G. Mehlman

Effect of pulse sharpening on imploding neon Z‐pinch plasmas

The radial implosion of hollow, cylindrical neon gas columns, driven by currents of up to 1.45 MA, produces a linear Z pinch with over 70% of the radiation in neon K lines. A plasma erosion opening switch (PEOS) is used to eliminate prepulse and to reduce the current rise time from ∼60 to ∼20 ns. Incorporation of the PEOS improves the uniformity of the Z pinch and increases the radiation yield.

Quantitative x‐ray emission from a DPF device

The x‐ray emission was measured from a Dense Plasma Focus (DPF) device. The high density plasma is generated by an electrical discharge in rarefied‐neon gas between electrodes in a Mather‐type plasma focus configuration. A curved‐crystal x‐ray spectrograph, a pinhole camera, and an active‐filtered photodiode were the diagnostics viewing the axial output of the pinched‐plasma region. The x‐ray pinhole images indicate a pinched volume roughly 8 mm in length with a nearly circular cross section of about 300 μm in diameter. The digitized spectral traces were computer processed to obtain absolute x‐ray line intensities. The neon plasma yielded 10–15 J of K‐shell radiation into 4π with the hydrogenlike and heliumlike alpha lines totaling 55%–65% of the total spectral emission. The x‐ray emission of the DPF device was studied as a function of discharge current and anode diameter.

Vacuum ultraviolet spectroscopy study of excimer‐laser‐generated plasmas

The dispersed emission in the vacuum ultraviolet (VUV) (1200–3000 A) from the plasma generated by the interaction of a KrF excimer laser with an Al and a YBa2Cu3O7 target has been measured. Emission spectra were collected as a function of distance above the target surface and as a function of laser fluence. The qualitative features of the plasmas from the two different targets were similar. The character of the emission spectra changed from a pseudocontinuous emission at the target surface to discrete emission from singly, doubly ionized species as well as neutrals at distances greater than ∼1.5 mm. The spatial variation indicated two regions: a high‐density sheath along the target where the core emission is close to blackbody; and beyond, a plasma with large opacity emitting a UV spectrum of intensity decreasing fast with distance. Estimates of the plasma temperature and density were between 2 and 4 eV and ∼1018/cm3, respectively.

Quantitative x‐ray spectroscopy of neon Z‐pinch plasmas

Spatially resolved soft x‐ray spectra were collected for neon plasmas produced by imploding hollow annular gas puffs with MA level driving currents. The Z‐pinch imploded plasmas were studied for different risetime currents produced with or without the use of a plasma erosion opening switch (PEOS). Selected spectrograms were processed and analyzed to obtain absolute energies for the radiation emitted in the Ne ix and Ne x discrete transitions, as well as for total emission over the spectral range 900–1600 eV. The neon plasmas radiate 1–2.5 kJ in this energy range predominantly in the α transitions of both ions. The plasma uniformity inferred from spatially resolved spectral lines improves significantly using a faster rise‐time driving current.

K-shell x-ray yield scaling for aluminum x-pinch plasmas

We report results from experiments performed to measure and characterize the intense K‐shell radiation from aluminum x‐pinch plasmas at peak driving currents ranging from 280 kA to 1.0 MA. Single pulse aluminum K‐shell (predominantly line radiation at 1.6–2 keV) x‐ray yields ranged from 7.6 J at 290 kA to 240 J at 1.0 MA. In the range from 280 to 470 kA, the yield scales with current to the power of 3.6, whereas nonoptimized K‐shell yields at 800 kA and 1.0 MA indicate a power of about 3 or higher.

Implosion of sodium‐bearing capillary‐discharge plasmas for x‐ray laser experiments

A NaF plasma from a capillary discharge has been imploded with a 1.2‐MA driving current to produce an intense source of sodium K‐shell x rays. A peak power of 25 GW in a 20‐ns pulse was measured for the sodium He‐α line which can be used as the pump radiation for a Na/Ne XUV laser scheme.

CHARACTERIZATION OF ELECTRON CYCLOTRON RESONANCE PLASMAS BY VACUUM ULTRAVIOLET SPECTROSCOPY

We have recorded, in the energy range of ∼4–11 eV, radiation from electron cyclotron resonance microwave plasmas generated when coupling microwave energy in a number of gases and gas mixtures appropriate for plasma deposition or etching. Molecular, atomic, and ionic emissions are observed in this range attesting to the major collisional processes (dissociation, excitation, and ionization) with hot electrons. The neutral atom temperature has been measured by standard spectroscopic techniques and found to be in the range of 0.1–0.3 eV, much lower than the electron temperature of the same plasmas. The main discharge parameters (i.e., gas pressure, flow rate, microwave power) have been varied to interpret their influence on the plasma radiation in terms of plasma parameters.

EARLY DYNAMICS OF LASER-ABLATED YBA2CU3O7 PLASMAS FROM THEIR VACUUM ULTRAVIOLET RADIATION TIME HISTORY

Simultaneous time and space resolution of the monochromatic vacuum ultraviolet emission from an excimer laser‐ablated plasma has yielded quantitative information on the plasma generation and velocity. From the time‐resolved spectral data, plasma velocities were measured and found to be ∼2.5×106 cm/s. The observed radiation time profile at the target surface reflects essentially the dynamics of the laser pulse absorption. Material continues to be ejected from the target for more than 100 ns after the end of the laser pulse.

Techniques for soft x‐ray absorption in laser‐produced plasmas

Interest in x‐ray absorption derives from soft x‐ray transport studies to characterize laser‐solid‐interaction effects. The object of this work was to determine whether x‐ray absorption features were measurable in laser‐produced plasmas from solid targets. X‐ray emission was made using high‐atomic‐number microsphere targets irradiated with a focused beam of the Pharos laser while the laser‐solid‐interaction plasmas were generated by a second beam in Al foil targets. High‐resolution, spatially resolved x‐ray spectra were collected with instrumentation and geometry similar to previous tracer‐dot spectroscopy measurements of plasma profile parameters. Soft x‐ray absorption lines, observed at different distances from the aluminum target surface, may be related to density gradients in the laser‐generated plasmas.

Soft X-ray continuum from laser-irradiated high-Z targets

Abstract X-ray data were acquired from intermediate- and high-Z element targets with the Pharos III laser system. Targets, mounted at the tip of thin glass stalks, were microscopic fragments or foils of single or multiple high-Z element composition. The laser irradiance was 4 × 1014W/cm2 with about 300J of a focused, 1.05 μm wavelenght laser beam. A convex KAP crystal spectrograph was used to collect high-resolution spectra in the 4–20 A soft X-ray region. The recorded spectral film densities were microdensitometered and converted by computer-processing to absolute continuum intensities. The spectral features were identified with the aid of ab initio atomic structure calculations. The continuum background was evaluated as a source of pseudo-continua for absorption studies of soft X-rays.