of the inception voltage.

A laser-accelerated flyer system

A system has been developed for the launch of miniature flyer plates, achieving velocities approaching 10 km/s. Laser-induced plasma is used to drive these flyers which are typically less than 1 mm in diameter and a few μm thick. High-speed streak photography was used to measure the key parameters of the flyers as a function of launch energy, mass and distance travelled. A fibre-optic beam shaping system has proved highly effective at manipulating the shape of the flyers and excellent reproducibility is found. Furthermore, a technique based on bi-layered films was found to significantly increase the performance of the system.

Plume dynamics by thin film carbon target by YAG laser

Plume dynamics induced by the ablation of thin carbon film of 200 nm deposited on a polymethylmethacrylate (PMMA) substrate has been studied. Laser beam was focused from rear side through PMMA substrate. We call this special target ‘thin film target’. Thin film target is found to achieve high-energy density states by absorbing the pulse laser energy. An Nd:YAG laser of infrared wavelength is used in this study due to the efficient absorption of energy by the ablated plasma plume. Ablation emission was observed by the high-speed streak camera. Plume is shown to be induced at the carbon film surface and also at the opposite PMMA surface. Several velocity groups are recognized in the plume. Stress waves in PMMA plate placed close to the thin film target was observed by the pulse laser shadowgraphy.