Streak Camera Images Research Articles

Design and first tests of the trapped electrons experiment T-REX.

Gyrotrons are essential for electron cyclotron resonance heating in fusion reactors, making efficient operation crucial for advancing fusion energy. Past experiments revealed instability issues due to trapped electrons in the magnetron injection gun (MIG) region, causing undesired currents and operational failures. To address this, tight manufacturing tolerances are required for the MIG geometry [Pagonakis etal., Phys. Plasmas 23, 023105 (2016)]. We present the initial findings of the trapped electrons experiment developed at the Swiss Plasma Center, designed to understand the physics of electron clouds in gyrotron MIGs. T-REX replicates MIG geometries, as well as their typical electric and magnetic fields, and it is supported by 2D particle-in-cell simulations with the FENNECS code [Le Bars etal., Phys. Plasmas 29, 082105 (2022); Le Bars, Ph.D. thesis, EPFL, Lausanne, 2023]. The setup includes two coaxial electrodes in a vacuum chamber atop a superconducting magnet, with a central electrode biased to negative DC voltages and an outer one at the ground, creating a radial electric field (1-2MV/m) and an axial magnetic field (B < 0.4T). This setup mimics Penning-Malmberg traps. We present the experimental device and first findings on current distribution and also a qualitative comparison with FENNECS simulations [Le Bars etal., Comput. Phys. Commun. 303, 109268 (2024)]. Planned diagnostics include optical emission spectroscopy, phosphor screen imaging, streak camera imaging, and potentially electric field distribution via the Stark effect. This research aims to enhance gyrotron performance and reliability in fusion energy systems.

A decomposition algorithm for streak camera data

We describe a novel reconstruction algorithm for time-resolved images obtained using a streak camera. This algorithm operates by decomposing a recorded image into a set of individual photoelectron-induced signals, thereby providing a powerful method for streak camera image reconstruction. This deconstruction allows for a standard statistical analysis of the resulting image. We demonstrate the effectiveness of this technique by analyzing the temporal spacing between the emitted fs-long laser pulse and its succeeding first, second, and third reflections within a thick glass captured by a streak image.

Incremental peritoneal dialysis and survival outcomes: a propensity-matched cohort study.

The advantages of an incremental dialysis start are not fully clear.We aimed to evaluate the association of incremental initiation of peritoneal dialysis with mortality. Incident peritoneal dialysis patients with a catheter placed at our hospital between 2008 and 2017 were included. All patients were followed up until December 31, 2019. Patients were categorized into different groups according to the initial daily dialysis exchanges, and were matched at a ratio of 1:2 with propensity score matching. Multiple variables including age, sex, residual kidney function, urine volume, hemoglobin, serum albumin and other important variables were included for the matching. Primary outcomes were all-cause and cardiovascular mortality. A total of 1315 patients with a mean age of 45.9years were enrolled. The mean glomerular filtration rate was 4.32ml/min/1.73 m2 at start ofdialysis. Two hundred eighty-fivepatientsin the incremental group and 502 in the full dose group were matched for age, sex, residual kidney function, urine volume, hemoglobin, serum albumin and other important variables. Patient survival and cardiovascular event-free survival were similar between the two groups. However, during the first 6 years of peritoneal dialysis, patients inthe incremental group had bettersurvival (P = 0.011) and cardiovascular event-free survival (P = 0.044) than the full dose group, while such advantages disappeared when dialysis vintage became longer. Further analysis showed that the incremental group (vs full dose dialysis) had a 39% lower risk (95% CI 0.42-0.90, P = 0.012) of all-cause mortality and a 41% decreased risk (95% CI 0.35-0.99, P = 0.047) of cardiovascular mortality during the first 6 years of dialysis. Additionally, the cumulative hazard for anuria was significantly lower in the incremental group versus the full dose group (P = 0.006). Our study shows a time-related survival advantage for incremental peritoneal dialysis patients, suggesting that an incremental regimen for startingperitoneal dialysis is feasibleand is not associated with worse outcomes. Graphical Abstract presenting schematically the measurements of the solvation response function by processing the relevant streak camera images and the time-correlated photon counting (TCSPC) data and appropriately combining them together.

