Kazuhiko Horioka

Optical guidance of terrawatt laser pulses by the implosion phase of a fast Z-pinch discharge in a gas-filled capillary

A new method of optical guidance by the implosion phase of a fast Z-pinch discharge in a gas-filled capillary is proposed. An imploding plasma column has a concave electron-density profile in the radial direction, just before a stagnation phase driven by a converging current sheet and a shock wave. The feasibility of optical guidance of a high-intensity (>1 x 10(17) W/cm(2)) Ti:sapphire laser pulse by use of this method over a distance of 2 cm, corresponding to 12.5 times the Rayleigh length, has been experimentally demonstrated. The guiding-channel formation process was directly probed with a He-Ne laser beam. The electron density in the fully ionized channel was estimated to be 2.0 x 10(17) cm(-3) on the axis and 7.0 x 10(17) cm(-3) on the peaks of the channel edge, with a diameter of 70 mum, as indicated by the experimental results, which were corroborated by a magnetohydrodynamics simulation.

Dependence on laser intensity and pulse duration in proton acceleration by irradiation of ultrashort laser pulses on a Cu foil target

The dependence on laser intensity and pulse duration in energetic proton acceleration by irradiation of ultrashort laser pulses on a 5μm thick copper tape target was measured. The laser intensity was varied from 8.5×1017W∕cm2 to 1.1×1019W∕cm2, and the pulse duration from 55 fs to 400 fs. The maximum proton energy increased as the pulse duration was increased while the laser intensity was kept constant. The dependence of the maximum proton energy on laser intensity and pulse duration was in good agreement with an analytical plasma-expanding model.

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.

Observation of multi-pulse soft x-ray lasing in a fast capillary 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. When operating the device with a current of 16 kA and dI/dt of 517.8 A ns-1 at gas pressure of 26.7 Pa, lasing has been confirmed. Multi-pulse laser output has also been observed with a slightly higher current of 17.5 kA, and this indicates that there exist several configurations suitable for the Ne-like Ar lasing during one pinch process. This is the first observation of multi-pulse laser output.

MeV-order proton and carbon ion acceleration by irradiation of 60 fs TW laser pulses on thin copper tape

We obtained high-energy ions by irradiating 60 fs laser pulses onto a copper tape target of 5 μm thickness. The proton was accelerated with energy of 1.2 MeV at laser intensity of 6.8×1018 W/cm2. The equivalent Boltzmann temperature of the proton was 185 keV which was less than the ponderomotive potential (650 keV) of an electromagnetic standing wave in a laser field. Moreover, we observed the acceleration of carbon ions with energy of more than 0.4 MeV at laser intensity of 3.4×1018 W/cm2.

Miniature hybrid plasma focus extreme ultraviolet source driven by 10 kA fast current pulse

A miniature hybrid plasma focus device, operated in xenon gas medium and driven by a 10kA fast current pulse, has been used to generate extreme ultraviolet radiation in the range of 6–15nm. At present the radiation characteristics from xenon plasma were mainly assessed qualitatively using standard tools such as visible light framing camera, extreme ultraviolet (EUV) pinhole camera, and EUV photodiode. Strong pinching of xenon plasma is indicative from both visible and EUV imagings. The maximum size of the EUV emitting zone is estimated to be of the order of 0.21×1.55mm and the estimated value is within the accepted value as benchmarked by industries. The EUV intensity measurement by photodiode showed fairly isotropic radiation at least in a half solid angle. This device can be developed further as a competent source for EUV metrology or lithography applications.

Estimation of optimum density and temperature for maximum efficiency of tin ions in Z discharge extreme ultraviolet sources

Extreme ultraviolet (EUV) discharge-based lamps for EUV lithography need to generate extremely high power in the narrow spectrum band of 13.5±0.135 nm. A simplified collisional-radiative model and radiative transfer solution for an isotropic medium were utilized to investigate the wavelength-integrated light outputs in tin (Sn) plasma. Detailed calculations using the Hebrew University-Lawrence Livermore atomic code were employed for determination of necessary atomic data of the Sn4+ to Sn13+ charge states. The result of model is compared with experimental spectra from a Sn-based discharge-produced plasma. The analysis reveals that considerably larger efficiency compared to the so-called efficiency of a black-body radiator is formed for the electron density ≃1018 cm−3. For higher electron density, the spectral efficiency of Sn plasma reduces due to the saturation of resonance transitions.

Magnetic core characteristics for high rep-rate induction modulator

Typical magnetic materials are experimentally characterized. Particularly our efforts are concentrated on deriving a core-loss scaling in operation region with minor B–H loops for highly repetitive voltage modulators. The core losses are evaluated as functions of magnetization rate and flux swing using semiempirical equations based on magnetization models. A prototype induction module has been successfully operated up to 1 MHz and the loss values are adapted well to the core-loss criterion.

Potential of discharge-based lithium plasma as an extreme ultraviolet source

Extreme ultraviolet (EUV) discharge-based lamps for EUV lithography need to generate extremely high power in the spectrum band of 13.5±0.135nm. A model was developed to investigate the wavelength-integrated Lyman-α lines light outputs in hydrogen-like lithium ion. The analysis reveals that the commonly observed low conversion efficiency is largely due to a transient nature of Z discharge-based plasma and that a magnetically confined lithium plasma is an efficient EUV source even at low electron temperature. Calculation shows necessary confinement time that raises the conversion efficiency up to half the spectral efficiency.

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.