Nobuhiro Sakamoto

Development and characterization of a low current capillary discharge for X-ray laser studies

Capillary discharge experiments were carried out for soft X-ray laser studies. A ceramic capillary, which has an inner diameter of 3 mm and a length of 150 mm, has been used for the end-on X-ray diode observation, and a Pyrex capillary has been used for side-view observation. Spike output has been observed, when operating the device with a predischarge current of 5 to 15 A, a current of 9 to 35 kA with a rise time of 55 ns in an argon gas pressure range from 100 to 800 mtorr. It is found that without a predischarge current, spike output has been hardly observed. Observation of spike output at a low discharge current of 9 kA provides us a possibility for design of a compact soft X-ray laser device. When the predischarge is turned off, the side-view observation of the capillary discharge clearly shows the growth of instabilities during the pinch process. This suggests that the predischarge is indispensable for achieving a uniform plasma, which is required by the soft X-ray lasing in a capillary discharge.

Performances of Ne-like Ar Soft X-ray Laser using Capillary Z-Pinch Discharge

We have designed, fabricated and tested a soft X-ray device, which uses a capillary discharge to achieve neon-like argon lasing. The ceramic capillary has an inner diameter of 3 mm and a length of 150 mm. Lasing has been confirmed when operating the device with a current of 9 to 32 kA with a rise time of 55 ns, in an argon gas pressure range from 100 to 800 mTorr. The relation between the observation of the laser spike and occurrence of the moving striation has been studied by a series of side-view observations. The appropriate starting time of the main discharge with respect to that of the predischarge current has been studied. When the predischarge is properly applied, constant laser amplification of the gain-length product of gl=12 (g=0.8 cm-1) was obtained, with a current of 32 kA at a pressure of 400 mTorr.

Effects of Plasma Dynamics on Lasing in Fast Capillary Discharge

The timing, intensity, and pulse width of lasing signals in fast capillary discharge plasma at a strong 2p53p–2p53s transition of Ne-like Ar are experimentally investigated. To clarify the lasing mechanism, the results are compared with those obtained by numerical calculation using a one-dimensional magneto-hydrodynamics (MHD) simulation code. The comparison indicates that the lasing occurs when a cylindrically imploding shock wave arrives at the axis of the capillary and it depends not only on the electron density and temperature but also strongly on the effects of time-varying refraction and the opacity of the pinching plasma.

On the time of lasing onset and the end-effect of a soft x-ray laser device using a capillary z-pinch discharge

We have measured the time of lasing onset of a capillary z-pinch discharge used for Ne-like Ar soft x-ray laser generation. It was found that for a relatively long plasma, the relationship between the time of lasing onset and the filling pressure of the Ar gas agrees, in trend, with that predicted by simulation. An interesting result was observed: the time of lasing onset becomes delayed when the length of the capillary becomes shorter, which is considered to be caused by an end-effect of the capillary on the pinching process. This also implies that a relatively long capillary is favourable for obtaining a plasma column of uniformity, and thus for reducing the relative absorption of laser power.

Study of low current capillary discharge for compact Soft X‐ray laser

Capillary discharge experiments were carried out to get a lasing of Ne‐like Ar. The relation between the appearance of the laser output and the instabilities of longitudinal modes has been studied by side‐view observation. And observation of lasing at a low discharge current of 9 kA provides us a possibility of designing a compact and high‐repetition‐rate soft X‐ray laser by use of a semiconductor switch instead of a gap switch.

Importance of the initial condition for high-energy lasing in fast-capillary-discharge lasers

The dependence of the lasing conditions of fast-capillary-discharge plasmas on the initial plasma parameters has been experimentally investigated. The results show that there is an optimum initial condition for lasings and that the initial condition is an important factor for high-energy lasing using capillary plasmas. The experimental results also indicate that the variation in lasing power can be correlated with the electron density of the initial plasmas.

Focusing of soft X-ray using a grazing incidence mirror

Attempts have been made to focus the light beam of soft X-rays generated from an Ar capillary discharge, using a grazing incidence Wolter-type mirror. A soft X-ray beam with an image of approximately 10 mm in diameter is focused to a spot less than a 1 mm. The result proves that the Wolter-type mirror is technically applicable for the focusing of the soft X-rays generated from a capillary discharge.

Plasma dynamics and lasing condition of fast capillary discharges

The dependence of the lasing condition of fast capillary discharge plasmas on the initial gas pressure and rising rate of the load current; dI/dt has been experimentally investigated. The results indicate that there is an optimum condition at suitable values of initial gas pressure and dI/dt. Based on the experimental results and a simplified scaling model for the compression process, the correlation between the plasma dynamics and the lasing condition in capillary pinch plasmas is discussed.

Consideration on the scaling of capillary pinch lasers

Summary form only given, as follows. Based on the experimental and theoretical results, we discuss the critical issues and the prospects for the energetic operation of discharge pumped capillary laser at shorter wavelength.

Measurement of gain-length product and output energy in a capillary discharge soft X-ray laser

Summary form only given, as follows. In 1994, J.J. Rocca et al. reported that a capillary discharge could produce a laser amplification at 3p-3s level of Ne-like Ar line (J = 0-1, /spl lambda/ = 46.9 nm). This has led to a successful construction of a tabletop soft X-ray laser, which is capable to generate saturated laser energy of 1 mJ at 4 Hz, with a discharge current of 26 kA. In our laboratory, research has also been performed on capillary discharge to achieve the Ne-like Ar soft X-ray gas lasing. A ceramic capillary, which has an inner diameter of 3 mm and a length of 150 mm, has been used for soft X-ray laser amplification. Spike output signal which has a characteristics of directivity, has been observed, when operating the device with a predischarge current of 5 to 15 A, a current of 9 kA to 35 kA with a half period of 110 ns in an argon gas pressure range from 100 mTorr to 800 mTorr. Gain-length product gl = 12 (g = 0.8 cm/sup -1/) was obtained with a current amplitude of 30 kA at the optimum pressure of 400 mTorr. But, we have not yet observed the beginning of gain saturation. Compared with the result of J.J. Rocca et al., the spike output observed in our experiment is quite possibly attribute to the Ne-like Ar ion soft X-ray lasing at the same wave length of 46.9 nm. However, spectroscopic measurement is needed to be conducted to confirm this. For the current of 9 kA with optimum pressure of 150 mTorr, estimated output energy is 300 nJ. And for the current of 30 kA and the optimum pressure of 400 mTorr, the estimated output energy of laser reaches 5-6 /spl mu/J. The output energy is significantly low because the lasing in our case does not reach the saturation level. Therefore, measurement of gain-length and output energy with longer capillary is needed to reveal the saturation of soft X-ray amplification and more output energy.