Quantitative assessment of fitting errors associated with streak camera noise in Thomson scattering data analysis.

Thomson scattering measurements in high energy density experiments are often recorded using optical streak cameras. In the low-signal regime, noise introduced by the streak camera can become an important and sometimes the dominant source of measurement uncertainty. In this paper, we present a formal method of accounting for the presence of streak camera noise in our measurements. We present a phenomenological description of the noise generation mechanisms and present a statistical model that may be used to construct the covariance matrix associated with a given measurement. This model is benchmarked against simulations of streak camera images. We demonstrate how this covariance may then be used to weight fitting of the data and provide quantitative assessments of the uncertainty in the fitting parameters determined by the best fit to the data and build confidence in the ability to make statistically significant measurements in the low-signal regime, where spatial correlations in the noise become apparent. These methods will have general applicability to other measurements made using optical streak cameras.

Spectrally filtered ps–ns emission dynamics of atmospheric-pressure nanosecond pulsed plasmas

The temporal dynamics of atmospheric-pressure nanosecond pulsed plasma discharges in a pin-to-pin electrode configuration are studied using streak-camera line imaging of the interelectrode gap with a time resolution as short as ∼25 ps. Discharge emission initiates homogeneously throughout the interelectrode gap with no detectable streamer propagation and then temporally decays in two distinct phases. Plasma emission bands attributed to various electronic transitions are tracked for single discharges in air and N2. Spectral filtering of the excited molecular states reveals that the N2(C–B) and N2(B–A) emission bands evolve in distinct early and late phases, respectively, with a time separation of ∼15–20 ns. Furthermore, significant differences in the temporal dynamics of plasma discharges in air and N2 are observed. High levels of excited-state atomic oxygen and NO appear after the initial decay of the N2(C) state and coincide primarily with the latter phases of plasma evolution in air environments. From temporal traces of discharge emission, the formation and relaxation timescales of the electronically excited states of N2 are quantified in pure N2 and air environments with sub-nanosecond resolution. The streak-OES (optical emission spectroscopy) technique enables quantitative time-resolved studies of key chemical species for model validation in ultra-short-pulsed plasmas.

Improved temporal contrast of streak camera measurements with periodic shadowing.

Periodic shadowing, a concept used in spectroscopy for stray light reduction, has been implemented to improve the temporal contrast of streak camera imaging. The capabilities of this technique are first proven by imaging elastically scattered picosecond laser pulses and are further applied to fluorescence lifetime imaging, where more accurate descriptions of fluorescence decay curves were observed. This all-optical approach can be adapted to various streak camera imaging systems, resulting in a robust technique to minimize space-charge induced temporal dispersion in streak cameras while maintaining temporal coverage and spatial information.

Multichannel Operation of a Multi-Gap, Multichannel Spark Gap Switch Using a Low-Magnitude Trigger Pulse and Its Performance Evaluation

This article describes the performance of a multi-gap, multichannel (MGMC) spark gap switch, evaluated numerically and experimentally, to achieve multichanneling using a relatively low-magnitude trigger pulse. First, an electrostatic analysis of the switch geometry was carried out using COMSOL Multiphysics 5.5 to determine the electric field distribution during high-voltage hold-off before triggering and to determine the coupling parameters between trigger and other intermediate electrodes of the switch. Second, a circuit model was developed in PSpice Lite 16.6 for predicting the process of channel formation and its influence on adjacent channels. Based on the simulation results obtained, a six-gap, eight-channel, planar MGMC switch operating in air has been designed and developed. The developed switch was, then, incorporated into a compact experimental setup to experimentally evaluate various switch parameters such as inductance, switching delay, jitter, and closing time when triggered using a negative-polarity trigger pulse of magnitude −40 kV, generated using a compact, in-house developed trigger generator. In contrast to most of the reported MGMC switches that use complicated Marx-based generators to produce ≥120-kV trigger pulses for multichannel formation, the experimental results, thus obtained, report comparable switch performance using a low-magnitude (−40 kV) trigger pulse. In addition, framing and streak camera images taken during the current discharge indicate the presence of ~6 luminous channels. In addition, short-circuit experiments for estimating switch inductance by consecutively shorting one or more channels also indicate multichanneling in the switch using a low-magnitude trigger pulse.

