A capillary gas cell for laser wakefield acceleration was developed with the aid of three-dimensional computational fluid dynamics simulations. The gas cell was specially designed to provide upward density tapering in the longitudinal direction, which is expected to suppress the dephasing problem in laser wakefield acceleration by keeping the accelerated electrons in the acceleration phase of the wake wave. The density-tapered capillary gas cell was fabricated by sapphire plates, and its performance characteristics were tested. The capillary gas cell was filled with a few hundred millibars of hydrogen gas, and a Ti:sapphire laser pulse with a peak power of 3.8 TW and a pulse duration of 40 fs (full width at half maximum) was sent through the capillary hole, which has a length of 7 mm and a square cross section of 350 × 350 µm2. The laser-produced hydrogen plasma in the capillary hole was then diagnosed two-dimensionally by using a transverse Mach–Zehnder interferometer. The capillary gas cell was found to provide an upward plasma density tapering in the range of 1018 cm−3–1019 cm−3, which has a potential to enhance the electron beam energy in laser wakefield acceleration experiments.

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Measurement of the electron density of the inductively-coupled plasma (ICP) was performed by using laser-plasma-produced THz (terahertz) pulses, which are generated by focusing laser beams in gas with DC bias. The generated THz pulses are sent to the ICP and the plasma density is measured by the THz time-domain spectroscopy (THz-TDS) method. By measuring the amount of the phase shift caused by the THz pulse passing through the plasma, the electron density of the plasma can be deduced. The result shows that the ICP device can produce argon plasmas with electron densities in the range of 10 13 ∼ 10 14 cm −3 , which is a typical density range for magnetic fusion plasmas. This implies that the laser-plasma-based THz source can be a new diagnostic tool for fusion plasmas as well as other plasmas. In addition to this result, more broadband THz pulses, which are generated by the two-color photoionization method, are also investigated for the application possibility for more diverse plasmas, for example, higher density plasmas.

We report the first experimental demonstration of a novel plasma diagnostics method using the single-shot THz detection method. In this experiment, a broadband single THz pulse is generated by focusing two-color (400 nm and 800 nm in wavelength) laser pulses in air and the THz pulse is sent through an inductively-coupled plasma with a density of 1013–1014 cm−3 which is in the modern tokamak plasma density range. Phase shift of the THz pulse is measured by the single-shot THz detection method and the instant plasma density information is obtained from the phase shift. The result is compared with that of the existing THz time domain spectroscopy method and it is found that both results are in good agreement. It implies that the proposed method in this paper can provide a new plasma diagnostics tool which can be used for dynamically changing plasmas.

Generation of broadband supercontinuum light has been of great interest in recent years. In this work, we investigated an interesting way for generation of broadband supercontinuum light by double-focusing of a femtosecond (fs) laser pulses in air. In this method, the fs laser pulse is focused in air and the produced supercontinuum light from the interaction of the laser pulse and air is focused again in air. In this way, we found that the spectral broadening of the supercontinuum light can be a lot more enhanced even with a relatively low laser energy of a few mJ/pulse, compared with the single-focusing method. In this paper, we present the experimental results for the broadband supercontinuum light by the double-focusing method, in addition to numerical simulation results.