N. H. Burnett

Harmonic generation in CO2 laser target interaction

We report the observation of an extended series of integral harmonic lines in the spectrum of direct backscatter of 10.6‐μm radiation incident at intensities ≳1014 W/cm2 onto planar solid targets. We have observed and spectrally resolved up to the eleventh harmonic (0.95 μm) at intensities well above the plasma continuum background.

Harmonic generation in CO/sub 2/ laser target interaction

We report the observation of an extended series of integral harmonic lines in the spectrum of direct backscatter of 10.6‐μm radiation incident at intensities ≳1014 W/cm2 onto planar solid targets. We have observed and spectrally resolved up to the eleventh harmonic (0.95 μm) at intensities well above the plasma continuum background.

Refraction effects associated with multiphoton ionization and ultrashort-pulse laser propagation in plasma waveguides

The problem of refraction of ultrashort, high-intensity laser pulses in underdense plasmas is discussed in the context of producing long plasma filaments by high f-number axial focusing of laser beams. For uniform gas targets, it is shown that refraction can clamp the intensity of the focused laser beam at a value near the threshold for multiphoton ionization. In order to make large volumes of underdense plasma with ultrashort pulses, it is necessary to preform a partially ionized filament with a suitable radial electron density profile. Such a structure can act as an indestructible waveguide for ultraintense, subpicosecond laser pulses. Indeed, highly ionized plasma represents the only practical optical material at wavelengths below 50 nm or at intensities above 10(13) W cm(-2).

Return‐current electron streams in high‐intensity laser target interaction

X‐ray streak photography of the interaction of a nanosecond CO2 laser pulse with large‐diameter foil targets shows the presence of an annular x‐ray emitting region which expands away from the focal spot with a veclocity up to 109 cm/sec. This region exhibits laser‐polarization‐dependent asymmetry, and it is postulated that it is formed by a return current of fast electrons bombarding the front of the target foil. It appears likely that this phenomenon is responsible in part for anomalous lateral energy transport and magnetic field generation in such experiments.

Backscatter and beam refraction in underdense gas breakdown with nanosecond CO2 laser

A 50‐J 2‐ns FWHM CO2 laser pulse focused to intensities in excess of 1014 W/cm2 has been used to induce breakdown in underdense He gas. The laser radiation scattered from these plasmas has been temporally and spectrally analyzed. It is observed that ion wave scattering can result in reflectivities in excess of 50% of the incident intensity. At filling pressures above 30 Torr the incident laser radiation is explosively refracted around the focal region by plasma formed a few mm upstream from focus thus quenching the high‐intensity laser plasma interaction less than 1 ns subsequent to breakdown. A weak but well‐defined line at 7.06 μm (ω=3/2 ω0) was observed in the backscatter at filling pressures above 40 Torr.

Passive image dissector for time‐resolved ir spectroscopy

A passive image dissection technique is described which is capable of switching adjustable slices of a linear ir image onto a single‐element detector with a fixed time delay between successive samples. The device allows good spatial resolution with state‐of‐the‐art detectivity and temporal resolution. The dissector has been employed in combination with a grating spectrograph to study the spectrum of backscattered radiation from CO2 laser‐produced plasmas, with 25 spectral channels and grating‐limited spectral channel width.

X‐ray emission characteristics of plasmas created by a high‐intensity CO2 laser

The spectrum of the x‐ray continuum and line emission emanating from Al, Mg, and (CH2)n plasmas created with a nanosecond CO2 laser pulse has been investigated at irradiance levels up to 2×1014 W/cm2.

Spark gap optical isolator for high power infrared lasers

A zero insertion loss optical isolator has been developed and tested in a short pulse CO2 laser oscillator–amplifier system. The device, which uses a narrow plasma column produced by a streamer discharge in air and situated at the common focus of two cylindrical mirrors, provides effective isolation of the oscillator and pulse forming system against retroreflected laser light. Using this arrangement a reflected 1 nsec CO2 laser pulse of ∼2 cm diam and energy density up to 1 J/cm2 has been completely blocked after an optical delay as short as 4 nsec. The device should also find application in other infrared laser systems for which no isolators at present exist.

Superthermal x‐ray emission from CO2‐laser‐produced plasmas

The high‐energy continuum x‐ray emission from plasma created by intense (1014 W cm−2) nanosecond 10‐μm laser pulses has been characterized. The temperature of the superthermal electron component deduced from this emission was found to be strongly dependent on focus position, while displaying a weaker dependence on irradiation angle, beam polarization, and target composition. The variation of the hot‐electron temperature as a function of Iλ2 has been examined in detail in a range of Iλ2 from 1015 to 2×1016 W μm2 cm−2 for various target materials and has been found to be in qualitative agreement with the predictions of current theories of hot‐electron production based on resonance absorption.

Compact 0.1 TW CO2 laser system

A 3 atm, 9×9×100 cm3 uv preionized self‐sustained discharge module has been used as the basis for a compact nanosecond pulse CO2 laser system. Up to 60 J has been extracted from the device in an ∼600 ps pulse. The high degree of gain uniformity and fast gain risetime demonstrated by the present device point to some potential advantages in the incorporation of large aperture uv preionized self‐sustained CO2 discharges into high irradiance target interaction laser systems.