Determination of the Charge per Micro-Bunch of a Self-Modulated Proton Bunch using a Streak Camera

The Advanced Wakefield Experiment (AWAKE) develops the first plasma wakefield accelerator with a high-energy proton bunch as driver. The 400 GeV bunch from CERN Super Proton Synchrotron (SPS) propagates through a 10 m long rubidium plasma, ionized by a 4TW laser pulse co-propagating with the proton bunch. The relativistic ionization front seeds a self-modulation process. The seeded self-modulation transforms the bunch into a train of micro-bunches resonantly driving wakefields. We measure the density modulation of the bunch, in time, with a streak camera with picosecond resolution. The observed effect corresponds to alternating focusing and defocusing fields. We present a procedure recovering the charge of the bunch from the experimental streak camera images containing the charge density. These studies are important to determine the charge per micro-bunch along the modulated proton bunch and to understand the wakefields driven by the modulated bunch.

Geometric distortion correction for streak camera imaging

As streak tube imaging is a concentric spherical electrostatic focusing system, geometric distortion is caused by the relative change in radial magnification. This reduces the spatial resolution of imaging. The physical principle of changing the radial magnification of an electronic optical imaging system is analyzed to correct the geometric distortion of the imaging system and to improve spatial resolution. Distortion is corrected using the calibration model of a spherical optical system. The contrast of a reticle intensity image is compared before and after correction, and the ultimate resolution is calculated. Contrast is increased by ∼4 % , and the ultimate resolution is increased by ∼2 lp / mm after correction.

A rate-doubled 10-GHz fiducial comb generator for precision optical timing calibration.

Solid-state optical fiducial timing pulse generators provide a convenient and accurate method to include timing fiducials in a streak-camera image for time-base correction. Current commercially available vertical-cavity surface-emitting lasers (VCSELs) emitting in the visible range can be amplitude modulated up to 5 GHz. An optically passive method is utilized to interleave a time-delayed path of the 5-GHz pulsed light with itself, producing a 10-GHz pulsed fiducial, or comb. Comb pulse rates at 0.5, 1.0, and 2.5 GHz can also be selected. The instrument presented is a self-contained and portable generator with primary use for streak-camera temporal calibration. Applications can also be extended to many other optical timing needs. The VCSEL output is fiber optic coupled at a wavelength of 680 nm (visible red) with a nearly Gaussian pulse shape. The peak power of each ∼50-ps full width at half maximum (FWHM) comb pulse at 5-GHz operation, or picket, is approximately 5 mW. The low phase noise of the internal microwave modulation drive source provides low pulse to pulse jitter. An external reference frequency standard can be utilized to synchronize the output to external timing equipment. A selectable internal reference frequency crystal oscillator is incorporated for stand-alone operation.

Application of M Sequence Family Measurement Matrix in Streak Camera Imaging

This study investigates the reduction in the resolution of the striations from the center to the edge through analysis of the imaging principle and the static experimental test of a streak tube. To improve the edge spatial resolution of the streak, we apply the compressed sensing to the X-ray streak camera imaging system and construct the compressed sensing (CS) reconstruction model for the streak camera and we implement the CS objective function by the orthogonal matching method. The reconstruction performance of the Gauss measurement matrix, Bernoulli measurement matrix, and M series family measurement matrix are compared, and the reconstruction parameters are optimized. A comparison between the original imaging results and the reconstruction results shows that the contrast ratio of the CS reconstruction is 12.2% higher than that of the original, and the limit resolution is 5 lp/mm higher than that of the original image. Furthermore, the improvement effect far from the central area is better than that at the central area. The CS reconstruction on the M series family measurement matrix can improve the image contrast ratio on the edge of the image, and, thus, static and dynamic spatial resolutions of the image are improved.

