W. Tighe

Harmonic generation from ions in underdense aluminum and lithium–fluorine plasmas

High harmonic generation from laser interactions with preformed underdense plasmas was observed with subpicosecond ultraviolet laser pulses focused to intensities up to 1018 W/cm2. The generation of seventh and ninth harmonics from aluminum plasmas was measured as well as harmonics to the nineteenth order from lithium–fluorine plasmas. The harmonic generation efficiency measured in these experiments is less than that from previous experiments that used neutral gases.

X-ray laser studies of recombining lithium plasmas created by optical field ionization

A high-intensity short-pulse KrF laser system is used to produce fast recombining plasmas by ionization of preformed lithium fluorine plasma targets. Soft-x-ray emission from these plasmas is measured, and population inversions in hydrogenic lithium are observed as the plasma recombines. For relatively low-temperature target plasma columns a nonlinear increase in the intensity of the 2–1 transition in Li III (135 A) is observed with respect to length.

Soft-x-ray spectroscopy of subpicosecond laser-produced plasmas

Experiments that examine the interaction of high-intensity subpicosecond laser radiation (∼1018 W/cm2) with solid targets are performed. Soft-x-ray spectra from interactions with Teflon and aluminum targets that imply the creation of high-temperature (>200 eV) high-density (>1023 cm−3) plasmas are obtained. Effects of a prepulse on the plasma characteristics are examined. The applicability of short, intense laser pulses for the formation of population inversions on x-ray transitions during recombination of high-density plasmas is discussed.

Powerful subpicosecond KrF laser for x-ray laser development in the 1-5-nm region

A high power, 300 fsec pulse duration, ultraviolet KrF laser system has been developed as a pump laser for short wavelength X-ray lasers. Additions and improvements to the laser system will be described. Attempts to reduce the effect of amplified spontaneous emission (ASE) through the use of spatial filters, saturable absorbers and target design will be discussed. Improvements to the optical system have been made in order to provide a 3 jtm focal spot size with care being taken not to introduce significant broadening of the pulse duration. It is estimated that focal spot intensities in excess of 1018 W/cm2 have been obtained on target. Soft X-ray spectra resulting from various laser-target experiments will be presented. Theoretical schemes for the development of X-ray lasing in the wavelength region of 1-5 nm will be discussed.

Toward small-scale x-ray lasers (Invited Paper)

An overview of the X-ray Laser project at Princeton University will be given. Emphasis will be on improvements being made to the small scale soft x-ray laser (SXL). New target designs to enhance cooling and others to reduce losses due to beam refraction have been introduced though results are stilt preliminary. Proof of principle experiments for the application of the SXL to both transmission and reflection microscopy have been performed. To generate shorter wavelength x-ray lasers on a reasonable scale-size, a high power, 300 fsec pulse duration, ultraviolet KrF laser system (the PSP-laser) has been developed. Of the several theoretical schemes which exist, the two-laser approach and a newly proposed recombination approach will be described. The new approach proposes to scale the existing 18.2 nm recombination x-ray laser to shorter wavelength (<4 nm) by making use of the short-pulse pump laser and rapid cooling associated with the adiabatic expansion of µsphere targets.

The design, construction and testing of the X-ray laser project krypton-fluoride laser amplifier

A large, UV preionized excimer krypton-fluoride (KrF) laser amplifier was designed and fabricated. This laser amplifier produced an on-target laser power density of 2*10/sup 18/ W/cm/sup 2/. Maximum output energy was determined to be 5 J in a 20-ns-long pulse, and maximum instantaneous power was achieved in a 250-350-mJ, 300-fs pulse with an injection (seed) pulse of approximately=3-4 mJ in a triple pass configuration. The output wavelength is 248 nm. The amplifier has a volume of approximately 5 liters. UV preionization was accomplished with a traveling spark board. Fast, low inductance switching of the high voltage was accomplished through the use of an electrically triggered rail-gap. Some of the design and tradeoffs of the electrical pulse discharge circuitry, considerations related to the materials used, the optics, gas handling, timing requirements, and some experimental results are discussed.<<ETX>>

Spectra from plasma created by very high intensity picosecond laser

Abstract The Powerful Picosecond Laser (PP-Laser) System with an output power level of 20–30 GW and focused power density of 10 16 W/cm 2 has been operating as a part of the X-ray laser program at Princeton University for more than 2 yr (presently the PP-Laser System is producing power density which is larger by almost two orders of magnitude). Such a laser system creates an electric field which is greater than the Coulomb field in atoms. This may lead to a number of new physical processes like multiphoton excitation and ionization in a very strong field. Several spectra from the plasma created by the PP-Laser interacting with different solid targets are presented. Spectra were obtained in the range of 6–300 A using the high resolution NRL-G SFC 3m XUV spectrograph. Very large broadenings of the lines from highly ionized ions were observed. Also presented are spectra recorded in a longer spectral range (up to visible), and some of the spectral line anomalies are discussed.

X-Ray Laser Microscopy

This paper describes work at Princeton on X-ray laser microscopy. Related to this work is a new system for the development of an X-ray laser in the wavelength region from 5 nm to 1 nm utilizing a Powerful Sub-Picosecond Laser (PP-Laser) of expected peak power up to 0.5 TW in a 300 fs pulse. Soft X-ray spectra generated by the interaction of the PP-Laser beam with different targets are presented and compared to the spectra generated by a much less intense laser beam (20-30 GW). The development of additional amplifiers for the recombining soft X-ray laser and the design of a cavity are presented from the point of view of applications for X-ray microscopy and microlithography.

Two‐laser approach to X‐ray lasers

Progress in X‐ray laser studies at Princeton University’s Plasma Physics Laboratory (PPPL) is proceeding along many fronts. Along with finding applications for the recombination soft X‐ray laser operating at 18.2 nm, there are intense efforts to generate shorter wavelength output and higher energy output. There are multi‐laser approaches involved in attaining both of these goals but here we will primarily discuss the effort to develop shorter wavelength X‐ray lasing by using two pump lasers. The motivation for this approach will be presented along with a description of the status of the experiment and initial spectra obtained from preliminary solid‐target interaction experiments.

Emission Of Lithiumlike Transitions From Dense Plasmas Created By Picosecond KrF Laser Pulses

Broad spectral lines have been observed in the high-resolution extreme ultraviolet (XUV) spectra from plasmas created by irradiating solid targets using picosecond laser pulses. Transitions of the type n=2-3 and n=2-4 in Li-like ions of the elements N, 0, F, and Al have linewidths up to 2 Å. Using quasistatic ion Stark broadening, the electron densities of the plasma emission regions have been derived from the linewidths. The derived electron densities are found to be inversely proportional to the lifetimes of the upper levels of the transitions. The electron densities range from 2 x 1022 cm-3 for levels with lifetimes less than 4 picoseconds to 1021 cm-3 for levels with lifetimes greater than 30 picoseconds. In effect, the XUV emission is time-resolved in the expanding plasma. The observation of emission from solid-density regions (greater than 4 x 1023 cm-3) will require transitions from levels in highly charged ions with lifetimes less than a picosecond.