Multi-channel FIR Interferometer for Electron Density Measurement on HT-7 Superconducting Tokamak

A five-channel far-infrared (FIR) hydrogen cyanide (HCN) laser interferometer was developed to measure plasma electron density profile on the HT-7 superconducting tokamak. The principle and structure of the five-channel FIR laser interferometer is described. The laser source used in the interferometer was a continuous wave glow discharge HCN laser with a 3.4 m cavity length and a 100 mW power output at 337μm wavelength. The temporal resolution was 0.1 ms and the detection sensitivity was 1/12 fringe. Preliminary experimental results measured by the interferometer on HT-7 tokamak are reported.

Measurement of time-varying electron density of the plasma generated from a small-size cylindrical RDX explosion by Rayleigh microwave scattering

It is challenging to measure the electron density of the unsteady plasma formed by charged particles generated from explosions in the air,because it is transient and on a microsecond time scale.In this study,the time-varying electron density of the plasma generated from a small cylindrical cyclotrimethylenetrinitramine(RDX)explosion in air was measured,based on the principle of microwave Rayleigh scattering.It was found that the evolution of the electron density is related to the diffusion of the detonation products.The application of the Rayleigh microwave scattering principle is an attempt to estimate the electron density in explosively generated plasma.Using the equivalent radius and length of the detonation products in the bright areas of images taken by a high-speed framing camera,the electron density was determined to be of the order of 10^(20)m^(−3).The delay time between the initiation time and the start of variation in the electron-density curve was 2.77–6.93μs.In the time-varying Rayleigh microwave scattering signal curve of the explosively generated plasma,the electron density had two fluctuation processes.The durations of the first stage and the second stage were 11.32μs and 19.20μs,respectively.Both fluctuation processes increased rapidly to a peak value and then rapidly attenuated with time.This revealed the movement characteristics of the charged particles during the explosion.

Forward modelling of the Cotton-Mouton effect polarimetry on EAST tokamak

Measurement of plasma electron density by far-infrared laser polarimetry has become a routine and indispensable tool in magnetic confinement fusion research.This article presents the design of a Cotton-Mouton polarimeter interferometer,which provides a reliable density measurement without fringe jumps.Cotton-Mouton effect on Experimental Advanced Superconducting Tokamak(EAST)is studied by Stokes equation with three parameters(s_(1),s_(2),s_(3)).It demonstrates that under the condition of a small Cotton-Mouton effect,parameter s_(2)contains information about Cotton-Mouton effect which is proportional to the line-integrated density.For a typical EAST plasma,the magnitude of Cotton-Mouton effects is less than 2πfor laser wavelength of 432μm.Refractive effect due to density gradient is calculated to be negligible.Time modulation of Stokes parameters(s_(2),s_(3))provides heterodyne measurement.Due to the instabilities arising from laser oscillation and beam refraction in plasmas,it is necessary for the system to be insensitive to variations in the amplitude of the detection signal.Furthermore,it is shown that non-equal amplitude of X-mode and O-mode within a certain range only affects the DC offset of Stokes parameters(s_(2),s_(3))but does not greatly influence the phase measurements of Cotton-Mouton effects.

Damage induced by femtosecond laser in optical dielectric films

Both the nature of avalanche ionization （AI） and the role of multi-photon ionization （MPI） in the studies of laser-induced damage have remained controversial up to now. According to the model proposed by Stuart et al., we study the role of MPI and AI in laser-induced damage in two dielectric films, fused silica （FS） and barium aluminum borosilicate （BBS）, irradiated by 780-nm laser pulse with the pulse width range of 0.01 - 5 ps. The effects of MPI and initial electron density on seed electron generation are numerically analyzed. For FS, laser-induced damage is dominated by AI for the entire pulse width regime due to the wider band-gap. While for BBS, MPI becomes the leading power in damage for the pulse width T less than about 0.03 ps. MPI may result in a sharp rise of threshold fluence Fth on r, and AI may lead to a mild increase or even a constant value of Fth on r. MPI serves the production of seed electrons for AI when the electron density for AI is approached or exceeded before the end of MPI. This also means that the effect of initial electron can be neglected when MPI dominates the seed electron generation. The threshold fluence Fth decreases with the increasing initial electron density when the latter exceeds a certain critical value.

Plasma channel formed by ultraviolet laser pulses at 193 nm in air

The propagation of picosecond deep ultraviolet laser pulse at wavelength of 193 nm in air is numerically investigated. Long plasma channel can be formed due to the competition between Kerr self-focusing and ionization induced defocusing. The plasma channel with electron density of above 10^13/cm^3 can be formed over 70 m by 50-ps, 20-mJ laser pulses. The fluctuation of laser intensity and electron density inside ultraviolet （UV） plasma channel is significantly lower UV laser by air is considered in the simulation and it the limit of the length of plasma channel. than that of infrared pulse. The linear absorption of is shown that the linear absorption is important for the limit of the length of plasma channel.

Spectral characteristics of arc plasma during laser-arc double-sided welding for aluminum alloy

In laser-arc double-sided welding, the spectral characteristics of the arc plasma are calculated and analyzed by spectroscopic diagnosis. The results show that, compared with conventional tungsten inert gas （TIG） welding, the introduction of a laser cilanges the physical characteristics of the arc plasma regardless of whether laser plasma penetration takes place, and that the influence of the laser mainly affects the near-anode region of the arc. When tile laser power is relatively low, the arc column tends to compress, and the arc spectral cilaracter- istics show no significant difference. When the arc root constricts, compared with pure TIG arc, the electron density increases by ~2.7 times and the electron temperature decreases by ~3000 K. When the arc column expands, the intensities of spectral lines of both the metal and Ar atoms are the strongest. But it is also observed that the electron density reduces, whereas there is no obvious decrease of electron temperature.