Time-Resolved Thomson Scattering on Laboratory Plasma Jets

Thomson scattering measurements were performed on plasma jets created from a 15- $\mu \text{m}$ -thick radial Al foil load on a 1-MA pulsed power machine. The laser used for these measurements has a maximum energy of 10 J at 526.5 nm. Using the full energy, however, significantly heats the $5\times 10^{18}$ cm−3 jet by inverse bremsstrahlung, creating a density bubble in the jet. To measure the evolving plasma parameters of this laser-heated jet, a streak camera was used to record the scattered spectrum, resulting in the sub-ns time-resolved Thomson scattering. Analysis of the streak camera image showed that the electron temperature of the jet was about 25 eV prior to the laser pulse. The laser then heated the plasma to 80–100 eV within about 2 ns. The electron temperature then stabilized for about 0.5 ns prior to falling at the end of the laser pulse. Jets made from a radial Ti foil showed more heating by the laser than the Al jets, going from 50 to over 150 eV, and heating was detected even when only 1 J of laser energy was used. Also, the ion-acoustic peaks in the scattered spectrum from the Ti jets were significantly narrower than those from Al jets, a result of several possible differences in the plasma created from these two materials.

Stochastic spin flips in polariton condensates: nonlinear tuning from GHz to sub-Hz

The stability of spin of macroscopic quantum states to intrinsic noise is studied for non-resonantly-pumped optically-trapped polariton condensates. We demonstrate flipping between the two spin-polarised states with >104 slow-down of the flip rate by tuning the optical pump power. Individual spin flips faster than 50 ps are time resolved using single-shot streak camera imaging. We reproduce our results within a mean-field model accounting for cross-spin scattering between excitons and polaritons, yielding a ratio of cross- to co-spin scattering of ∼0.6, in contrast with previous literature suggestions.

Impact of volume and surface processes on the pre-ionization of dielectric barrier discharges: advanced diagnostics and fluid modeling

The phenomenology and breakdown mechanism of dielectric barrier discharges are strongly determined by volume and surface memory effects. In particular, the pre-ionization provided by residual species in the volume or surface charges on the dielectrics influences the breakdown behavior of filamentary and diffuse discharges. This was investigated by advanced diagnostics such as streak camera imaging, laser photodetachment of negative ions and laser photodesorption of electrons from dielectric surfaces in correlation with 1D fluid modeling. The streak camera images show that an increasing number of residual charges in the volume changes the microdischarge breakdown in air-like gas mixtures from a cathode-directed streamer to a simultaneous propagation of cathode- and anode-directed streamers. In contrast, seed electrons are important for the pre-ionization if the density of residual charges in the volume is low. One source of seed electrons are negative ions, whose density exceeds the electron density during the pre-phase of diffuse helium–oxygen barrier discharges as indicated by the laser photodetachment experiments. Electrons desorbed from the cathodic dielectric have an even larger influence. They induce a transition from the glow-like to the Townsend-like discharge mode in nominally pure helium. Apart from analyzing the importance of the pre-ionization for the breakdown mechanism, the opportunities for manipulating the lateral structure and discharge modes are discussed. For this purpose, the intensity and diameter of a diffuse discharge in helium are controlled by an illuminated semiconducting barrier.

Streak camera imaging of single photons at telecom wavelength

Streak cameras are powerful tools for temporal characterization of ultrafast light pulses, even at the single-photon level. However, the low signal-to-noise ratio in the infrared range prevents measurements on weak light sources in the telecom regime. We present an approach to circumvent this problem, utilizing an up-conversion process in periodically poled waveguides in Lithium Niobate. We convert single photons from a parametric down-conversion source in order to reach the point of maximum detection efficiency of commercially available streak cameras. We explore phase-matching configurations to apply the up-conversion scheme in real-world applications.

