Guanglong Chen

Pressure dependence of argon cluster size for different nozzle geometries

We experimentally study Rayleigh scattering from a cluster jet produced by high pressure argon gas expanding into vacuum through four different nozzles (a supersonic slit nozzle, a slit nozzle, a conical nozzle, and a sonic nozzle). The scattering signal intensity and the scattering image are recorded by photomultiplier tube and charge-coupled device camera, respectively. Based on the scattering image, the atom density in the gas flow is estimated. This allows for the comparison of the dependence of average cluster size on argon gas backing pressure between the nozzles. The experimental results show that the planar expansion developed from the supersonic slit and the slit nozzles exhibits the higher atom density than the axisymmetric expansion from the conical and the sonic nozzles. The slit nozzle is shown to have the highest pressure dependence of average cluster size. It is found that the supersonic slit nozzle is more favorable to the large clusters than the slit nozzle under the backing pressure of u...

Understanding of cluster size deviation by measuring the dimensions of cluster jet from conical nozzles

This work aims to understand the cluster size deviation from the prediction by an existing scaling law for conical nozzles. The dimensions of cluster jet at different heights above a nozzle along the direction of gas flow are measured. This study indicates that the dimension of cluster jet is underestimated in the existing scaling law and this under-estimation leads to the over-estimation of the equivalent diameter of conical nozzle. Thus the underestimation of the dimension of cluster jet may be one of possible factors responsible for the cluster size deviation (the degree of the deviation depends on details of cluster jet).

Experimental investigation on argon cluster sizes for conical nozzles with different opening angles

Using Rayleigh scattering measurement, we experimentally studied the effect of the opening angle of conical nozzles on the average sizes of argon clusters produced by high-pressure argon gas (up to 50 bars) expanding into vacuum. Both the scattering signal intensity and the scattering image were synchronically recorded by a photomultiplier tube and a charge-coupled device camera. These measurements allow for the comparison of average cluster sizes among conical nozzles of different opening angles. The experimental results indicate that, as expected by Hagena’s scaling law, the argon cluster size is dependent on the opening angle. However, it is also found that (1) the cluster size exhibits a larger deviation from Hagena’s scaling law at high backing pressure for a nozzle of a smaller opening angle and (2) the smaller the opening angle of conical nozzle gets, the weaker the pressure dependence of cluster size becomes.

Controlling the length of plasma waveguide up to 5 mm, produced by femtosecond laser pulses in atomic clustered gas

We report the observation of longitudinally uniform plasma waveguide with a controlled length of up to nearly 5 mm, in argon clustered gas jet. This self-channeling plasma is obtained using a 35 mJ, 30 fs FWHM pulse as a pump laser pulse to create the plasma channel. A 1 mJ pulse of the same laser is used for probing the plasma channels using interferometric diagnostics. The radial distribution of the electron density confirms the formation of a plasma waveguide. Clustered argon enhances the absorption efficiency of femtosecond pulses which enables the use of pump pulses of only 35 mJ, approximately 10 times less energy than required for heating conventional gas targets. The plasma channel length is controlled by the laser focus point (F), the laser intensity (I), the pump-probe delay time (t) and the laser height from a nozzle (z). The variation of the electron density for these parameters is also studied. We found that the highest density of 1.2 x 10(19) cm(-3) was obtained at I = 5.2 x 10(16) W/cm(2), z = 2 mm and t = 7.6 ns. It was demonstrated that by using a clustered jet, both the plasma waveguide length and the plasma density could be controlled.

Two-color field for the generation of an isolated attosecond pulse in water-window region

For the investigation of various ultrafast electron dynamics, an isolated attosecond pulse in a broad spectral range is necessary. The generation of isolated attosecond pulses demands the manipulation of the electric field of a laser. We propose a two-color field scheme for generating an isolated attosecond pulse in the water-window region. Two-color fields are generated by mixing two equally-strong pulsed color fields. The investigation shows that an isolated attosecond pulse with a photon energy of near 500 eV and a pulse duration of 125 - 188 attoseconds can be generated using 10 - 15 fs FWHM laser fields.

Investigation of the on-axis atom number density in the supersonic gas jet under high gas backing pressure by simulation

The supersonic gas jets from conical nozzles are simulated using 2D model. The on-axis atom number density in gas jet is investigated in detail by comparing the simulated densities with the idealized densities of straight streamline model in scaling laws. It is found that the density is generally lower than the idealized one and the deviation between them is mainly dependent on the opening angle of conical nozzle, the nozzle length and the gas backing pressure. The density deviation is then used to discuss the deviation of the equivalent diameter of a conical nozzle from the idealized deq in scaling laws. The investigation on the lateral expansion of gas jet indicates the lateral expansion could be responsible for the behavior of the density deviation. These results could be useful for the estimation of cluster size and the understanding of experimental results in laser-cluster interaction experiments.

Simulations of a polar molecule (sulfur dioxide) in a supersonic jet

Clustering of polar molecules (SO2) in a supersonic jet was investigated by simulation. The cluster size Nc was calculated under different gas backing pressures P0 and source temperatures T0. Based on these results, the dependence of Nc on P0 or T0 was compared with that for the gases of non-polar molecules. It is found that SO2 molecules condense very easily into clusters even when P0 is very low. This result implies that large clusters can be produced using polar molecules, which could be useful in the studies of laser-cluster interactions. Also, SO2 gas is an important component of air pollution, so the knowledge of the clustering properties of SO2 could be helpful in the study of air pollution and environment.

The radial dimension of a supersonic jet expansion from conical nozzle

In a laser-cluster interaction experiment, the radial dimension of a supersonic gas jet is an important parameter for the characterization of interaction volume. It is noted that due to the lateral gas expansion, the diameter of a supersonic gas jet is larger than the idealized diameter of a gas jet from a conical nozzle. In this work the effect of the lateral expansion on the radial dimension of gas jet was investigated by simulations. Based on the simulation results, the diameter of gas jet l was compared in detail with the corresponding diameter lT in the idealized straight streamline model and the diameter lH at a half of maximum atom density of gas jet. The results reveal how the deviation of l from lT (lH) changes with respect to the opening angles of conical nozzles, the heights above the nozzle, the nozzle lengths and the gas backing pressures. It is found that the diameter of gas jet l is close to the idealized diameter lT and lH in the case where a long conical nozzle with a large opening angle ...

Manifestation of attosecond XUV fields temporal structures in attosecond streaking spectrogram

The features of an attosecond extreme ultraviolet (XUV) field are encoded in the attosecond XUV spectrogram. We investigate the effect of the temporal structures of attosecond XUV fields on the attosecond streaking spectrogram. Factors such as the number of attosecond XUV pulses and the temporal chirp of attosecond XUV pulses are considered. Results indicate that unlike the attosecond streaking spectrogram for an attosecond XUV field with two pulses of a half-cycle separation of streaking field, the spectrogram for the attosecond XUV field with three pulses demonstrates fine spectral fringes in separated traces.