压缩感知在条纹相机成像中的应用

压缩感知在条纹相机成像中的应用

Dynamics of Laser Triggered, Gas-Insulated Spark Gaps During Repetitive Operation

The Electra pulsed power system at the U.S. Naval Research Laboratory uses laser triggered SF6 gas-insulated spark gaps to transfer energy to a high power electron beam diode. These 1-MV spark gaps have a charge transfer of 10 mC per shot operating at 5 pules per second (pps) utilizing electrodes with the potential durability of 300 000 pulses. During continuous repetitive switch operation, after a few thousand shots intermittent nonfire events occur with greater frequency approaching 100 000 shots. The nonfire events and the lack of reproducibility and reliability reduce the potential applications for laser triggered spark gaps. Therefore, the temporal evolution and the spatial evolution of the switch dynamics have been examined. Gated camera images, streak camera images, and interferometry are used to characterize the dynamics of single shots to directly compare to the dynamics during continuous operation. The streak camera images indicate that the switch dynamics are dependent on the repetition rate. For single shots and 1 pps operation (9000 continuous shots), no misfires (mistimings or nonfires) are observed in the switch. This observation is concomitant with a consistent and reproducible breakdown position and breakdown dynamics. At 5 pps, the variations in the laser breakdown position (preferentially toward cathode) and the laser breakdown emission intensity are observed. After several thousands of continuous shots, nonfire events are detected in small groups, before normal operation resumes. A temperature differential of at least 22.2 K (40 F) between the SF6 gas at the top of the housing and the bottom of the spark gap housing is observed. The onset of nonfire events is correlated with the hydrodynamic changes, from increased temperature, of the SF6 gas flow within the spark gap. A redesign of the spark gap along with adequate cooling to reduce the hydrodynamic flow and forces of the insulating SF6 gas would mitigate if not eliminate nonfire events.

Streak Tubes for Diagnostics of Lasers and Plasmas

Designing a facility for laser fusion research requires sufficient advancement in diagnostics techniques for lasers and plasmas, including those involving streak camera imaging. Maximum specifications of streak cameras depend on the parameters of streak tubes. The paper illustrates how these devices function, and which of their parameters are limiting. The paper presents a novel technological platform designed at VNIIA, which was used to develop a new generation of streak tubes. Using these streak tubes in streak cameras, the efficiency of streak camera imaging techniques can be improved by several orders of magnitude, and new techniques can be designed.

Laser irradiations of advanced targets promoting absorption resonance for ion acceleration in TNSA regime

Advanced targets based on Au nanoparticles embedded in polymers films show high absorption coefficient in the UV–visible and infrared region. They can be employed to enhance the proton and ion acceleration from the laser-generated plasma in TNSA regime. In conditions of “p” polarized laser irradiations at 1015W/cm2 intensity, in these films can be induced resonant absorption due to plasma wave excitation. Plasma on-line diagnostics is based on SiC detectors, Thomson spectrometry and X-ray streak camera imaging. Measurements of kinetic energy of accelerated ions indicate a significant increment using polymer targets containing gold nanoparticles and “p” polarized laser light with respect to pure polymers and unpolarized light irradiation.

Optical Measurement of Peak Air Shock Pressures Following Explosions

AbstractHigh speed video and streak camera imaging are used to measure peak pressures for explosions of spherical charges of the high explosive C‐4 (92 % trimethylenetrinitramine, C3H6N6O6). The technique measures the velocity of the air shock produced by the detonation of the explosive charges, converts this velocity to a Mach number, and uses the Mach number to determine a peak shock pressure. Peak pressure measurements are reported from a few millimeters to approximately one meter from the charge surface. Optical peak pressure measurements are compared to peak pressures measured using piezoelectric pressure transducers, and to peak pressure measurements estimated using the blast computer code CONWEP. A discussion of accuracy of peak pressures determined optically is provided.